Quarkonium production in pA: collider vs fixed target experiments

E. ScomparinINFN 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)

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 ?

A matter of time evolution• Quarkonium production: a two-step process

• Perturbative QCD production of the cc pair• Non-perturbative binding (color neutralization)

p

c

cg

J/, c, ...

• 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 ?

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-xF production resonance forms outside the nucleus• Low-xF production 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

c

cg

J/, c, ...c

cg

J/, c, ...

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

ApppA

• ..or calculate the “effective” absorption cross section for quarkonium

LpppA

abseA

• ...or, more rigorously, in the frame of the Glauber model

absAAAz

zsdzA

AAApppA ezsdzsdA,1

2 ,

• ...or, finally, in terms of nuclear modification factor RpA

pp

pA

collpA NR

1

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

2 rabs

• When measuring various resonances, understanding of feed-down fractions is essential• In the charmonium sector one has

%3.01.8/

/222

J

XJSBSSR

(from various pA measurements, extrapolated to L=0,P. Faccioli et al., JHEP 10 (2008) 004.)

%525cR

absabsabsabs

S c 4.2,7.32

J/ production in fixed target experimentsWhat have we learned ?

a) The ancestors (NA3,...)b) Systematic studies (E866/FNAL, NA50/SPS,...)c) 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 GeVE866 (M.J.Leitch et al., PRL84(2000) 3256)p-Be p-Fe p-W 800 GeV,-0.10<xF<0.93, pT<4 GeVNA50 (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 GeVHERA-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 GeVNA60 (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 GeVP-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

Larger number of nuclei

2 different energiesin the sameexperiment

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

NA3

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)

Fully formed quarkonia ?(2S) > J/

open charm: no A-depat mid-rapidity

AbsorptionPre-resonant cc pair

(2S) = J/• Strong dependence of on kinematics suggests the presence of effects different from pure cc break-up

pA 800 GeV

Result 2: J/ enhanced at xF<0

• 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

E866

HERA-B

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

Result 4: J/ vs (2S)• At y~0, at SPS energy, the nucleus realizes if a (2S) or a J/ •is passing through it

• Stronger absorption for (2S) as expected, but effect not scaling as r2

J// r2(2S) only a fraction of the resonances formed in the nucleus

NA50

A cocktail with many ingredients

• Lots of interesting physics• Can we disentangle the various effects ?• Can we calculate them in a reliable way ?

• 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

If we try to put together all the available fixed-target results, do we reach a satisfactory understanding of what’s going on ?

(first systematic study, R.Vogt, Phys. Rev. C61(2000)035203)

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

• Assuming a certain production approach (i.e. fixing the kinematics), the shadowing contribution to quarkonium production can be separated from other nuclear effects

K.Eskola et al., JHEP04(2009)065

(from C. Salgado)

Is shadowing + absorption enough?

• 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 (21 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

C. Lourenco, R. Vogt and H.K.Woehri, JHEP 02(2009) 014

Initial-state energy loss

• Energy loss of incident partons shifts x1

• √s of the parton-parton interaction changes (but not shadowing)

11

'1 1 collN

gqxx 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!

H.K.Woehri, “3 days of Quarkonium production...”, Palaiseau 2010

• New theoretical approaches (Peigne’, Arleo): coherent energy loss, may explain small effect in DY and large for charmonia

Fixed target – where are we?

• 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)

• 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

• Which kind of new data may help clarifying the picture ?

•What did we learn at collider energy ?

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:

rTb

pA: rT ~ b

rT’s 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

RHIC

Consequences• Centrality classes do not probe completely unique regions and have a large amount of overlap

• Also shadowing estimates are less precise (need b-dependence, proportionality of effect with L usually assumed)

zdz

zrdzQxRNzrQxR

A

AAiA

ALi

,0

,1,1,,, 22

,

(see S.R.Klein and R.Vogt, Phys. Rev. Lett. 91 (2003) 142301)

rT

L.A.LindenLevy,“3 days of Quarkonium production...”, Palaiseau 2010

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!

FermiMotionanti-

shadowing

EMCeffectshadowin

g

x

forward y x~0.005mid y x~0.03backward y x~0.1

• In spite of RHIC starting its data taking in 2000, first high statistics dAu took place in 2008

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 ?

(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!

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

ApproximatelyxF=2mT sinh y

CM=-3.5xF=-0.89 @ pT=0xF<-1 @ pT>1.6 GeV/c

pPb Pbp

With this set-up, for J/

CM =-3.04

xF=0.89 @ pT=0xF>1 @ pT>1.6 GeV/c

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

•A “rotating target” system is easy to realize and of great help here to minimize systematics between series of measurements

…only possible at a fixed-target experiment!

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/)

More in general,backward hemispherenot covered at fixedtarget, physics to be

explored

AFTER

•Backward hemisphere studied at RHIC, but only down to xF ~ -0.15 (y=-2.2, pT=0) where a suppression is still seen

J/ vs (2S) vs c

•Studies on heavier charmonia much less accurate than for J/ In particular c almost unexplored in pA

Most accurate result on = c - J/ = 0.05 0.04from HERA-B

•Notoriously difficult measurement, but important•Realistic MC calculations needed!• Influence of polarization on the acceptances!

•Especially interesting at negative xF (more time spent by resonances in the medium)

•Forward production interesting too, see unexpected PHENIX result on (2S)

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), = 0.980.08, |y|<0.5

E772(800GeV), = 0.9620.006 0.0080<xF<0.6, 0<pT<4 GeV/c

•More info from RHIC, but still statistics-limited!

Conclusions

• Study of quarkonium production in pA collisions 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!

Collider vs fixed target

• 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 ?

A.Frawley,ECT*,Trento, 2009

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

Not the end of the story...• Recent studies from PHENIX: RdAu

(direct normalization to p-p)

dy

dNiN

dyidN

iR pp

coll

Aud

dAu

• Nuclear effects should depend on the (density weighted) longitudinal thickness through the gold nucleus

TT rzdzr ,1

0

• Can the data tell us which kind of dependence on (rT) is occurring for CNM effects?

A. Adare et al., arXiv:1010.1246

Scaling of CNM effects

• The correlation RCP vs RdAu turns out to be sensitive to the functional dependence of CNM effects on (rT)

21

1

TT

TT

raT

rarM

rarM

erM T

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 ?

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-unitsat top LHC energy

(and also at present energy)

21

212AA

ZZEs pNN

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 !

(difference in energies and yCM taken into account)

J/

• Very relevant for the interpretation of Pb-Pb data !• Shadowing vs CGC-inspired models

R. Vogt, Phys.Rev.C81:044903,2010

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=5.5 TeV

√s=14 TeV

• Description in terms of shadowing + cc breakup may hold

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 ~ 9300t ~ 15/9300 fm/c ~ 10-3 fm/c

• In such a case no significant cc breakup ?

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

What about c ?

• Nuclear effects on c are studied through

• No significant difference between (c) and (J/) is observedsimilar “global” CNM effects on both resonances in the covered kinematical range (average value =0.05±0.04)

Jc

I. Abt et al.,HERA-B, Phys.Rev.D79:012001,2009

• Much more difficult measurement

Shadowing vs centrality

E866HERA-BNA50NA60, 400 GeVNA60, 158 GeV

...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)

cc/J

gluon

• However, it was soon realized that this effect is due to a broadening of pT distributions connected with initial state gluon scattering (Cronin effect)

Some systematics• Fit pT

2 for various nuclei measured by various experiments as

<pT2>= <pT

2>pp+ (A1/3-1)

• <pT2>pp shows a roughly linear increase vs s

L

• The slope is fairly constant, with a decrease at low s

Look for x2 scaling

400 GeV158 GeV

• Shadowing in the target nucleus only depends on x2

• But also √sJ/-N can be expressed as a function of x2

2

21~

x

xms JNJ

• So at fixed x2 should be independent on incident energy..... .... which is clearly not the case

NA60

R. Arnaldi et al., arXiv:1004.5523

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