Francesco Prino INFN – Sezione di Torino DNP Fall Meeting, Newport Beach, October 25 th 2011 Heavy flavours in heavy ion collisions at the LHC

2 Heavy Ion Collisions Study nuclear matter at extreme conditions of temperature and density  Collect evidence for a state where quarks and gluons are deconfined (Quark Gluon Plasma) and study its properties  Phase transition predicted by Lattice QCD calculations T C ≈ 170 MeV   C ≈ 0.6 GeV/fm 3 3 flavours; (q-q)=0 Basic idea: compress large amount of energy in a very small volume  produce a “fireball” of hot matter:  temperature O(10 12 K) ~ 10 5 x T at centre of Sun ~ T of universe 10 µs after Big Bang  F. Karsch, Nucl.Phys.A698 (2002) 199

Heavy quarks as probes of the medium Hard probes in nucleus-nucleus collisions:  Produced at the very early stage of the collisions in partonic processes with large Q 2  pQCD can be used to calculate initial cross sections  Traverse the hot and dense medium  Can be used to probe the properties of the medium 3 D K  B e,  D D e,  c quark b quark

Parton energy loss and nuclear modification factor Parton energy loss while traversing the medium  Medium induced gluon radiation  Collisions with medium constituents Observable: nuclear modification factor If no nuclear effects are present -> R AA =1 Effects from the hot and deconfined medium: -> breakup of binary scaling -> R AA  1 But also cold nuclear matter effects give rise to R AA  1  e.g. Shadowing, Cronin enhancement  Need control experiments: pA collisions 4 pp reference PbPb measurement Production of hard probes in AA expected to scale with the number of nucleon-nucleon collisions N coll (binary scaling)

Heavy quark energy loss Energy loss  E depends on  Properties of the medium: density, temperature, mean free path  Path length in the medium (L)  Properties of the parton: Casimir coupling factor (C R ) Mass of the quark (dead cone effect) 5  Wicks, Gyulassy, Last Call for LHC predictions  Dokshitzer and Kharzeev, PLB 519 (2001) 199

Azimuthal anisotropy Re-scatterings among produced particles convert the initial geometrical anisotropy into an observable momentum anisotropy  Collective motion (flow) of the “bulk” (low p T ) In addition, path-length (L) dependent energy loss in an almond- shaped medium induces an asymmetry in momentum space  Longer path length -> larger energy loss for particles exiting out-of-plane Observable: Fourier coefficients, in particular 2 nd harmonic v2, called elliptic flow 6 Initial geometrical anisotropy in non-central heavy ion collisions  The impact parameter selects a preferred direction in the transverse plane

Heavy flavour v 2 Due to their large mass, c and b quarks should take longer time (= more re-scatterings) to be influenced by the collective expansion of the medium  v 2 (b) < v 2 (c) Uniqueness of heavy quarks: cannot be destroyed and/or created in the medium  Transported through the full system evolution 7  J. Uphoff et al., arXiv:

PbPb collisions at the LHC 8 Pb-Pb collisions at the LHC  √s NN =2.76 TeV ( ≈ 14x√s NN at RHIC)  Delivered Integrated luminosity: 10  b -1 in  b -1 in 2011  3 experiments (ALICE, ATLAS, CMS)

Heavy flavour reconstruction 9 L xy B J/  ++ -- Full reconstruction of D meson hadronic decays Displaced J/  (from B decays)Semi-leptonic decays (c,b) jet b-tagging D 0  K - π + D +  K - π + π + D* +  D 0 π + D s +  K - K + π + B,D Primary vertex e, 

ALICE + ATLAS + CMS 10 Complementary rapidity and p T coverage DISCLAIMER: acceptance plots refer to published measurements in pp

How to: displaced tracks Lower mass heavy flavour hadrons decay weakly:  Lifetimes: ≈0.5-1 ps for D and ≈1.5 ps for B  c  ≈  m for D and ≈ 500  m for B Possibility to detect decay vertices/displaced tracks  Tracking precision plays a crucial role 11 Track impact parameter: distance of closest approach of a track to the interaction vertex  ALICE, JHEP 09 (2012) 112

12 How to: particle identification ALICE TPC dE/dx vs. p ALICE TOF time (n  ) vs. p ALICE EMCAL E/p for TPC e  ALICE MUON ARM  ALICE, JHEP 09 (2012) 112  ALICE, arXiv:

... before going to the results 13

Is there evidence for parton energy loss? 14 Charged particle spectra suppressed in AA w.r.t. pp (R AA <1)  Larger suppression at LHC than at RHIC  Maximum suppression for charged particles at p T ≈6-7 GeV/c First results from pilot pPb run confirm that it comes from a final state effect  CMS, EPJC 72 (2012) 1945  ALICE, arXiv:

Are heavy flavours well calibrated probes? 15  CMS, EPJC 71 (2011) 1575  ALICE, arXiv:  ALICE, JHEP 1201 (2012)  CMS, PRL 106 (2011) Do we understand their production in pp? YES! pQCD predictions agree with data within uncertainties

Nuclear modification factor 16 E E-  E

Heavy flavour decay electrons 17 Inclusive electron spectrum with two different PID analyses: TPC+TOF+TRD and TPC+EMCAL Subtract background electrons  Electron pair invariant mass method  Cocktail method Inclusive-background = c+b pp reference:  7 TeV pp data sacled to 2.76 TeV for p T <8 GeV/c  FONLL for p T >8 GeV/c e

Heavy flavour decay electrons 18 Inclusive electrons – cocktail  = c+b pp reference:  7 TeV pp data sacled to 2.76 TeV for p T <8 GeV/c  FONLL(pQCD) for p T >8 GeV/c e Clear suppression in the p T range GeV/c -> amounts to a factor of in 3<p T < 10 GeV/c

Heavy flavour decay muons at forward rapidity 19 Single muons at forward rapidity (-4<  <-2.5)  Punch-through hadrons rejected by requiring match with trigger chambers  Subtract background  from  /K decay Extrapolated from mid-rapidity measurement with an hypothesis on the rapidity dependence of R AA pp reference measured at 2.76 TeV  Suppression by a factor 2-4 in 0- 10% centrality Less suppression in peripheral collisions  ALICE, PRL 109 (2012)

Heavy flavour decay muons at midrapidity 20 Single muons in |  |<1.05, 4<p T <14 GeV/c  Match tracks from Inner Detector and Muon Spectrometer  Use discriminant variables with different distribution for signal and background Background: p/K decays in flight, muons from hadronic showers, fakes Approximately flat vs. p T  Trend difficult to evaluate due to fluctuations in peripheral bin

Electrons vs. muons 21 Similar R AA for heavy flavour decay electrons (|  |<0.6) and muons (2.5<y<4) in 0-10% centrality Direct comparison between R AA and R CP not possible  Assuming ~no suppression for 60-80% centrality -> same size of suppression also for muons in |  |<1.05

Can we separate charm and beauty? 22

D mesons 23 Analysis strategy  Invariant mass analysis of fully reconstructed decay topologies displaced from the primary vertex Feed down from B (10-15 % after cuts) subtracted using pQCD (FONLL) predictions  Plus in PbPb hypothesis on R AA of D from B K  D 0  K - π + D +  K - π + π + D* +  D 0 π +

D meson R AA 24 pp reference from measured D 0, D + and D* p T -differential cross sections at 7 TeV scaled to 2.76 TeV with FONLL  Extrapolated assuming FONLL p T shape to highest p T bins not measured in pp D 0, D + and D* + R AA agree within uncertainties Strong suppression of prompt D mesons in central collisions -> up to a factor of 5 for p T ≈ 10 GeV/c

Charm + strange: D s + 25 Strong D s + suppression (similar as D 0, D + and D* + ) for 8< p T <12 GeV/C R AA seems to increase (=less suppression) at low p T  Current data do not allow a conclusive comparison to other D mesons within uncertainties First measurement of D s + in AA collisions Expectation: enhancement of the strange/non-strange D meson yield at intermediate p T if charm hadronizes via recombination in the medium  Kuznetsova, Rafelski, EPJ C 51 (2007) 113  He, Fries, Rapp, arXiv:

D vs. heavy flavour leptons and light flavours 26 To properly compare D and leptons the decay kinematics should be considered  p T e ≈0.5·p T B at high p T e Similar trend vs. p T for D, charged particles and  ±  Maybe a hint of R AA D > R AA π at low p T

Data vs. models 27 Models of in-medium parton energy loss can describe reasonably well heavy flavour decay muons at forward rapidity and D mesons at midrapidity Little shadowing at high p T  suppression is a hot matter effect  need pPb data to quantify initial state effect HF muons D mesons  ALICE, PRL 109 (2012)

J/  from B feed-down 28 J/  from B decays to access beauty in- medium energy loss  Long B-meson lifetime -> secondary J/  ’s from B feed-down feature decay vertices displaced from the primary collision vertex  Fraction of non-prompt J/  from simultaneous fit to  +  - invariant mass spectrum and pseudo-proper decay length distributions L xy B J/  ++ --

R AA of non-prompt J/  29 Slow decrease of R AA with increasing centrality Hint for increasing suppression (-> smaller R AA ) with increasing p T  CMS, PAS HIN

Beauty vs. charm 30 In central collisions, the expected R AA hierarchy is observed: R AA charm < R AA beauty Caveat: different y and p T range

b-jet tagging 31 Jets from b quark fragmentation identified (tagged) for the first time in heavy ion collisions by CMS jets are tagged by cutting on discriminating variables based on the flight distance of the secondary vertex  Enrich the sample in b-jets  An alternative tagger based only the impact parameter of the tracks in the jet is used as cross check b-quark contribution extracted using template fits to secondary vertex invariant mass distributions  CMS, PAS HIN

Beauty vs. light flavours 32 Low p T : different suppression for beauty and light flavours  BEWARE: 1) not the same centrality 2) B->J/  decay kinematics High p T : similar suppression for light flavour and b- tagged jets

Azimuthal anisotropy 33

D meson v 2 34 First direct measurement of D anisotropy in heavy-ion collisions Yield extracted from invariant mass spectra of K  candidates in 2 bins of azimuthal angle relative to the event plane -> indication of non-zero D meson v 2 (3  effect) in 2<p T <6 GeV/c

Challenge the models 35 The simultaneous description of D meson R AA and v 2 is a challenge for theoretical models

Challenge the models 36 The simultaneous description of heavy flavour decay electrons R AA and v 2 is a challenge for theoretical models

37 Heavy flavours: what have we learned so far? Abundant heavy flavour production at the LHC  Allow for precision measurements Can separate charm and beauty (vertex detectors!)  Indication for R AA beauty >R AA charm and R AA beauty >R AA light  More statistics needed to conclude on R AA charm vs. R AA light Indication (3  ) for non-zero charm elliptic flow at low p T Hadrochemistry of D meson species  First intriguing result on D s + R AA, not enough statistics to conclude

38 Heavy flavours: what next? So far, an appetizer What will/can come in next years ( ):  pPb run -> establish initial state effects  Separate charm and beauty also for semi-leptonic channels  Improved precision on the comparison between charm and light hadron R AA  More differential studies on beauty And even more with the upgrades (2018):  High precision measurements of D meson v 2 and comparison to light flavours -> charm thermalization in the medium?  Charm baryons (  c ) -> study baryon/meson ratio in the charm sector  High precision measurement of D s + R AA and v 2 ...

Backup 39

D meson dN/dp T 40

D and charged particle R AA 41  ALICE, JHEP 09 (2012) 112

D meson R AA : LHC vs RHIC 42

Heavy Flavour electrons: LHC vs RHIC 43

D s /D 0 and D s /D + 44

R AA of non-prompt J/  45 Hint of slow decrease of R AA with increasing rapidity  Non-prompt J/  at midrapidity slightly less suppressed than at forward rapidity

b-jet tagging 46 Jets from b quark fragmentation identified (tagged) for the first time in heavy ion collisions by CMS jets are tagged by cutting on discriminating variables based on the flight distance of the secondary vertex  Enrich the sample in b-jets  An alternative tagger based only the impact parameter of the tracks in the jet is used as cross check b-quark contribution extracted using template fits to secondary vertex invariant mass distributions

b-jet fraction vs. centrality 47 Fraction of b-jets over inclusive jet  Does not show a strong centrality dependence