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

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Presentation transcript:

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

IV Convegno Nazionale Fisica ALICE, Palau, 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, Andrea Dainese 3 Parton densities at small-x DIS ep collisions probe distributions of partons in the proton: HERA: strong rise of F 2 (x,Q 2 ) at small-x: Q2Q2 Q 2 = “resolving power” F 2,F 3,F L = proton structure functions, (y = inelasticity) Bjorken x = momentum fraction carried by parton ∂lnF 2 /∂lnQ 2  gluons  couples directly only to (sea) quarks

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 4 (x,Q 2 ) evolution of PDFs Q 2 - DGLAP (k T -ordered emission): F 2 (Q 2 )~  s ln(Q 2 /Q 0 2 ) n, Q 0 2 ~1 GeV 2 x - BFKL (p L -ordered emission): F 2 (x)~  s ln(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, Andrea Dainese 5 Saturation scale Q s 2 Onset of non-linear QCD when gluons are numerous enough (low-x) & extended enough (low-Q 2 ) to overlap: Pb/Au nucleus (larger parton tranverse density) amplifies saturation effects by factor about 6 (200 1/3 ) Q s 2 ~2 GeV 2 (RHIC, 200 GeV)  x < Q s 2 ~5 GeV 2 (LHC, 8.8 TeV)  x <  ~0.3) Saturation for: low x, large  s, large y, large A [G A (x,Q 2 )=A g(x,Q 2 )] poorly known PDFs in p and A

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

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 7 Low-x (<10 -2 ) gluon PDF (nucleus) Current data on low-x gluons from: nuclear F 2, nuclear Drell-Yan (eA) DGLAP analysis: linear evolution + nuclear shadowing Shadowing  low-x gluon “fusion” or “recombination”: g x1 + g x2  g x1+x2 Shadowing factor for PDFs: xG A (x,Q 2 ) = A xg(x,Q 2 ) R G A (x,Q 2 ) Most of data in non-pert. range (Q 2 factor 2!) D.d’Enterria,JPG30 (05)S767 Armesto, JPG32(06)R367

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 8 D.d’Enterria,JPG30 (05)S767 Q s 2 RHIC =2 Experimental study of saturation: (x,Q 2 ) acceptances & probes Probes at RHIC (Q 2 <2): bulk dN ch /dy forward hadrons (p T <3-4 GeV)

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 9 LHC: study “saturation” with perturbative probes LHC (Pb): unexplored region (A=208, x<10 -3, Q 2 < 5 GeV 2 )  large uncertainties in p and Pb PDFs D.d’Enterria,JPG30 (05)S767 Q s 2 RHIC =2 Experimental study of saturation: (x,Q 2 ) acceptances & probes Probes at RHIC (Q 2 <2): bulk dN ch /dy forward hadrons (p T <3-4 GeV) Q s 2 LHC =5 LHC y=0LHC y=4 Probes at LHC (Q 2 <5): bulk dN ch /dy forward J/  charm new!

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 10 Heavy quark production at LHC: a probe of small-x gluons Probe small-x region with HQs at low p T and/or forward y Charm production at low p T : Q 2 (4m c 2 ~ 5 GeV 2 ) ~ Q s 2 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 Cartoon charm beauty PYTHIA charm

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 11 Low-x at LHC: forward J/  in ALICE (2.5 <|  |< 4) J/  measurement in  -spectrometer  xg(x) at x 2 ~10 -5 : d  /dy J/  : NLO CEM, varying PDFs QQbar: Sensitive to different PDFs and DGLAP versus CGC predictions (Note: m J/  ~Q s at the LHC) 14 TeV D.Stocco

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 12 Heavy quark production at LHC: a probe of small-x gluons (pp) Large pQCD uncertainties for charm p T  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 BK: Charm suppression due to non-linear effects DGLAP: Charm enhancement due to non-linear effects (GLR-MQ term) |y| < 1

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 13 Saturation scale Q s 2 (x) ~ xg(x)A/R A 2 ~ xg(x)A 1/3 At LHC for x~10 -4, Q s ~2 GeV > m c For m T,c ~ Q s, charm prod. CGC-dominated: scales with  N part in pA (not N coll ) harder p T spectra, since typical k T ~ Q s ~2 GeV, while in standard factorization k T ~  QCD ~0.2 GeV 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) Kharzeev, Tuchin, NPA 735 (2004) LHC Predictions Workshop

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

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

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 16 Status of software tools for open charm analysis

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

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 18 AliAnalysisVertexingHF class single-track cuts on p t and d 0, (PID...) [common cuts for all analyses] build all (+,-) pairs and compute secondary vtx create AliAODRecoDecayHF2Prong apply reco cuts store D 0 loop on all tracks (+ & -): build triplets, create AliAODRecoDecayHF3Prong, apply reco cuts (common for the 3 particles?) loop on all tracks (+ & -) … create AliAODRecoDecayHF … store D +, D s +,  c store D 0 for D +,D s +,  c + for D 0  K  for D 0  K , J/  ee store J/  tight D0 mass cut loop on all tracks for D* candidates store D *+ BA,LNL,TO

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 19 Output in AliAODEvent All the I/O managed by AliAODHandler & AliAODInputHandler Macro to read the candidates: ReadAODVertexingHF.C AliAODEvent AliAODTrack... AliAODVertex... tracks vertices (from ESD) aodTree in AliAOD.root (standard AOD, unchanged ) AliAODRecoDecay... AliAODVertex... verticesHFD0toKpi,... friend aodTree in AliAOD.VertexingHF.root not yet...

IV Convegno Nazionale Fisica ALICE, Palau, 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, 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 p T and forward rapidity Preparation of D 0 / charm analysis in progress (need to focus on background estimate and on corrections)

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

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 23 Charm at  s = 14 TeV: x,Q 2 range (1) Simple estimate: c c

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 24 How to detect the enhancement due to nonlinear effects? The idea is that the effect (enh. only at very low p t ) 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 Data: m c = 1.2 GeV, Q 2 = 4m T 2 and enhancement Data: m c = 1.3 GeV, Q 2 = m T 2 and enhancement

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

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 26 D.d’Enterria,JPG30 (05)S767 Q s 2 RHIC =2 Saturation in heavy-ion collisions: (x,Q 2 ) acceptances & probes Probes at RHIC (Q 2 <Q s 2 =2): bulk dN ch /dy forward hadrons (p T <3-4 GeV)

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

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

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

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

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

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 32 Effect of ITS misalignment on d 0 resolution Impact parameter resol:  track = a scatter /p t  b meas  c misalign Effect studied with full simulation of exptected initial (full+) and residual (after realignment) misalignments null residual full full+

IV Convegno Nazionale Fisica ALICE, Palau, Andrea Dainese 33 Corrections & Errors: Beauty feed-down (~10%) Methods: 1.Monte Carlo with state-of-the-art pQCD input 2.use single  measurement from ALICE? Systematic error: uncertainty on b-bbar cross section from theory (estimated to be ~8-10%)

IV Convegno Nazionale Fisica ALICE, Palau, 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 {p t, y} grid use ALICE common correction framework needed tuned MC (good descr. of tracking effs and resols)  d 0 resol. to be evaluated from data (cosmics and pp) vs p t,  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?