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the alice experiment at cern is optimized to study heavy - ion collisions at center - of - mass energy @xmath4 5.5 gev at the lhc  @xcite .
the high momentum particle identification detector  @xcite ( hmpid ) in alice is dedicated to the inclusive measurement of charged particles , both in heavy - ion and proton - proton collisions .
the single arm design of the hmpid covers 5% of the central barrel region of alice at mid - rapidity .
the hmpid enhances the particle identification capabilities of alice , extending the momentum range further than is accessible to individual central tracking detectors : its , tpc and tof .
the hmpid can identify charged pions and kaons in the momentum range @xmath5 and protons up to @xmath6  5  gev/@xmath7 with 3@xmath8 separation .
this paper focuses on the as - built description of the hmpid and the first commissioning results .
further details of hmpid hardware and physics goals can be found in  @xcite .
the high momentum particle identification detector consists of seven identical proximity focusing type ring imaging cherenkov counter ( rich ) modules .
the modules are located on an independent support cradle and mounted at the two oclock position of the alice space frame as shown in fig .  [
fig : hmpidonframe ] . the seven modules ( 1.5 m @xmath9 1.5 m each ) cover @xmath10  @xmath11  0.6 in pseudo - rapidity and 57.6@xmath12 in azimuth .
the chambers are tilted and positioned in a cupola - like structure to focus on the nominal interaction point , at @xmath13  4.7  m distance from the modules .
the layout of the chambers on the support cradle maximizes the hmpid acceptance for two - particle correlation studies at high-@xmath14 .
fast charged particles , emerging from the collisions , create cherenkov photons traversing the low chromacity liquid @xmath1 radiators ( with index of refraction of @xmath15 at @xmath16 nm ) .
the choice of the radiator liquid determines the momentum threshold ; for liquid @xmath1 the momentum threshold is given by @xmath17 1.21  @xmath18 , where @xmath19 is the particle mass .
each hmpid module is equipped with three radiator vessels made of neoceram@xmath20 , providing 15  mm radiator thickness .
the neoceram ( a glass - ceramic material ) is thermally compatible to the fused silica plates , used as uv - transparent windows .
the radiator vessels are filled and emptied at a constant flow by the liquid circulation system . to ensure safe remote operation
the system is built on the gravity flow principle . to minimize the contamination of water and oxygen ( less than 5 ppm ) , and to achieve the best transparency of the liquid @xmath1 radiator for cherenkov photons ,
filters are implemented for purification .
the liquid circulation system is scheduled to be completed and installed in its final location at the beginning of 2008 .
the cherenkov photons created in the liquid radiator refract out from the fused silica windows and enter the proximity gap .
the proximity volume is filled with pure @xmath21 at ambient pressure and temperature .
the cherenkov photons are detected on the pad cathodes of multi - wire pad chambers ( mwpc ) .
each module consists of 6 caesium iodide ( csi ) covered photocathodes , 60 cm @xmath9 40 cm each .
quantum efficiency @xmath13 25% ( at @xmath16 nm ) is achieved with the 300 nm thick csi covered photocathodes .
the pad cathodes are segmented into 8 mm @xmath9 8.4 mm pads to provide position sensitive information .
detailed description of the hmpid csi photocathode production can be found in  @xcite .
the total gas gain achievable is 5 @xmath9 10@xmath22 , with the cathodes kept at ground and a positive voltage of 2050 v applied to the anode wires .
the front - end electronics is located at the back side of the hmpid modules .
the closure box is flushed with nitrogen and closed by a cooling panel , that removes the dissipated heat of the electronics , 450 w per module .
the front - end electronics is based on two dedicated asic chips , gassiplex and dilogic  @xcite , processing the analogue signals from the pad cathodes .
the gassiplex and the dilogic chips were successfully developed within the framework of the hmpid project in the alcatel - mietec 0.7  @xmath23 m technology .
the gassiplex chip is a 16-channel analogue multiplexed low - noise signal processor .
preceding the hmpid commissioning , the noise of the front - end chips and the map of dead or noisy pads have been surveyed .
the noise is found to be 1000e rms with a dispersion of less than 50e , and the number of dead / noisy pads is found to be 200 out of 161280 pads .
the dilogic chip is a sparse data scan readout processor used for zero suppression and pedestal subtraction .
individual threshold and pedestal values can be processed for up to 64 channels .
data from the 7 hmpid modules are readout via 14 standard alice optical links ( ddl )  @xcite .
digitization is performed using a commercial 12-bit adc .
the digitization and zero - suppression time does not depend on the occupancy , only on the multiplexing rate of 3 chips ( 48 channels ) , which makes the event overlap negligible even in the highest luminosity proton - proton collisions . since particle identification is not possible in the hmpid without momentum information , only events with tpc information are of interest .
the readout time after l2 trigger arrival ( for the estimated 12% module occupancy ) will be of the order of 300 @xmath23s , hence the hmpid can perfectly cope with the readout rates foreseen for the tpc  @xcite .    in total ,
10080 gassiplex and 3360 dilogic chips have been installed in the seven hmpid modules .
all hmpid modules have been successfully installed on the supporting cradle and moved into the alice magnet on 26 september 2006 , as shown in fig .  [
fig : hmpidinstall ] . preceding the installation
, five hmpid modules were tested in test beam and two modules in cosmic ray data taking  @xcite .        while the hmpid is not operational ,
the modules are flushed with argon .
after the installation of the hmpid modules on the support cradle in the experimental cavern and prior to moving the assembly into the alice magnet , a leak was detected in one of the radiator trays ( in module 6 ) .
the loss of a single radiator ( 1 out of 21 ) represents @xmath13 5% loss of acceptance . due to the small impact on the physics potential of the detector and the not negligible risks , including delays , involved in the repair
, it has been decided to install the detector inside the alice solenoid with 20 operational radiators while flushing the leaking one with methane .
the safe operation of the detector under commissioning and in normal operation is achieved via the detector control system ( dcs ) , developed within the hmpid project . since july 2007
all hmpid modules are fully powered and controlled by dcs .
figure  [ fig : dcs1 ] shows the detector - oriented view of the fully powered hmpid , with most subsystems operational .
the hmpid dcs controls the subsystems of gas , cooling , liquid circulation and transparency , high - voltage , low - voltage and the front end electronics as shown in fig .
[ fig : dcs1 ] . during the commissioning phase the
alarm and interlock levels have been set for the full scale cosmic test of alice . for expert operation ,
the dcs provides access down to a single hardware element of the hmpid , in the device oriented view .
figure  [ fig : dcs2 ] shows the device oriented view of the high voltage modules during the high voltage ramp up of hmpid module 0 .
a long term stability test of the fully powered hmpid has been performed under dcs control , archiving all relevant detector information in the central alice database .
the continuous monitoring revealed a loose high voltage channel ( in module 5 ) . the standard high voltage ( 2050 v )
can not be applied on this channel ; a lower high voltage setting is used .
this may only allow the detection of charged particles , and not cherenkov photons , in that high voltage sector .
this represents a @xmath13 2.4% ( 1 out of 42 high voltage channels ) acceptance loss .
after the long term stability test , the systematic survey of the read - out electronics for noisy and dead channels has been repeated .
the first results of the hmpid module 0 and photocathode 0 are shown in figs .
[ fig : pedmean ] and [ fig : pedsigma ] .
figure  [ fig : pedmean ] shows the mean pedestal value for each photocathode pad .
the average pedestal value is found to be @xmath13 110 out of 4092 adc channels .
figure  [ fig : pedsigma ] shows the width of the pedestal distribution for each photocathode pad .
the average width is @xmath13 1.1 , equivalent to a charge 1000e rms , with small number of outlying pads .
this agrees with the expected value of the noise from the read - out electronics as reported in  @xcite .
the as - built high momentum particle identification detector has been presented , along with its commissioning and installation details .
a leak has been found in one of the 21 radiators and one loose high voltage channel out of 42 high voltage channels has been discovered . however , the physics goals of hmpid are not significantly compromised by the small acceptance loss .
all seven hmpid modules were mounted on the support cradle and installed in the alice magnet in september 2006 .
full control of the hmpid modules has been achieved via the detector control system in july 2007 .
the liquid circulation system is scheduled to be completed at the beginning of 2008 .
hence , the hmpid will be ready for the data taking by the expected start of the lhc in the summer of 2008 .
alice has been specifically optimized to study heavy - ion collisions at the lhc , up to a charged particle density of 8000 per unit of rapidity in central heavy - ion collisions at @xmath0 = 5.5 tev . 
 the high momentum particle identification detector ( hmpid ) has a proximity focusing geometry with a liquid @xmath1 cherenkov radiator coupled to multi - wire pad chambers ( mwpc ) equipped with csi photocathodes , over a total active area of 11 @xmath2 . 
 it has been designed to identify charged pions and kaons in the range 1 @xmath3 3  and protons in the range 2 @xmath3 5 . 
 the as - built detector and all relevant subsystems ( gas , liquid @xmath1 , cooling and control ) are described . 
 installation issues and first commissioning results are also presented .    
 hmpid , alice , rich , csi photocathode    25.6 , 34.8.a , 29.40.ka