Abstract
A description is provided of the software algorithms developed for the CMS tracker both for reconstructing charged-particle trajectories in proton-proton interactions and for using the resulting tracks to estimate the positions of the LHC luminous region and individual primary-interaction vertices. Despite the very hostile environment at the LHC, the performance obtained with these algorithms is found to be excellent. For tt̄ events under typical 2011 pileup conditions, the average track-reconstruction efficiency for promptly-produced charged particles with transverse momenta of pT > 0.9GeV is 94% for pseudorapidities of |η| < 0.9 and 85% for 0.9 < |η| < 2.5. The inefficiency is caused mainly by hadrons that undergo nuclear interactions in the tracker material. For isolated muons, the corresponding efficiencies are essentially 100%. For isolated muons of pT = 100GeV emitted at |η| < 1.4, the resolutions are approximately 2.8% in pT, and respectively, 10μm and 30μm in the transverse and longitudinal impact parameters. The position resolution achieved for reconstructed primary vertices that correspond to interesting pp collisions is 10–12μm in each of the three spatial dimensions. The tracking and vertexing software is fast and flexible, and easily adaptable to other functions, such as fast tracking for the trigger, or dedicated tracking for electrons that takes into account bremsstrahlung.
Highlights
At an instantaneous luminosity of 1034 cm−2 s−1, typical of that expected at the Large Hadron Collider (LHC), with the proton bunches crossing at intervals of 25 ns, the Compact Muon Solenoid (CMS) tracker is expected to be traversed by about 1000 charged particles at each bunch crossing, produced by an average of more than twenty proton-proton interactions
Compared to the results shown in the previous section for isolated particles, the tracking performance discussed is affected by an additional important feature of LHC events: the large number of hits produced in the tracker at each LHC bunch crossing
These hits originate from the hundreds of primary particles and their interactions selected by the CMS triggers
Summary
At an instantaneous luminosity of 1034 cm−2 s−1, typical of that expected at the Large Hadron Collider (LHC), with the proton bunches crossing at intervals of 25 ns, the Compact Muon Solenoid (CMS) tracker is expected to be traversed by about 1000 charged particles at each bunch crossing, produced by an average of more than twenty proton-proton (pp) interactions. These multiple interactions are known as pileup, to which prior or later bunch crossings can contribute because of the finite time resolution of the detector. The tracking software must run sufficiently fast to be used for offline event reconstruction (of ≈109 events per year), and for the CMS High-Level Trigger (HLT), which processes events at rates of up to 100 kHz
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