Reichelt, P.

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Prospects of Low-Mass Dielectron Measurements in ALICE with an upgradedCentral Barrel Detector∗P. Reichelt† 1 and the ALICE Collaboration1IKF, Goethe-Universita¨t Frankfurt, GermanyThe measurement of electron-positron pairs in the lowinvariant mass region allows to study the vacuum and in-medium properties of light vector mesons. Dielectrons alsoprobe the production of thermal photons in heavy-ion col-lisions. ALICE is well-suited to perform this measurementdue to its excellent tracking and particle identiﬁcation ca-pabilities at very low momenta. However, Dalitz decaysand photon conversions lead to a high combinatorial back-ground. Additionally, coincident semi-leptonic decays ofcharm and anti-charm hadrons produce a continuum sig-nal, which dominates over a thermal dielectron signal.Both contributions can be reduced by an improved InnerTracking System, to be installed during LHC’s long shut-down 2 (2018). It will further improve the tracking efﬁ-ciency at low pT and provide excellent detection capabili-ties for electrons from secondary vertices like conversionsand heavy-quark decays. Additionally, an upgrade of theTPC readout will substantially increase the data taking rate[1]. We present the expected impact on the low-mass di-electron measurement in Pb–Pb collisions at full LHC en-ergy.)2 (GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2 1.4)-1dy (GeVeedN/dM-410-310-210-110110Bkg Curr ITSBkg New ITSSignal Sum)2 (GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2 1.4SumRapp in-medium SFRapp QGP 10%)± (ρcocktail w/o  20%)± ee (→ cc2.5E7 ’measured’ 0.25%)±Syst. err. bkg. ( = 5.5 TeVNNsPbPb @ 0 - 10%, 2.5E7| < 0.84e|y > 0.2 GeV/ceTp < 3.0t,ee0.0 < p  ALICE  Simulations  ALICE  SimulationsFigure 1: Left: modelled combinatorial background in cen-tral Pb–Pb collisions. Right: composition of the expectedsignal and sampled data points.The electron selection is simulated using data-driven pa-rameterizations of the TPC and TOF detectors. A reducedsolenoid magnetic ﬁeld of B = 0.2 T is assumed to in-crease the low-pT acceptance. ITS standalone tracks areused to reject electrons from unwanted sources. The re-sulting combinatorial background (invariant mass distribu-tion of like-sign pairs) is shown left in Figure 1 for boththe current and planned ITS version. The right side showsthe composition of the real signal, consisting of a hadronic∗Work supported by BMBF, GSI, EMMI and HGS-HIRe† preichelt@ikf.uni-frankfurt.decocktail, open charm decays, and a thermal radiation pre-diction [2]. Also included are ’data’ points sampled ac-cording to the signiﬁcance expected with the current ITSfor 25M central Pb–Pb events and their systematic error.The thermal excess spectrum is obtained by subtractingcocktail and charm and assigning estimated uncertainties asgiven in Figure 1. Figure 2, left panel, shows the resultingexcess spectrum for the current ITS, which is dominated bysystematic errors.The planned ITS is expected to reduce the background andthus its error by a factor of about 2. Pairs from open charmdecays can be suppressed by a factor of 5 using a vertexcut, at the cost of some statistics. This will be compensatedwith the additional TPC upgrade, as 100 times more datacan be taken. The result of these improvements is seen inthe right panel of Figure 2.)2 (GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2 1.4)-1dy (GeVeedN/dM-410-310-210-110110Rapp SumRapp in-medium SFRapp QGP - cockt.c2.5E7 ’meas.’ - cSyst. err. bkg. + cocktailcSyst. err. c = 5.5 TeVNNsPbPb @ 0 - 10%, 2.5E7| < 0.84e|y > 0.2 GeV/ceTp < 3.0t,ee0.0 < p)2 (GeV/ceeM0 0.2 0.4 0.6 0.8 1 1.2 1.4-510-410-310-210-110Rapp SumRapp in-medium SFRapp QGP - cockt.c2.5E9 ’meas.’ - cSyst. err. bkg. + cocktailcSyst. err. c = 5.5 TeVNNsPbPb @ 0 - 10%, 2.5E9| < 0.84e|y > 0.2 GeV/ceTp < 3.0t,ee0.0 < p1  ALICE  Simulations  ALICE  SimulationsFigure 2: Left: thermal excess spectrum with current ITSfor 25M events. Right: same with planned ITS for 2.5Bevents, using a vertex cut to reduce the error from charm.Further implications of such a precise measurement ofthe thermal excess yield are the possibility to determinethe QGP temperature by a ﬁt to the mass region above1 GeV/c2, and we ﬁnd the uncertainties to be within about10 %. A more differential analysis even allows to deter-mine the elliptic ﬂow v2 of dielectrons as function of massor pT, and the absolute uncertainty is expected to be of theorder of σv2 = 1 %.The variety of these measurements will give deeper insightinto the early phase of the heavy ion collisions.References[1] ALICE Collaboration, Upgrade of the ALICE ExperimentLetter Of Intent, CERN-LHCC-2012-012, (2012).[2] R. Rapp and J. Wambach, Eur. Phys. J. A6 (1999) 415, andprivate communication.GSI SCIENTIFIC REPORT 2012 PHN-NQM-EXP-6293

Prospects of Low-Mass Dielectron Measurements in ALICE with an upgraded Central Barrel Detector

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Prospects of Low-Mass Dielectron Measurements in ALICE with an upgraded Central Barrel Detector

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