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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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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

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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 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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)

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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, dAu @ 200 GeV) x < 10 -3 -10 -2 Q s 2 ~5 GeV 2 (LHC, pPb @ 8.8 TeV) x < 10 -4 -10 -3 ~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

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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 F 2 “scaling violations”: Large uncertainties below x~10 -2 at moderate Q 2 (<5 GeV 2 ) R.D. Ball et al. arXiv:0808.1231 Q 2 = 2 GeV 2

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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 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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)

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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!

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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) pp @ 14 TeV D.Stocco

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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 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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) 248 + LHC Predictions Workshop

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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) 034022 K.Eskola et al. JHEP 0807 (08)102

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese 15 Low-p T charm in ALICE (|η|<1) Open charm reconstruction down to p T =0 in the ALICE central barrel D0K-+D0K-+ 1 LHC year (2009) R.Grosso

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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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...

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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)

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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 p T and forward rapidity Preparation of D 0 / charm analysis in progress (need to focus on background estimate and on corrections)

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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese 23 Charm at s = 14 TeV: x,Q 2 range (1) Simple estimate: c c

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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

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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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
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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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) 022002

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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:

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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!

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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+

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IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 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%)

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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 {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?

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