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Charm as a probe of small-x gluons at LHC

Charm as a probe of small-x gluons at LHC. Andrea Dainese (INFN Legnaro). Layout. Small-x gluon dynamics, the onset of saturation and nuclear shadowing Study of small-x effects with charm in ALICE Appendix: status of open charm reconstruction code open items

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Charm as a probe of small-x gluons at LHC

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  1. Charm as a probe of small-x gluons at LHC Andrea Dainese (INFN Legnaro) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  2. Layout • Small-x gluon dynamics, the onset of saturation and nuclear shadowing • Study of small-x effects with charm in ALICE • Appendix: • status of open charm reconstruction code • open items • more details: presentation at PWG3 meeting, Sept 15th IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  3. Parton densities at small-x • DIS ep collisions probe distributions of partons in the proton: • HERA: strong rise of F2(x,Q2) at small-x: Q 2= “resolving power” Bjorken x = momentum fraction carried by parton Q2 F2 ,F3 ,FL= proton structure functions, (y = inelasticity) g couples directly only to (sea) quarks ∂lnF2/∂lnQ2 gluons IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  4. (x,Q2) evolution of PDFs • Q2 - DGLAP (kT-ordered emission): F2(Q2)~asln(Q2/Q02)n, Q02 ~1 GeV2 • x - BFKL (pL-ordered emission): F2(x)~asln(1/x)n • Linear equations (single parton splitting) fail at low x (even more for multi-parton systems = nuclei) Onset of Saturation: (i) Standard (linear) approach to PDF evolution invalid nonlinear gluon-gluon fusion balances gluon splitting (ii) Factorisation assumptions (pert. QCD) invalid parton scattering not incoherent Color Glass Condensate: Collision of 2 classical saturated fields (full parton coherence) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  5. Saturation scale Qs2 • Onset of non-linear QCD when gluons are numerous enough (low-x) & extended enough (low-Q2) to overlap: • Pb/Au nucleus (larger parton tranverse density) amplifies saturation effects by factor about 6 (2001/3) [GA(x,Q2)=A g(x,Q2)] (l~0.3) Saturation for: low x, large s, large y, large A Qs2~2 GeV2 (RHIC, dAu @ 200 GeV)  x < 10-3-10-2 Qs2~5 GeV2 (LHC, pPb @ 8.8 TeV)  x < 10-4-10-3 poorly known PDFs in p and A IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  6. Low-x (<10-2) gluon PDF (proton) • Most of our current knowledge of low-x gluons comes indirectly from F2 “scaling violations”: • Large uncertainties below x~10-2 at moderate Q2 (<5 GeV2) R.D. Ball et al. arXiv:0808.1231 Q2 = 2 GeV2 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  7. Low-x (<10-2) gluon PDF (nucleus) • Current data on low-x gluons from: nuclear F2, nuclear Drell-Yan (eA) • DGLAP analysis: linear evolution + nuclear shadowing • Shadowing  low-x gluon “fusion” or “recombination”: gx1 + gx2 gx1+x2 • Shadowing factor for PDFs: xGA(x,Q2) = A xg(x,Q2) RGA(x,Q2) • Most of data in non-pert. range (Q2<1-2 GeV2)  limited applicability of pQCD analysis  large uncertainties (> factor 2!) Armesto, JPG32(06)R367 D.d’Enterria,JPG30 (05)S767 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  8. Experimental study of saturation:(x,Q2) acceptances & probes • Probes at RHIC (Q2<2): • bulk dNch/dy • forward hadrons (pT<3-4 GeV) Qs2RHIC=2 D.d’Enterria,JPG30 (05)S767 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  9. LHC y=4 LHC y=0 Qs2LHC=5 • Probes at LHC (Q2<5): • bulk dNch/dy • forward J/y • charm new! Experimental study of saturation:(x,Q2) acceptances & probes • LHC: study “saturation” with perturbative probes • LHC (Pb): unexplored region (A=208, x<10-3, Q2 < 5 GeV2) large uncertainties in p and Pb PDFs • Probes at RHIC (Q2<2): • bulk dNch/dy • forward hadrons (pT<3-4 GeV) Qs2RHIC=2 D.d’Enterria,JPG30 (05)S767 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  10. charm beauty Cartoon Heavy quark production at LHC:a probe of small-x gluons • Probe small-x region with HQs at low pT and/or forward y • Charm production at low pT: Q2 (4mc2 ~5 GeV2) ~ Qs2 • Down to x~10-4 with charm at y=0, x~10-6 at y4 • Large pert. yields: ~0.1 cc/ev in pp, ~100 cc/ev in Pb-Pb PYTHIA charm IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  11. D.Stocco Low-x at LHC: forward J/y in ALICE (2.5 <|h|< 4) • J/ measurement in m-spectrometer  xg(x) at x2~10-5 : ds/dy J/: NLO CEM, varying PDFs pp @ 14 TeV QQbar: Sensitive to different PDFs and DGLAP versus CGC predictions (Note: mJ/~Qs at the LHC) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  12. Heavy quark production at LHC:a probe of small-x gluons (pp) • Large pQCD uncertainties for charm pT0 • onset of non-linear effects / saturation ? • Two attempts to include non-linear terms in evolution equations • DGLAP+GLRMQ • BK indicate charm as a sensitive probe in pp Eskola et al., NPB660 (2003) 211 Kutak, Kwiecinski, Martin, Stasto |y| < 1 DGLAP: Charm enhancement due to non-linear effects (GLR-MQ term) BK: Charm suppression due to non-linear effects IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  13. CGC: very similar RpA for p, D, B, ~indep. of pt and rapidity Heavy quark production at LHC:a probe of small-x gluons (pPb) • Saturation scale Qs2(x) ~ xg(x)A/RA2 ~ xg(x)A1/3 • At LHC for x~10-4, Qs~2 GeV > mc • For mT,c~Qs, charm prod. CGC-dominated: • scales with Npart in pA (not Ncoll) • harder pT spectra, since typical kT~Qs~2 GeV, while • in standard factorization kT~LQCD~0.2 GeV IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese Kharzeev, Tuchin, NPA 735 (2004) 248 + LHC Predictions Workshop

  14. Heavy quark production at LHC:a probe of small-x gluons (pPb) • Charm production at low pT sensitive to shadowing parametrisations • Sensitive to multi-parton interactions in pPb: • cccc / cc ~ 10% • probe “many-body” PDFs K.Eskola et al. JHEP 0807 (08)102 Cattaruzza, Del Fabbro, Treleani, PRD 70 (2004) 034022 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  15. Low-pT charm in ALICE (|η|<1) • Open charm reconstruction down to pT=0 in the ALICE central barrel D0K-+ R.Grosso 1 LHC year (2009) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  16. Status of software toolsfor open charm analysis IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  17. Charm Analysis Scheme Charm “production” (ESD/AOD  AOD for Charm) Signal selection Invariant mass analysis and significance maximization (in bins of pt, y, f  FRP…) Correction for efficiencies, acceptance, BR Cross section normalisation IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  18. AliAnalysisVertexingHF class BA,LNL,TO single-track cuts on pt and d0, (PID...) [common cuts for all analyses] build all (+,-) pairs and compute secondary vtx for D+,Ds+,Lc+ for D0Kp, J/yee loop on all tracks (+ & -): build triplets, create AliAODRecoDecayHF3Prong, apply reco cuts (common for the 3 particles?) create AliAODRecoDecayHF2Prong apply reco cuts tight D0 mass cut loop on all tracks for D* candidates for D0Kppp loop on all tracks (+ & -) … create AliAODRecoDecayHF … store D0 store D0 store D*+ store J/y store D+, Ds+, Lc IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  19. Output in AliAODEvent AliAODEvent • All the I/O managed by AliAODHandler & AliAODInputHandler • Macro to read the candidates: ReadAODVertexingHF.C aodTree in AliAOD.root (standard AOD, unchanged ) friend aodTree in AliAOD.VertexingHF.root vertices (from ESD) AliAODVertex AliAODVertex AliAODVertex ... AliAODTrack AliAODTrack AliAODTrack ... tracks AliAODVertex ... AliAODRecoDecay AliAODRecoDecay ... verticesHF D0toKpi, ... not yet ... IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  20. Open items (hadronic charm) • AliAnalysisVertexingHF: • possibility to start from AOD (AODs will be are smaller than ESDs  faster analysis; will be replicated in more Tiers  faster access) • ITS alignment & impact parameter resolution • alignment result from cosmics suggest that we can reach the target precision ( S.Moretto) • estimate residual misalignment & impact parameter resolution from data • User analysis tools • event mixing for background subtraction: try using official event mixing framework (AliAnalysisTaskME) • PID • optimize cuts on TOF PID & play with Bayesian method (priors) • Correction Framework • possibility start from AOD, adapt to charm (selection cuts...) • Correction for b feed-down • prepare subtraction using MC with QCD cross section and errors (collab. muon people and theorists) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  21. Summary • Gluon saturation & non-linear evolution must set-in at (some) low-x in the hadrons → Basic info on high-energy limit of QCD • LHC = unique lab to study high parton density dynamics in p,Pb down to x~10-6 using perturbative probes (charm) • some hints already in pp • crucial to have p-Pb run • LHeC (Large Hadron electron Collider – under discussion within EFCA) would be “the” machine for this physics • ALICE has good capality with charm(onia) at low pT and forward rapidity • Preparation of D0 / charm analysis in progress (need to focus on background estimate and on corrections) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  22. EXTRA SLIDES IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  23. c c Charm at s = 14 TeV: x,Q2 range (1) • Simple estimate: IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  24. Data: mc = 1.2 GeV, Q2 = 4mT2 and enhancement Data: mc = 1.3 GeV, Q2 = mT2 and enhancement How to detect the enhancement due to nonlinear effects? • The idea is that the effect (enh. only at very low pt) cannot be mimicked by NLO pQCD • In practice: consider ratio “Data/Theory” for all reasonable choices of theory parameters Dainese, Bondila, Eskola, Kolhinen, Vogt, JPG30 (2004) 1787 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  25. Experimental study of saturation:acceptances & probes A=200 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  26. Saturation in heavy-ion collisions:(x,Q2) acceptances & probes • Probes at RHIC (Q2<Qs2=2): • bulk dNch/dy • forward hadrons (pT<3-4 GeV) Qs2RHIC=2 D.d’Enterria,JPG30 (05)S767 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  27. Saturation hints at RHIC (1):hadron multiplicity dNch/dh • AuAu (200 GeV) 0-5% most central collisions: • Reduced multiplicity predicted by saturation models: gluon recombination reduces incoming parton flux Predicted multiplicites: dNch/dh ~ 650 charged particles at y=0 dNch/dh IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  28. Saturation hints at RHIC (2): geometrical scaling of dNch/dh • CGC: final hadron multiplicity µ initial number of released gluons µ Qs2: • Centrality & √s dependence well described: Collision of 2 classical (saturated) fields + “local parton-hadron duality” (1 gluon = 1 final hadron) Armesto, Salgado, Wiedemann, PRL94 (2005) 022002 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  29. Saturation hints at RHIC (3): forward hadron suppression • h~0 (x~10-2): hard hadroprod. described by NLO pQCD (+little shadowing) • h=3.2 (x~10-3): suppressed hadron production (pT<4 GeV/c) better described by CGC than pQCD: reduced partonic flux in Au at low-x |h| < 0.5 h = 3.2 pQCD+shadowing CGC IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  30. Saturation at LHC:charged multiplicity CGC: + “local parton-hadron duality” (1 gluon = 1 final hadron) • Saturation-driven • predictions: • dNch/dh = 1500 • day-1 measurement for all experiments LHC Predictions Workshop, compiled by N.Armesto IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  31. GLR-MQ vs BK: gluons R = 5 GeV-1 = 1 fm R = 4 GeV-1 = 0.8 fm Caveat: LO! Linear (dashed) vs non-linear (solid) in BK with R = 4 GeV-1 Linear (blue) vs non-linear (red) in DGLAP Kutak, Kwiecinski, Martin, Stasto Eskola, Honkanen, Kolhinen, Qiu, Salgado, NPB660 (2003) 211 IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  32. Effect of ITS misalignment on d0 resolution • Impact parameter resol: strack = ascatter/pt  bmeas  cmisalign • Effect studied with full simulation of exptected initial (full+) and residual (after realignment) misalignments null residual full full+ IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  33. Corrections & Errors:Beauty feed-down (~10%) • Methods: • Monte Carlo with state-of-the-art pQCD input • use single m measurement from ALICE? • Systematic error: uncertainty on b-bbar cross section from theory (estimated to be ~8-10%) IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

  34. Corrections & Errors:Acceptance, Reco & Selection Efficiency • Embedding of MC signal in MC (or real?) events, and calculate all corrections in one go (selected  in-acceptance). Average correction in {pt, y} grid • use ALICE common correction framework • needed tuned MC (good descr. of tracking effs and resols) • d0 resol. to be evaluated from data (cosmics and pp) vs pt, q • Alignment and calibration: • simulation: same as for data reconstruction • reconstruction: same as for data reconstruction + estimated residual misalignment and miscalibration • Systematic error: • compare weights in different runs, and with the two field orientations (+z and -z) • check stability of extracted yield VS variation of cuts • repeat everything with different sets for alignment corrections? IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese

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