Andronic, A.

Fasel, M.

Feldkamp, L.

Lohner, D.

Lu, X.-G.

Pachmayer, Y.

GSI Repository

The electron identification performance of ALICE TRD∗A. Andronic1, M. Fasel1,2, L. Feldkamp3, D. Lohner2, X.-G. Lu2, Y. Pachmayer2 for the ALICECollaboration1EMMI & GSI Darmstadt; 2Universita¨t Heidelberg; 3Universita¨t Mu¨nsterThe Transition Radiation Detector (TRD) is designed toenhance electron identification in the ALICE central barrel(|η| <0.9) and to provide a level-1 hardware trigger (about8μs after a hadron-hadron collision) on electrons with hightransverse momenta and on jets [1]. The electron identifi-cation is achieved by the detection of the specific energyloss and transition radiation (TR). Transition radiation isproduced when a relativistic charged particle (γ & 1000)traverses many interfaces of two media of different dielec-tric constants composing a radiator [2]. On average, in theALICE TRD, for each electron above 1 GeV/c momentum,one TR photon (of 1-30 keV) is absorbed in the high-Z gasmixture (Xe-C02 [85-15]) of the 3.7 cm thick detector.Figure 1: Pion efficiency as a function of electron effi-ciency for the momentum range 1-2 GeV/c for variousmethods (see text).To quantify the TRD identification performance we usethe data collected in proton-proton collisions at√s =7 TeV. Using topological cuts we select clean samples ofelectrons from γ conversions and pions from K0s decaysand employ also TPC and TOF particle identification. Weexpress the performance in terms of the pion efficiency,which is the fraction of pions wrongly identified as elec-trons. The pion suppression factor is the inverse of the pionefficiency. We employ the following methods: (i) trun-cated mean; (ii) one-dimensional likelihood on the total in-tegrated charge (LQ1D); (iii) bidimensional likelihood onintegrated charge in two intervals of drift time (LQ2D);and (iv) neural networks (NN) [4]. The results are com-pared in Fig. 1, where we show, for tracks with signals inall six layers of the TRD, the pion efficiency as a func-∗Work supported by GSI, BMBF, Helmholtz Alliance HA216/EMMI,HQM, and HGS-HIRetion of the electron efficiency. While the truncated meanand the LQ1D are the simplest and, consequently, more ro-bust methods, the full exploitation of the TRD capabilityis reached taking advantage of the temporal pattern of thesignal, utilized in the LQ2D and NN methods. Those bringa further pion suppression factor of about two comparedto the LQ1D method. The present pion suppression fac-tors obtained from collision data confirm the design valuemeasured in testbeams with prototypes [3].Figure 2: Momentum dependence of the pion efficiencyfor the LQ1D, LQ2D and NN methods. The results are for90% electron efficiency.The momentum dependence of the pion efficiency isshown in Fig. 2. The pion suppression improves with in-creasing momentum up to about 1 GeV/c as a consequenceof the strong onset of TR production by electrons in thismomentum range. Above 1 GeV/c TR production starts tosaturate and the relativistic rise of the specific energy lossof pions leads to a reduction of the pion rejection factor.The TRD particle identification is at present successfullyused for the measurement of electrons from heavy-flavourdecays [5] and for the study of J/ψ mesons.References[1] J. Klein, arXiv:1112.5110; see also this Report.[2] A. Andronic, J.P. Wessels, Nucl. Instrum. Meth. A 666 (2012)130.[3] R. Bailhache and C. Lippmann [ ALICE TRD Collab. ], Nucl.Instrum. Meth. A 563 (2006) 310.[4] C. Adler et al. [ALICE Collab.], Nucl. Instrum. Meth. A552,364 (2005).[5] B. Abelev et al. [ALICE Collab.], Phys. Rev. D 86 (2012)112007.PHN-NQM-EXP-73 GSI SCIENTIFIC REPORT 2012104

The electron identification performance of ALICE TRD

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The electron identification performance of ALICE TRD

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