MEASUREMENTS OF HEAVY-FLAVOUR DECAY ELECTRONS IN PB-PB COLLISIONS - PowerPoint PPT Presentation

MEASUREMENTS OF HEAVY-FLAVOUR DECAY ELECTRONS IN PB-PB COLLISIONS WITH ALICE AT LHC DEEPA THOMAS UNIVERSITY OF TEXAS AT AUSTIN 2015 US LHC USERS ASSOCIATION MEETING 11-13 NOVEMBER 2015

INTRODUCTION Discovery of asymptotic freedom lead to the prediction of a deconfined state of quarks and gluons at high temperature and pressure à Quark Gluon Plasma. A unique way to study QCD matter in lab is by colliding heavy-ions at relativistic energies. • Reaching energy density above 1 GeV/fm 3 Currently at LHC, Pb ions collide at √ s NN = 2.76 TeV. • Run 2 : √ s NN = 5.02 TeV. 2

WHY STUDY HEAVY FLAVOUR? Charm and beauty quarks (heavy quarks) • Mainly produced in hard scattering processes in the initial stage of the collisions with high Q 2 values. • Traverse the medium undergoing elastic and inelastic collisions in the QGP. • Sensitive to the transport properties of the medium. Heavy quarks lose less energy compared to light quarks and gluons in the QCD medium • Color coupling factor à Δ E g > Δ E q • Due to Dead cone effect [1] : suppression of gluon radiation at forward angles θ < M/E . Harder fragmentation à measured hadron properties [1]. Yu. Dokshitzer and D.E. Kharzeev, Phys.Lett. B 519 199 ‐ 206 (2001). are closer to parton properties. 3

HEAVY FLAVOUR HADRONS One way to study heavy-quark production is using its semi-leptonic decay channel. • c, b à l (e, µ) + X. • Large branching ratio (10%). • Leptons can be used as trigger particles. • High momentum electrons à clean signature in electromagnetic calorimeter. • Present here heavy-flavour decay electron (HFE) measurements. 4

� ALICE DETECTOR Detectors used for analysis: Inner Tracking System à tracking, primary vertex reconstruction. Time Projection Chamber à tracking, momentum and dE/dx measurement. Time of Flight à PID. Transition Radiation Detector à Electron ID and trigger. Electromagnetic calorimeter à Energy measurement, EMC trigger.  TPC --- measures dE/dx US Detector contribution : EMCal + VZero DCal (from Run 2) à MinBias trigger, centrality and event plane estimation. Silicon Pixel Detector à MinBias trigger. 5 � �

ELECTRON MEASUREMENT Electron identification • TOF : ±3 σ on electron hypothesis. p K K p π π e e • TPC d E /d x : 0-3 σ of electron Bethe-Bloch TPC π π band. TOF • EMCal : 0.8 < E/p < 1.2 Important background electron sources : Entries 2000 • Photon conversion, Dalitz decays of neutral mesons 20-40% central Pb-Pb, s = 2.76 TeV 1800 NN 1.5 < p < 2 GeV/c and quarkonium decays. T 1600 Background electrons subtracted using : 1400 05/07/2013 electrons 1200 hadrons • Cocktail method 1000 EMCal • Background calculated based on measured 800 pion p T -differential yield ( η and J/ Ψ spectra 600 used when available). 400 • Invariant mass method 200 • Reconstruction of electron-positron pairs from 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 E/p decays of neutral mesons and photon ALI − PERF − 52231 conversions. 6

NUCLEAR MODIFICATION FACTOR ( R AA ) the nuclear < ‘QCD medium’ <N Coll > à average number of binary nucleon-nucleon collision dN PbPb /dp T à measured p T differential cross section in Pb-Pb collisions d σ pp /dp T à reference p T differential cross section in pp collisions at the same √ s as Pb-Pb collisions. R AA = 1 à Absence of nuclear matter effects R AA < 1 à Indicates suppression of the observed yield in Pb-Pb collisions relative to pp collisions. 7

HFE DN/DP T DISTRIBUTION 1 -1 10 ) ) -2 -2 Heavy flavour decay electrons Heavy flavour decay electrons dy ((GeV/c) dy ((GeV/c) 2.76 TeV Pb-Pb (0-10%), | η |<0.6 2.76 TeV Pb-Pb (40-50% central), |y|<0.6 -1 10 -2 pp 7 TeV → 2.76 TeV × 〈 T 〉 (ALICE) 10 pp 7 TeV 2.76 TeV <T > (ALICE) AA → × pp 7 Tev 2.76 TeV T (ATLAS) AA → × 〈 〉 AA -2 FONLL 2.76 TeV (Uncertainty band) 10 FONLL 2.76 TeV (Uncertainty band) -3 10 T /dp 0-10% central -3 T 10 40-50% central /dp 2 Pb-Pb events dN -4 10 Pb-Pb events 2 -4 T dN 10 p π 1/2 T -5 10 p -5 10 π 1/2 -6 -6 10 10 pp reference -7 10 -7 pp reference 10 -8 10 -8 0 2 4 6 8 10 12 14 16 18 10 0 2 4 6 8 10 12 14 16 18 p (GeV/c) p (GeV/c) T T ALI − PREL − 52792 ALI − PREL − 31884 dN/dp T distribution for Pb-Pb collisions and pp references (<T AA > scaled) • pp reference : • p T < 8 GeV/c : 7 TeV data scaled with p-QCD FONLL scaling • p T > 8 GeV/c : p-QCD FONLL prediction • Comparison with FONLL prediction for pp reference (M.Cacciari et al. JHEF 0103 (2001) 006) 8

HFE R AA IN 0-10% AND 40-50% CENTRAL PB- PB COLLISIONS 2 2 AA AA Pb-Pb, s = 2.76 TeV, 0-10% central, |y|<0.6 Pb-Pb, s = 2.76 TeV, 40-50% central, |y|<0.6 Heavy flavour decay electrons R Heavy flavour decay electron R NN NN 1.8 1.8 with pp ref. from scaled cross section at s = 7 TeV with pp ref. from scaled cross section at s = 7 TeV 1.6 1.6 with pp ref. from FONLL calculation at s = 2.76 TeV with pp ref. from FONLL calculation at s = 2.76 TeV 1.4 1.4 1.2 1.2 40-50% central Pb-Pb 0-10% central Pb-Pb 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 • 0 0 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 p (GeV/c) p (GeV/c) T T 1 / Clear suppression of heavy flavour decay dN dp ALI − PREL − 31917 ALI − PREL − 52742 � � PbPb T R electrons (R AA ~ 0.4) w.r.t reference pp � AA / T d dp reference in (0-10%) central Pb-Pb events. AA pp T R AA (0-10%) for p T e < 18 GeV/c R AA of ~ 0.6 observed for semi-central Pb-Pb collisions. • R AA (40-50%) for p T e < 10 GeV/c R AA (0-10%) < R AA (40-50%). 9 • • σ same ¡√s ¡

AZIMUTHAL ANISOTROPY ( V 2 ) OF ELECTRONS One observables sensitive to the dynamics of the early stages of Pb-Pb collision is the azimuthal distribution of the emitted particles in the plane perpendicular to the beam direction. When nuclei collide at non-zero impact parameter the initial matter distribution is anisotropic (almond shaped). • If matter is strongly interacting à spatial asymmetry converted into an anisotropic momentum distribution. • Anisotropy characterized by Fourier co-efficient. • Second moment called elliptic flow (v 2 ). v 2 = h cos[2( φ � Ψ RP )] i ϕ is the azimuthal angle of the particle • Hydrodynamical models can describe the measurements of elliptic flow for light hadrons at low p T (p T < 2 − 3 GeV/c). 10

AZIMUTHAL ANISOTROPY ( V 2 ) heavy-flavour decay electrons (|y|<0.7) compared with heavy- dN d ϕ = N 0 2 π (1 + 2 v 1 cos( ϕ − Ψ 1 ) + 2 v 2 cos( ϕ − Ψ 2 ) + ... ) flavour decay muons (2.5 < y <4) 0.5 0.5 0.5 2 2 2 v v v ALICE Preliminary ALICE Preliminary ALICE Preliminary 0.4 0.4 0.4 ± ± Heavy-flavour decay e , {EP, | ∆ η | > 0.9} , |y| < 0.7 Heavy-flavour decay e ± , {EP, | | > 0.9} , |y| < 0.7 Heavy-flavour decay e , {EP, | ∆ η | > 0.9} , |y| < 0.7 v ∆ η v v 2 2 2 ± ± Heavy-flavour decay µ ± , {2} , 2.5 < y < 4 Heavy-flavour decay µ , {2} , 2.5 < y < 4 Heavy-flavour decay µ , {2} , 2.5 < y < 4 v v v 2 0.3 2 2 0.3 0.3 Pb-Pb, = 2.76 TeV Pb-Pb, = 2.76 TeV Pb-Pb, = 2.76 TeV s s s NN NN NN 20-40% 0-10% Centrality Class 0.2 20-40% Centrality Class 10-20% Centrality Class 0-10% 10-20% 0.2 0.2 0.1 0.1 0.1 0 -0.1 0 0 -0.2 -0.1 -0.1 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 (GeV/ ) p c (GeV/ ) (GeV/ ) T p c p c ALI − PREL − 77612 T T ALI − PREL − 77620 ALI − PREL − 77628 Non-zero v 2 observed in semi-central Pb-Pb collisions. Indication for v 2 (20-40%) > v 2 (10-20%) > v 2 (0-10%). v 2 of heavy-flavour decay electrons consistent with that of HF-decay muons. Confirms strong interaction of heavy quarks with the medium. Supports that charm quarks participate in the collective expansion of the medium. 11

COMPARISON OF R AA AND V 2 WITH THEORETICAL MODELS 0.5 2 2 AA v ALICE Preliminary Heavy flavour decay electron R ALICE Preliminary 1.8 Heavy flavour decay electron ALICE, {EP, | | > 0.9} ∆ η v 2 Pb-Pb, = 2.76 TeV, 0-10% central s 0.4 syst error NN 1.6 with pp ref. from scaled cross section at s = 7 TeV BAMPS el. with pp ref. from FONLL calculation at s = 2.76 TeV BAMPS el. + rad. BAMPS el. 1.4 0.3 POWLANG BAMPS el. + rad. TAMU MC@sHQ+EPOS, Coll+Rad(LPM) 1.2 POWLANG TAMU MC@sHG+EPOS,Coll+Rad(LPM) 0.2 20-40% 1 0.8 0-10% 0.1 0.6 0.4 0 Pb-Pb, = 2.76 TeV s NN 0.2 20-40% Centrality Class, |y| < 0.7 -0.1 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 16 18 p (GeV/c) (GeV/ ) p c T T ALI − PREL − 77686 ALI − PREL − 77576 POWLANG: Eur. Phys. J. C 71 (2011) 1666, J. Phys. G 38 (2011) 124144. BAMPS: Phys. Lett. B 717 (2012) 430 TAMU elastic: arXiv: 1401.3817 MC@ sHQ+EPOS, Coll + Rad (LPM): Phys. Rev. C 89 (2014) 014905 Simultaneous description of HF-decay electron R AA and v 2 is challenging. à Can provide constraints to energy-loss models. Similar picture for heavy-flavour decay muons (and D mesons). 12