Charmonium production in pA collisions: results and perspectives R.Shahoyan, IST Lisbon

1. Charmonium production in pA collisions:results and perspectivesR.Shahoyan, IST Lisbon • Motivation • Production models and absorption parametrizations • Last results from NA50 on J/ and ’, comparison with E866 and NA3 • Prospects: NA60 experiment HIC03, June 25-28 2003

2. Introduction • Charmonium suppression was predicted as a signature of QGP formation in nucleus- nucleus collisions (T.Matsui and H.Satz, Phys. Lett. B178 (1986) 416) • Was found already in the normal nuclear matter due to interactions • NA50 observed in 158 AGeV/c PbPb collisions the anomalous suppression of J/y (Phys.Lett B477(2000) 28). Its step pattern may be interpreted as initial melting of c states at the onset of QGP (T~Tc,) with subsequent melting of the J/y at higher temperatures. • Large fraction of J/ comes from the c (~32% HERA-B,hep/ex0211033) and ’ (~10% PDG, Phys. Rev. D 54 (1996) 1). Need: Better understanding of J/, ’ and c production and absorption in normal nuclear matter (pA collision) Detailed study of the Open Charm production and J/anomalous suppressionpattern in heavy ion interactions. • E866/NuSea collaboration observed stronger suppression of y’ compared with J/y already in interactions of 800 GeV/c protons on Be, Fe and W targets (Phys.Rev.Lett.84(2000) 3256). • The motivation of the present study is to look for similar effect in NA50 pA data.

3. Color Singlet Model – perturbative creation of the static pair in color singlet state with subsequent binding to final meson without changing of quantum numbers (C-H. Chang, Nucl. Phys. B 172 (1980) 425.; R. Baier and R. Rückl, Phys. Lett. B 102 (1981) 364; Z. Phys. C 19 (1983) 251)Direct J/ and ’ are suppressed (hard gluon emission), main contribution from c decays. Describes high pT ISR data, but fails by the factor ~30 for the J/ and ~60 for the ’ to reproduce CDF data at =1.8 GeV (F.Abe et al., CDF Collab., Phys. Rev. Lett. 79 (1997) 572) Color Evaporation Model – perturbative creationof the pair in the color octet state with subsequent non-perturbative hadronization to color singlet via unsuppressed soft gluon emission (H. Fritzsch, Phys. Lett. B67, 217 (1977); F. Halzen, Phys. Lett. B69, (1977),105). Predicts unpolarized charmonium production. Color Octet Model – uses NRQCD formalism to describe the non-perturbative hadronizationof the color octet to the color singlet state via soft gluon emission (E.Braaten et al., Phys.Lett B 333 (1994) 548). Predicts transverse polarization at high Pt (but recently BaBar observed longitudinal polarization: Phys.Rev.Lett, 90, 162001 (2003) ). CEM and COM correctly reproduce the energy dependence of the charmonium cross-section, although require parameters extracted from the experiment. Charmonium Production Models Both assume production of non-singlet pair (without identity of final meson) with significant hadronic break-up cross-section, which evolves into charmonium state after some formation time (~1 fm). This may introduce the dependence of the absorption cross-sections at given XF on the charmonium type and collision energy: at smaller Lorentz factors higher fraction of pairs forms the final charmonium state still traveling in the nucleus.

4. Glauber Model:meson is produced in binary nucleon-nucleon interaction with cross section s0and absorbed in nuclear matter with cross section sabs TA(s) - nuclear thickness function at impact vector s. <rL> parametrization:meson is absorbed with cross section sabsseeing in average<rL> amount of matter from its production point to exit from nucleus: (from expansion of Glauber formula: ) Aa parametrization:(widely used but rough) Connected with previousformulae as: Parametrizations of Charmonium Absorption

5. J/ acceptance Analyzed domain:-0.5<ycm< 0.5|cos|<0.5(-0.1<XF<0.1) Fits to cross-sections integrated over rapidity (HI + LI data) Glauber = 4.6 ± 0.8 mb= 7.7 ± 1.1 mb exp(-L) = 4.3 ± 0.7 mb= 6.6 ± 0.8 mb From ’/  - ’= 2.4 ± 0.05 mb A = 0.928 ± 0.015’= 0.888 ± 0.018 ’- = -0.041 ± 0.009 NA50 results for J/ and ’p @ 450 GeV on Be, Al, Cu, Ag and W targets Submitted to Physics Letters B High intensity beam (1996-1998) [R.S at XXXVII Rencontres de Moriond, hep-ex/0207014] and Low intensity beam (1998-2000) [P.Cortese at QM2002, Phys.Lett. B 553 (2003) 167]

6. XF - dependence of the absorptionfrom the absolute cross-sections of J/ and ’(errors are dominated by the systematics of the normalization)

7. ’ with respect to J/ from the cross-sections ratios(free of normalization errors, but full Glauber fit is not possible) Evidence for the stronger suppression of slower ’ ? Would be expected due to the faster formation of the final state

8. Does normal nuclear absorption depend on or scales with XF? It does not scale neither with x of the struck parton in the target (parton energy loss scenario) nor with Plab (~charmonium formation time) Comparison with E866/NuSea and NA3 E866 at 800 GeV finds (Phys.Rev.Lett 84 (2000) 3256)  ~0.95at XF~0, vs ~0.92 of NA50 at 450 GeV. Also, NA50 shows larger difference between the J/ and ‘. NA3 at 200 GeV reported (Z.Phys.C 20 (1983) 101) value close to E866 and similar XF behaviour. NA3 used p and Pt targets while NA50 and E866: Be ... W. ’s may be misleading => use Glauber model

9. E866,800 GeV Absorption decreases as pT increases and turns to enhancement: understood as an effect of the partons rescattering before interaction amplified by the absorption: J/ produced in the end of the nucleusby rescattered gluon sees less matter and vice-versa. But <pT2> increases already at the p-p level: can this be the reason of the stronger suppression at SPS thanat FNAL? Oliver Drapier, Mémoire de l’habilitation

10. Fraction of from gg fusion EKS 98 from H.Wöhri, CINANP03 shadowing at XF~0with EKS 98 anti-shadowing Can modification of PDF’s in the nucleus or Initial State interactions affect charmonium suppression? SPS and FNAL experiments are far from the strong shadowing x domains, at least for small XF.Open Charm cross-section/nucleon in pA does not show dependence on A at XF ~ 0:  = 1.02 ±0.03 ±0.02 (E789 800GeV, Phys.Rev.Lett. 72 (1994) 2542) => suppression dueto the structure functions nuclear modifications and initial state interactions at SPS-FNAL energies may be relevant only at large XF.

11. HERA-B, hep-ex/0211033 c production • CEM and COM (NRQCD) predict different A-dependence for the c1,2production (R.Vogt, Nucl.Phys. A 700 (2002) 539) • CEM: all charmonia are produced from the color octet Only at very negative XF it is slow enough to form the final meson still inside the nucleus. The suppression observed at XF ~0 is dominantly due to the color octet absorption => J/, ’ and c1,2 should have the same A-dependence • COM: c1,2 production is dominated by the point-like color singlet contribution (in opposite to J/ and ’) => c1,2 should suffer much less absorption. • Due to the large contribution to the observed J/ cross-section the c1,2 A-dependence may be crucial for the understanding of the charmonium suppressionpattern in heavy ion collisions • A-dependence was not measured yet: • Most recent measurement of HERA-B: 920 GeV/c p on C and Ti targets. But obtained relative error on the fraction of the J/ from the c1,2 decays is still ~30%... E771, Phys.Rev.D62(2000)03206

12. vertex tracker Pixels Overview of NA60 Experiment Beam Tracker Muon Spectrometer 2.5 T dipole magnet beam ZDC Quartz Blade Interaction Counter Muon track matching through the absorber m vertex pm offset } < 1 mm Micro-Strips D m Muon track offset measurement :Separate charm from prompt dimuons

13. pA runs 2002 : 400 GeV (low statistics) after addition of vertex detector Planed for 2004 2 months of beam requested.Primary aim:cA-dependence study~ 1% precision on nuclear absorption cross-section to be achieved

14. Heavy Ion runs ~20mm XY resolution h J/ survival probability by M.Nardi 2002:Pb-Pb 30 and 20 GeV/A 3 pixel planesMuon Spectrometer was not operated. Centrality Bin hmax (dN/dh)hmax 0-10 % 2.2 ± 0.1 166 ± 5 10-20% 2.2 ± 0.1 128 ± 7 20-35% 1.9 ± 0.2 90 ± 4 Planed: October 2003, In-In 158 GeV/A Primary aims:study ofcharmonium anomalous suppression onset,Intermediate mass region (open charm vs prompt dimuons), Low mass region

15. Separating charm decays from prompt dimuons 0 100 200 300 400 500 600 700 offset (mm) muon track offset resolutionbetter than 35 mm for p  15 GeV/c Background Signal D+ : ct = 317 mmD0 : ct = 124 mm promptdimuons offset  90 mm open charm Background Signal 90  offset  800 mm and muons away from each other  180 mm in the transverse plane at Zvertex