Dietel, T.

Kirsch, S.

Klein, J.

Lindenstruth, V.

Pachmayer, Y.

Rettig, F.

Stachel, J.

Wessels, J. P.

Westerhoff, U.

GSI Repository

Triggering with the ALICE TRD∗T. Dietel1, S. Kirsch3, J. Klein2, V. Lindenstruth3, Y. Pachmayer2, F. Rettig3, J. Stachel2,J. P. Wessels1, U. Westerhoff1 for the ALICE Collaboration1Institut fu¨r Kernphysik, Mu¨nster; 2Physikalisches Institut, Heidelberg; 3Frankfurt Institute for Advanced StudiesIntroductionThe Transition Radiation Detector (TRD) in ALICE pro-vides hardware triggers within 7 μs after an interaction, e.g.for electrons and jets. The TRD is azimuthally segmentedinto 18 supermodules, 13 of which are currently installed.Each supermodule consists of 30 individual tracking cham-bers, arranged in 6 layers with 5 chambers each. Besidesthe specific energy deposit, electrons can be identified bythe additional detection of transition radiation produced ina fibre-foam radiator. The signals are used for both trackingand particle identification.The front-end electronics digitizes the amplified andshaped signal. In a digital processing chain, a straight-line fit on cluster positions is calculated. The track seg-ments are characterized by fit parameters and accumulatedcharge. The latter is translated into an electron likelihoodby a tuned look-up table. These data are transferred tothe Global Tracking Unit (GTU). Within 2 μs the tracksegments are combined into TRD global tracks by a 3-dimensional matching algorithm. For every track the trans-verse momentum with a resolution of 20 % down to 10 %in the p⊥ range from 2 to 8 GeV/c and the electron prob-ability (PID) are calculated. Based on this information, avariety of triggers can be implemented.Electron TriggersThe TRD single electron triggers require at least onetrack exceeding configurable p⊥ and PID thresholds. Onlytracks with a contribution in the innermost TRD layer andat least four others are considered in order to maximize therejection. Two combinations of p⊥ and PID thresholds areprovided to the central trigger processor. They have beenin production since September 2012.One trigger is optimized to select electrons from J/ψ →e+ e−. Due to the low mass of 3.1 GeV/c2 of the J/ψ,this trigger has a low transverse momentum threshold of2 GeV/c. A high electron threshold is chosen in order tokeep the rejection sufficiently high, approximately 1500 inpp collisions at 8 TeV.The second trigger extends the p⊥ reach for electronsfrom semi-leptonic decays of heavy-flavor hadrons (opencharm and beauty). The transverse momentum threshold ofthis trigger is set to 3 GeV/c. To achieve a comparable trig-ger rate, the threshold for the electron likelihood is lowerallowing for a higher electron efficiency. Figure 1 showsthe enhancement of electrons above the trigger threshold.∗Work supported by GSI, BMBF, HGS-HIRe, and Helmholtz AllianceHA216/EMMI  	            !" #$%&ﬀ&ﬁ)*ﬂ,Figure 1: Ionization energy loss in the TPC as a functionof momentum for tracks which fulfilled the TRD triggerconditionJet TriggerJets are characterized by an increased local multiplicity.A typical jet area in η−φ used in jet finding, e.g. with anti-kt algorithm with R = 0.4, corresponds approximately tothe area covered by a TRD stack. Thus, events containing3 tracks with transverse momentum above 3 GeV/c withinany TRD stack are triggered, see Figure 2. Despite beingsensitive to charged tracks only, the trigger becomes fullyefficient for jets of ∼ 100 GeV/c.Figure 2: Event display of a di-jet event triggered by theTRD in pp collisions at 7 TeVReferences[1] J. Klein et al., Triggering with the ALICE TRD, NIM A,doi: 10.1016/j.nima.2012.05.011GSI SCIENTIFIC REPORT 2012 PHN-NQM-EXP-74105

Triggering with the ALICE TRD

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Triggering with the ALICE TRD

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