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Quarkonium production in pA: collider vs fixed target experiments

Quarkonium production in pA: collider vs fixed target experiments. E. Scomparin INFN Torino(Italy). pA: learning about production and in–medium properties Lessons from (a recent) past fixed target (SPS, FNAL, HERA) collider (RHIC, LHC)

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Quarkonium production in pA: collider vs fixed target experiments

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  1. Quarkonium production in pA: collider vs fixed target experiments E. Scomparin INFN Torino(Italy) • pA: learning about production and in–medium properties • Lessons from (a recent) past • fixed target (SPS, FNAL, HERA) • collider (RHIC, LHC) • Prospects for future fixed target measurements (AFTER)

  2. Using the nucleus as a target • Indeed the first experiment observing J/ in hadronic collisions • was performed at BNL using a (light) nucleus as a target... • But what can we actually learn about • J/ production mechanisms • Cold nuclear matter effects • (both initial/final state) • by studying p-A collisions ?

  3. c J/, c, ... c p g A matter of time evolution • Quarkonium production: a two-step process • Perturbative QCD production of the cc pair • Non-perturbative binding (color neutralization) • What happens when all (or part) of this process occurs inside the • nuclear medium ? • Can the interaction of the “pre-resonant” state with the nucleus • significantly depend on its properties (color octet, color singlet...) ? • Can such a interaction be modeled in a reliable way ? • Can we learn anything on production from the disappearance • (appearance) of bound states, due to the interaction with nuclear • matter ?

  4. c c J/, c, ... J/, c, ... c c g g Kinematics • By properly selecting the kinematics of the quarkonium states • it is in principle possible to select events where resonance • formation occurs inside (or outside) the nucleus • A study vs xF is particularly relevant • High-xFproduction  resonance forms outside the nucleus • Low-xFproduction  resonance forms inside the nucleus • By varying the size of the target nucleus (i.e. performing • systematic studies as a function of A) one can vary the thickness of • nuclear matter L crossed by the cc pair • (or the fully formed resonance) p

  5. What do the experiments measure ? • Use various target nuclei (or a single heavy nucleus, splitting the • event sample in centrality bins) to study the dependence of nuclear • effects on the thickness of nuclear matter • Parameterize nuclear effects on quarkonium production in terms • of the  parameter • ..or calculate the “effective” absorption cross section for quarkonium • ...or, more rigorously, in the frame of the Glauber model • ...or, finally, in terms of nuclear modification factor RpA

  6. Comparing different resonances • Different resonances correspond to different mixtures of • intermediate color octet/singlet states • Different states could be affected in a different way by the • nuclear medium compare nuclear effects on various • resonances • If the resonance hadronizes inside the medium, it is then • expected to interact with • When measuring various resonances, understanding of • feed-down fractions is essential • In the charmonium sector one has (from various pA measurements, extrapolated to L=0, P. Faccioli et al., JHEP 10 (2008) 004.)

  7. J/ production in fixed target experimentsWhat have we learned ? • The ancestors (NA3,...) • Systematic studies (E866/FNAL, NA50/SPS,...) • The third generation (NA60/SPS, HERA-B/HERA) NA3 (J. Badier et al., ZPC20 (1983) 101) p-p p-Pt, 200 GeV, 0<xF<0.6, pT<5 GeV E866(M.J.Leitch et al., PRL84(2000) 3256) p-Be p-Fe p-W 800 GeV,-0.10<xF<0.93, pT<4 GeV NA50 (B. Alessandro et al., EPJC48(2006) 329) p-Be p-Al p-Cu p-Ag p-W p-Pb, 400/450 GeV, -0.1<xF<0.1, pT<5 GeV HERA-B(I. Abt et al., Eur. Phys. J. C 60 (2009) 517) p-Cu (p-Ti) p-W, 920 GeV, -0.34<xF<0.14, pT<5 GeV NA60 (R. Arnaldi et al., Nucl. Phys. A830(2009) 345c) p-Be p-Al p-Cu p-In p-W p-Pb p-U, 158 GeV, 0.1<xF<0.35, 0<pT<4 GeV P-Be p-Cu p-In p-W p-Pb p-U, 400 GeV, -0.1<xF<0.1, 0<pT<4 GeV Wider xF coverage Access negative xF 2 different energies in the same experiment Larger number of nuclei

  8. First hints • Already in first studies (NA3) it was clear that non-trivial effects • were at play NA3

  9. Result 1: strong dependence on xF • Tevatron experiments E772/E789/E866 • p-Be, p-Fe, p-W • First systematic studies, including both J/ and  (2S) pA 800 GeV open charm: no A-dep at mid-rapidity Absorption Pre-resonant cc pair (2S) =  J/ • Strong dependence • of  on kinematics • suggests the • presence of effects • different from pure • cc break-up Fully formed quarkonia ? (2S) >  J/

  10. Result 2: J/ enhanced at xF<0 E866 HERA-B • HERA-B: pA 920 GeV • First surprise: J/ production is enhanced at negative xF • Should correspond to slow cc pairs which hadronize inside the • nucleus, so a suppression might be expected

  11. Result 3: s-dependence at fixed xF • Main result from NA60: nuclear effect stronger at lower √s • Tendency to have a lower  at large xF visible also in • low-energy data

  12. Result 4: J/ vs (2S) • At y~0, at SPS energy, the nucleus realizes if a (2S) or a J/ • is passing through it NA50 • Stronger absorption for (2S) as expected, but effect not scaling as r2J// r2(2S)  only a fraction of the resonances formed in the nucleus

  13. A cocktail with many ingredients • The break-up of the cc pair because of the interactions with CNM • is an important effect, but other effects may also play a role • Nuclear shadowing • Initial state energy loss • Final state energy loss • Intrinsic charm in the proton (first systematic study, R.Vogt, Phys. Rev. C61(2000)035203) • Lots of interesting physics • Can we disentangle the various effects ? • Can we calculate them in a reliable way ? If we try to put together all the available fixed-target results, do we reach a satisfactory understanding of what’s going on ?

  14. Nuclear shadowing • Various parameterizations developed in the last ~10 years • Significant spread in the results, in particular for gluon PDFs • More recent analysis (EPS09), include uncertainty estimate K.Eskola et al., JHEP04(2009)065 • Assuming a certain production • approach (i.e. fixing the kinematics), • the shadowing contribution to • quarkonium production can be • separated from other nuclear effects (from C. Salgado)

  15. Is shadowing + absorption enough? C. Lourenco, R. Vogt and H.K.Woehri, JHEP 02(2009) 014 • Assume that the dominant effects are shadowing and cc breakup • cc break-up cross section should depend only on √sJ/-N • Correct the results for shadowing (21 kinematics), using EKS98 • Even after correction, there is still a significant spread of the • results at constant √sJ/-N Effects different from shadowing and cc breakup are important

  16. Initial-state energy loss H.K.Woehri, “3 days of Quarkonium production...”, Palaiseau 2010 • Energy loss of incident partons  shifts x1 • √s of the parton-parton interaction changes (but not shadowing) q(g): fractional energy loss • q =0.002 (small!) seems enough to reproduce Drell-Yan results • But a much larger (~factor 10) energy loss is required to • reproduce large-xF J/ depletion from E866! • New theoretical approaches (Peigne’, Arleo): coherent energy loss, • may explain small effect in DY and large for charmonia

  17. Fixed target – where are we? • Clearly a superposition of many effects has been observed • Good progress in the last few years • Availability of new sets of data • Progress in the understanding of some of the effects • Data show • Shadowing + absorption NOT OK • Initial state energy loss constrained by Drell-Yan •  small effect, can hardly play a role in the quarkonium sector • (but see progress on theoretical side) • Which kind of new data may help clarifying the picture ? • What did we learn at collider energy ?

  18. Moving to higher energies:dAu at RHIC • Much larger √s at colliders, but: • Integrated luminosity smaller than at fixed target • Difficult to accelerate several different nuclei • Use one nucleus and select on impact parameter, but: RHIC b b rT rT’s pA: rT ~ b dAu: due to the size of the deuteron <r>~2.5fm the distribution of transverse positions are not very well represented by impact parameter

  19. Consequences • Centrality classes do not probe • completely unique regions • and have a large amount of • overlap L.A.LindenLevy, “3 days of Quarkonium production...”, Palaiseau 2010 rT • Also shadowing estimates are less precise • (need b-dependence, proportionality of effect with L usually assumed) (see S.R.Klein and R.Vogt, Phys. Rev. Lett. 91 (2003) 142301)

  20. J/ suppression in d-Au • Regions corresponding to very different strength of shadowing • effects have been studied (-2.2<y<-1.2, |y|<0.35, 1.2<y<2.2) •  good test of our understanding of the physics! FermiMotion anti-shadowing forward yx~0.005mid yx~0.03backward yx~0.1 EMCeffect shadowing x • In spite of RHIC starting its data taking in 2000, first high • statistics dAu took place in 2008

  21. A “selection” of PHENIX RAA results • Also at RHIC energies a • superposition of • shadowing+absorption is not • satisfactory, compared to data • In particular the relative • suppression between peripheral • and central events (RCP) is not • reproduced • Best fit obtained with • “abnormally” steep centrality • dependence of the absorption Issue related to the centrality selection ? Genuine physical effect ?

  22. (2S) suppression in d-Au • Shadowing effects for J/ and (2S) should be very similar • At RHIC energy the final meson state should form outside the • nucleus absorption effects expected to be similar • In contrast to these expectations, • much stronger (2S) suppression!

  23. High-energy fixed target ? • With the present (rather extended) set of results, is it • meaningful/interesting to study heavy quarkonium • in pPb in a fixed target/high-energy environment ? • Which lessons can we get from previous experiments ? • Where do we lack data ? pPb @ s ~ 115 GeV Pbp @ s ~ 72 GeV Approximately xF=2mT sinh y With this set-up, for J/ xF=0.89 @ pT=0 xF>1 @ pT>1.6 GeV/c Pbp pPb xF=-0.89 @ pT=0 xF<-1 @ pT>1.6 GeV/c CM =-3.04 CM=-3.5

  24. Nuclear geometry • We know nuclear geometry quite well • See e.g.: • Landolt-Börnstein DB – Nuclear radii Springer-Verlag 1967 • DeVries, DeJager and DeVries, Atomic Data and • Nuclear Data Tables 36 (1987) 495 • A set of pA experiments with 5-6 nuclear targets can give a much • more precise handle on the dependence of quarkonia-related • observables on the amount of nuclear matter seen in the collision • than ANY attempt of correlating multiplicity,… with the centrality • itself …only possible at a fixed-target experiment! • A “rotating target” system is easy to realize and of great help • here to minimize systematics between series of measurements

  25. Covering the backward region Higher fixed-target s: HERA-B,s=41.6 GeV Observed >1! (while one would expect stronger dissociation effects, slower J/) AFTER More in general, backward hemisphere not covered at fixed target, physics to be explored • Backward hemisphere studied at RHIC, but only down to • xF~ -0.15 (y=-2.2, pT=0) where a suppression is still seen

  26. J/ vs (2S) vs c • Studies on heavier charmoniamuch less accurate than for J/ • In particular c almost unexplored in pA Most accurate result on  = c - J/ = 0.05  0.04 from HERA-B • Especially interesting at • negative xF(more time spent • by resonances in the medium) • Forward production interesting • too, see unexpected PHENIX • result on (2S) • Notoriously difficult measurement, but important • Realistic MC calculations needed! • Influence of polarization on the acceptances!

  27. Bottomonium studies • Not covered here, but much less advanced than charmonium • Fixed-target experiments: E772, NA50 • Weaker nuclear effects compared to charmonia NA50 (450 GeV),  = 0.980.08, |y|<0.5 E772(800GeV),  = 0.9620.006 0.008 0<xF<0.6, 0<pT<4 GeV/c • More info from RHIC, but still • statistics-limited!

  28. Conclusions • Study of quarkonium production in pAcollisions is a very • interesting tool to learn about quarkonium production • mechanisms, and to “calibrate” effects observed in AA collisions • For J/ a considerable amount of data exist at fixed-target energy, • still large kinematical “holes” are present and our understanding • of the involved processes is not satisfactory • For (2S) and c, as well as for bottomonia, the information is • much less complete • Collider studies have added info, but did not prove to be decisive • in enlightening the picture • A high-energy fixed target experiment, with large kinematic • coverage, has plenty of space to do excellent physics in this • field and help solving some of the several quarkonium puzzles!

  29. Collider vs fixed target A.Frawley, ECT*,Trento, 2009 • At forward rapidity the same strong rise in nuclear effects , • already observed at fixed target (E866) is present • However, still no convincing explanation • Initial state energy loss ? • Effect related to parton saturation (CGC) ? • Shouldn’t this depend on √s ?

  30. An “intermezzo”: CNM and A-A • p-A data useful to calibrate the size of CNM effects in A-A collisions • Strong dependence on rapidity and √s • In spite of that, when CNM effects are taken into account and • “removed” from A-A J/ suppression data, a coherent picture of • “anomalous” suppression emerges R. Arnaldi, A. Frawley, ECT*, Trento, 2009

  31. Not the end of the story... • Recent studies from PHENIX: RdAu • (direct normalization to p-p) • Nuclear effects should depend • on the (density weighted) • longitudinal thickness through • the gold nucleus • Can the data tell us which • kind of dependence on (rT) • is occurring for CNM effects? A. Adare et al., arXiv:1010.1246

  32. Scaling of CNM effects • The correlation RCP vs RdAuturns out • to be sensitive to the functional • dependence of CNM effects on (rT)  cc break-up  shadowing • Forward y results NOT compatible • neither with linear nor exp. behaviour • “Traditional” description in terms of • shadowing +cc break-up does not • hold • Different cc break-up dependence ? • Gluon saturation effects ?

  33. What can we expect at the LHC? • p-A collisions surely not top priority for a machine as the LHC • 1 month/year dedicated to “nuclear” collisions • 2010: first Pb-Pb (Lint ~ 10 b-1) • 2011: >5 times Lint (2010) expected • 2012: either • significantly larger Lint in Pb-Pb • first p-A collisions! • (with √s similar to nominal Pb-Pb) • d-Pb not foreseen • (machine-related constraints) • p-Pb • Easier control of centrality selection • Rapidity shift of detector acceptance • with respect to symmetric collisions (p-p, Pb-Pb) ~ 0.5 y-units at top LHC energy (and also at present energy)

  34. Shadowing at the LHC • Use J/ production for the investigation of the low-x pdf region • Present uncertainties from EPS09 fits clearly ask for an • experimental measurement! J/  R. Vogt, Phys.Rev.C81:044903,2010 (difference in energies and yCM taken into account) • Very relevant for the interpretation of Pb-Pb data ! • Shadowing vs CGC-inspired models

  35. Other effects ? • Initial-state energy loss could of course be present • But the high-xF region (where this process is important) is • essentially inaccessible at the LHC, being pushed towards too • foward rapidities √s=14 TeV √s=5.5 TeV • Description in terms of shadowing + cc breakup may hold

  36. What about cc break-up ? • No stringent predictions, for the moment • Since at very high energy the crossing of the nucleus is • almost instantaneous, the cc pair may not have time to • significantly evolve, and still be almost point-like • after it has crossed nuclear matter • (at least at y~0) In p-Pb collisions at √s=8.8 TeV   ~ 9300 t ~ 15/9300 fm/c ~ 10-3 fm/c • In such a case no significant cc breakup ?

  37. Conclusions • The production of quarkonia in nuclear matter has been now • studied for a long time, both at fixed target and at colliders • Rather extended (statistics, √s, kinematic coverage) sets of • data are available • Many competing effects have been singled out •  Modeling difficult, slow but constant progress • As of today a coherent description is unfortunately still lacking • Could be a very important tool for • production mechanisms • understanding of A-A collision data • LHC energy domain • Different mixture of initial/final state effects • Study gluon pdfs in a still unexplored x-range

  38. What about c ? • Much more difficult measurement • Nuclear effects on c are studied through I. Abt et al.,HERA-B, Phys.Rev.D79:012001,2009 • No significant difference between (c) and (J/) is observed • similar “global” CNM effects on both resonances in the covered kinematical range (average value =0.05±0.04)

  39. Shadowing vs centrality

  40. E866 HERA-B NA50 NA60, 400 GeV NA60, 158 GeV

  41. ...but let’s still have a look at the pT • All the experiments observe weaker nuclear effects at high pT • (even turning towards an enhancement) • However, it was soon realized • that this effect is due to a • broadening of pT distributions • connected with initial state • gluon scattering • (Cronin effect)

  42. Some systematics • Fit pT2 for various nuclei measured by various experiments as  L <pT2>= <pT2>pp+  (A1/3-1) • <pT2>pp shows a roughly linear increase vs s • The slope is fairly constant, with a decrease at low s

  43. Look for x2 scaling 400 GeV NA60 158 GeV R. Arnaldi et al., arXiv:1004.5523 • Shadowing in the target nucleus only depends on x2 • But also √sJ/-N can be expressed as a function of x2 • So at fixed x2 should be independent on incident energy..... • .... which is clearly not the case

  44. STAR, PHENIX: indication for suppression, but not a precise measurement

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