QUESTIONS ON QUARKONIUM PRODUCTION IN NUCLEAR COLLISIONS

International Workshop Quarkonium Working Group QUESTIONS ON QUARKONIUM PRODUCTION IN NUCLEAR COLLISIONS ALBERTO POLLERI TU München and ECT* Trento CERN - November 2002 Outline What do we know for sure? What do we claim to know, but we are not sure? What we don t know, but we can soon find out? What we really don t know and we don t have a clue?

QUARKONIUM (Q) FROM pp COLLISIONS We need to understand the process pp (γp) Q + X Is the CSM really wrong for our purposes? If yes, how do we improve it in a practical way? Why do we need to understand pp for nuclear collisions? = Nuclei resolve the space-time evolution of the process = The production mechanism is modified For our purposes we need to know the amplitude A pp Q+X (x F, p T ) In a wide enough interval in x F around zero With the correct p T behavior from zero up to few GeV Why is the cross section not enough? Nuclei resolve the production vertex in a p T - and x F -dependent way

THE pa BASELINE pa collisions give the baseline to study medium effects They are necessary in order to isolate anomalous behavior in AB pa Process dependent nuclear effects dn pa Q = dσ pp Q = dσ pp Q d 2 r T A ( r) S A ( r, p) A S A ( p) dn AB Q (b) = dσ pp Q d 2 r T A ( r + ) T B ( r ) S A ( r +, p) S B ( r, p) dσ pp Q T AB (b) S A ( p) S B ( p) y r ± = r ± b/2 b r x Not useful to extract S from fitting data in pa! Nuclear effects must be calculated!

SOME KNOWN FACTS ABOUT pa Yield of Q is reduced as compared to pp because of inelastic collisions with nucleons This normal absorption takes place on a length scale of the order of the radius R A of the nucleus Some very impostant features: t Formation time t f = 2 E Q M 2 Q M 2 Q Explains difference Q vs. Q x F < 0 states fully developed x F > 0 states are pre-resonances Energy dependence σ tot ΨN s 0.2 Faster growth than for light hadrons Because Q is a small object z R 2 A

LESS KNOWN FACTS ABOUT pa Initial state energy loss (Inferred from DY at E lab = 800 GeV) κ = E / L 2.5 GeV/fm R(x 1 ) = G( x 1) G(x 1 ) x 1 = E G E p E Q + κ L E p = x 1 + x 1 Effects due to large Coherence time t c = 2 E Q M 2 QQ λ mfp 2 fm Production amplitudes on different nucleons add up coherently and interfere destructively = Shadowing M 2 larger for QQG than for QQ fluctuation t c smaller but grows with collision energy More shadowing in pa than in γa production of Q! 1 Expected x F dependence in AA just from nuclear effects 1 x F +1

COMPARING SCALES A comparison of different scales λ mfp, R A, t f and t c 1e+00 1e-01 1e-02 x 2 1e-03 1e-04 1e-05 1e-06 Charmonium SPS RHIC LHC t f, t c [fm/c] 6 5 4 3 2 1 R A λ mfp 0-1 -0.8-0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1 x F Many effects combine, difficult to compute and to disentangle them. Hard task for theory!

ANOMALOUS (MEDIUM) EFFECTS Anomalous effects are those not present in pa collisions Large impact = produced medium is very dense y ɛ( r, b) q n( r, b) b r x Need more information, otherwise any reasonable model works! Time dependence, medium evolution What are the active degrees of freedom? Phase space distribution f i ( x, p, t) Interface between Q (QQ) and f i = interaction Cross sections σ in πq, σin ρq, σin GQ, σin qq? Mean field effects? Other?

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INTERACTION OF Q WITH THE MEDIUM Eventually, the deconfined medium must hadronize (This we know for sure!) During time evolution, what happens to Q? It binds in the medium and it can be dissociated by collisions? It does not bind right away because of screening of the QQ interaction due to thermalization? 100 T c = 170 MeV 10 n o? LHC Note orders of magnitude! n(t) [fm -3 ] 1 n c n o SPS n o RHIC 0.1 n f.o. hadrons QGP gluons 0.01 0 100 200 300 400 500 600 T [MeV] What about Q and Q initially produced? Are they abundant enough to have a significant probability to be close in phase space at some time in the deconfined stage and coalesce? Is the process QQ Q at the hadronization stage similar to what occurs in pp collisions? What is the role of fluctuations at maximal centrality?

SCANNING THE MEDIUM Use more differential information in x F and p T x F dependence of cross section ratio in AA to pp collisions h dn AA dy y 1 R Q AA y The medium is more dense in the central region A dip appears on top of nuclear effects! Study anomalies in p T distributions on top of Cronin effect and look at centrality dependence of p 2 T. Consider spectra of high-p T hadrons together with those of D and B mesons (jet quenching) as a cross-check.