Open charm detection in the ALICE central barrel

1. Open charm detection in the ALICE central barrel Francesco Prino INFN – Sezione di Torino credits: Elena Bruna, Andrea Dainese, Carlos Salgado Alessandria, Dimuon Net, March 29th 2006

2. Physics motivation

3. D s /2 x1 Q2 c x2 s /2 c D q Q Q g g Q g q g Q Q Q Q g Q g Charm production: pp collisions • Hard partonic processes (q-qbar annihilation, gluon fusion) • pQCD phenomenon taking place on short time-scale (≈1/mQ) • Factorized pQCD approach Parton Distribution Functions xa , xb= momentum fraction of partons a, b in hadrons cross-section at hadron level fragmentation z = pD /pc cross-section at parton level

4. Charm production: AA collisions • Hard primary production in parton processes (pQCD) • Binary scaling for hard process yield: • long lifetime of charm quarks allows them to live through the thermalization phase of the QGP and be affected by its presence • Secondary (thermal) c-cbar production in the QGP • mc (≈1.2 GeV) only 10%-50% higher than predicted temperature of QGP at the LHC (500-800 MeV) • Thermal yield expected much smaller than hard primary production • can be observed if the pQCD production in A-A is precisely understood

5. c antishadowing A D shadowing SPS RHIC u D LHC c A Binary scaling break-up • Initial state effects • PDFs in nucleus different from PDFs in nucleon • Anti-shadowing and shadowing • kT broadening (Cronin effect) • Parton saturation (Color Glass Condensate) • Final state effects (due to the medium) • Energy loss • Mainly by gluon radiation • In medium hadronization • Recombination vs. fragmentation Present also in pA (dA) collisions Concentrated at lower pT Only in AA collisions Dominant at higher pT

6. Final state effects: energy loss • BDMPS formalism for radiative energy loss  Baier et al., Nucl. Phys. B483 (1997) 291) • Energy loss for heavy flavours is expected to be reduced by: • Casimir factor • light hadrons originate predominantly from gluon jets, heavy flavoured hadrons originate from heavy quark jets • CR is 4/3 for quark-gluon coupling, 3 for gluon-gluon coupling • Dead-cone effect • gluon radiation expected to be suppressed for q < MQ/EQ • Dokshitzer & Karzeev,Phys. Lett. B519 (2001) 199 • Armesto et al., Phys. Rev. D69 (2004) 114003 average energy loss distance travelled in the medium Casimir coupling factor transport coefficient of the medium

7. y x y x x z Another medium effect: flow • Flow = collective motion of particles (due to high pressure arising from compression and heating of nuclear matter) superimposed on top of the thermal motion • Flow is natural in hydrodynamic language, but flow as intended in heavy ion collisions does not necessarily imply (ideal) hydrodynamic behaviour • Isotropic expansion of the fireball: • Radial transverse flow • Only type of flow for b=0 • Relevant observables: pT (mT) spectra • Anisotropic patterns: • Directed flow • Generated very early when the nuclei penetrate each other • Expected weaker with increasing collision energy • Dominated by early non-equilibrium processes • Elliptic flow (and hexadecupole…) • Caused by initial geometrical anisotropy for b ≠ 0 • Larger pressure gradient along X than along Y • Develops early in the collision ( first 5 fm/c )

8. Experimental observables

9. Observables: RAA • Nuclear modification factor • RAA≠1 binary scaling violation • Low pT  main effect = nuclear shadowing • High pT  main effect = energy loss RHIC LHC

10. Observables: RDh • Heavy-to-light ratio: • sensitive to color charge and mass dependence of parton energy loss • compare gluon ( light hadrons) and charm quark (D) energy loss Charm mass effect  only for pT<10 GeV where also other effects are present Colour charge effect  RDh > 1 due to DEq < DEg  Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027

11. Observables: v2 • Anisotropy in the observed particle azimuthal distribution due to correlations between azimuthal angle of outgoing particles and the direction of the impact parameter

12. Sources of charmed meson v2 • Elliptic flow • Requires strong interaction among constituents to convert the initial spatial anisotropy into an observable momentum anisotropy • Probes charm thermalization • Parton energy loss • Smaller in-medium length L in-plane (parallel to reaction plane) than out-of-plane (perpendicular to the reaction plane) • Drees, Feng, Jia, Phys. Rev. C71, 034909 • Dainese, Loizides, Paic, EPJ C38, 461 • Scattering on pions • Due to elliptic flow, azimuthal distribution of pions is anisotropic

13. 130 GeV Au+Au (0-10%) D from PYTHIA D from Hydro B from PYTHIA B from Hydro e from PYTHIA e from Hydro Charm flow - 1st idea • Batsouli at al., Phys. Lett. B 557 (2003) 26 • Both pQCD charm production without final state effects (infinite mean free path) and hydro with complete thermal equilibrium for charm (zero mean free path) are consistent with single-electron spectra from PHENIX • Charm v2 as a “smoking gun” for hydrodynamic flow of charm

14. Charm flow and coalescence • Hadronization via coalescence of constituent quarks successfully explains observed v2 of light mesons and baryons at intermediate pT • hint for partonic degrees of freedom • Lin Molnar, Phys. Rev. C68 (2003) 044901 • Coalescence of quarks with similar velocities • Charm quark carry most of the D momentum • v2(pT) rises slower for asymmetric hadrons (D, Ds) • non-zero v2 for D mesons even for zero charm v2 (no charm thermalization) • Greco Ko Rapp, Phys. Lett. B595 (2004) 202 • Prediction for v2 of electrons from D decay

15. What to learn from v2 of D mesons? • Low/interediate pT (< 2-5 GeV/c) • recombination scenario • estimate v2 of c quarks • degree of thermalization of charm in the medium • Large pT (> 5-10 GeV/c): • path-length dependence of in-medium energy loss • energy loss in an almond-shaped partonic system  Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257, PLB to appear

16. Parenthesis: J/Y v2 SOURCES of charmonium v2: • Charm elliptic flow • for J/Y formed by c-cbar recombination at hadronization • if charm quarks are early-thermalized • zero J/Y v2 if charm v2 is zero • unlike D mesons which have the contribution from light quark v2 • J/Y nuclear absorption • L (length in nuclear matter) depends on f • L larger out-of-plane than in-plane • J/Y break-up on co-moving hadrons • J/Y break-up by QGP hard gluons • parton density is azimuthally anisotropic • Heiselberg Mattiello, Phys. Rev. C60 (1999) 44902 • Wang Yuan, Phys. Lett. B540 (2002) 62

17. (1) q_hat = 0 GeV2/fm Greco,Ko,Rapp: PLB595(2004)202 (4) dNg / dy = 1000 (2) q_hat = 4 GeV2/fm (3) q_hat = 14 GeV2/fm RHIC results: non-photonic electrons Nuclear modification factor Azimuthal anisotropy The medium is so dense that c quarks lose energy (by gluon radiation) The medium is so strongly interacting that c quarks suffer significant rescattering and develop azimuthal anisotropy

18. Charm in ALICE central barrel

19. muon arm central barrel Charm at the LHC (I) • Large cross-section • Much more abundant production with respect to SPS and RHIC • Small x • unexplored small-x region can be probed with charm at low pT and/or forward rapidity • down to x~10-4 at y=0 and x~10-6 in the muon arm

20. charm 12% sc(DGLAP+non-linear) sc(DGLAP) Charm at the LHC (II) • p-p collisions • Test of pQCD in a new energy and x regime • Test for saturation models • Enhancement of charm production at low pT due to non-linear gluon evolution ? • Reference for Pb-Pb (necessary for RAA) • p-Pb collisions • Probe nuclear PDFs at LHC energy • Disentangle initial and final state effects • Pb-Pb collisions • Probe the medium formed in the collision • WARNING: pp, pPb and PbPb will have different s values • Need to extrapolate from 14 TeV to 5.5 TeV to compute RAA • Small (≈ 10%) theoretical uncertainty on the ratio of results at 14 and 5.5 TeV

21. primary vertex decay length = L q track impact parameter decay vertex Charmed mesons and baryons • Weakly decaying charm states • Mean proper length ≈ 100 mm • Main selection tool: displaced-vertex • Tracks from open charm decays are typically displaced from primary vertex by ≈100 mm • Need for high precision vertex detector (resolution on track impact parameter ≈ tens of microns)

22. ALICE at the LHC Time Of Flight (TOF) Transition Radiation Detector (TRD) Muon arm Time Projection Chamber (TPC) Inner Tracking System (ITS) L3 magnet

23. Heavy-flavours in ALICE • ALICE channels: • electronic (|h|<0.9) • muonic (-4<h<-2.5) • hadronic (|h|<0.9) • ALICE coverage: • low-pT region • central and forward rapidity regions • Precise vertexing in the central region to identify D (ct ~ 100-300 mm) and B (ct ~ 500 mm) decays

24. D mesons: hadronic decays • Most promising channels for exclusive charmed meson reconstruction

25. D mesons in central barrel • No dedicated trigger in the central barrel  extract the signal from Minimum Bias events • Large combinatorial background (benchmark study with dNch/dy = 6000 in central Pb-Pb!) • SELECTION STRATEGY: invariant-mass analysis of fully-reconstructed topologies originating from displaced vertices • build pairs/triplets/quadruplets of tracks with correct combination of charge signsandlarge impact parameters • particle identification to tag the decay products • calculate the vertex (DCA point) of the tracks • good pointing of reconstructed D momentum to the primary vertex

26. Silicon Pixel Detectors (2D) Silicon Drift Detectors (2D) Silicon Strip Detectors (1D) R= 43.6 cm L= 97.6 cm Inner Tracking System • 6 cylindical layers of silicon detectors: provide also dE/dx for particle idetification

27. Time Projection Chamber • Main tracking detector • Characteristics: • Rin 90 cm • Rext 250 cm • Length (active volume) 500 cm • Pseudorapidity coverage: -0.9 < h < 0.9 • Azimuthal coverage: 2p • # readout channels ≈560k • Maximum drift time: 88 ms • Gas mixture: 90% Ne 10% CO2 • Provides: • Many 3D points per track • Tracking efficiency > 90% • Particle identification by dE/dx • in the low-momentum region • in the relativistic rise

28. Time Of Flight • Multigap Resistive Plate Chambers • for pion, kaon and proton PID • Characteristics: • Rin 370 cm • Rext 399 cm • Length (active volume) 745 cm • # readout channels ≈160k • Pseudorapidity coverage: -0.9 < h < 0.9 • Azimuthal coverage: 2p • Provides: • pion, Kaon identification (with contamination <10%) in the momentum range 0.2-2.5 GeV/c • proton identification (with contamination <10%) in the momentum range 0.4-4.5 GeV/c TOF Pb-Pb, dNch/dy=6000

29. Tracking: momentum resolution without vertex constrain with vertex constrain (s=50 mm)

30. rf: 50 mm z: 425 mm central Pb–Pb PIXEL CELL < 60 mm (rf) for pt > 1 GeV/c Two layers: r = 4 cm r = 7 cm Track impact parameter in Pb-Pb • Resolution on track impact parameter mainly provided by the 2 layers of Silicon Pixel Detectors • Interaction point (primary vertex) • x and y coordinates known with high precision from beam position given by LHC (sbeam=15 mm) • z coordinate measured from cluster correlation on the two layers of SPD

31. pp low multiplicity pp high multiplicity Track impact parameter in p-p • Interaction point (x and y) known from LHC with less precision • Due to the need of reduce the luminosity by beam defocusing (sbeam=150 mm instead of 15 mm) • 3D reconstruction of primary vertex with (primary) tracks • Contribution to track impact parameter resolution from primary vertex uncertainty not negligible (especially for low multiplicity events)

32. Particle Identification • Hadron identification in ALICE barrel based on: • Momentum from track parameters • Velocity related information (dE/dx, time of flight, Čerenkov light...) specific for each detector • Different systems are efficient in different momentum ranges and for different particles

33. D meson simulation and reconstruction

34. Charm production at the LHC • ALICE baseline for charm cross-section and pT spectra: • NLO pQCD calculations (Mangano, Nason, Ridolfi, NPB373 (1992) 295.) • Theoretical uncertainty = factor 2-3 • Average between cross-sections obtained with MRSTHO and CTEQ5M sets of PDF • ≈ 20% difference in scc between MRST HO and CTEQ5M • Binary scaling + shadowing (EKS98) to extrapolate to p-Pb and Pb-Pb

35. D0 K-p+ : selection of candidates

36. D0 K-p+: Results (I) central Pb-Pb With dNch/dy = 3000 in Pb-Pb, S/B larger by  4 and significance larger by  2

37. D0 K-p+: Results (II) inner bars: stat. errors outer bars: stat.  pt-dep. syst. not shown: 9% (Pb-Pb), 5% (pp, p-Pb) normalization errors 1 year at nominal luminosity (107 central Pb-Pb events, 109 pp events) + 1 year with 1month of p-Pb running (108 p-Pb events) Down to pt ~ 0 in pp and p-Pb (1 GeV/c in Pb-Pb) • important to go to low pT for charm cross-section measurement

38. D+ K-p+p+ : motivation • Determination of charm cross section • D0/D+ ratio puzzle • Expected value = 3.08 (from spin degeneracy of D and D* and decay B.R.) • Measured value = 2.32 (ALEPH at LEP) • Different systematics • Different selection strategies due to: • D+ has a “longer” mean proper length (ct ~312mm compared to ~123 mm of the D0) • D+ fully reconstructable from a 3-charged body decay instead of the 2 (or 4) body decay of D0

39. D+ → K-p+p+ vs. D0 → K-p+ Advantages • D+ has a longer mean proper length (ct~312 mm compared to ~123 mm of the D0) • D+ → K-p+p+ has a larger branching ratio (9.2% compared to 3.8% for D0 → K-p+) • Possibility to exploit the resonant decay through Kbar0* to enhance S/B Drawbacks • Larger combinatorial background (3 decay products instead of the 2 of the D0→ K-p+) • Smaller <pT> of the decay products (~ 0.7 GeV/c compared to ~ 1 GeV/c of the D0 decay products) • D+ less abundant than D0 (factor 2-3)

40. D+ → K-p+p+: selection of candidates • Single track cuts (pT and d0) • Build Kp pairs • cut on the distance between the DCA point of the 2 tracks and the primary vertex • Build Kpp triplets from accepted Kp pairs Signal Background d0K x d0p2 d0K x d0p2 Cut on d0… d0K x d0p1 d0K x d0p1

41. Straight Line Approximation PT = 0.5 GeV/c d (μm) track Secondary vertex d (μm)= distance between the secondary vertex and the tangent line Primary vertex Decay dist (μm) D+→K-p+p+: decay vertex reconstruction • Calculate the point of minimum distance from the 3 tracks • Tracks approximated as straight lines (analytical method) • Minimize the quantity D2=d12+d22+d32 with:

42. y y x’ y’ rotated x x D+ → K-p+p+: decay vertex resolution π+ π+ K- bending plane D+

43. D+ → K-p+p+: selection of vertices BLACK: signal vertices RED: BKG Kpp vertices BLACK: signal vertices RED: BKG Kpp vertices Track dispersion around decay vertex Distance primary - secondary vertex Cosine of pointing angle BLACK: signal vertices RED: BKG Kpp vertices The histograms are normalized to the same area

44. D+ → K-p+p+: preliminary results • Preliminary because: • Limited statistics of simulated events • Perfect PID assumed • Cuts tuned using all BKG triplets and not only the ones with invariant mass within 1 (or 3) s from D+ mass

45. Ds+ K+K-p+ : motivation I. Kuznetsova and J. Rafelski • Ds probe of hadronization: • String fragmentation: Ds+ (cs) / D+ (cd) ~ 1/3 • Recombination: Ds+ (cs) / D+ (cd) ~ N(s)/N(d) (~ 1 at LHC?) • Chemical non-equilibrium may cause a shift in relative yields of charmed hadrons: • Strangeness oversaturation (gs>1) is a signature of deconfinement • Ds v2 important test for coalescence models • Molnar, J. Phys. G31 (2005) S421.

46. Ds+ K+K-p+ vs. D+ → K-p+p+ Advantages • Smaller combinatorial background if particle identification is efficient (kaons are less abundant than pions) • Larger fraction of Ds+ → K+K-p+ from resonant decays (through Kbar0* or f) with respect to D+ Drawbacks • Ds+ has a smaller mean proper length (ct =147 mm compared to 312 mm of the D+) • Ds+ → K+K-p+ has a smaller Branching Ratio (4.3%) with respect to D+ → K-p+p+ (BR=9.2%) Analysis in progress by Rosetta Silvestri

47. Perspective for D0 energy loss

48. Low pT (< 6–7 GeV/c) Also nuclear shadowing ‘High’ pT (6–15 GeV/c) Only parton energy loss D0 K-p+ : RAA • 1 year at nominal luminosity • 1 month  107 central Pb-Pb events • 10 months  109 pp events

49. D0 K-p+ : heavy-to-light ratios • 1 year at nominal luminosity • 1 month  107 central Pb-Pb events • 10 months  109 pp events

50. Perspective for D+ v2