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Paper text:
ultrarelativistic heavy ion collisions explore extreme conditions of strongly interacting matter .
these studies connect to a diverse set of forefront topics ranging from saturation of gluonic fields to string theory @xcite .
experiments at the brookhaven national laboratory rhic facility indicate that a new state of matter is formed in a - a collisions with @xmath0 200  gev / nucleon based on a number of key observations .
this picture is consistent with extensive lattice qcd studies predicting a phase transition as well as the results from cern experiments at lower energies suggesting the onset of a phase transition . among the key observations , the discovery of _ jet quenching _ is perhaps the clearest indication of the formation of an ultra - dense system - the `` disappearance '' of one of the two jets in back - to - back jet production in central collisions , together with the associated suppression of high-@xmath1 jet events .
further exploration of this phenomenon , as well as systematic investigations of these collisions , will be possible at the lhc . with a planned ultimate energy of 5.5 tev / nucleon for pb - pb collisions , the anticipated program benefits from a factor of 30 increase in @xmath2 as well as from highly hermetic detector systems with essentially complete coverage in precision calorimetry .
the complementary heavy ion studies planned for the atlas , cms , and alice experiments at lhc together cover the full spectrum in @xmath1 and pseudorapidity @xmath3 needed to understand the complex and potentially surprising collision events that are anticipated . in the following
is a brief discussion of the planned program for the atlas experiment , with an overview of the detector and the heavy ion program followed by the prospects for advances in jet quenching studies .
atlas and cms are the two broad - focus experiments at lhc , with complementary designs that provide excellent resolution and highly hermetic coverage in calorimetry , tracking , and muon systems .
the atlas calorimetry is a particular strong point for jet quenching studies , with an angular coverage down to @xmath4 5 ( 0.001@xmath5 ) and fine - grained segmentation .
atlas is comprised of three major detector component types : the inner tracking systems , the electromagnetic and hadronic calorimeters , and the muon systems . each is made up of several subsystems ; for an in - depth discussion of these as well as of the superconducting magnets , trigger , readout , data acquisition , controls and other system components , see @xcite .
the inner tracking systems , which operate in a 2 t solenoidal magnetic field , consist of three subsystems with a total of more than 86 million electronic channels .
the calorimeters , which represent nearly 190,000 channels , are configured in four subsystems , three of which are based on liquid argon technology .
the muon systems , which have in total one million channels , make use of toroidal magnetic fields of up to 2.5 and 3.5 t in the barrel and endcap region , respectively .
an average of 99% of the channels in each of these subsystems were operational well in advance of the 2009 data taking .
a complex , three - level trigger and data acquisition system selects and transports the data to disk ; calibration , reconstruction , and simulation software are exercised on the worldwide computing grid of lhc , a network of @xmath6 computers located in more than 50 countries around the world .
the expectations for the heavy ion data are based on extrapolations from rhic energies and from the first p - p data acquired at lhc in 2009 and 2010 .
because the energy available in the collisions is so much greater than for previous measurements , the first order of business is to check the validity of these extrapolations @xcite .
of particular interest are the energy density and degree of collectivity of the response , as expressed by global observables such as the charged particle multiplicity and the elliptic flow @xcite . these observables are critical for validating the physical picture extrapolated from rhic measurements , and they can begin to be determined with a relatively small integrated luminosity from the first few days of heavy ion collision data .
subsequent topics that can be addressed with a one - month run at the planned luminosity of @xmath7 include studies of quarkonia production and of hard probes such as jets and high-@xmath8 hadrons .
in addition to light quark jets , heavy quark jets and heavy mesons can be accessed .
the expected sensitivity of atlas to heavy ion events has been fully explored using the hijing @xcite event generator and the full simulation and reconstruction chain of atlas .
some highlights of these studies are mentioned here , and the results are more fully documented elsewhere @xcite . the first variable to be determined from the data is the event centrality , on which most other observables depend .
the event centrality can be estimated from the response of the calorimeters ; according to the simulations , the number of collisions within the event , the number of collision participants , and the impact parameter are all monotonically related to the total energy deposits in the forward calorimeter , the electromagnetic calorimeters , and the hadronic calorimeters .
the estimation of the event centrality using such a primitive measure from several calibrated detector systems will facilitate consistency checks over a wide range in pseudorapidity @xmath3 .
r0.5     a fundamental observable depending on the event centrality is the charged particle multiplicity @xmath9 .
it has been shown that @xmath9 can be independently estimated by counting the number of pixel hits in the first , second , and third layers of the atlas pixel detector @xcite .
the method requires application of a simple correction function that is determined primarily from geometry .
other methods have also been explored , such as the `` tracklet '' method [ 8 ] that combines an event vertex with hits in the first two pixel layers .
the determination of @xmath9 will allow a first estimate of the energy density and will eliminate significant uncertainties in extrapolations of this quantity from lower energy collisions .
the distribution of the transverse energy with angle @xmath10 has been shown to be accurately determined in atlas using a simple energy sum over the electromagnetic and hadronic calorimeter cells . according to simulations ,
this method reproduces the event - by - event distribution in @xmath11 to the few percent level over the wide angular range @xmath12 .
elliptic flow @xmath13 is a parameter characterizing the azimuthal angular distribution of particles emitted with respect to the reaction plane defined by the impact parameter vector and the beam axis .
it is defined through the fourier expansion of this angular distribution    @xmath14    where @xmath15 is the angle of the reaction plane within atlas and @xmath16 is the azimuthal angle of an individual particle .
measurement of elliptic flow in atlas has been explored using three different methods , the reaction plane method , the two - particle correlation method , and the lee - yang zeros method @xcite .
hijing events have been simulated for impact parameters b=2.3 , 7.0 , and 10.7 fm , and the three methods give results that are consistent within approximately 5% for the most central collisions and somewhat better for the most peripheral collisions as seen in fig .
the resolution for determination of the reaction plane is particularly good for atlas since this can be performed independently in several different detectors systems , all of which have complete coverage in azimuthal angle @xmath16 . in the simulation studies , the resolution correction averaged over seven subsystems for central events was 0.86 , very close to unity , demonstrating the excellent event plane resolution available in atlas .
the capability for heavy quarkonium reconstruction has been studied in the dimuon channel . as an example , the mass resolution found for the @xmath17 1s state was found to be approximately 120 mev , easily adequate for separation of the 1s and 2s @xmath17 states @xcite .
this capability will significantly extend the reach of the color screening studies with quarkonia that have been performed previously at rhic and cern .
additional studies in the dimuon channel have been performed to determine the atlas performance for identification of @xmath18 and @xmath19 bosons in the heavy ion environment . in these studies ,
a mass resolution for @xmath19 bosons of approximately 2.2 gev was found , comparable to that found in the p - p environment , with a background level of less than 10% for central collisions .
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jet suppression is an important component of the atlas heavy ion physics program , and an area where the excellent calorimetry and hermetic coverage of the calorimeters is very beneficial .
simulations have been performed for light quark jets and photon - tagged jets as well as for heavy quark signatures .
the background from the low - energy qcd interactions is significant , thus , methods have been developed for identifying light quark jets using a background subtraction procedure , and to identify photons with good efficiency while rejecting signals from other neutral particles @xcite .    in the heavy ion environment ,
jets in atlas after background subtraction are reconstructed with essentially full efficiency above 70 - 80 gev , with no additional corrections required @xcite as seen in fig .
2 ( left panel ) . further , according to simulations , there is essentially no distortion of the extracted light quark jet fragmentation function above @xmath20 , and the distortions due to the heavy ion background for @xmath21 can be corrected reliably @xcite .
extensive studies of direct photons and photon - jet correlations have been performed with hijing simulated data . in order to reject background , isolation cuts and shower shape cuts
have been developed which can be optimized to preserve good reconstruction efficiency while rejecting hits from neutral hadrons .
these studies show that excellent direct photon statistics will be obtained as seen in fig .
2 ( right panel ) . further , photon - jet correlations are clearly accessible as seen in fig . 3 ( left panel ) @xcite .
heavy quark jets have been studied in the dimuon channel , using the semileptonic decay of b and c quarks . according to these simulations ,
the purity of the dimuon sample above 50 gev is substantial as can be seen in fig .
3 ( right panel ) . heavy quark suppression at rhic remains an unsolved puzzle @xcite , although it can be explained in alternative pictures @xcite , and thus it is of high scientific interest to address it at the lhc .
further studies relevant to heavy quark jet suppression in atlas are planned , e.g. , in the semileptonic decay to @xmath22 and @xmath23 , and in reconstruction of heavy mesons using detached vertex tagging .
a broad program of measurements is planned for heavy ion collisions in atlas . with up to a factor of 30 increase in collision energy compared to existing data , 
 significant new insights are anticipated to be obtained with the first data measured . 
 global features of the lhc collisions will be accessible with the early data and will set the stage for the precision measurements to follow . 
 atlas is particularly well suited for exploration of `` jet quenching , '' the extinction of energetic jets in the hot dense medium . 
 observations of heavy quark jet suppression will be possible with unprecedented energy reach and statistical precision , potentially yielding new insights into the basic mechanisms involved .     
 address = institute for advanced studies in physics and engineering + valparaso center for science and technology + universidad tcnica federico santa mara + valparaso , chile