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lattice calculations of quantum chromodynamics ( qcd ) , predict that at a critical temperature ( @xmath1 170 mev ) and energy density ( @xmath1 1 gev/@xmath2 ) , the nuclear matter undergoes a phase transition to a deconfined state of quarks and gluons  @xcite , also known as quark gluon plasma ( qgp ) . in the laboratory , the qcd phase diagram and the expected formation of the qgp can be studied using ultra - relativistic heavy - ion collisions .
the suppression  @xcite or enhancement  @xcite of heavy quark resonances in heavy - ion collisions at lhc is predicted to be a strong signature of qgp as the temperature of the fireball produced in pb - pb collisions is expected to be about three times higher than the critical temperature .
alice ( a large ion collider experiment  @xcite ) is a general purpose experiment whose detectors identify and measure hadrons , electrons , photons and muons produced in p - p and pb - pb collisions at the cern lhc .
alice is optimized for heavy - ion reactions and thus is capable to track and identify particles from very low ( @xmath1 100 mev / c ) to fairly high ( @xmath1 100 gev / c ) transverse momentum , to reconstruct short lived particles like hyperons , d and b mesons and the heavy quark resonances [ j/@xmath3 ( @xmath4 ) , @xmath5 ( @xmath6 ) ] in an environment of extreme particle densities .
the j/@xmath3 and @xmath5 are reconstructed by alice in the central rapidity region ( @xmath7 ) from their dielectron decay channel and at large rapidity ( @xmath8 ) from their dimuon decay channel using a spectrometer .
this muon spectrometer [ fig .
[ fig : ms_pt_cut_efficiency ] ( top ) ] is designed to run at the highest dimuon rate in heavy - ion collisions at lhc ( @xmath9  @xmath10  s@xmath11 for pb - pb beam ) .
it consists of the following components : a passive front absorber to absorb hadrons and photons from the interaction vertex ; high granularity tracking system of 5 stations each with two detection planes ; a large warm dipole magnet ; a passive muon filter wall , followed by four planes of trigger chambers and a inner beam shield surrounding the beam pipe to protect the chambers from high particle flux at large rapidities .
each tracking chamber provides two dimensional hit information by measuring the charge distributions on two segmented cathode planes .
the cathode which has higher resolution in the direction ( y - direction ) perpendicular to the plane containing the magnetic field ( x - direction ) and beam axis ( z - direction ) is referred to as the bending cathode , while the other along the magnetic field is referred to as non - bending cathode .
two tracking stations are placed before , one inside and two after the dipole magnet .
the total number of readout pads is about 1.1 million and covers an area of about 100 m@xmath12 .
the trigger system consists of four resistive plate chamber ( rpc ) planes arranged in two stations which are placed behind the muon filter .
the trigger system has to select events containing a muon pair coming from the decay of j/@xmath3 or @xmath5 resonances from all possible background contaminations .
the main background comes from low-@xmath13   muons of pion and kaon decays .
thus , the l0 trigger is generated if at least two tracks with opposite charge ( or the same charge used in this case for background subtraction ) , both above a predefined @xmath13   cut are detected in an event .
two different @xmath13   thresholds of 1 gev / c and 2 gev / c have been chosen for j/@xmath3 and @xmath5 measurement , respectively , according to simulation studies .
however , the coarse grained segmentation of the rpcs and the presence of the iron wall before the trigger stations , do not allow a sharp @xmath13   cut .
this is demonstrated in fig .
[ fig : ms_pt_cut_efficiency ] ( bottom ) , through a simulation study carried out using the aliroot framework  @xcite .
it is evident that the l0 trigger passes a substantial number of events which are below the @xmath13-threshold .
in addition , the trigger efficiency is around 60% near the threshold value of 1 gev / c and 2 gev / c .
thus , the primary task of the dimuon high level trigger ( dhlt ) is to refine the @xmath13   cut in order to increase its selectivity . for this purpose , in the addition to the trigger chambers , the slower but more accurate tracking chambers have been used and a real - time event reconstruction scheme for the entire muon spectrometer has been developed .
this scheme should satisfy the following criteria :    \(a ) the event processing should be done at a rate of 1 khz for pb - pb collisions ; ( b ) the real - time reconstruction should produce results of appreciable quality without losing any signal event ; ( c ) it should be robust and should not stop processing due to any data corruption in the input buffer .
the event reconstruction in tracking chambers is a two step process , which involves the reconstruction of charge clusters and the track formation using the reconstructed hit points . in offline reconstruction ,
these two steps are implemented by mathieson fitting of the charge clusters which are defined by the nearest neighbour algorithm and kalman filtering , respectively . however
, none of them can be applied for real - time reconstruction due to the time constrain set by the expected event rate of 1 khz .
thus , new algorithms for fast reconstruction have been developed for dhlt  @xcite .
these are described below .
* hit - reconstruction : * : :    this new algorithm does not identify the charge clusters by nearest    neighbour search . instead ,
it searches for pads with maximum charge    whose immediate neighbours have nonzero charge .
the two schemes are    equivalent since every cluster has an unique central pad whose charge    is greater than other members . once the central pads are identified ,    the hit positions along the bending and non - bending directions are    given by the centre of gravity of the charges measured on the central    pad and the two pads around it in y and x directions , respectively .    finally the information on bending ( y - direction ) and non - bending    ( x - direction ) cathodes are merged to generate hit position .
* partial tracking : * : :    in the first attempt to improve the accuracy of the l0 trigger , the    trigger track segments are extended upto the fourth tracking    station  @xcite .
the straight line tracks in the two tracking    stations after the magnetic field give a better    @xmath13   estimation than the value obtained from    the trigger tracks .
the result of this improvement is shown in    fig .
[ fig : ms_pt_cut_efficiency ] ( bottom ) , by the solid lines which are    marked by ` hlt partial tracker ' .
it is evident that there is a marked    improvement in accuracy over the l0 trigger , but the trigger    efficiencies at @xmath13   = 1 gev / c and 2 gev / c    are about 20% and the desired efficiency ( @xmath1490% ) is    achieved only around 1.5 gev and 2.5 gev , respectively .
thus , this    method is not suitable for dhlt and the full tracking through the    magnetic field is essential .
* full tracking : * : :    the full tracking formalism creates tracks from the last trigger    station to the first tracking station .
various tracking stages has    been shown in fig .
[ fig : ms_pt_cut_efficiency](top ) .
these are    described below .
+    _ tracking in station 4,5 : _ ; ;      this part is identical to the partial tracking method where the      straight line tracks are extrapolated upto the fourth tracking      station .    _
tracking in station 1,2 : _ ; ;      in this part , the small track segments are formed in station 1 and 2      using cellular automata  @xcite ( ca ) method .
the ca formalism has      been followed since it does not require any a priori knowledge of      the seed for the tracking .    _
kalman @xmath15-test : _ ; ;      once the track segments are formed before and after the dipole      magnet , they are matched through the magnetic field using      @xmath15 test of kalman filtering  @xcite .
if the track      segments are not matched , the @xmath13   of the      track is estimated as in the case of partial tracking .
this ensures      that no physics event is lost in the full tracking scheme .
_ track extrapolation : _ ; ;      in case of complete tracks , the @xmath13       estimation is further improved by incorporating the corrections due      to energy loss and multiple coulomb scattering in the front      absorber .
the accuracy and efficiency of the @xmath13-cut for the full tracker is shown in fig .  [
fig : ms_pt_cut_efficiency ] ( bottom ) , by the solid line marked as ` hlt full tracker ' and have been compared with the offline reconstruction .
it can be observed from this simulation study that the estimated @xmath13 s of the muon tracks match closely with the offline analysis and the desired improvement of the l0 can be achieved .
the framework of the full tracker was successfully implemented on the hlt computing farm and all the validation tests were completed before the pp collisions at @xmath0  tev data taking in march , 2010 .    in fig .
[ fig : offline_hlt_trigger_cut ] , the effect of the @xmath13-cut from the offline analysis and from the real - time full tracker has been compared for pp ( `` real data '' ) collisions .
the top and bottom panels of the figure show the invariant mass plot around the j/@xmath3 peak when the @xmath13   of one muon was found to be greater than 1 gev / c by the offline and online reconstruction , respectively .
the invariant spectra are fitted with a gaussian signal and a double exponential functions .
the width of the j/@xmath3 peak was found to be 93.8 and 95.5 mev / c@xmath12 for offline and hlt triggers , respectively .
this shows that a sufficient accuracy of the @xmath16-cut for the full tracker is achieved .
however , the counts in the j/@xmath3 peak for the offline reconstruction is higher by 7% with respect to the online reconstruction .
this indicate a small inefficiency of the online trigger .
studies are ongoing to correct this effect .
it has been demonstrated both from simulation and data , that the hlt for the muon spectrometer is capable to improve the trigger momentum resolution of the tracks and validate the l0 muon candidates by a sharp @xmath13   cut . in the coming years of lhc operation ,
when the luminosity of the beams will reach its nominal values , the dhlt will play a crucial role in background rejection and allow the muon spectrometer to run at higher l0 trigger rates .
in addition , it is estimated that the use of dhlt , with a loosening of the l0 @xmath13   cut , will allow recovering about 20% of the low-@xmath13   muon events .
the muon spectrometer ( ms ) of the alice experiment at lhc is equipped with a hlt ( high level trigger ) , whose aim is to improve the accuracy of the trigger cuts delivered at the l0 stage . 
 a computational challenge of real - time event reconstruction is satisfied to achieve this software trigger cut of the hlt . 
 after the description of the online algorithms , the performance of the online tracker is compared with that of the offline tracker using the measured pp collisions at @xmath0 tev .    
 alice , hlt , muon spectrometer , online reconstruction , software trigger