aa r X i v : . [ nu c l - e x ] S e p Preparation for heavy-flavour measurements with ALICEat the LHC
Andrea Dainese, for the ALICE Collaboration
INFN - Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy
Abstract
ALICE [1] will study nucleus–nucleus and proton–proton collisions at the LHC. The main goalof the experiment is to investigate the properties of QCD matter at the extreme energy densitiesthat will be reached in Pb–Pb collisions. Heavy quarks (charm and beauty) are regarded aspowerful tools for this study. After briefly reviewing the ALICE heavy-flavour program, we willdescribe the preparation for the first measurements to be performed with pp collisions.
1. Introduction: heavy quarks at the LHC
The measurement of open charm and beauty production in Pb–Pb collisions at √ s NN = . √ s = ff ects in Pb–Pb collisions, are interesting per se ,as a test of QCD in a new domain, 5–7 times above the present energy frontier at the Tevatron.The cc and bb production yields assumed as the baseline for ALICE simulation studies are:for pp collisions at 14 TeV, 0.16 and 0.007, respectively [2] (and lower by about 25% at 10 TeV,the envisaged energy of the first long pp run); for the 5% most central Pb–Pb collisions at5 . ff ect’ is expected to reduce the energy loss of massive quarks withrespect to light partons [4]. Therefore, one should observe a pattern of decreasing suppres-sion when going from the mostly gluon-originated light-flavour hadrons ( h ± or π ) to D and Bmesons. In terms of the Pb–Pb-to-pp nuclear modification factors of the p t -di ff erential yields: R h AA ( p t ) < ∼ R DAA ( p t ) < ∼ R BAA ( p t ) [5].
2. Heavy-flavour measurements in preparation
The ALICE experimental setup, described in detail in [1, 6], allows for the detection of opencharm and beauty hadrons in the high-multiplicity environment of central Pb–Pb collisions at
Preprint submitted to Nuclear Physics A November 21, 2018
HC energy, where a few thousand charged particles might be produced per unit of rapidity. Theheavy-flavour capability of the ALICE detector is provided by: • Tracking system; the Inner Tracking System (ITS) [7], the Time Projection Chamber(TPC) [8] and the Transition Radiation Detector (TRD) [9], embedded in a magnetic fieldof 0 . | η | < . p t resolution better than 2% up to 20 GeV / c and a transverse impact parameter resolutionbetter than 60 µ m for p t > / c (the two innermost layers of the ITS, r ≈ • Particle identification system; charged hadrons are separated via d E / d x in the TPC andvia time-of-flight in the TOF detector; electrons are separated from charged hadrons in thededicated TRD, in the TPC, and in the electromagnetic calorimeter (EMCAL); muons areidentified in the muon spectrometer covering the pseudo-rapidity range − < η < − . • Open charm: fully reconstructed hadronic decays D → K − π + , D + → K − π + π + , D ∗ + → D π + , D → K − π + π − π + , D + s → K − K + π + , Λ + c → pK − π + (under study) in | η | < .
9; singlemuons and di-muons in − < η < − . • Open beauty: inclusive single leptons B → e + X in | η | < . → µ + X in − < η < − .
5; inclusive displaced charmonia B → J /ψ ( → e + e − ) + X (under study); b-tagging ofjets reconstructed in the tracking detectors and in the EMCAL (under study). At present, the installation of most of the ALICE detector is completed and, since December2007, the di ff erent sub-systems are being commissioned and calibrated with cosmic-ray tracks(atmospheric muons) [6, 7]. In view of the heavy-flavour measurements, a crucial part of thecommissioning is represented by the alignment of the ITS, that is the determination of the actualposition and orientation in space of its 2198 Silicon sensors. The alignment, which has to reacha precision well below 10 µ m in order to guarantee a close-to-design tracking resolution, will beperformed using tracks from cosmics and first pp collisions. The first results [7], obtained withcosmics collected during summer 2008, indicate that the target precision is within reach.On the o ffl ine software side, an intense activity for the preparation and refinement of all theanalysis tools is ongoing. In particular, the analysis model using the Grid distributed computingenvironment is being tested. As an example, in a recent campaign, more than 10 pp events,corresponding to about 1 /
10 of the expected yearly statistics, have been simulated by running upto 15,000 simultaneous processes at almost 100 centres worldwide. The simulated data are nowbeing analyzed remotely by several tens of single users. In the following Sections, we report ona selection of results extrapolated to the expected statistics collected by ALICE per LHC year . Among the most promising channels for open charm detection are the D → K − π + ( c τ ≈ µ m, branching ratio ≈ . + → K − π + π + ( c τ ≈ µ m, branching ratio ≈ . d , defined as the track distance of closest approach to the interaction vertex, in the plane transverse to the beams. central (0–5% σ inel ) Pb–Pb events in 1 month at L Pb − Pb = cm − s − and 10 pp events in 8 months at L ALICEpp = × cm − s − , in the barrel detectors; the forward muon arm will collect about 40 times larger samples ofmuon-trigger events (i.e. 4 × central Pb–Pb events); safety factors are included. [GeV/c] t p0 2 4 6 8 10 12 14 16 18 20 d y [ m b / G e V / c ] t / dp D s d -4 -3 -2 -1
101 [GeV/c] t p0 2 4 6 8 10 12 14 16 18 20 d y [ m b / G e V / c ] t / dp D s d -4 -3 -2 -1 FO NLO (MNR)FONLL p K fi D = 14 TeVspp, -4 -3 -2 -1
101 [GeV/c] mint
B p0 5 10 15 20 25 30 ) / d y [ m b ] m i n t > p t ( p B s d -4 -3 -2 -1 FO NLO (MNR)FONLL
B semi-electronic decays = 14 TeVspp,
Figure 1: D p t -di ff erential (left) and B p mint -di ff erential (right) production cross sections in | y | <
1, in pp at 14 TeV,compared to NLO pQCD predictions (MNR [3] and FONLL [10]). Inner error bars represent the statistical errors, outererror bars represent the quadratic sum of statistical and systematic errors. A normalization error of 5% is not shown. decays, reconstructed in the TPC and ITS, in the rapidity range | y | <
1. The detection strategy tocope with the large combinatorial background from the underlying event is based on the selectionof displaced-vertex topologies, i.e. separation of the decay tracks from the primary vertex andgood alignment between the reconstructed D meson momentum and flight direction [2]. Theaccessible p t range for the D is 1–20 GeV / c in Pb–Pb and 0 . / c in pp, with statisticalerrors better than 15–20% at high p t . Similar capability is expected for the D + . The systematicerrors (acceptance and e ffi ciency corrections, centrality selection for Pb–Pb) are expected to besmaller than 20%. The production of open beauty at central rapidity, | y | <
1, can be studied bydetecting the semi-electronic decays of b-hadrons (branching ratio ≃ d ≃ µ m, it is possible to obtain a high-purity sample with a strategy that relies on electron identification (TPC and TRD) and impactparameter cut (to reduce the semi-electronic charm-decay component and reject misidentified π ± and e ± from Dalitz decays and γ conversions). As an example, with 10 central Pb–Pb events,this strategy is expected to allow for the measurement of the b-decay electron p t -differentialcross section in the range 2 < p t <
20 GeV / c with statistical errors lower than 15% at high p t . Similar performance figures are expected for pp collisions. In Fig. 1 we superimpose thesimulated results for D d σ/ d p t d y and B d σ ( p t > p mint ) / d y in pp collisions to the predictionsfrom the MNR [3] and FONLL [10] calculations. The comparison shows that ALICE will beable to perform a sensitive test of the pQCD predictions for c and b production at LHC energy.By comparing to theoretical predictions the expected ALICE precision for the measure-ment of the nuclear modification factors R DAA and R e from BAA , and for the heavy-to-light ratio R e from BAA / R e from DAA , it has been shown [11] that the charm and beauty measurements describedabove can be used to test the expected colour-charge and mass dependence of parton energy loss. Charm and beauty production can be measured also in the forward muon spectrometer ( − <η < − .
5) by analyzing the single-muon p t and di-muon invariant-mass distributions [2]. Themain background to the ‘heavy-flavour muon’ signal is π ± and K ± decays. The cut p t > . / c is applied in order to increase the signal-to-background ratio. Then, a technique that performs a3 [GeV/c] mint p0 5 10 15 20 25 30 ) [ m b ] m i n t > p t ( p D s -3 -2 -1 input distribution (sys. err.~20%) + m - m ‹ reconstructed distribution (sys. err.~15%) m ‹ reconstructed distribution charm [GeV/c] mint p0 5 10 15 20 25 30 ) [ m b ] m i n t > p t ( p B s −3 −2 −1 input distribution (sys. err.~15%) + m − m ‹ reconstructed distribution (sys. err.~20%) m ‹ reconstructed distribution bottom Figure 2: Charm (left) and beauty (right) production measurements in − < y < − .
5, using single muons and di-muons,in pp at √ s =
14 TeV. Boxes represent the systematic uncertainties. Error bars represent the statistical uncertainties. simultaneous fit of the single-muon and di-muon distributions with the charm and beauty compo-nents, using the predicted shapes as templates, allows to extract a p mint -di ff erential cross sectionfor D and B mesons. The expected performance for pp collisions is shown in Fig. 2. Sinceonly minimal cuts are applied, the statistical errors are expected to be lower than 5% up to muon p t ≈
30 GeV / c . The systematic errors, mainly due to the fit assumptions, are expected to belower than 20%. High- p t single muons could provide the first observation of b-quark energy lossat LHC. Indeed, the single-muon p t distribution at LHC energies is expected to be dominatedby b decays in the range 3 < ∼ p t < ∼
25 GeV / c and by W-boson decays above this range. Therefore,the central-to-peripheral muon nuclear modification factor of R CP ( p t ) would be suppressed in theregion dominated by beauty, due to parton energy loss, and would rapidly increase to about one(binary scaling), where the medium-blind muons from W decays dominate [12].
3. Summary
Heavy quarks will provide ways to test di ff erent aspects of QCD under extreme conditionsat the LHC: from the predictions of pQCD at a new energy scale, in pp collisions, to the mech-anism of energy loss in a QCD medium, in Pb–Pb. The ALICE detectors and analysis toolsare being commissioned, with the aim of achieving the excellent tracking, vertexing and particleidentification performance that will allow to accomplish the rich heavy-flavour physics program. References [1] ALICE Collaboration,
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