1. Padova, 18 dicembre 2003 Andrea Dainese 1 Charm production and energy loss in nucleus-nucleus collisions with ALICE Andrea Dainese Università degli Studi di Padova Dottorato di Ricerca in Fisica – Ciclo XVI cc c D0D0 K-K- ++ c

2. Padova, 18 dicembre 2003 Andrea Dainese 2 Outline Introduction: physics of hot and dense QCD matter Heavy-ion physics at the LHC Parton energy loss in the quark-gluon plasma (QGP) the case of heavy quarks (charm) Experimental feasibility for exclusive D meson reconstruction in Pb-Pb (and pp) collisions Can we measure D meson p t distribution in Pb-Pb? energy loss of c quarks in the QGP?

3. Padova, 18 dicembre 2003 Andrea Dainese 3 Physics of hot and dense QCD matter QCD phase diagram hadronic matter exists in different states at high energy density (high temperature and/or high density) nuclear matter undergoes a phase transition to a deconfined Quark-Gluon Plasma (QGP) Temperature Baryonic density U. Heinz “Is it possible to de-confine quarks?”

4. Padova, 18 dicembre 2003 Andrea Dainese 4 Physics of hot and dense QCD matter Lattice QCD (  B ~ 0) estimates the phase transition at: T c ~ 170 MeV and  c ~ 1 GeV/fm 3 ~ 5  nucleus phase transition: pion gas  QGP 3 massless flavours

5. Padova, 18 dicembre 2003 Andrea Dainese 5 Physics of hot and dense QCD matter Temperature Baryonic density U. Heinz 10-  s-old Universe ? Neutron stars ?

6. Padova, 18 dicembre 2003 Andrea Dainese 6 The Little Bang in the lab In high-energy heavy-ion collisions large energy densities (> 2–3 GeV/fm 3 ) are reached over large volumes (> 1000 fm 3 ) QGP evidence at CERN-SPS (up to: Pb-Pb, ) BNL-RHIC extending these results at Next step: LHC with Pb-Pb @ deep deconfinement: “ideal gas” of gluons and quarks large production of hard partons and heavy quarks excellent tools to study properties of QGP

7. Padova, 18 dicembre 2003 Andrea Dainese 7 Deep deconfinement at the LHC LHC: factor 30 jump in w.r.t. RHIC much larger initial temperature hotter, bigger, longer-living study of hotter, bigger, longer-living ‘drops’ of QGP ‘Deep de-confinement’  closer to ‘ideal’ QGP  easier comp. with theory SPS 17 GeV RHIC 200 GeV LHC 5.5 TeV initial T~ 200 MeV~ 300 MeV> 600 MeV volume10 3 fm 3 10 4 fm 3 10 5 fm 3 life-time< 2 fm/c2-4 fm/c> 10 fm/c

8. Padova, 18 dicembre 2003 Andrea Dainese 8 Hard Processes in AA at the LHC Main novelty of the LHC: large hard cross section Hard processes are extremely useful tools large virtuality Q  happen at t = 0  small “formation time”  t ~ 1/ Q (for charm:  t < 1/2m c ~ 0.1 fm/c <<  QGP ~ 5–10 fm/c) Initial yields and p t distributions in AA can be predicted using pp measurements + pQCD + collision geometry + “known” nuclear effects (e.g. PDF nuclear shadowing  suppression of low-x (low-p t ) production) Deviations from such predictions are due to the medium medium formed in the collision Q Pb c c time

9. Padova, 18 dicembre 2003 Andrea Dainese 9 Hard partons probe the medium Partons travel ~ 5 fm in the high colour-density medium Energy loss by gluon bremsstrahlung modifies momentum distributions depends on medium properties PROBE medium probe IN (known from pp, pA + pQCD) probe OUT IN vs. OUT

10. Padova, 18 dicembre 2003 Andrea Dainese 10 QCD process: emitted gluon itself radiates   E  L 2 Parton Energy Loss Due to medium-induced gluon emission Average energy loss (BDMPS model): hard parton path length L Casimir coupling factor: 4/3 for quarks 3 for gluons Medium transport coefficient  gluon density and momenta R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne' and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997) 291. C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [arXiv:hep-ph/0302184]. cold matter QGP @ LHC LHC E  , L = 5 fm Kinematic constraint  E  E significantly reduces effective average  E

11. Padova, 18 dicembre 2003 Andrea Dainese 11 Q Heavy Quarks: dead cone Heavy quarks with momenta < 20–30 GeV/c  v << c Gluons radiation is suppressed at angles < m Q /E Q “dead cone” effect Due to destructive interference Contributes to the harder fragmentation of heavy quarks Yu.L.Dokschitzer and D.E.Kharzeev: dead cone implies lower energy loss Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [arXiv:hep-ph/0106202]. D mesons quenching reduced Ratio D/hadrons (or D/  0 ) enhanced and sensitive to medium properties

12. Padova, 18 dicembre 2003 Andrea Dainese 12 Experimental study of energy loss Quenching can be studied by comparing p t distributions of leading particles in pp and AA Nuclear modification factor: Energy loss R AA < 1 PHENIX @ RHIC  s NN = 130 GeV dead cone ? PYTHIA pp + binary scaling PHENIX

13. Padova, 18 dicembre 2003 Andrea Dainese 13 D 0  K -  + in ALICE Exclusive reconstruction direct measurement of the p t distribution ideal tool to study R AA Weak decay with mean proper length c  = 124  m STRATEGY: invariant-mass analysis of fully-reconstructed topologies originating from (displaced) secondary vertices Measurement of Impact Parameters Measurement of Momenta Particle identification to tag the two decay products

14. Padova, 18 dicembre 2003 Andrea Dainese 14 Background multiplicity in Pb-Pb What is the background to hadronic D decays? combinatorial background given by pairs of uncorrelated tracks with large impact parameter in central Pb-Pb at LHC huge combinatorial background! Simulations performed using need excellent detector response and good selection strategy

15. Padova, 18 dicembre 2003 Andrea Dainese 15 D 0  K -  + : Detection strategy with ALICE Inner Tracking System Measurement of Impact Parameters & Momentum Res. Improvement Time Projection Chamber Track Finding & Measurement of Momenta Time Of Flight Particle Identification (K/  separation) Magnetic field (B = 0.4 Tesla in this study)

16. Padova, 18 dicembre 2003 Andrea Dainese 16 ALICE Barrel |  |<0.9: B = 0.4 T TOF TPC ITS with: - Si pixels - Si drifts - Si strips PIXEL CELL z: 425  m r  : 50  m Two layers: r = 4 cm r = 7 cm  9.8 M

17. Padova, 18 dicembre 2003 Andrea Dainese 17 Tracking Tracking efficiency ~70% with dN ch /dy=6000 pions kaons p t resolution = 1% at 1 GeV/c D 0 invariant mass resolution:

18. Padova, 18 dicembre 2003 Andrea Dainese 18 Impact parameter resolution < 60  m for p t > 1 GeV/c Crucial for heavy-quark ID Systematic study of resolution was carried out

19. Padova, 18 dicembre 2003 Andrea Dainese 19 TOF PID TOF Pb-Pb, dN ch /dy=6000 Optimization for hadronic charm decays was studied: minimize probability to tag K as 

20. Padova, 18 dicembre 2003 Andrea Dainese 20 D 0  K -  + : Signal and background Signal: charm cross section from NLO pQCD (MNR program), average of results given by MRS98 and CTEQ5M PDFs (with EKS98 in Pb-Pb) signal generated using PYTHIA, tuned to reproduce p t distr. given by NLO pQCD contribution from b  B  D 0 (~5%) also included Background: Pb-Pb: HIJING (dN ch /dy=6000 ! we expect 2500 !); pp: PYTHIA; systemshadowing pp 14 TeV 11.210.160.0007 Pb-Pb 5.5 TeV (5% cent) 6.60.651150.5 MNR Program: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295.

21. Padova, 18 dicembre 2003 Andrea Dainese 21 D 0  K -  + : Selection of D 0 candidates Main selection: displaced-vertex selection pair of tracks with large impact parameters good pointing of reconstructed D 0 momentum to the primary vertex d 0 K  d 0  <<0 cos  pointing  1

22. Padova, 18 dicembre 2003 Andrea Dainese 22 S/B initial (M  3  ) S/evt final (M  1  ) S/B final (M  1  ) Significance S/  S+B (M  1  ) Pb-Pb 5  10 -6 1.3  10 -3 11 % 37 (for 10 7 evts, ~1 month) pp 2  10 -3 1.9  10 -5 11 % 44 (for 10 9 evts, ~1 year) D 0  K -  + : Results Note: with dN ch /dy = 3000, S/B larger by  4 and significance larger by  2 0–14 GeV/c1–14 GeV/c Pb-Pb, 5.5 TeV 10 7 events pp, 14 TeV 10 9 events

23. Padova, 18 dicembre 2003 Andrea Dainese 23 D 0  K -  + : d 2  (D 0 )/dp t dy and d  (D 0 )/dy d  (D 0 )/dy for |y| 1 GeV/c (65% of  (p t > 0)) statistical error = 7 % systematic error = 19 % from b = 9 % MC correction = 10% B.R. = 2.4 % from AA to NN = 13 % d  (D 0 )/dy for |y| 0 statistical error = 3 % systematic error = 14 % from b = 8 % MC correction = 10% B.R. = 2.4 %  inel = 5 % inner bars: statistical outer bars: systematic

24. Padova, 18 dicembre 2003 Andrea Dainese 24 Sensitivity on R AA for D 0 mesons ‘High’ p t (6–15 GeV/c) here energy loss can be studied (it’s the only expected effect) Low p t (< 6–7 GeV/c) Nuclear shadowing

25. Padova, 18 dicembre 2003 Andrea Dainese 25 Energy-loss simulation Energy loss simulated using one of the most advanced models* My contributions: path length of partons in the medium dead-cone for heavy quarks estimate transport coefficient for central Pb-Pb collisions at LHC, on the basis of the hadron suppression observed at RHIC *C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [arXiv:hep-ph/0302184]. Q

26. Padova, 18 dicembre 2003 Andrea Dainese 26 Average relative energy loss cold matter QGP @ LHC

27. Padova, 18 dicembre 2003 Andrea Dainese 27 R AA with Quenching R AA ~ 0.4–0.5 increasing at high p t R AA ~ 0.7–0.8 decreasing at high p t

28. Padova, 18 dicembre 2003 Andrea Dainese 28 D/hadrons ratio (1) Ratio expected to be enhanced because: D comes from (c) quark, while , K, p come mainly (~80% in PYTHIA) from gluons, which lose  2 more energy w.r.t. quarks dead cone for heavy quarks Experimentally use double ratio: R AA D /R AA h almost all systematic errors of both Pb-Pb and pp cancel out! D/h ratio: R D/h = R AA D / R AA h R D/h ~ 2–3 in hot QGP sensitive to medium density

29. Padova, 18 dicembre 2003 Andrea Dainese 29 Summary D 0 mesons can be exclusively reconstructed with the ALICE detector despite the high-multiplicity environment of central Pb-Pb coll. This will allow to: measure charm production in 0–15 GeV/c with statistical uncertainty better than 10–15% systematic uncertainty better than 15–20% study the mass and flavour dependence of QCD energy loss

30. Padova, 18 dicembre 2003 Andrea Dainese 30 Back-up Slides

31. Padova, 18 dicembre 2003 Andrea Dainese 31 Heavy ion Physics at the LHC LHC: factor 30 jump in w.r.t. RHIC hotter bigger study of: longer-living ‘drops’ of QGP What are the properties of deconfined matter? > 102 - 4< 2  QGP (fm/c) ~ 10 5 ~ 10 4 ~ 10 3 V f [fm 3 ] 100103  [GeV/fm 3 ] ~ 2500650400dN ch /dy 550020017s 1/2 [GeV] LHCRHICSPS Central collisions  3  10

32. Padova, 18 dicembre 2003 Andrea Dainese 32 Initial- and final-state effects In absence of nuclear effects: Hard processes, and charm production in particular, allow to study and disentangle: Initial-state (cold medium) effects, by comparing pA with pp and with pQCD predictions Nuclear modification of PDFs Final-state (hot medium) effects, by comparing AA, pA, pp (and pQCD) Partonic Energy Loss Thermal Charm Production (?) at low p t (also) at high p t

33. Padova, 18 dicembre 2003 Andrea Dainese 33 Initial-state effects: Shadowing Bjorken-x: fraction of the momentum of the proton ( ) carried by the parton entering the hard scattering At the LHC Pb ion @ LHC ~ 10 5 -10 6 partons (mainly gluons) q q Q charm xaxa xbxb xa+xbxa+xb g Pb (x)/g p (x) Shadowing: reduces initial hard yield at low p t scales trivially from pA to AA “they are so close that they fuse”

34. Padova, 18 dicembre 2003 Andrea Dainese 34 BDMPS model  ktkt vacuum medium

35. Padova, 18 dicembre 2003 Andrea Dainese 35 Dead cone effect Dokshitzer-Kharzeev: energy distribution  dN/d  of radiated gluons suppressed by angle-dependent factor high-energy part of gluon radiation more suppressed strong reduction of quenching effect vanishes for E Q >> m Q (high-p t charm = standard quenching) Dokshitzer D mesons quenching reduced Ratio D/hadrons (or D/  0 ) enhanced and sensitive to medium properties

36. Padova, 18 dicembre 2003 Andrea Dainese 36 Exclusive charm reconstruction D 0  K -   decay allows the exclusive reconstruction of open charm mesons and a direct measurement of the c quark p t distribution In order to probe:  additional cc production  energy loss of c quarks Essential to measure:  total cc production cross-section  p t distribution of charm particles channel D0  K-D0  K- c  D  e c low p t acceptance ~ 0 ~ 2-3 GeV/c (electron ID) c high p t acceptance ~15 GeV/c (statistics) probably > 15 GeV/c S/B separation clean (inv. mass) non-trivial c/b disentangling c  D 0 c  D  e

37. Padova, 18 dicembre 2003 Andrea Dainese 37 LayerDetectorradius [cm] resolution r  [  m]resolution z [  m] 1Si Pixels412120 2Si Pixels712120 3Si Drifts14.93828 4Si Drifts23.83828 5Si Strips39.120830 6Si Strips43.620830 ITS layers

38. Padova, 18 dicembre 2003 Andrea Dainese 38 Signal generation Charm cross section from NLO pQCD (MNR program), average of results given by MRST and CTEQ5M PDFs (with EKS98 in Pb-Pb) Signal generated using PYTHIA, tuned to reproduce p t distr. given by NLO pQCD Also the contribution from beauty was included: systemshadowing pp 14 TeV 11.210.160.0007 Pb-Pb 5.5 TeV (5% cent) 6.60.651150.5

39. Padova, 18 dicembre 2003 Andrea Dainese 39 Classification of the processes Hard scattering: LO graph Processes classified w.r.t. # HVQs in hard scattering final state No double counting because hard scattering is the process with largest virtuality Q Q Q Pair Creation (PC)Flavour Excitation (FE)Gluon Splitting (GS) hard scattering Q QQ parton shower

40. Padova, 18 dicembre 2003 Andrea Dainese 40 Hard cross section in pQCD

41. Padova, 18 dicembre 2003 Andrea Dainese 41 Extrapolation to AA Binary scaling: Glauber model of collision geometry: Shadowing: “a nucleon in the Pb nucleus contains less small-x partons than a free nucleon” b Pb-Pb, b=0 at LHC:

42. Padova, 18 dicembre 2003 Andrea Dainese 42 Tuning of PYTHIA parameters Comparison at the bare quark level Heavy Quarks in PYTHIA: MSEL = 4/5  Leading Order processes settings corresponding to MNR good agreement with MNR LO MSEL = 1  initial and final state Parton Shower processes describe contributions above LO agreement with MNR NLO less good parton shower processes  NLO processes massless Matrix Elements! cross section diverges at p t hard  0 Tuning of parameters less “physics inspired” Main parameter tuned: min. p t hard (2.1 GeV/c for c, 2.75 GeV/c for b)

43. Padova, 18 dicembre 2003 Andrea Dainese 43 Charm NLO: (Pb-Pb)/N coll 5.5 TeV MNRPythia Include shadowing pair creation flavour excitation gluon splitting TOTAL

44. Padova, 18 dicembre 2003 Andrea Dainese 44 Heavy quark fragmentation M. Cacciari, CTEQ School 03

45. Padova, 18 dicembre 2003 Andrea Dainese 45 Fragmentation in PYTHIA Use default parameters Lund string fragmentation model longitudinal fragmentation:  Lund symmetric fragmentation function  Modified to account for harder spectra in HVQ fragmentation transverse momentum pick-up   (p x ) =  (p y ) = 230 MeV/c Qq qqQq z=E+p z z'=f(z) (p x, p y ) (-p x, -p y )

46. Padova, 18 dicembre 2003 Andrea Dainese 46 Effect of fragm. on spectra Average p t -reduction: 25% for charm 15% for beauty Charm Beauty bare quark meson

47. Padova, 18 dicembre 2003 Andrea Dainese 47 3D reconstruction with tracks Track reconstruction in TPC+ITS track seeding uses the position of the primary vertex:  (x, y) from beam position (resolution ~ 150  m)  z from pixels information (resolution ~ 150  m) Vertex reconstruction in 2 steps: VERTEX FINDING: using DCA for track pairs VERTEX FITTING:  give optimal estimate of the position of the vertex  give vertex covariance matrix  give a  

48. Padova, 18 dicembre 2003 Andrea Dainese 48 Expected resolutions Average # rec. tracks = 7 (average on events with # > 1) Average p T of rec. tracks = 0.6 GeV/c Resolutions of track position parameters @ 0.6 GeV/c:  (d0(r  ))  100  m [ d0(r  ) is  to the track! ]  (d0(z))  240  m

49. Padova, 18 dicembre 2003 Andrea Dainese 49 Vertex Finding Algorithm Aim: get a first estimate of the vertex position in (x,y) to be used as a starting point for vertex fitter independent of beam size improved w.r.t. beam size (hopefully) Method: 1.propagate tracks to vertex nominal position 2.calculate DCA (in space) for each possible pair of tracks (using straight line approximation) 3.get estimate of x vtx and y vtx from mean of results from all pairs

50. Padova, 18 dicembre 2003 Andrea Dainese 50 Vertex Fitting Algorithm Tracks are propagated to the point given by the vertex finder (at the moment nominal position used) A  2 is written as the sum of the single track  2 s w.r.t. a generic vertex position r vtx : where W i is track “covariance matrix in global ref. frame” The solution that minimizes this  2 is analytic: vertex covariance matrix

51. Padova, 18 dicembre 2003 Andrea Dainese 51 Tuning of the algorithm Criterion used to reject mismeasured and secondary tracks from the fit: cut on the maximum contribution to the  2  i 2 <   max if   max is too low too many tracks are rejected and we loose resolution if   max is too high bad or secondary tracks enter the fit and we loose resolution This cut is tuned, as a function of event multiplicity, in order to optimize the resolution

52. Padova, 18 dicembre 2003 Andrea Dainese 52 Results: resolutions  (x) = 55  m  (y) = 55  m  (z) = 90  m

53. Padova, 18 dicembre 2003 Andrea Dainese 53 Results: pulls The covariance matrix of the vertex describes correctly the resolutions

54. Padova, 18 dicembre 2003 Andrea Dainese 54 Resolutions VS multiplicity

55. Padova, 18 dicembre 2003 Andrea Dainese 55 Interaction vertex reconstruction Position of the beam in (x,y) given by the machine with very high precision (stable for a long time) “Nominal” size of the beam:  = 15  m in Pb-Pb  = 15  m in pp ( L ~ 10 31 cm -2 s -1 )   150  m in pp (if L is reduced at ALICE IP to ~ 10 29 cm -2 s -1 ) In pp the vertex position has to be reconstructed in 3D using tracks as in Pb-Pb pp with charm pp min. bias

56. Padova, 18 dicembre 2003 Andrea Dainese 56 Fast Simulation Techniques The selection strategy of D 0 mesons has to be optimized using 10 4 -10 5 events for Pb-Pb and 10 6 -10 7 for pp 1 Fully simulated Pb-Pb event takes 1.2 GB and ~12h! A number of fast simulation techniques were used in order to achieve these statistics fast response of the ITS (“fast points”, reduce ITS-time by factor 25) TPC tracking parameterization, specifically developed for charm and beauty studies (reduces total time by factor 40) Checks were done that these approximations don’t affect track resolutions (efficiencies were corrected)

57. Padova, 18 dicembre 2003 Andrea Dainese 57 Results for Pb-Pb (K,  ) Invariant Mass distribution (p T –integrated) (corresponding to 10 7 Pb-Pb events ~ 1 month of ALICE) S/event = 0.0013 (1.3  10 4 D 0 in 1 month) B/event = 0.0116 Statistical Significance of the Signal: after background subtraction The history of signal … Total D 0 / event 141 decaying in K  5.4 with K and  in acc. 0.5 after track rec. 0.14 after ( ,  ) rejection 0.13 after selection cuts 0.0013 … and of background Selection cuts reduce background by a factor ~10 -7 10 -2

58. Padova, 18 dicembre 2003 Andrea Dainese 58 What if multiplicity in Pb-Pb is lower? We used dN ch /dy = 6000, which is a pessimistic estimate Recent analyses of RHIC results seem to suggest as a more realistic value dN ch /dy = 3000 (or less) Charm production cross section: estimate from NLO pQCD (only primary production, no collective effects) average of theoretical uncertainties (choice of: m c,  F,  R, PDF) BKG proportional to (dN ch /dy) 2 We can scale the results to the case of dN ch /dy = 3000: S/B = 44 % SGNC = 74 (this only from scaling, obviously better with retuning of cuts)

59. Padova, 18 dicembre 2003 Andrea Dainese 59 Estimate of the errors Statistical error on the selected signal = 1/Significance Main systematic errors considered: correction for feed-down from beauty (B.R. B  D 0 is 65%!): error of ~8% assuming present uncertainty (~80%) on @ LHC Monte Carlo corrections: ~10% B.R. D 0  K  : 2.4% extrapolation from N(D 0 )/event to d  (D 0 )/dy:  pp: error on (~5%, will be measured by TOTEM)  Pb-Pb: error on centrality selection (~8%) + error on T AB (~10%) Pb-Pb pp

60. Padova, 18 dicembre 2003 Andrea Dainese 60 Comparison with pQCD for pp

61. Padova, 18 dicembre 2003 Andrea Dainese 61 Interpolation pp 14  5.5 TeV In pQCD calculations the ratio of the differential cross sections at 14 and 5.5 TeV is independent of the input parameters within 10% up to 20 GeV/c pQCD can be safely used to extrapolate pp @ 14 TeV to 5.5 TeV Necessary to compare Pb-Pb and pp by R AA

62. Padova, 18 dicembre 2003 Andrea Dainese 62 Effect of shadowing

63. Padova, 18 dicembre 2003 Andrea Dainese 63 Transverse path length L (in Pb-Pb, b<3.5 fm i.e. 5% central) Partons produced at mid-rapidity assumed to travel in the transverse plane Parton production points (x 0,y 0 ) sampled according to density of collisions  coll (x,y) and their azimuthal propagation directions (u x,u y ) sampled uniformly This definition of L is exact only for cylindric profile  coll (r)=  0  (R-r) No exact definition for generic  coll profile! MC sampling varying b in [0,3.5 fm] according to dN/db  b

64. Padova, 18 dicembre 2003 Andrea Dainese 64 Energy-loss probability distribution For given q and parton species, convolution of P(  E|L) and L distribution Energy loss can be sampled from this P(  E) distribution

65. Padova, 18 dicembre 2003 Andrea Dainese 65 Inclusion of dead-cone effect First approximation: dead-cone effect accounted for by folding the energy-loss probability P(  E) with the heavy/light suppression factor F H/L Folding done as function of the quark energy This procedure yields same result as recalculating the quenching weights with Single-gluon emission dominates

66. Padova, 18 dicembre 2003 Andrea Dainese 66 Transport coefficient choice Require for LHC suppression of hadrons as observed at RHIC: R AA ~ 0.2-0.3 for 4<p t <10 GeV/c p t distributions of hadrons at LHC: partons (p t >5 GeV/c) generated with PYTHIA pp, 5.5 TeV (average parton composition: 78% g + 22% q) energy loss: p t ’ = p t –  E (independent) fragmentation with KKP LO F.F. R AA = (p t distr. w/ quenching) / (p t distr. w/o quenching)

67. Padova, 18 dicembre 2003 Andrea Dainese 67 L distribution vs. constant L

68. Padova, 18 dicembre 2003 Andrea Dainese 68 Quenching of charm quarks … Procedure similar to that for light quarks and gluons Differences: kinematic constraint: if  E > p t then c quark is thermalized; assign p t ’ according to thermal m t distribution with T = 300 MeV; in this way the total charm cross section is conserved fragmentation to D mesons via PYTHIA string model

69. Padova, 18 dicembre 2003 Andrea Dainese 69 … and of D mesons

70. Padova, 18 dicembre 2003 Andrea Dainese 70 D/hadrons ratio (2) R D/h is enhanced only by the dead-cone effect Enhancement due to different quark/gluon loss not seen It is compensated by the harder fragmentation of charm p t hadron = z p t parton (p t parton )’ = p t parton –  E (p t hadron )’ = p t hadron – z  E Energy loss observed in R AA is not  E but z  E z c  D  0.8; z gluon  hadron  0.4 (for p t >5 GeV/c)  E c =  E gluon /2.25 (w/o dead cone) z c  D  E c  0.9 z gluon  hadron  E gluon Without dead cone, R AA D  R AA h

71. Padova, 18 dicembre 2003 Andrea Dainese 71 B  D “a` la CDF” Impact parameter of D 0 can be used to separate primary and secondary D 0 Background shape from side-bands in inv. mass Background subtracted pp Pb-Pb