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Guru Nanak In Iraq
Thread starter Dalvinder Singh Grewal
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baghdad guru nanak iraq pir dastgir
Dalvinder Singh Grewal
SPNer
Dr Dalvinder Singh Grewal
The city of Baghdad (33° 20'N, 44° 30'E), is the capital of the Republic of Iraq which has remained a great centre of learning in the History of Islamic world. The old name of this ancient region is Mesopotamia located in south-west Asia with two main rivers flowing by known as Tigris (Dajla) and Euphrates (Furat). Al-Mustamik of Cairo ruled from 902-914 Hijri (1497-1508 AD) and Al-Mutawakkil III ruled from 914-923 Hijri (1508-1517 AD). A historical Sikh shrine dedicated to Guru Nanak in the graveyard of Karkh area of Baghdad existed till recently when it was destroyed during the period of attack by US Forces on Iraq which the Iraq Government has promised to reconstruct it recently.
Guru Nanak arrived in Baghdad and along with Mardana and he stayed towards east of the city down a hill in a garden where existed the place of Wali Hind. The Caliph of Baghdad then was Bakar, the great grandson of Caliph Walid. He troubled saints and faqirs as well as his people. He kept many saints to show miracles.
While one proceeds from main Baghdad city on Shar-e-Shaikh Omer about one kilometer away there is a graveyard on the right hand side where the holy tomb of Hazrat Imam Ghazali (R.A.), the holy Saint Hazrat Ghaus-ul-Azam (R.A.). Near the holy shrine of Hazrat Ghaus Pak (R.A.) is the eternal resting place of his family members. Next is Al-Qadereyya Shrine. It was originally a religious school built by the Hanbali scholar and pious man Sheikh Abu Said Al-Mubarak bin Ali Al-Mukharrami (died 1119 AD - 513 AH), later improved and enlarged by his pupil Sheikh Abdul Qadir Al-Gilani also known as Pir Dastgir, where he lived, contemplated, and taught until his death in 1165 AD. A huge dome was constructed over the indoor praying section of the mosque by the Ottoman Sultan Suleiman the Magnificent on the mausoleum and the Mosque of Sheikh Abdul Qader Al-Gilani (famous as Pir Dastgeer) in 1534 AD (941 AH). This shrine owns a library of 35,000 volumes, including a fine collection of Holy Qu'rans.
Further up on the Shaikh Umar highway is the Grand Palace of the famous Caliph Harunul-Rashid. Further ahead on the road is a major crossing known as Bab-ul-Moazzam (Old centre of Baghdad city). From here one road leads to Azamiya and another to Al-Kirkh with eight bridge-crossings in between. Al Karakh is the area visited by Guru Nanak
Going on from central Baghdad to Mansura, there is a big Museum near the International Railway Station, built upon the location where the famous King Shaddad had built heaven and hell on earth. Nearby is the tomb of Zubaida Khatoon, the wife of Abbasi King Harun-ul Rashid. The holy shrines of Hazrat Maroof Kirkhee (R.A.), Hazrat Usha (A.S.), Hazrat Junaid Baghdadi (R.A.), Hazrat Sirri Saqti (R.A.), Hazrat Bahlol Dana (R.A.), Hazrat Zun-nun Misri (R.A.), Hazrat Ibrahim (R.A.) and Hazrat Shah Mansoor Khawas (R.A.). Near the old holy graveyard of ancient times, there is the sitting-place of Guru Nanak, the founder of the Sikhism [1]
On reaching Baghdad, Guru Nanak Dev sat down in a garden under the hill towards east of Baghdad whir platform of Wali Hind was erected. Guru Nanak sang a hymn in Sri Raag : Sun(i) man mitar piaria mil wela he eh. Jab lag joban sas hai tab lag(u) ih(u) tan(u) deh. (SGGS p. 20;11)
Listen, O my mind, my friend, my darling: now is the time to meet the Lord. As long as there is youth and breath, give this body to Him. Without virtue, it is useless; the body shall crumble into a pile of dust. O my mind, earn the profit, before you return home. The Gurmukh praises the Naam, and the fire of egotism is extinguished. Again and again, we hear and tell stories; we read and write and understand loads of knowledge, but still, desires increase day and night, and the disease of egotism fills us with corruption. That Carefree Lord cannot be appraised; His Real Value is known only through the Wisdom of the Guru's Teachings. Even if someone has hundreds of thousands of clever mental tricks, and the love and company of hundreds of thousands of people—still, without the Saadh Sangat, the Company of the Holy, he will not feel satisfied. Without the Name, all suffer in sorrow. Chanting the Name of the Lord, O my soul, you shall be emancipated; as Gurmukh, you shall come to understand your own self. I have sold my body and mind to the Guru, and I have given my mind and head as well. I was seeking and searching for Him throughout the three worlds; then, as Gurmukh, I sought and found Him. The True Guru has united me in Union, O Nanak, with that God. (SGGS: p.20)
The people who heard the singing reported the matter to Caliph Bakar the grandson of Caliph Walid. He was a tyrant who had troubled his people especially the fakirs. Every fakir was supposed to show miracles. He had many fakirs got tortured as they failed to show miracles. As his follower Abdul Rehman heard the hymn singing, he said, "This is not the Azan, but some other call. I will send someone to check who is giving this call and why?" Abdul Rehman sent his disciple Abdul Wajid to check. Abdul Wajid saw the Guru singing the hymn along with rebec being played by Mardana. He was so mesmerized that he forgot questioning Guru Nanak. Instead he got lost in the lilting song and started singing himself. The Pir sent more men but whosoever came joined Guru Nanak's singing of hymn. As the singing stopped Abdul Wajid said "In Islam singing and music are banned." Guru Nanak said, Mohammad Sahib took Bibi Aisha to the camp of Bloch to listen to their songs." Abdul Wajid asked. "Whom do you worship?' Guru Nanak said, "Only One God! Who is the Greatest of all". Abdul Wajid retorted, "Do you mean 'Allah hu Akbar?'
Guru Nanak said: "Yes! 'Allah hu Akbar' means 'God the Greatest'. I say the same in my own language and words 'Ik Ongkar, Sat(i)nam(u), Karata Purukh(u), Nirbhau Nnirvair(u) Akaal Moort(i) Ajoonee Saibhang, Gurparsad(i)' meaning "There is Only One God of the Entire Universe." You consider it 'Tawhid'[2]Quran, (Sura 112 (Al-Ikhlas), ayat 1-2) says: "He is God, the One and Only God, the Eternal, Absolute"
"But you said something more than this also?" Wazid questioned further. To this, Guru Nanak replied: "Yes, I said Sat(i) Nam: The One whose Name is God is the ultimate Truth. Quran mentions it as Al-Haqq (Ayats 6:62, 22:6, 23:116, 24:25.) The Truth: The Reality as God. Further I said 'Karta Purukh' meaning This Great person is the Creator of the entire universe.Al-Khāliq, (Sura 6:102, 13:16), [14] (36:81, 39:62, 40:62, 59:24) Quran (Sura 35 (Fatir) ayat 1) says: "Praise is to God, Creator of the heavens and the earth, [who] made the angels messengers having wings, two or three or four. He increases in creation what He wills. Indeed, Allah is the only competent over all the things of universe". Then I said: 'Nirbhau, Nirvair': meaning: 'He is fearless and has no enemies': and 'Akaal Moort(i), Ajooni Saibhang' meaning 'The God is beyond Time: He is not into birth and death as He is Self-Created". Quran, (Sura 112 (Al-Ikhlas), ayat 3) [27]says: 'He begetteth not, nor is He begotten.' The last I said was 'Gurparsad (i)' meaning He can be reached by the Grace of the Guide/Guru/God.' Quran mentions of 'Al-hadi', 'Ar-Rashīd', (Quran 2:256, 72:10). Quran (sura 35(Fatir) ayat 1) says [34] 'God showers His grace upon the people ...' (2:244, 251).
When the meanings and their link to Quran was explained Abdul Wajid said: "Great: You are a real philosopher of Islam. Which lineage of religion do you belong to?" Guru Nanak replied, "I do not follow any other religion or lineage than that of God." Abdul Wajid was surprised. He said, "It appears that you are not a Muslim? If you are not a Muslim, teaching other than Islam will land you in to trouble. Someone may even order your killing". Guru Nanak sang: Ek tui, Ek tui (pp.143-144)
'All the spiritual teachers, their disciples and the rulers of the world shall be buried under the ground. The emperors shall also pass away; God alone is Eternal. You alone, Lord, You alone (shall remain forever). Neither the angels, nor the demons, nor human beings, nor the Siddhas, nor the seekers shall remain on the earth. Who else is there? You alone, O Lord, You alone (shall remain forever). Neither the just, nor the generous, nor any humans at all, nor the seven realms beneath the earth, shall remain. The One Lord alone exists. Who else is there? You alone, O Lord, You alone (shall remain forever). Neither the sun, nor the moon, nor the planets, nor the seven continents, nor the oceans, nor food, nor the wind—nothing is permanent. You alone, O Lord, You alone. Our sustenance is not in the hands of any person. The hopes of all rest in the One Lord. The One Lord alone exists—who else is there? You alone, O Lord, You alone (shall remain forever). The birds have no money in their pockets. They place their hopes on trees and water. He alone is the Giver. You alone, Lord, You alone (shall remain forever). Nanak says that the destiny which is pre-ordained and written on one's forehead—no one can erase it. The Lord infuses strength, and He takes it away again. You alone, O Lord, You alone (shall remain forever).' (pp. 143-144).
Meanwhile the people had gathered at the strange hymn and thinking it to be against Islam, they were ready to stone the Guru and his companion Mardana. Hearing such proclamations of God's supremacy and the Lord's praises, the crowd was sobered as if under a hypnotic spell, and the stones fell off their hands. As Abdul Wajid did not return and public gathered to listen to the discussion, Caliph Bakar and Abdul Rehman came to the place. As they listened to the hymn themselves, they too were mesmerized. They understood that he is none other than Guru Nanak whose discussions had impressed even the king Selim at Medina.
Who was Caliph Bakar? If we go through the details of Caliphs during Guru Nanak's period it was Abbasid Dynasty of Egypt (A.H. 659-923/ AD 1261-1517) which also ruled Baghdad who had declared themselves as Caliphs. Following were the Caliphs during this period: (a) al-Mustasmik (Ist reign) A.H. 903-914/A.D. 1497-1508, (b) al-MutaWakkil III (Ist reign) A.H. 914-922/ A.D 1508-1517, (c) al- Mustasmik (2nd reign) A.H. 922-923 /A.D. 1516-1517, (d) al-MutaWakkil III (2nd reign) A.H. 923/ A.D 1517. Ottoman Dynasty won over Egypt and Iraq and had the following rulers who too declared themselves as Caliph: (a) Bayezid II A.H. 886-918 /A.D. 1481-1512 (b) Selim I, Yavuz A.H. 918-926/ A.D. 1512-1520 (c) Suleiman I, Kanuni 926-974/A.D. 1520-1566.[3] Aq Quyunly Dynasty A.H. 780-914/A.D. 1378-1508 also had influence in Iraq especially in Baghdad for a short duration; none of them however was Bakar. It is most likely that the person was Sheikhof the Qadiriyya who was appointed chief Sufi of Baghdad. Caliph was Selim I whom Guru Nanak must have met him in Baghdad as well.
This event has been described by Bhai Gurdas in his ballad Vaar 1 Paudi 35.
'Next (from Mecca) Baba went to Baghdad and stayed outside the city. Baba himself was in the form of Timeless and secondly, he had his companion Mardana, the rebec player. While praying, Baba gave out call to the Divine (like in Namaz). Listening to this the people around went into absolute silence. The whole city became quiet. The Peer also got wonderstruck. Observing minutely he found (in the form of Baba Nanak) an exhilarated saint. Peer Dastgeer asked him, which category of saints you belong to and what is your lineage. (Mardana told) He is Nanak, who has come into this Dark Age (Kaliyug); he recognises God and sainthood as one. He is known in all the directions besides earth and sky.'
Bhai Gurdas has quoted Pir dastgir who met Guru Nanak at Baghdad. [4] The grave of Pir Hazrat Dastgir Abdul Qadir Jilani is exiting in Baghdad, but it was his heir who too was called Dastgeer.
Vaaran of Bhai Gurdas is accepted as the key to understand Gurbani by the majority of the Sikh theologians, scholars and researchers. The Dharam Parchar Committee of the Shiromani Gurdwara Parbandhak Committee (SGPC) has recommended preaching from Vaaran of Bhai Gurdas in the Gurdwaras [Reht Maryada]. Many Sikh scholars think that no article/paper on Sikhi (Sikhism) is considered complete unless some Pauris are not quoted from Vaaran of Bhai Gurdas. In view of this we must examine as to who was this Pir Dastgeer?
History records one Pir Hazrat Dastgir Abdul Qadir Jilani, who flourished at around 1075--1167 CE. Hazrat Dastgir Abdul Qadir Jilani (1075--1167 CE) is the founder of the Qadiriyya sect. Qadir Jilani, was a respected scholar and preacher. [5]Having been a pupil at the school (madrass) of Abu Sa'id al-Mubarakhe became leader of this school after his death in 1119 CE. Being the new Sheikhhe and his large family lived comfortably in themadrasauntil his death in 1166, when his son, Abdul Razzaq, succeeded his father as Sheikh. Gilani's son, Abdul Razzaq Jilani, published a hagiography of his father, emphasizing his reputation as founder of a distinct and prestigious Sufi order.[6] The mausoleum confirms his pre-existence. Since he was not alive then, there has to be some of his 'gaddinashin' as identifiying with the name of the elders have been a prevalent tradition globally.
Guniyate Salahine, [16]an autobiography by Pir Kadar Jilani details this incident. The disciples of the pir had issued the fatwa to kill Nanak. When the pir and his disciples met Nanak, the pir bowed in front of Nanak and said in Arabic:
"Eh Pire Dasatgir Tu Mara Bagir
Dastam Duna Bagir Ke Goind Dasatgir
Taksire Nafas Ma Ra Raza Buay Name Risad"
Meaning "My respected audience, do not be surprised that I am bowing before him. I am doing so after recognizing him. You too should bow and offer salam. He is not a mere pir, he is a prophet. If you see him through my eyes, he is the God himself."
Prof Espiritu [7] as quoted by Dr Chahal has also provided some more information about the use of word "Dastgir."
"Just 3 days ago, I was informed by a pious Naqshbandi Muslim who is living in Van, Eastern Turkey, by the name of Nevzat Varlik Vanli (via email) that the custodians (khidmatgar) of Qadri Sufi dargahs among Kurdish speaking areas in Van, Turkey are collectively called "Dastagir". In short, Dastagir also can mean Hazrat Abdul Qadir Jilani Sheikh Dastagir Piran Pir or the mutawwali (attendant) or khidmatgar (religious custodians) of any Qadri Sufi Dargahs (dargahs associated with Hazrat Jilani) in Kurdish areas around Southern Turkey (Anatolia) and in Kurdish areas in Iraq. So I feel from this input of Nevzat Varlik Vanli Hazretleri that Dastagir can mean also custodian of a shrine; that ][8]Guru Nanak Padshah may had been to Baghdad Sharif and had a conversation with a dastagir (custodian) of a Qadri dargah. There is one famous dargah of Baghdad Sharif--and that is the blessed tomb of Sheikh Abdul Qadir Jilani Dastagir Pir. So, I feel that there may be a possibilitythat Guru Nanak Padshah went to Baghdad Sharif; but he talked to the Dastagir custodian and not to Hazrat Dastagir Jilani himself . But then again his having been to Baghdad can be a possibility--but he talked to a Dastagir shrine custodian and not to Hazrat Dastagir Jilani himself. Dastagir thus can also mean the direct kith and kin of Hazrat Jilani who oftentimes are the custodians of the grave of Piran Pir in Baghdad Sharif."[7]
By the end of the fifteenth century the Qadiriyya had distinct branches and had spread to Morocco, Spain,Turkey, India, Ethiopia, Somalia and present-day Mali. [9][4]Established Sufi sheikhs often adopted the Qadiriyya tradition without abandoning leadership of their local communities. During the Safavidrule of Baghdad, from 1508 to 1534, the Sheikhof the Qadiriyya was appointed chief Sufi of Baghdad and the surrounding lands. Shortly after, the Ottoman Turksconquered Baghdad in 1534, Suleiman the Magnificent commissioned a dome to be built on the tomb of Jilani, establishing the Qadiriyya as his main allies in Iraq. Sultan Bahoo, a Sufi saint, contributed to the spread of Qadiriyya in western India. [2] Bhai Kahn Singh [3] also mentions that Pir Dastgir Abdul Qadir of Jilan City breathed his last on February 22, 1166 CE. He explained the meaning of 'Dastgir' as the one who holds the hands of others, helpful and supporter. Giani Gian Singh mentions him to be Juljalali son of Pir Jalal in the lineage of Pir Dastgir. [4] [8]
Guru Nanak and Pir Dastgir at Baghdad
Guru Nanak Dev Ji came to the outskirts of the city of Baghdad. It was early morning. The cool breeze was blowing. The sky was blue and the sun was shining. Many people were passing by. For them it was the time for early Naamaz (Muslim Pray). Guru Ji started singing Divine Hymns, with Mardana playing the rebec. When some Muslim who was listening to it and understood its purport went and reported the blasphemy of the utterance - inasmuch as the Qu'ran had mentioned seven earths and seven skies only - to the Sajjdanashin of the Shrine of Pir Dastagir, Abdul Qadir Jilani. An agitated crowd gathered on the spot. The people were on the verge of throwing stones when they heard the Guru's divine invocation. The people in remorse went to their Pir and told him of the presence of a holy man.
Guruji then called "Satnam" loudly in such a style that listening to which the whole city went into absolute silence in astonishment. Guru Ji's voice, full of Divine Melody, drew the attention of surrounding people of Bagdad.
Bhai Gurdas Ji said… It was a pure Divine bliss that calmed people's mind and soul. They were absorbed within their own consciousness. The news spread in the city like fire in the forest. Now where there I s Amrit (ambrosial nectar), also present is the poison. This is the truth. [10]
Gurbani says: Poison and nectar dwell together. Sandalwood trees are beautiful but snakes full of poison encircle them too.
Long discussions with the heir apparent of Pir Dastgir with Guru Nanak were in the form of question and answers. He was so satisfied that tears flowed from his eyes. Guru Ji's messages saturated his heart with Divine feelings and he fell at Guruji's feet. He requested Guru Nanak to occupy the high chair on which he himself sat. This chair was called "Amar Singhasan" and is stated to be situated in Bagdad. Following label was affixed in the chair
"Rabul MaJid Hazrat Baba Nanak"
Its translation: The Blessed Messenger of God, Hazarat Baba Nanak
In Baghdad Guru Nanak stayed at a graveyard outside the town, and Mardana as usual was with him. Early in the morning before dawn, Mardana played the rebec and the Master recited hymns about the infiniteness of God and His Creation wherein occurred the expression: there are numerous patals (earths) and innumerable akashs (sky), when some Muslim who was listening to it and understood its purport went and reported the blasphemy of the utterance, inasmuch as the Qu'ran had mentioned seven earths and seven skies only, to the Sajjdanashin of the Shrine of Pir Dastagir Abdul Qadir Jilani.
The Pir ordered that the offender be stoned to death and numerous people armed themselves with stones and rushed out. In the meantime Guru Nanak concluded his morning prayers with the greeting "Sat Kartar", in a voice divinely electrifying, which immediately disarmed the mob.
On this, a pir [saint], Bahlol by name, came forward and had a discourse with Guru Nanak on the subject of the morning prayer. Accompanying the pir was his son and such was the effect of the Guru's answers that first the son and then the father confessed their conversion and admitted the truth of the maxim that creation was not confined to seven earths and seven skies only. Guru Nanak stayed in Baghdad for four months and had many discourses with other holy men. Before his departure from there he was presented a chola [robe] as a token of respect on which verses in Arabic are inscribed. This chola lies preserved in the Gurdwara at Dera Baba Nanak, in Pakistan. [11]
The Pir, having reached the place, enquired from Mardana who the holy man was. He was told that he was Nanak who had rejected all others except one God who was all pervading on earth, sky and in all four directions. Guru Nanak Dev Ji stayed in Baghdad for four months and had many discourses with other holy men.
The shrine of Guru Nanak is located in Karkh District in Sheikh Maaruf Neighbourhood. This shrine can be reached from a road that goes from Sheikh Maruf Cemetery towards 14 July Road. In the shrine, the following inscription is engraved: "Guru Nanak headed for Baghdad as a traveler, and there he took a house for himself at its gates." Karkh, or Al-Karkh, is historically the name of the western half of Baghdad in Iraq, or, alternatively, the western shore of the Tigris River as it runs through Baghdad. The eastern shore is known a Al-Rasafa.[12] Today, it is also a neighborhood between the international zone and the Tigris. Karkh is one of nine administrative districts in Baghdad.[12]
Map of Baghdad 1931 shows the location of Guru Nanak's shrine. It is one mile to the right of Tigris River and a mile and a half from Baghdad
Map of Baghdad in 1931 shows the location of Guru Nanak's shrine. It is one mile to the right of river Tigris and a mile-and-half from Baghdad Railway Station West, between two railway lines. To the northwest is an old and extensive graveyard, extending from the town of Zubaida Khatum. To its east stands a magnificent edifice (118 feet by 55 feet) commemorating the famous Jewish saint, Nabiullah Usha, and to the northeast is the shrine of Sheikh Ibrahim forming a square of 27 feet. It is believed that during his stay in Baghdad Guru Nanak had a large following including the successors of Sheikh Bahlol Dana (the Wise) and those of Sheikh Muhy-ud-din Abdul Qadir Jilani. [13]
A front view photograph of the shrine was taken in December 1931. On the platform where the Guru sat is a plaque in Arabic. The tomb of Bahlol is toward the west. Another tomb is in the center and the platform is 7 feet by 4 feet. In the mausoleum of Bahlol is a small rectangular garden (26 feet 5 inches by 12 feet 5 inches) in the center of the courtyard with a masonry pavement round it. There are a few tut (mulberry) and palm trees.
The text on a pillar on the shrine reads as under
Behold! How a wish has been fulfilled by Holy and High Providence. That the building of Baba Nanak has been newly built with the help of seven autat (great valis). That the happy murid of God (Baba Nanak) has started a fountain of grace issuing new water in the land. 917 Hijri.
The inscribed stone slab was found in 1931. It measures 21 feet 14 inches. Its inscription was slightly damaged during the collapse of the building after 1920. The date given as 917 is supported by the Abjad system. 917 Hijri is equivalent to 1511 A.D. and 1568 Sammat Bikarmi.
A memorial platform was raised on the spot where the Guru and his companion, Mardana, the Muslim bard, had stopped. A few years later, a room was constructed there and a stone slab with an inscription in Ottoman Turkish was installed in it. Guru Nanak held discourses with some local Sufi saints at this site. The plan of the Guru's shrine shows that it is situated within a walled square, with the gateway in the southeast corner. It measures 54 feet and 4 inches on the east and west; 54 feet, 6 inches on the north and 56 feet, 7 inches on the south. The platform on which the Guru sat is in the northeast corner with a plaque on which some words were written in Arabic. The tomb of Bahlol Dana stands to the west. Another tomb is in the center and the platform is about 7 feet by 4 feet to the east with an inscribed slab in the wall to the north about 4 feet above the ground. The slab made of sandstone, measures 21 inches by 15 inches.
During the World War I, when the British and Indian armies conquered Baghdad, they discovered the place where Guru Nanak had his discourse was held with Bahlol's descendant. It lies to the west of the town and between the old graveyard to the north and the present Baghdad-Samara railway line to the south. Dr Kirpal Singh, then a Captain in the Indian Medical Service, also saw it during the war, and he, in his letter, [12] dated October 15, 1918, described it as follows:
"It is really a humble looking building and known to very few people except Sikhs. To some Arabs it is known as well by the name of tomb of Bahlol. You enter the building by a small door, on which something is written in Arabic, not visible to a casual visitor. Even with attention it is difficult to read. I could not read it hence could not copy it. I have taken the photograph of the outside, which I shall forward to you in due course. Entering the building, you come to a brick paved passage going to your right straight into the room (with a veranda), wherein you find the tomb and the raised platform. In the courtyard there are a few trees, mostly pomegranates".
The room that has the tomb and the platform has two doors, one of which is open whilst the other is barred. As you enter the room, you come face to face with the platform, which is roughly 2 to 2.5 feet high and about 3' by 4' in dimensions. It is now covered with handkerchiefs of various colors presented by Sikhs. In the center close to the wall you find a picture of Sri Guru Nanak, presented by some energetic Sikh, above which you find the slab with the writing which I reproduced in this letter for you. The name of the man in charge is Sayed Yusuf.
The mutvali in charge of the shrine told Sardar Kartar Singh Kartar [13] who visited the shrine in 1931AD that he had an old Arabic manuscript containing an account of Guru Nanak's visit, but unfortunately it was stolen in 1920. In the same year, another stone inscription related to the Guru's visit was found in the wall to the east of Sheikh Abdul Qadir Jilani's shrine near Baghdad Railway Station East. It had been seen by many, but was missing in 1926, when the wall collapsed. It is possible that similar relics may be found there.
The Queen of Baghdad had no child. By the blessings of Guru Nanak, a son was born to the Queen and she offered a chola (robe) to Guru Nanak prepared by her, having the hymns of Quran on it. That chola is still lying in Gurdwara Chola Sahib at Dera Baba Nanak, Gurdaspur district, Punjab. [14] From Baghdad, Before his departure from there he was presented a chola (robe) as a token of respect on which verses in Arabic are inscribed. This chola lies preserved in the Gurdwara at Dera Baba Nanak, in Pakistan. This chola is sometimes sited by Muslims that Guru Nanak Dev Ji was somehow a follower of Islam. The chola was given to Guru Sahib Ji by his devotees in Baghdad and Guru Sahib Ji took it in the spirit in which it was given. Pir Bahlol, a Sufi saint was deeply influenced by Guru Nanak Dev and continued to live where the Guru had stayed.
People of Baghdad depend on river Tigris for water. Wells, in and around Baghdad are brackish. It is said that the Guru's disciples together with others who visited the takia complained to the Guru about the difficulty in procuring drinking water. Guru Nanak got a well dug in the southeast corner and it produced sweet water. Even now, it is the only well with sweet drinking water. Its diameter is about 21 feet and the date of its construction is 917 Hijri as given on the plaque. The well and the compound were reinforced in 1320 A.H. (1942 AD) by Qasim Pasha, Beg-Bashi, son of Mohammad Beg..The monument erected near this well makes a mention of the Guru.
Bahlol Dana's tomb and other buildings were in a sad state of disrepair, but in 1120 Hijri, Qazim Pasha practically rebuilt the tomb. The roof and verandah of Guru Nanak's shrine had also collapsed.
About the inscription on the stone slab, Sardar Kartar Singh Kartar, the late president of Central Sikh Committee, Baghdad wrote to S. Manjeet Singh, on 6 August 1932,
"Sikhs desired to rebuild [the shrine]. Once the Central Sikh Committee applied to the Auqaf Department for repair of the shrine but no reply was received. At last the Central Sikh Committee, Baghdad again sent an application to His Excellency the High Commissioner of Iraq in 1931, who, … has very kindly given sanction to carry out the repairs at the expense of the Committee."
Again on 1 February 1952, S. Kartar Singh wrote to S. Manjeet Singh: "The Sikh committee, Baghdad was very anxious to repair Guru Nanak Dev's shrine as early as possible, but it was not an easy job to approach the Iraqi Government for the purpose. All such buildings are under the control of Ministry of Auqaf whose sanction was absolutely necessary to carry out the necessary repairs. At last the Iraqi Government was kind enough to give the sanction during 1934 and repairs were done.
"In addition to the repairs of the existing building a room is also added in front of the room where Bahlol Dana's tomb exists. It is really gratifying to note that the Sikh community in Iraq has done a splendid job in repairing the shrine of their Guru who is well known among Arabs as Baba Nanak or Hindu Pir". In 1934 I was glad to receive the information that repairs to the Guru's shrine in Baghdad had been completed. On the 23rd of November, 1969, his followers celebrated his 500th birthday by erecting a memorial statue at his tomb.[14]
It is said, "Even today there are several disciples of Guru Nanak in Iraq. These people live on the banks of the TigrisRiver, particularly in the cities of Al Kutand Baghdad. They are called Sobiand generally they are gold-smiths by occupation. They are experts in their trade. They keep long hair and do not cut their beards and remember the Guru by names of Baba Nanak or Baba Nana."
[1] His Holiness Shaikh-Al-Mashaikh Syed Tahir Alauddin Gilani, 2014, Darbar Ghous ul Aalmeen, Peeran e Peer Hazrat Syed Abdul Qadir Gilani at Baghdad known as Darbar E Ghousia (Gilania), September 25, 2014 .
[2] Omer Tarin, Hazrat Ghaus e Azam Shaykh Abdul Qadir Jilani sahib, RA: Aqeedat o Salam, Urdu monograph, Lahore, 1996.
[3] Jonathan Neaman Lipman (1 July 1998).Faimiliar strangers: a historyof Muslims in Northwest China. University of Washington Press. pp. 88–. ISBN978-0-295-80055-4.
[4]Qadiriyya - Wikipedia
[5] Vaaran by Bhai Gurdas (In Punjabi), Giani Hazara Singh Pundit and Padam Bhushan Bhai Sahib Dr Vir Singh (Interpreters), 1984. Pub: Manager, Khalsa Samachaar, Hall Bazar, Amritsar
[5] Singh, Kahn (Bhai), 1981. Mahan Kosh (Punjabi), Bhasha Vibhag, Punjab, Patiala, page 831.
[6] Dr. Kirpal Singh
[7] Chahal, D. S. 2007, How Long was Guru Nanak's Travel Towards Middle East? Understanding Sikhism, Res. J. 9 (2): 34-36, 26, 2007.
[8] Shri Ananda Acharya, "On Reading an Arabic Inscription in a Shrine Outside the Town of Baghdad dated 912 Hijira," 'Snow Birds', Macmillan and Co., 1919 [8]
[9] Afridi, Shah Alam, 1922, Mazaraat-e-Auliya-e-Delhi, Delhi-6, India
[10] Mohammad Najib-ur-Rehman, Hazrat Sakhi Sultan (2012).Sawane Hayat Syed Abdullah Shah Madni Jilani: A biography of Syed Abdullah Shah Madni Jilani.Sultan-ul-Faqr Publications Regd.(Lahore, Pakistan).ISBN9789699795022.
[11] Devinder Singh Chahal, Dr. 2010, Did Guru Nanak Meet Pir Dastgir and Pir Bahlol Dana? Understanding Sikhism –Research Journal, January - December 2010, Vol. 12, No 1-2 page 49-51
[12] Singh, Trilochan, 1969, Guru Nanak: Founder of Sikhism. Gurdwara Parbandhak Committee, Sis Gang, Chandni Chowk, Delhi.
[13] The account of Guru Nanak's visit to Baghdad was taken from (a) The Divine Master, Lahore 1930 by Sewaram Singh, published in Punjab: Past and Present and Guru Nanak's Visit to Baghdad, by Manjeet Singh, The Sikh Review, Oct-Nov. 1969 -- Editor
(b) Guru Nanak And Pir Dasatgir at Baghdad | Discover Sikhism
(c)Nanak in Baghdad Tribune India, Friday, July 25, 2003, Chandigarh, India
(d) Twarikh Guru Khalsa of Giani Gian Singh
(e) http://www.sikhsangat.com/index.php?/topic/17645-guru-nanak-in-baghdad/
[14] Macauliffe, M.A (1909).The Sikh Religion: Its Gurus Sacred Writings and Authors.Low Price Publications.ISBN 8175361328.
[15] Singh, Manjeet (Oct-Nov 1969). "Past and Present and Guru Nanak's Visit to Baghdad".The Sikh Review': [16] "Gurudwara in Iraq damaged, Amarinder writes to US",[news.indiainfo.com], (April 22, 2003)
[17] "Gurdwara in Baghdad."New light on Guru Nanak's Visit to Baghdad (Prof.Kulraj Singh).Accessed onMay 22,2005.
[18] Bhai Bala (Sandhu) (2010).Sri Guru Nanak Dev ji Janamsakhi Pages: 671-675.CSJS- Chattar Singh Jeevan Singh.ISBN 81-7601-000-6.
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\section{Introduction}

A hot and dense state of matter, called quark gluon plasma (QGP),
is believed to have formed during the first few microseconds after the big bang. Its degrees of freedom above a critical temperature $T_c$ consist of deconfined quarks and gluons. The Relativistic Heavy Ion Collider (RHIC) was built to study the formation and properties of quark gluon plasma in collisions of heavy ions at very large energies. We have overwhelming evidence from the first few years of running of RHIC that the temperatures reached are well above $T_c$ and that the new matter is indeed partonic \cite{whitepaper:05}.

One of the key observations at RHIC is the suppression of hadrons with large
transverse momentum $p_T$ \cite{adcox,adler1}, most impressively manifested
in reduced nuclear modification factors $R_{AA} \approx 0.2$ for hadrons in
the most central collisions for $p_T > 5$ GeV/$c$. $R_{AA}$ measures the ratio
of yields in nuclear collisions ($A+A$) vs nucleon-nucleon collisions ($N+N$)
at the same
energy scaled with the appropriate number of binary collisions. High-$p_T$
hadrons above 5 GeV/$c$ in $A+A$ collisions are believed to come
predominantly from fragmentation off QCD jets. QCD jets originate from
high-$p_T$ partons created in the initial hard collisions between the
nucleons in the nuclei. The jet quenching phenomenon at
RHIC has been attributed to loss of energy of the high-$p_T$ partons on
their way through the medium
\cite{BDMPS:96,Zakharov:96,gyulassy,wiedemann,wang,AMY:02}.
It is often characterized by a transport coefficient $\hat q = \mu^2/\lambda$,
a ratio of the average squared momentum transfer $\mu^2$ per collision and
the mean free path $\lambda$.

Initially, energy loss had been attributed to induced gluon radiation
by scattering of the leading jet parton with thermal partons from the
medium. Perturbative calculations of this process yield results
for $R_{AA}$ compatible with data after fixing one parameter characterizing
the strength of the interaction, the transport coefficient $\hat q$ or
an equivalent quantity. However, recently several inconsistencies have led
to considerable efforts to revisit this topic. First, the values for
the transport coefficients extracted from measurements of $R_{AA}$ have
large uncertainties and differ between the various theoretical models
\cite{Samu:07}. Second, the description of observables beyond those based
on the simple single inclusive spectrum is far more problematic.
As an example we mention the azimuthal anisotropy $v_2$ at high $p_T$
which is larger than predicted by theory \cite{Shuryak:01}. Third,
the application of radiative, perturbative energy loss to heavy quarks
leads to the prediction of much smaller quenching than for light quarks,
in contradiction with data from RHIC
\cite{STARcharm:06}.

The partial failure of radiative, perturbative energy loss led to renewed
interest in energy loss from elastic collisions \cite{mustafa, djordjevic1,
weiliu}, although a self-consistent treatment of elastic and radiative
energy loss in one proper unified theoretical framework is still under
construction. It has also ushered in a period of renewed interest in
non-perturbative mechanisms. Examples are energy loss through bound states in
the plasma \cite{vHR:04} --- successfully applied to heavy quarks --- and energy loss in the infinite coupling limit of QCD, modeled through the AdS/CFT correspondence \cite{adscft:06}.

In this work, we discuss the potential of flavor dependent measurements of jet
quenching observables. It is obvious that the interaction of a jet with the
medium can change the flavor of the jet --- defined here as the flavor of the
leading parton. We will show that this leads to observable differences
in the final jet hadron chemistry. Our generalized definition of flavor here includes light
quarks $q$ ($u$,$d$,$s$), gluons $g$, and photons $\gamma$.
Heavy flavors $Q$ ($c$ and $b$) should also be included.
There is no doubt that flavor changing processes exist for fast partons
coupling to a medium. Indeed, flavor changing channels have been included
in computations of medium-modified fragmentation functions
\cite{Schafer:2007xh}, and quark-gluon conversions of the leading jet
particle are also implemented in the AMY formalism \cite{AMY:02}.
Nevertheless, flavor changing processes are often neglected in
phenomenological studies.

This situation led to the wide-spread opinion that quark and gluon jets
are well-defined concepts in a medium and that they ought to exhibit a relative
suppression by a color factor $9/4$.
This claim might have some validity in the case that the mean-free path
of the leading particle is of the order of (or larger than) the size of
the medium, $\lambda \approx L$. Energy loss is then dominated by emission of
only one gluon. In general, however, the situation is different. We want
to emphasize that the flavor of a leading parton might only be well-defined
locally and that it can change along the trajectory.
Recently, Sapeta and Wiedemann argued that changes in jet chemistry could
also arise from the increased multiplicity in medium-modified jets
\cite{Sapeta:2007ad}. In a model using enhanced parton branching they
find effects similar to those discussed by us here.

Nevertheless, most studies of jets in nuclear matter still focus solely on the
\emph{kinematical} effects, based on quenching of longitudinal momentum
and broadening of transverse momentum of the leading parton. E.g.,
observables like $R_{AA}$ and $v_2$ are sensitive to various integrals
over the differential energy loss $dE/dx$ and ultimately measure the momentum
transfer per path length, i.e.\ $\hat q$. We can summarize this as the
effect of jets coupling to the \emph{thermal} properties of the medium.
We like to advocate a second, complementary approach which looks at
\emph{flavor} effects from jets coupling to the \emph{chemical} properties
of the medium. Measurements of identified hadrons at high-$p_T$ can
constrain the rate of conversions. With sufficient experimental
sensitivity this would lead to estimates for the mean free path
$\lambda$ of the jet in the medium, complementary to measurements
of $\hat q$. We expect such measurements to provide additional
stringent tests for the validity of any model for the jet-medium coupling.
Of course, with any novel observation different explanations have to
be taken into account.

This work is organized as follows. In the next section we will discuss
all relevant conversion channels on a qualitative level and assess their
relative importance. We focus on jets
interacting with a deconfined quark gluon plasma in nuclear collisions,
but our arguments can be easily applied to cold or hot hadronic matter as well.
We then proceed to present numerical results for RHIC and LHC in a consistent model based on rate equations for conversions and a Fokker-Planck equation for
energy loss, using perturbative leading-order (LO) cross sections.
The formalism is introduced in Sec.\ \ref{propagate} while a discussion of
the results can be found in Secs.\ IV and V.
We focus on light and strange quarks, and real photons. Conversions including
heavy quarks will not affect the abundance of light species and photons
measurably. Therefore, the effect on charm and bottom spectra will be
addressed in
a separate publication \cite{weiliu3}. We will also introduce some novel double-ratio
observables that might be particularly sensitive to conversion processes.
Finally, a summary and discussion are presented in Sec.\ \ref{summary}.

\section{Flavor Conversions in Quark Gluon Plasma}

Let us start by discussing possible flavor conversion processes in a
leading order perturbative approach. Annihilation and pair creation
processes (i) $q+\bar q \leftrightarrow g+g$ can lead to a conversion of quarks into gluons and vice versa.
In the following we agree that the first parton mentioned on either side of an
arrow $\leftrightarrow$ or $\to$ is the leading jet parton in the sense that
it is the one with the larger momentum with respect to the local rest frame of
the medium among all other partons on the same side of the arrow. $q$ can
be a quark or antiquark and $\bar q$ is its antiparticle.

Photons can be easily created through (ii) $q+\bar q \to \gamma + g$ and (iii)
$q +\bar q \to \gamma+\gamma$. Note that we can neglect the opposite processes
and generally all processes with photons in the initial state for obvious
reasons. It is also safe to ignore $q+\bar q \to g + \gamma$ which in our
notation contributes to the yield of gluon jets, but is much suppressed
compared to the annihilation contribution (i).
On the other hand, flavor changing Compton processes
(iv) $q+g \leftrightarrow g +q$ are as important as annihilation and pair
creation. Compton processes also contribute to photon production via
(v) $q+g \to \gamma+q$.

Let us comment on the fate of heavy quarks. Heavy quark annihilation is
negligible at realistic temperatures, but the opposite process (vi)
$g+g \to Q + \bar Q$ might be an important source of excess heavy quarks.
The Compton process for heavy quarks (vii) $Q+g \to g+ Q$ is
interesting as well since it can effectively accelerate heavy quarks from
the medium or decelerate existing fast heavy quarks. We will
report on the quantitative effects of
processes involving heavy quarks in a forthcoming publication \cite{weiliu3}.

Some of the conversion channels (i) to (v) have been investigated in the
past. In \cite{weiliu1} the authors discussed the conversion of quark
into gluon jets and vice versa through annihilation and Compton channels
(i) and (iv), respectively. In the absence of flavor conversions the relative
suppression of gluon relative to quark jets is expected to be 9/4.
Conversions reduce the difference between the nuclear modification factors
for high-$p_T$ quarks and gluons. Assuming gluon dominated proton fragmentation
functions, as given in modern parameterizations \cite{AKK:05}, this leads to
an enhancement of the $p/\pi^+$ and ${\bar p}/\pi^-$ ratios at high transverse momentum. This is
supported by recent data from the STAR Collaboration at RHIC \cite{adams1,
adams} which shows rather large $p/\pi^+$ ratios in Au+Au at high $p_T$.
Models without conversion predict consistently lower $p/\pi^+$ and
$\bar p/\pi^-$ ratios \cite{wang1}.

Interestingly, in \cite{weiliu1} the required conversion rates were found to
be much larger than those given by the leading order perturbative
calculation. An enhancement factor ($K\approx 4$) was necessary to describe the
data. Although there are some uncertainties from the fragmentation functions
this either points to large higher order corrections \cite{weiliu2} or
to a non-perturbative mechanism favored in a strongly coupled QGP scenario
\cite{whitepaper:05}.

It was first pointed out in \cite{fries1} that annihilation and Compton
processes can also lead to the conversion of a quark or gluon jet into a
photon. Perturbative estimates of this process at leading order indicate a
sizable contribution to the total direct photon yield at intermediate $p_T$
between 3 and 6 GeV/$c$ at RHIC energies \cite{FMS:05,simon,GAFS:04,
Turbide:2007mi}. The same was found for the production of virtual photons
and lepton pairs \cite{SGF:02,Turbide:2007mi}.
Measurements of single inclusive direct photon production from the
PHENIX experiment \cite{phenix:photons} are not conclusive so far. The data
has been described equally well within error bars both by calculations
taking into account only prompt hard photons and thermal photons,
and by calculations adding jet-photon conversions as well
\cite{Isobe:07}.

Recently, it was also noticed that jet-photon
conversions lead to photons with an azimuthal asymmetry $v_2$ which is
negative w.r.t.\ the $v_2$ of hadrons and other photon sources
\cite{simon1}. In Ref.\ \cite{simon1} it was estimated that the sum
of all direct photon sources leads to small and negative direct photon
$v_2$ between 3 and 5\% at intermediate $p_T$. However, later it was noted
that the result depends crucially on details of the fireball simulation
and might be much closer to zero \cite{Turbide:2007mi}. First experimental
results are compatible with zero with sizable uncertainties
\cite{phenix:05v2,phenix:07v2}.

The current experimental situation can be summarized by noting that
both for conversions into photons and for quark-gluon conversions no
final conclusion has been reached about a direct observation of these
channels. This should change in the future with extended hadron
identification capabilities at RHIC and increased statistics for
direct photons. Of course, the experimental confirmation of one
conversion signal would be a strong indication that conversion in
general, as advocated here, is an important process.

In the remainder of the paper we present a calculation of conversion
channels (i),(ii),(vi) and (v) in one consistent framework. We are going
to compute
nuclear modification factors $R_{AA}$ for pions, kaons and direct photons,
and the elliptic flow $v_2$ of direct photons. This leads to a
direct quantitative connection between the conversions into photons
\cite{fries1} and conversions into gluons \cite{weiliu1}.

Furthermore, we suggest the relative yield of strange hadrons at high $p_T$
as a new signature for jet medium coupling. The ratio of $s$ quarks to
the sum of $u$ and $d$ quarks,
\begin{equation}
  w = \frac{s}{u+d}
\end{equation}
is about 5\% for the initial leading jet particle at RHIC energies at a
typical $p_T$ of about 10 GeV/$c$. It can be easily traced back to the
small fraction of strange quarks in the incident nucleons. A rough estimate
with CTEQ5M parton distributions \cite{cteq5:99}, assuming dominance of Compton
channels in the initial hard scattering would give
\begin{align}
  w_\text{jet}(p_T = 10 \text{ GeV/}c) &\approx w_\text{pdf}(x\approx 0.1,
  Q\approx 10 \text{ GeV}) \nonumber \\
  &\approx \frac{0.07}{1.1} \approx 6.4 \% \, ,
\end{align}
consistent with the result from the full calculation.
On the other hand, in a chemically equilibrated quark gluon plasma at a
given temperature $T$ the ratio is
\begin{equation}
  w_\text{ce}(T) \approx \frac{m_s^2}{4T^2} K_2(m_s/T) \, .
\end{equation}
assuming massless $u$ and $d$ quarks. For a strange quark mass of $m_s = 100$
MeV the ratio is almost 1/2 even at $T_c \approx 180$ MeV:
$w_\text{ce}(T_c) = 0.47$. The difference between $w_\text{jet}$ and
$w_\text{ce}$ is rather large and we have to expect that the relative
abundance of strange quark jets will rise with time.

For an infinite medium the particle ratios of the jets
would equilibrate to that of the medium (the same will happen to their
momentum distribution, making them indistinguishable from the
medium). For a finite path length $L$, the rate of
equilibration should be a good measure of the strength of the coupling
to the medium. In particular, it should give a good estimate of
the mean free path $\lambda$ between flavor changing scatterings.
The approach to equilibrium will be determined by the ratio $\lambda/L$.

The final parton abundances, once a jet leaves the medium, will translate
into hadron abundances as given by the fragmentation process. Our poor
knowledge of fragmentation functions limits our ability to make precise
predictions for absolute hadron ratios. Hence, we propose to use double
ratios of abundances measured in $A+A$ and $p+p$ collisions. The first ratio
we are going to look at is that for direct photons relative to pions
\begin{equation}
  r_{\gamma/\pi^+} = \frac{(\gamma/\pi^+)_{AA}}{(\gamma/\pi^+)_{pp}}.
\end{equation}
Note that formally this is the same as $R_{AA}^\gamma/R_{AA}^{\pi}$.
In such double ratios uncertainties from the determination of the number
of $N+N$ collisions (needed for $R_{AA}$) cancel. On the theoretical side
the large uncertainties from fragmentation functions tend to cancel between
the $A+A$ and the $p+p$ contribution. On the experimental side, we would
hope that the systematic errors for a direct extraction of double ratios
are smaller than for an a posteriori reconstruction from single ratios.
We will also take a fresh look at the relative abundances of protons and
pions
\begin{equation}
   r_{p/\pi^+} = \frac{(p/\pi^+)_{AA}}{(p/\pi^+)_{pp}}.
\end{equation}

Generally, in $A+A$ collisions we expect the relative abundances of flavors
to tend toward equilibrium values. Therefore, we expect an increase of
photons compared to quark and gluons and we expect an increase of gluons
relative to quarks \cite{weiliu1}. Under the assumption that protons
are dominated by fragmentation from gluons we expect the $p/\pi^+$ ratio
to increase in $A+A$ collisions, while $\gamma/\pi^+$ will show
an even steeper increase. A similar increase should also be observed in the
relative abundance of strange hadrons as high-$p_T$ strange quarks tend
toward equilibrium.
In the remainder of the paper we will try to verify our claims numerically.

\section{Jet Propagation in an Expanding QGP}\label{propagate}

Our treatment of a fast quark or gluon in a QGP medium has
already been discussed in Ref.\ \cite{weiliu,weiliu1}. The behavior is
governed by coupled Fokker-Planck equations and rate equations
\begin{eqnarray}\label{fpeq}
\frac{d\langle p^a_T \rangle}{d\tau} &=& -\langle \gamma_a(p_T,T) p^a_T
\rangle \approx -\gamma_a(\langle p^a_T\rangle,T))\langle p^a_T\rangle \, ,
\\
\frac{dN^a}{d\tau} &=& -\sum_{b}\Gamma^{a\to b}(p_T,T)N^a+\sum_{c}\Gamma^{c\to a}(p_T,T)N^c,\nonumber
\end{eqnarray}
with letters $a,b,c$ denoting quark, antiquark, gluon, or photon.
We have dropped the diffusion term and also neglected the
dispersion of jet momentum, i.e.\ $\langle p^2_T\rangle = \langle p_T\rangle^2$
since we want to focus on high momentum jets. The drag coefficient $\gamma_a$
is obtained from Ref.\ \cite{weiliu1}, including all elastic processes
between quarks and gluons.
To mimic the contributions from higher order radiative processes, we introduce an enhancement factor $K$.
The conversion widths $\Gamma$ for quark to gluon and vice versa (channels
(i) and (iv)) are extracted from Ref.\ \cite{weiliu1} as well, multiplied by the
same enhancement factor $K$ to account for higher order corrections. The widths
for the quark to photon processes (ii) and (v) at leading order are obtained
from Ref.\ \cite{fries1}.

For a quark or gluon jet moving through the QGP, the rate of change
for its mean transverse momentum $\langle p_T\rangle$ can be obtained from the
Fokker-Planck equation in (\ref{fpeq}). A quark or gluon can be
converted to a gluon, quark or photon with a rate given by the corresponding
collisional width. These effects are modeled by introducing a large number of
test quark and gluon jets that are distributed in the transverse plane
according to the underlying binary nucleon-nucleon collisions. Their
transverse momentum distribution is taken to be uniform in azimuthal angle
$\phi$. A test jet of a given kind with certain transverse momentum is
assigned a probability that is proportional to the corresponding jet momentum
spectrum with the proportionality constant determined by requiring that the
sum of the probabilities for all test jets of this kind is equal to their
total number.
More details can be found in Ref.\ \cite{weiliu1}.

Let us quickly discuss the possible lessons that can be learned by looking
at a simplified situation. We assume a rare jet flavor which can be produced
from a plentiful reservoir of a second jet flavor via a single
channel with width $\Gamma\sim 1/\lambda$. As long as the first
flavor stays rare the back-reaction is negligible.
A real world example close to this hypothetical situation could be photons
produced from light quark jets at RHIC.
Then the rate to produce an additional excess of the rare flavor is
\begin{equation}
  \frac{dN_\text{rare}}{d\tau} \approx \frac{1}{\lambda} N_\text{res}
\end{equation}
Let us further assume that $N_\text{res}$ is roughly constant during the
lifetime of jets in the system (as for the real world example: up to 30\%
of quark jets at RHIC leak to gluon jets \cite{weiliu1}).
Then we arrive at a simple formula connecting yields and the mean
free path for the single channel.
\begin{equation}
  \frac{\lambda}{L} \approx \frac{N_\text{res}}{N_\text{rare,excess}}.
\end{equation}

Obviously, realistic systems are more complicated with multiple channels,
changing rates $\Gamma$ due to the temperature evolution of the system,
and a whole range of possible path lengths $L$.
Therefore, in the following section we will apply our test particle Monte Carlo simulation to study the propagation of jets in an expanding QGP fireball.

\section{Results for Spectra , $R_{AA}$ and $v_2$ at RHIC} \label{nucl-mod}

\begin{figure}[t]
\centerline{
\includegraphics[width=3.0in,height=3.0in,angle=-90]{photon_pp.eps}}
\caption{(Color online)Photon spectrum in $p+p$ collisions at
  $\sqrt{s_{NN}}=200$ GeV as function of transverse momentum.
  Data are taken from the PHENIX Collaboration \cite{adler2}.}
\label{spectra_pp}
\end{figure}

In $p+p$ collisions, direct photons come from initial hard
collisions and vacuum bremsstrahlung off jets.
Bremsstrahlung photons can be obtained by folding the QCD
with photon fragmentation functions
$D_{\gamma/(q,g)} (z,Q^2)$ from Ref.\ \cite{owens}. The spectra of quark and gluon jets are taken from
\cite{weiliu2} which fit both pion and proton spectra measured in $p+p$
collisions from STAR, while the prompt initial photons are calculated
at leading order according to \cite{owens} with a $K$ factor of 1.5.
The resulting $p_T$ spectrum of photons in $p+p$
collisions at $\sqrt{s_{NN}}$=200 GeV is shown in Fig.\ \ref{spectra_pp}, and it
is found to be in good agreement with data from PHENIX \cite{adler2} at
transverse momenta above 4 GeV/$c$. This provides a well controlled baseline
for calculations of the nuclear modification factor $R^\gamma_{AA}$.

\begin{figure*}[ht]
\includegraphics[width=3.0in,height=3.0in,angle=-90]{photon_kc4.eps}
\hspace{0.2cm}
\includegraphics[width=3.0in,height=3.0in,angle=-90]{photon_kc0.eps}
\caption{(Color online) Photon spectra in central Au+Au collisions at
$\sqrt{s_{NN}}=200$ GeV as functions of transverse momentum with (left panel)
and without conversions (right panel).  Direct photons are from initial hard
collisions (Solid line), final state jet fragmentation (Dotted line), and
jet-photon conversions (dashed line). Data are taken from PHENIX \cite{adler4}.}
\label{spectra}
\end{figure*}

The yield of additional direct photons in Au+Au collisions from jets in the
medium is computed by
applying the test particle Monte Carlo method discussed above. For the
dynamics of the fireball in central collisions we assume that it evolves
boost invariantly in the longitudinal direction, but with an accelerated
transverse expansion. The parameters for the fireball formed in Au+Au
collisions at $\sqrt{s_{NN}}=$ 200 GeV are taken from Refs.\
\cite{weiliu, weiliu1, chen} where one of us studied energy loss of
heavy quark, and the $p/\pi^+$ and $\bar p/\pi^-$ ratios due to jet flavor conversions, as well as production of pentaquark baryons.
The volume expands in the proper time $\tau$ according to
$V(\tau)=\pi R(\tau)^2\tau$, where $R(\tau) = R_0 + a(\tau-\tau_0)^2/2$ is the
transverse radius with an initial value $R_0$=7 fm/$c$, $\tau_0$=0.6 fm/$c$ is
the QGP formation time, and $a=0.1 \,c^2$/fm is the transverse
acceleration. Starting with an initial temperature $T_0=350$ MeV, the time
dependence of the temperature is obtained from entropy conservation, leading
to the critical temperature $T_c=175$ MeV at proper time $\tau_c=5.0$ fm/$c$.

We show all results with two extreme assumptions for the $K$-factor of
the jet-plasma interactions. We explore the scenario $K=0$ which simply
switches off all flavor conversions. We also show computations with
$K=4$. This rather large value is likely to establish an
upper bound for the effect, but it is also compatible with the previous
work in \cite{weiliu1}. Of course, taking such a large value would imply
that a leading order perturbative treatment is not sufficient to begin
with. Note that we keep a separate value of $K = 1$ for conversions
with a photon in the final state. We have fixed this value with the available
data on direct photons.

The resulting spectra of direct photons in Au+Au collisions at
$\sqrt{s_{NN}}$ = 200 GeV are plotted in Fig.\ \ref{spectra} as functions of
transverse momentum. The left panel shows the scenario with jet flavor and jet-photon conversions included,
while the right panel neglects conversions. The results show that direct photons from jet-photon conversions (dashed line in the left panel)
make a sizable contribution below $p_T= 6$ GeV/$c$ at RHIC, in accordance with
previous results \cite{fries1}. Jet flavor conversions reduce the number of
high momentum quark jets by 30\% \cite{weiliu1}. Therefore, conversions lead to
a decreased production of direct photons from final state jet fragmentation
(dotted lines in both left panel and right panel) by roughly the same amount.

\begin{figure*}[tb]
\includegraphics[width=2.3in,height=2.2in,angle=-90]{photon_raa.eps}
\hspace{0.2cm}
\includegraphics[width=2.3in,height=2.2in,angle=-90]{photon_doubraa.eps}
\hspace{0.2cm}
\includegraphics[width=2.3in,height=2.2in,angle=-90]{raa_ppi.eps}
\caption{(Color online) Nuclear modification factor $R^\gamma_{AA}$
of direct photons (left panel) and the double ratios of $r_{\gamma/\pi^+} = R^\gamma_{AA} / R^{\pi^+}_{AA}$ (middle panel) and $r_{p/\pi^+} =
R^p_{AA}/R^{\pi^+}_{AA}$ in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV as functions of transverse momentum $p_T$. Results are with (solid line) or without (dotted line) conversions.}
\label{raa}
\end{figure*}

The nuclear modification factor $R^\gamma_{AA}$ for the total direct
photon spectrum is shown in the left panel of Fig.\ \ref{raa} (the number of binary collisions is taken to be $\approx$ 960 for the most cental bin).
The result with all conversion turned off ($K=0$) is plotted with a dotted line
while that with jet conversions included is indicated
by the solid line. We find a noticeable difference between both cases. In our
calculation the result with conversions is more compatible with the data
from PHENIX. Note that the difference between both scenarios
decreases with increasing transverse momentum because prompt hard photons from
initial scatterings, unchanged by the nuclear environment, dominate at higher
$p_T$.

As we discussed above, it might be interesting to look at double ratios of nuclear modification factors to extract jet conversion effects.
In the middle panel of Fig.\ \ref{raa} we present our expectation for
$r_{\gamma/\pi^+}= R^\gamma_{AA}/R^{\pi^+}_{AA}$ at intermediate and high
$p_T$. Note that no recombination effects are taken into account in this study
\cite{reco}, so that deviations in quantities involving hadrons can be
expected below $p_T \approx 6$ GeV/$c$. We find that conversion processes lead
to a large signal in $r_{\gamma/\pi^+}$. Our results with and without conversion
differ by a factor 2. We also show the double ratio $r_{p/\pi^+} =
R^p_{AA}/R^{\pi^+}_{AA}$ in the right panel of Fig.\ \ref{raa}. Again, we find a
large separation between the extreme cases with and without conversions,
opening the door for a possible measurement at RHIC.

\begin{figure*}[t]
\centerline{
\includegraphics[width=2.5in,height=2.5in,angle=0]{v2_kc4.eps}
\hspace{-0.2cm}
\includegraphics[width=2.5in,height=2.5in,angle=0]{v2_kc0.eps}}
\caption{(Color online) Elliptic flow $v^\gamma_2$ of direct photons in
  peripheral Au+Au collisions with centrality of $20 -30\% $ at
  $\sqrt{s_{NN}}=200$ GeV as functions of transverse momentum for two
  different scenarios of with (left panel) or without conversions (right
  panel), respectively. Data is taken from the PHENIX collaboration.
\cite{phenix:07v2}}
\label{v_2}
\end{figure*}

\begin{figure*}[t]
\includegraphics[width=3.0in,height=3.5in,angle=-90]{v2_diff_acce.eps}
\caption{(Color online) Elliptic flow $v^\gamma_2$ of direct photons in
  peripheral Au+Au collisions with centrality of $30 -40\% $ (left) and $40 - 50\% $ (right) at $\sqrt{s_{NN}}=200$ GeV as a function of transverse momentum.}
\label{v_2_diff_acce}
\end{figure*}

An azimuthal anisotropy is believed to be generated in a partonic stage in semi-central and peripheral heavy ion collisions when the fireball has
the maximum geometric asymmetry. This anisotropy is studied in terms of Fourier coefficients $v_2$ defined from particle yield $dN/(p_Tdp_Td\phi)$ as
\begin{eqnarray}
  \frac{dN}{p_Tdp_Td\phi} = \frac{dN}{2\pi p_Tdp_T}\left[1+ 2v_2(p_T)\cos
  (2\phi)+ \mathcal{O}(4\phi)\right]
\end{eqnarray}
where the angle $\phi$ is defined with respect to the reaction plane.
The coefficient $v_2$ can be extracted from the transverse momentum $p_T = (p_x,p_y)$ of measured particles via
\begin{eqnarray}\label{eq-v2}
v_2 = \left\langle \frac{p^2_x-p^2_y}{p^2_x+p^2_y}\right\rangle,
\end{eqnarray}
where the direction of the $x$-axis is in the reaction plane with
$\phi$ = 0 and the direction of the $y$-axis is out of the reaction plane.
Here, $\langle \cdots\rangle$ means averaging over all measured particles
for a given species.

The parameters of the expanding fireball formed in peripheral collisions
are extracted from information about centrality, number of participants, and
number of binary collisions in Au+Au collisions at $\sqrt{s_{NN}}$= 200 GeV
given in \cite{adler3}. For example, in collisions in the centrality bin 20--30\%, the fireball forms an almond-shaped cylinder with the initial length of the long transverse axis being $l^0_L$ = 6.1 fm and that of the short transverse axis being $l^0_S$ = 3.5 fm.
The acceleration in the direction of the long axis is $a_L$ = 0.1 $c^2$/fm and that in the direction of the short axis is $a_S=l_L/l_S a_L$. We also
assume the fireball is boost-invariant and
the temperature dependence is obtained from entropy conservation with an
initial temperature $T_0$ = 340 MeV at proper time $\tau_0$ = 0.6 fm/$c$ and critical temperature $T_c$ = 175 MeV.

\begin{figure*}[tb]
\includegraphics[width=3.0in,height=3.0in,angle=-90]{pp_k0.eps}
\includegraphics[width=3.0in,height=3.0in,angle=-90]{raa_k0.eps}
\caption{(Color online) $K^0_S$ (left panel) spectra from quark and gluon jet
  fragmentation via AKK fragmentation function \cite{AKK:05} in $p+p$
  collisions at $\sqrt{s_{NN}}=$200 GeV. Data are from the STAR Collaboration
  \cite{abelev}. Nuclear modification factor $R_{AA}$ (right panel) for
  $K^0_S$ in Au+Au collisions at same center-of-mass energy as a function of transverse momentum.}
\label{raa_k0}
\end{figure*}

The obtained elliptic flow $v^\gamma_2$ of direct photons is shown in Fig.\
\ref{v_2} as a function of $p_T$. We plot the results with conversions in the
left panel, while those in the right panel have $K=0$. The elliptic flow of
photons from jet-photon conversions is negative with a magnitude of about 2\%,
because the production of direct photons in the direction out of the reaction
plane is favored by the longer propagation path of jets through the medium
\cite{simon1}. This makes it more likely that a rare jet-photon conversion
occurs. $v^\gamma_2$ of photons from final state jet fragmentation is
positive, following the pattern of $v_2$ for hadrons at large $p_T$. Adding
all contributions together leads to a positive total elliptic flow
$v^\gamma_2$ of direct photons. When jet conversions are switched off, the elliptic flow
$v^\gamma_2$ of direct photons from final state jet fragmentation decreases
because of the larger number of quark jets leaving the fireball. The effects
of missing negative conversion $v_2$ and increased positive bremsstrahlung
$v_2$ roughly cancel. Thus we find that the total elliptic flow $v^\gamma_2$
of direct photons is roughly the same in cases whether or not jet conversions
are present.
Preliminary data from PHENIX \cite{phenix:07v2} is shown in the left panel.
Both cases are compatible with the data within the large experimental
uncertainties.

To make this study more complete, it is interesting to explore the centrality dependence of the novel double ratio $R^\gamma_{AA}/R^\pi_{AA}$ and elliptic flow of direct photons at RHIC. When the centrality of the collisions increases, the resulting decreased average path length will lead to reduction of both energy loss and jet conversion rates. Hence we expect that the difference  between the scenarios with and without jet conversions for the double ratio $R^\gamma_{AA}/R^\pi_{AA}$ will shrink with increasing centrality. For the elliptic flow of direct photons, the centrality dependence is not so straight forward to estimate because changes in energy loss and jet conversion rates try to sway $v_2$ in different directions. We calculate the elliptic flow of direct photons produced in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV in the more peripheral centrality bins of 30-40\% and 40-50\%, respectively, and show the results in Fig.\ \ref{v_2_diff_acce}. The resulting total elliptic flow of direct photons is just as small as for the 20-30\% centrality bin. Thus direct photon $v_2$ is not sensitive to the impact parameter in Au+Au collisions due to the cancelation between jet conversions and jet energy loss.

Let us now proceed to discuss the impact of conversions on strange hadrons,
in particular kaons. In Ref.\ \cite{weiliu1} only $u$ and $d$ quarks had been
included alongside gluons, and the resulting $p/\pi^+$ and $\bar p/\pi^-$
ratios reproduce the data from STAR \cite{adams}. We neglect the relatively
small current mass of the strange quark and replace it solely with the thermal
mass $\sim gT$ in the medium when calculating its drag coefficient and
conversion width. This might slightly overestimate conversion rates to
strange quarks, but it simplifies the treatment \cite{weiliu1}.

In the left panel of Fig.\ \ref{raa_k0} we show the spectrum of $K^0_s$ in
$p+p$ collisions calculated as a baseline. On the right hand side
we plot the nuclear modification factor $R^K_{AA}$ of the $K^0_S$ as a
function of transverse momentum $p_T$. We observe that conversions greatly
enhance the yield of $K^0_S$ in nuclear collisions. In fact,
conversions could lead to a $R_{AA}$ that is up to a factor 2 larger at high
$p_T$ than that for pions or protons. Note again, that a recombination
contribution is not included here, so caution has to be exercised at lower
$p_T$. We predict that the measurement of the nuclear modification factor
of kaons and Lambdas at high $p_T$ will provide an unique signal for
jet conversions in the QGP formed at RHIC.

\section{A Look at LHC}
\label{lhc}

At LHC energies jet physics is expected to play an even bigger role than
at RHIC. Using the procedures developed previously we compute the
expected nuclear modification factors $R^\gamma_{AA}$ and $R^K_{AA}$, and the
elliptic flow $v^\gamma_2$ for direct photons as well as various double ratios
in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.5$ TeV. We want to gauge the effect of jet conversions on high-$p_T$ measurements at LHC.

\begin{figure*}[t]
\centerline{
\includegraphics[width=3.0in,height=3.0in,angle=-90]{lhc_pho_4.eps}
\hspace{0.2cm}
\includegraphics[width=3.0in,height=3.0in,angle=-90]{lhc_pho_0.eps}}
\caption{(Color online) Photon spectra in central Pb+Pb collisions at
$\sqrt{s_{NN}}=5.5$ TeV as functions of transverse momentum $p_T$ with
(left panel) or without conversions (right panel).}
\label{lhc_spectra}
\end{figure*}

\begin{figure*}[t]
\centerline{
\includegraphics[width=2.4in,height=2.2in,angle=-90]{lhc_raa.eps}
\hspace{0.2cm}
\includegraphics[width=2.4in,height=2.2in,angle=-90]{lhc_raa_ratio.eps}
\hspace{0.2cm}
\includegraphics[width=2.4in,height=2.2in,angle=-90]{lhc_raa_ppi.eps}}
\caption{(Color online) Same quantities as those in Fig.\ref{raa} in central Pb+Pb collisions at $\sqrt{s_{NN}}=5.5$ TeV as functions of transverse momentum with or without conversions.}
\label{lhc_raa}
\end{figure*}

We use jet spectra at 5.5 TeV by multiplying those obtained from PYTHIA in $p+p$ collision at the same energy by the number of binary collisions
\cite{weiliu2} ($\langle N_{coll}\rangle\approx$ 1700 \cite{david}).
We also assume that the fireball produced in central collisions at LHC is
cylindrical and Bjorken boost-invariant. The parameters of the fireball are
taken from Ref.\ \cite{zhang} where thermal charm production at LHC was
studied. Specifically, we choose initial proper time $\tau_0=0.2$ fm/$c$ for
the formation of the equilibrated QGP with initial temperature $T_0=700$ MeV,
and flow acceleration $a=0.1$ $c^2$/fm. This yields reasonable final
transverse flow velocities with initial radius $R_0=7.0$ fm. Fixing the
parameters at the critical temperature $T_c=175$ MeV via entropy conservation we obtain $\tau_c=6.3$ fm/c. We apply the same test particle
Monte Carlo method as above with the same set of $K$ factors.

Our results for direct photons are shown in Fig.\ \ref{lhc_spectra} for $K=4$
(1 for photons) (left panel) and $K=0$ (right panel), respectively.
Direct photons
from jet-photon conversions and jet fragmentation dominate below $p_T = 16$
GeV/$c$ when conversions are switched on, with photons from conversion being
the largest source below 10 GeV/$c$. Without jet conversions the direct
photons from jet fragmentation are always brighter than those from initial
hard collisions up to 20 GeV/$c$, because of the reduced conversion of quarks.

It is straightforward to calculate the nuclear modification factor
$R^\gamma_{AA}$, which is shown in the left panel of Fig.\ \ref{lhc_raa}. The
difference between both scenarios can be as large as a factor of 2 at $p_T =
4$ GeV/$c$, and decreases to a factor of about 1.25 at 20 GeV/$c$. We also
observe a large increase in the double ratio $r_{\gamma/\pi^+} = R^\gamma_{AA} /
R^{\pi^+}_{AA}$ if conversions are included, just as seen for RHIC as well. For comparison, we also show the result of the double ratio $r_{p/\pi^+}$ at LHC .

\begin{figure*}[ht]
\centerline{
\includegraphics[width=3.0in,height=3.0in,angle=-90]{lhc_v2_kc4.eps}
\hspace{-0.2cm}
\includegraphics[width=3.0in,height=3.0in,angle=-90]{lhc_v2_kc0.eps}}
\caption{(Color online) Elliptic flow $v^\gamma_2$ of direct photons in
  peripheral Pb+Pb collisions with centrality of $20 -30\% $ at $\sqrt{s_{NN}}=5.5$ TeV as functions of transverse momentum with (left panel) or without conversions (right panel).}
\label{lhc_v_2}
\end{figure*}

\begin{figure*}[t]
\includegraphics[width=3.0in,height=3.5in,angle=-90]{lhc_v2_diff_acce.eps}
\caption{(Color online) Elliptic flow $v^\gamma_2$ of direct photons in
  peripheral Pb+Pb collisions with centrality of $30 -40\% $ (left) and $40 - 50\% $ (right) at $\sqrt{s_{NN}}=5.5$ TeV as functions of transverse momentum.}
\label{lhc_v_2_diff_acce}
\end{figure*}

Since we have seen that direct photons at LHC are mostly from
jet-photon conversions and final state jet fragmentation, they are
strongly influenced by jet conversions. Photon elliptic flow
$v^\gamma_2$ might reveal those large effects. The parameters of the fireball formed in peripheral collisions at LHC are extracted from information about centrality, number of participants, and number of binary collisions in Pb+Pb collisions at $\sqrt{s_{NN}}$= 5.5= TeV \cite{david}. For simplicity, in collisions at LHC within the centrality bin $20-30$\%, we use the same geometric parameters (the length of the long and short axes) for the initial fireball as at RHIC, but we set the initial temperature and proper time of QGP formation to $T_0$ = 700 MeV and $\tau_0$ = 0.2 fm/c, respectively, consistent with central collisions. The time dependence of the temperature is also fixed via entropy conservation.

In Fig.\ \ref{lhc_v_2} we show the resulting elliptic flow $v^\gamma_2$ of
direct photons in Pb+Pb collisions at $\sqrt{s_{AA}}$ = 5.5 TeV with and
without conversions. The elliptic flow from photon
fragmentation off jets is positive with a magnitude of ~2\% in the
case of jet conversions, but it is reduced by a factor of 2 in the case of
missing jet conversions due to the reduced quenching for quark.
The resulting total elliptic flow of direct photons without jet
conversions differs only slightly from that of jet fragmentation alone, since
direct prompt photons from the initial state are subdominant below 20
GeV/$c$. With jet conversions included, we again observe a cancellation
between the negative anisotropy from jet-photon conversions and the increase
in jet fragmentation with positive anisotropy, leading to a total elliptic
flow $v^\gamma_2$ of almost the same magnitude as that without jet
conversions. Together with our previous result at RHIC energies this might
indicate that photon elliptic flow, despite its early promise, might need
very large experimental precision to be used as a probe.
The situation might be much improved, however, if measurements with additional
isolation cuts could be made \cite{simon1}.

We also predict the centrality dependence of the elliptic flow for direct photons in Pb+Pb collisions at $\sqrt{s_{NN}}$=5.5 TeV and show the results in Fig.\ \ref{lhc_v_2_diff_acce}. The numerical results confirm once more that the total elliptic flow of direct photons at high transverse momentum varies only weakly with the centrality of the collision.

\begin{figure}[ht]
\centerline{
\includegraphics[width=3.0in,height=3.0in,angle=0]{lhc_raa_k0.eps}}
\caption{(Color online) Nuclear modification factor $R_{AA}$ for $K^0_S$ in $Pb+Pb$ collisions at $\sqrt{s_{NN}}$=5.5 TeV as function of transverse momentum.}
\label{lhc_raa_k0}
\end{figure}

In Fig.\ \ref{lhc_raa_k0} we show $R^K_{AA}$ of $K^0_S$ in central collisions
at LHC. We observe that jet conversions have only a small impact on
$R^K_{AA}$. This had to be expected, because initial jet production is
gluon dominated and strange quark jets are not very much suppressed to
begin with, very unlike the situation found at RHIC.
The impact of jet conversions on the $R_{AA}$ of kaons is similar to the
effect on pions at LHC \cite{weiliu3}. We conclude that strange hadrons
might not be a good probe of jet conversions any more at LHC energies.

\section{Summary and Discussion}\label{summary}

We studied the impact of flavor conversions of jets on observables in
heavy ion collisions. We argued that the relaxation of jet flavors
toward chemical equilibrium can be an independent measure for the
strength of the jet-medium coupling. We presented a computation of
several observables which are sensitive to conversion
processes at RHIC and LHC using a Fokker-Planck approach and coupled rate
equations for conversions. Rates were based on leading order cross sections
with a $K$ factor.

We find a large change in the nuclear modification factor $R^\gamma_{AA}$
when jet conversions are switched on, while the effects on photonic elliptic
flow $v^\gamma_2$ tend to cancel.  We find our results to be consistent with
the available experimental data from PHENIX \cite{adler2}. We also calculated
the impact of jet conversions on the double ratios of nuclear modification
factor $r_{\gamma/\pi^+} = R^\gamma_{AA}/R^{\pi^+}_{AA}$ and $r_{p/\pi^+} = R^p_{AA}/R^{\pi^+}_{AA}$ and find that these quantities are quite sensitive to the
presence of jet conversions. The difference can be as large as a factor of two.
At the same time such double ratios might be extracted from experimental data
with rather small systematic error bars. Large $K$-factors in
our leading order treatment would hint at non-perturbative mechanisms, or
to the presence of additional effects, like the influence of increased
multiplicities in modified jets \cite{Sapeta:2007ad}.
We also advocate the measurement of strange hadrons at high $p_T$ as a
possible signature for the jet-medium coupling. The low initial strange
quark content of jets at RHIC makes kaons and other strange hadrons a
a very sensitive probe for the coupling to the medium.

\begin{acknowledgments}
We thank Che-Ming Ko for useful discussions and suggestions.
This work was supported by RIKEN/BNL, DOE grant DE-AC02-98CH10886, and the
Texas A\&M College of Science.
\end{acknowledgments}

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