Lippmann, C.

Thäder, Jochen

GSI Repository

Operation and performance of the ALICE Time Projection Chamber, ahigh-resolution detector for the highest particle multiplicities∗C. Lippmann1 and J. Tha¨der1 for the ALICE Collaboration1ExtreMe Matter Institute EMMI and Research Division, GSI Helmhotzzentrum fu¨r Schwerionenforschung, Darmstadt,GermanyIntroductionThe unique feature of ALICE [1], the general-purposeheavy-ion experiment at the LHC, is the optimization ofthe detector for the reconstruction and identification of par-ticles over a broad momentum range and in an environmentwith the highest particle multiplicities. Using a large cylin-drical Time Projection Chamber (TPC) [2] embedded in asolenoid magnet with a magnetic field of 0.5 T as the mainelement, ALICE is able to provide precision tracking andparticle identification (PID) from 100 MeV/c to 50 GeV/c.Running ConditionsDuring 2012 the LHC was performing outstandingly well.ALICE and the TPC were in continuous data taking mode,while the LHC delivered collisions of protons at high lu-minosity (∼1031cm−2s−1). Compared to the other LHCexperiments the luminosity in ALICE was limited by us-ing special filling schemes, where some of the up to 1,368main bunches per ring collide with the much less populatedsatellite bunches in the interaction region of ALICE. How-ever, this scheme made the experiment quite sensitive tovacuum problems, which increase the rate of backgroundevents to a level that reached a significant fraction of thetotal event rate. Instabilities of the TPC readout chambershad already been an issue in the previous years (in partic-ular in the 2011 Pb-Pb run) and were coped with by re-moving the high voltage capacitors from all TPC readoutchambers in 2011 and by investing in higher performancehigh voltage power supplies in the beginning of 2012.PerformanceFig. 1 shows the relative momentum resolution σ(pT)/pTas a function of the transverse momentum pT. pT is re-constructed from the track curvature, measured in the mag-netic field, and combines the information from the TPCand from the silicon Inner Tracking System (ITS). At lowmomentum σ(pT)/pT is dominated by multiple scattering,but stays at only ∼1% at pT ≈ 1GeV/c, thanks to thesmall material budget (10% of a radiation length betweenthe vertex and the outer wall of the TPC). At higher mo-menta the resolution increases linearly and reaches ∼10%at pT = 50GeV/c for the data sample used here.Charged particles are identified in the TPC in a broadmomentum range. The PID in the TPC is based on themeasurement of the momentum and the specific energy lossper unit path length dE/dx. For a particle with a givenmomentum dE/dx depends only on its charge and mass.∗Work supported by GSI, BMBF, and Helmholtz AllianceHA216/EMMI.Figure 1: The relative pT resolution σ(pT)/pT as a func-tion of pT for combined tracking in TPC and ITS.The functional dependence of the specific energy loss onparticle momenta is shown as lines together with data fornegatively charged particles in Fig. 2. The particle separa-tion capabilities are excellent; the method even allows forthe observation of rare (with charge |z| > 1) anti-nuclei(these particles were enhanced in the data sample used forFig. 2 by the use of an offline trigger).Figure 2: The TPC dE/dx vs p/z in Pb–Pb collisions at√sNN = 2.76 TeV, highlighting the PID performance.The TPC shows excellent performance. After the PbPbruns of November 2010 and 2011 the next highlight andmilestone is the pPb run that is set to take place in Januaryand February 2013. After the two-year LHC shutdown,where the LHC and the experiments will be consolidated,further Pb-Pb runs at nominal LHC energy are foreseen.References[1] K. Aamodt et al. (ALICE Collaboration), JINST 3 (2008)S08002.[2] J. Alme et al., Nucl. Instr. Meth. A 622 (2010) 316.PHN-NQM-EXP-71 GSI SCIENTIFIC REPORT 2012102

Operation and performance of the ALICE Time Projection Chamber, a high-resolution detector for the highest particle multiplicities

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Operation and performance of the ALICE Time Projection Chamber, a high-resolution detector for the highest particle multiplicities

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