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Heavy flavours in heavy ion collisions at the LHC

Heavy flavours in heavy ion collisions at the LHC. Francesco Prino INFN – Sezione di Torino. DNP Fall Meeting, Newport Beach, October 25 th 2011. 3 flavours; (q-q)=0. Heavy Ion Collisions. Basic idea: compress large amount of energy in a very small volume

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Heavy flavours in heavy ion collisions at the LHC

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

  2. 3 flavours; (q-q)=0 Heavy Ion Collisions • Basic idea: compress large amount of energy in a very small volume • produce a “fireball” of hot matter: • temperature O(1012 K) • ~ 105 x T at centre of Sun • ~ T of universe 10 µs after Big Bang • 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 • TC ≈ 170 MeV C≈ 0.6 GeV/fm3 • F. Karsch, • Nucl.Phys.A698 (2002) 199

  3. Heavy quarks as probes of the medium K p e,m D • Hard probes in nucleus-nucleus collisions: • Produced at the very early stage of the collisions in partonic processes with large Q2 • 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 n e,m D D B b quark c quark

  4. 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 -> RAA=1 • Effects from the hot and deconfined medium: -> breakup of binary scaling -> RAA1 • But also cold nuclear matter effects give rise to RAA1 • e.g. Shadowing, Cronin enhancement • Need control experiments: pA collisions • Production of hard probes in AA expected to scale with the number of nucleon-nucleon collisions Ncoll (binary scaling) PbPb measurement pp reference

  5. Heavy quark energy loss • Energy loss DE depends on • Properties of the medium: density, temperature, mean free path • Path length in the medium (L) • Properties of the parton: • Casimir coupling factor (CR) • Mass of the quark (dead cone effect) gluonstrahlung probability • Dokshitzerand Kharzeev, PLB 519 (2001) 199 • Wicks, Gyulassy, Last Call for LHC predictions

  6. Azimuthal anisotropy • Re-scatterings among produced particles convert the initial geometrical anisotropy into an observable momentum anisotropy • Collective motion (flow) of the “bulk” (low pT) • 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 2nd harmonic v2, called elliptic flow • Initial geometrical anisotropy in non-central heavy ion collisions • The impact parameter selects a preferred direction in the transverse plane

  7. Heavy flavour v2 • 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 • v2(b) < v2(c) • Uniqueness of heavy quarks: cannot be destroyed and/or created in the medium • Transported through the full system evolution • J. Uphoff et al., arXiv:1205.4945

  8. PbPb collisions at the LHC Pb-Pb collisions at the LHC  √sNN=2.76 TeV(≈ 14x√sNN at RHIC)  Delivered Integrated luminosity: 10 mb-1 in 2010 166 mb-1 in 2011 3 experiments (ALICE, ATLAS, CMS)

  9. Heavy flavour reconstruction m+ J/y m- Semi-leptonic decays (c,b) Displaced J/y (from B decays) B Lxy e,m Primary vertex B,D jet b-tagging Full reconstruction of D meson hadronic decays • D0K-π+ • D+K-π+π+ • D*+ D0π+ • Ds+K-K+π+

  10. ALICE + ATLAS + CMS • Complementary rapidity and pT coverage DISCLAIMER: acceptance plots refer to published measurements in pp

  11. How to: displaced tracks • Lower mass heavy flavour hadrons decay weakly: • Lifetimes: ≈0.5-1 ps for D and ≈1.5 ps for B • ct:≈100-300 mm for D and ≈ 500mm for B • Possibility to detect decay vertices/displaced tracks • Tracking precision plays a crucial role • 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, arXiv:1205.5423 ALICE MUON ARM ALICE TPC dE/dx vs. p ALICE TOF time (ns) vs. p ALICE EMCAL E/p for TPC e • ALICE, JHEP 09 (2012) 112

  13. ... before going to the results

  14. Is there evidence for parton energy loss?  CMS, EPJC 72 (2012) 1945  ALICE, arXiv:1210.4520 • Charged particle spectra suppressed in AA w.r.t. pp (RAA<1) • Larger suppression at LHC than at RHIC • Maximum suppression for charged particles at pT≈6-7 GeV/c • First results from pilot pPb run confirm that it comes from a final state effect

  15. Are heavy flavours well calibrated probes?  ALICE, arXiv:1205.5423  CMS, EPJC 71 (2011) 1575 Do we understand their production in pp? YES! pQCD predictions agree with data within uncertainties  ALICE, JHEP 1201 (2012)  CMS, PRL 106 (2011) 112001

  16. Nuclear modification factor E E-E

  17. Heavy flavour decay electrons e • 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 TeVpp data sacled to 2.76 TeVfor pT<8 GeV/c • FONLL for pT>8 GeV/c

  18. Heavy flavour decay electrons e • Inclusive electrons – cocktail • = c+b • pp reference: • 7 TeVpp data sacled to 2.76 TeV for pT<8 GeV/c • FONLL(pQCD) for pT>8 GeV/c • Clear suppression in the pT range 3-18 GeV/c • -> amounts to a factor of 1.5-3 in 3<pT<10 GeV/c

  19. Heavy flavour decay muonsat forward rapidity • Single muons at forward rapidity (-4<h<-2.5) • Punch-through hadrons rejected by requiring match with trigger chambers • Subtract background m from p/K decay • Extrapolated from mid-rapidity measurement with an hypothesis on the rapidity dependence of RAA • pp reference measured at 2.76 TeV • Suppression by a factor 2-4 in 0-10% centrality • Less suppression in peripheral collisions m • ALICE, PRL 109 (2012) 112301

  20. Heavy flavour decay muonsat midrapidity • Single muons in |h|<1.05, 4<pT<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. pT • Trend difficult to evaluate due to fluctuations in peripheral bin

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

  22. Can we separate charm and beauty?

  23. D mesons K p • 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 RAA of D from B • D0K-π+ • D+K-π+π+ • D*+ D0π+

  24. D meson RAA • pp reference from measured D0, D+ and D* pT-differential cross sections at 7 TeV scaled to 2.76 TeV with FONLL • Extrapolated assuming FONLL pT shape to highest pT bins not measured in pp • D0, D+ and D*+ RAA agree within uncertainties • Strong suppression of prompt D mesons in central collisions • -> up to a factor of 5 for pT≈10 GeV/c

  25. Charm + strange: Ds+ • Strong Ds+ suppression (similar as D0, D+ and D*+) for 8< pT <12 GeV/C • RAA seems to increase (=less suppression) at low pT • Current data do not allow a conclusive comparison to other D mesons within uncertainties • First measurement of Ds+ in AA collisions • Expectation: enhancement of the strange/non-strange D meson yield at intermediate pT if charm hadronizes via recombination in the medium • Kuznetsova, Rafelski, EPJ C 51 (2007) 113 • He, Fries, Rapp, arXiv:1204.4442

  26. D vs. heavy flavour leptons and light flavours • To properly compare D and leptons the decay kinematics should be considered • pTe≈0.5·pTB at high pTe • Similar trend vs. pT for D, charged particles and p± • Maybe a hint of RAAD > RAAπ at low pT

  27. Data vs. models • Little shadowing at high pT • suppression is a hot matter effect • need pPb data to quantify initial state effect • Models of in-medium parton energy loss can describe reasonably well heavy flavour decay muons at forward rapidity and D mesons at midrapidity D mesons HF muons • ALICE, PRL 109 (2012) 112301

  28. J/y from B feed-down m+ J/y m- B • J/y from B decays to access beauty in-medium energy loss • Long B-meson lifetime -> secondary J/y’s from B feed-down feature decay vertices displaced from the primary collision vertex • Fraction of non-prompt J/y from simultaneous fit tom+m- invariant mass spectrum and pseudo-proper decay length distributions Lxy

  29. RAAof non-prompt J/y • Slow decrease of RAA with increasing centrality • Hint for increasing suppression (-> smaller RAA) with increasing pT  CMS, PAS HIN-12-014

  30. Beauty vs. charm Caveat: different y and pT range • In central collisions, the expected RAA hierarchy is observed: • RAAcharm<RAAbeauty

  31. b-jet tagging • 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-12-003

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

  33. Azimuthal anisotropy

  34. D meson v2 • First direct measurement of D anisotropy in heavy-ion collisions • Yield extracted from invariant mass spectra of Kp candidates in 2 bins of azimuthal angle relative to the event plane -> indication of non-zero D meson v2 (3s effect) in 2<pT<6 GeV/c

  35. Challenge the models • The simultaneous description of D meson RAA and v2 is a challenge for theoretical models

  36. Challenge the models • The simultaneous description of heavy flavour decay electrons RAA and v2 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 RAAbeauty>RAAcharm and RAAbeauty>RAAlight • More statistics needed to conclude on RAAcharm vs. RAAlight • Indication (3s) for non-zero charm elliptic flow at low pT • Hadrochemistry of D meson species • First intriguing result on Ds+ RAA, not enough statistics to conclude

  38. Heavy flavours: what next? • So far, an appetizer • What will/can come in next years (2013-2017): • 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 RAA • More differential studies on beauty • And even more with the upgrades (2018): • High precision measurements of D meson v2 and comparison to light flavours -> charm thermalization in the medium? • Charm baryons (Lc) -> study baryon/meson ratio in the charm sector • High precision measurement of Ds+ RAA and v2 • ...

  39. Backup

  40. D meson dN/dpT

  41. D and charged particle RAA • ALICE, JHEP 09 (2012) 112

  42. D meson RAA: LHC vs RHIC

  43. Heavy Flavour electrons: LHC vs RHIC

  44. Ds/D0 and Ds/D+

  45. RAAof non-prompt J/y • Hint of slow decrease of RAA with increasing rapidity • Non-prompt J/y at midrapidity slightly less suppressed than at forward rapidity

  46. b-jet tagging • 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

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

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