We present a wide study on the comparison of different shadowing models and their influence on J/{psi} production. We have taken into account the possibility of different partonic processes for the c{bar c}-pair production. We notice that the effect of shadowing corrections on J/{psi} production clearly depends on the partonic process considered. Our results are compared to the available data on dAu collisions at RHIC energies. We try different break up cross section for each of the studied shadowing models

/SLAC

/SPhN, DAPNIA, Saclay

Ferreiro, E.G.

Fleuret, F.

Lansberg, J.P.

Polytechnique, /Ecole

Rakotozafindrabe, A.

U., /Santiago de Compostela

UNT Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515COLD NUCLEAR MATTER EFFECTS ON J/ψ PRODUCTION AT RHIC:COMPARING SHADOWING MODELSE. G. FERREIRO1, F. FLEURET2, J. P. LANSBERG3, A. RAKOTOZAFINDRABE41Departamento de F́ısica de Part́ıculas, Universidade de Santiago de CompostelaE-15782 Santiago de Compostela, Spain2Laboratoire Leprince Ringuet, CNRS-IN2P3, École Polytechnique, 91128 Palaiseau, France3SLAC National Accelerator Laboratory, Theoretical Physics, Stanford University,Menlo Park, CA 95025, USA4IRFU/SPhN, CEA Saclay, 91191 Gif-sur-Yvette Cedex, FranceWe present a wide study on the comparison of different shadowing models and their influenceon J/ψ production. We have taken into account the possibility of different partonic processesfor the cc̄-pair production. We notice that the effect of shadowing corrections on J/ψ pro-duction clearly depends on the partonic process considered. Our results are compared to theavailable data on dAu collisions at RHIC energies. We try different break up cross section foreach of the studied shadowing models.1 IntroductionThe interest devoted to J/ψ production has not decreased for the last thirty years. It is motivatedby the search of the transition from hadronic matter to a deconfined state of QCD matter, theso-called Quark-Gluon Plasma (QGP). The high density of gluons in the QGP is expected tohinder the formation of quarkonium systems, by a process analogous to Debye screening of theelectromagnetic field in a plasma 1.The results on J/ψ production measured by the PHENIX experiment at the BNL RelativisticHeavy Ion Collider (RHIC) show a significant suppression of the J/ψ yield in AuAu collisionsat√sNN=200 GeV2. Nevertheless, PHENIX data on dAu collisions 3 have also pointed outthat Cold Nuclear Matter (CNM) effects play an essential role at these energies.All this reveals that, in fact, the interpretation of the results obtained in nucleus-nucleuscollisions relies on a good understanding and a proper subtraction of the CNM effects, knownto impact the J/ψ production in proton(deuteron)-nucleus collisions, where the deconfinementcan not be reached.It is our purpose here to develop an exhaustive study of these effects. This includes shado-wing, i.e. the modification of the parton distribution of a nucleon in a nucleus, and final-statenuclear absorption.Moreover, we have recently noticed4 that the impact of gluon shadowing on J/ψ productiondoes depend on the partonic process producing the cc̄ and then the J/ψ. Because of this, we haveincluded here two different production mechanism: g + g → cc̄ → J/ψ (+X), that correspondsto the picture of the Colour Evaporation Model (CEM) at LO, and g + g → J/ψ + g, as in theColor Singlet Model (CSM), but including the s-channel cut contributions.In practice, we have proceeded as follows: we have developed a Glauber Monte-Carlo codewhere we have interfaced the two different partonic processes for cc̄ with the CNM effects,namely the shadowing and nuclear absorption, in order to get the J/ψ production cross sectionsfor pA and AA collisions. We have considered four different models for the shadowing effects.We shall compare our results with the experimental measurements on dAu collisions presentlyavailable at RHIC.SLAC-PUB-13660June 2009Contributed to the 44th Rencontres De Moriond On QCD And High Energy Interactions, 03/14/2009 -- 3/21/2009, La Thuile, Valle d'Aosta, Italy2 The ModelTo describe the J/ψ production in nucleus collisions, our Monte-Carlo framework is based on theprobabilistic Glauber model, the nuclear density profiles being defined with the Woods-Saxonparameterisation for any nucleus A > 2 and the Hulthen wavefunction for the deuteron. Thenucleon-nucleon inelastic cross section at√sNN = 200 GeV is taken to σNN = 42 mb and theaverage nucleon density to ρ0 = 0.17 nucleons/fm3. For each event (for each AB collision) at arandom impact parameter b, the Glauber Monte-Carlo model allows us to determine the numberof nucleons in the path of each incoming nucleon, therefore allowing us to easily derive thenumber Ncoll of nucleon-nucleon collisions and the total number Npart of nucleons participatinginto the collision. In order to study the J/ψ production, we need to implement in our MonteCarlo the following ingredients: the partonic process for the cc̄ production and the CNM effects.2.1 Partonic process for the cc̄ productionMost of the studies of J/ψ production rely on the assumption that the cc̄ pair is produced bythe fusion of two gluons carrying some intrinsic transverse momentum kT . The partonic processbeing a 2 → 1 scattering, the sum of the gluon intrinsic transverse momentum is transferred tothe cc̄ pair, thus to the J/ψ since the soft hadronisation process does not modify the kinemat-ics. This corresponds to the picture of the Colour Evaporation Model (CEM) at LO. In suchapproaches, the transverse momentum of the J/ψ entirely comes from the intrinsic transversemomentum of the initial gluons.However, such an effect is not sufficient to describe the PT spectrum of quarkonia producedin hadron collisions5. Most of the transverse momentum should have an extrinsic origin, i.e. theJ/ψ’s PT would be balanced by the emission of a recoiling particle in the final state. The J/ψwould then be produced by gluon fusion in a 2 → 2 process with emission of a hard final-stategluon. This emission, which is anyhow mandatory to conserve C-parity, has a definite influenceon the kinematics of the J/ψ production. Indeed, for a given J/ψ momentum (thus for fixed yand PT ), the processes discussed above, i.e. g+ g → cc̄→ J/ψ (+X) and g + g → J/ψ + g, willproceed on the average from gluons with different Bjorken-x. Therefore, they will be affectedby different shadowing corrections. From now on, we will refer to the former scenario as theintrinsic scheme, and to the latter as the extrinsic scheme.In the intrinsic scheme, the initial gluons carry a non-zero intrinsic transverse momentum,which is transferred to the J/ψ . Note that, following6, we do not neglect the value of the J/ψ’sPT in this simplified kinematics. In fact, in this scheme, we use the fits to the y and PT spectrameasured by PHENIX 7 in pp collisions at√sNN = 200 GeV as inputs of the Monte-Carlo.The measurement of the J/ψ momentum completely fixes the longitudinal momentum fractioncarried by the initial partons:x1,2 =mT√sNNexp (±y) ≡ x01,2(y, PT ), (1)with the transverse mass mT =√M2 + P 2T , M being the J/ψ mass.On the other hand, in the extrinsic scheme, information from the data alone – the y and PTspectra – is not sufficient to determine x1 and x2. Indeed, the presence of a final-state gluonauthorizes much more freedom to choose (x1, x2) for a given set (y, PT ). The four-momentumconservation explicitely results in a more complex expression of x2 as a function of (x1, y, PT ):x2 =x1mT√sNNe−y −M2√sNN (√sNNx1 −mT ey). (2)Equivalently, a similar expression can be written for x1 as a function of (x2, y, PT ). Even if thekinematics determines the physical phase space, models are anyhow mandatory to compute theproper weighting of each kinematically allowed (x1, x2). This weight is simply the differentialcross section at the partonic level times the gluon Parton Distribution Functions (PDFs), i.e.g(x1, µf )g(x2, µf ) dσgg→J/ψ+g/dy dPT dx1dx2. In the present implementation of our code, weare able to use the partonic differential cross section computed from any theoretical approach.For now, we use the one from8 which takes into account the s-channel cut contributions9 to thebasic Color Singlet Model (CSM) and satisfactorily describes the data down to very low PT .2.2 ShadowingTo get the J/ψ yield in pA and AA collisions, a shadowing-correction factor has to be applied tothe J/ψ yield obtained from the simple superposition of the equivalent number of pp collisions.This shadowing factor can be expressed in terms of the ratios RAi of the nuclear Parton Distri-bution Functions (nPDF) in a nucleon of a nucleus A to the PDF in the free nucleon. We willconsider three different shadowing models for comparation: EKS98 10, EPS08 11 and nDSg 12at LO.These models provide the nuclear ratios RAi at a given initial value of Q20 which is assumedlarge enough for perturbative evolution to be applied: Q20 = 2.25 GeV2 for EKS98, Q20 =1.69 GeV2 for EPS08 and Q20 = 0.4 GeV2(0.26 GeV2) for nDSg and perform their evolutionthrough the DGLAP evolution equations to LO accuracy in the case of EKS98 and EPS08 andto LO and NLO accuracy in the case of nDSg. The spatial dependence of the shadowing is notgiven in the above models. However, it has been included in our approach, assuming that theinhomogeneous shadowing is proportional to the local density 13.The nuclear ratios of the PDFs are then expressed by:RAi (x,Q2) =fAi (x,Q2)Afnucleoni (x,Q2), fi = q, q̄, g . (3)The numerical parameterisation of RAi (x,Q2) is given for all parton flavours. Here, we restrainour study to gluons since, at high energy, J/ψ is essentially produced through gluon fusion 5.2.3 The nuclear absorptionThe second CNM effect that we are going to take into account concerns the nuclear absorption.In the framework of the probabilistic Glauber model, this effect refers to the probability for thepre-resonant cc̄ pair to survive to the propagation through the nuclear medium and is usuallyparametrised by introducing an effective absorption cross section σabs. It is our purpose hereto compare different absorptive σ within the two partonic cc̄ production mechanisms –intrinsicand extrinsic– and for the three shadowing models cited above.3 ResultsIn the following, we present our results for the J/ψ nuclear modification factor:RAB =dNJ/ψAB〈Ncoll〉dNJ/ψpp. (4)dNJ/ψAB (dNJ/ψpp ) is the J/ψ yield observed in AB (pp) collisions and 〈Ncoll〉 is the average numberof nucleon-nucleon collisions occurring in one AB collision. In the absence of nuclear effects,RAB should equal unity.We will restrict ourselves to dAu collisions, since only CNM matter effects are at play here,so they provide the best field for the study of the shadowing and the nuclear absorption.We have used PHENIX measurements of RdAu3 in order to compare the different shadowingmodels. In Fig. 1, we have computed our results in the different shadowing frameworks for fourσabs for each of the shadowing models considered in both the intrinsic and extrinsic scheme.We have also evalute the best fit for σabs, following the method used by PHENIX in3 and14. By using the data on RdAu versus rapidity, we have obtained the best χ2 for the EPS08model, computed in the extrinsic scheme for a σabs = 3.6 mb.Figure 1: J/ψ nuclear modification factor in dAu collisions at√sNN = 200 GeV versus y for the differ-ent shadowing models in the intrinsic (up) and extrinsic (down) scheme. For complementary figures see:http://phenix-france.in2p3.fr/software/jin/index.htmlAcknowledgmentsE. G. F. thanks Xunta de Galicia (2008/012) and Ministerio de Educacion y Ciencia of Spain(FPA2008-03961-E/IN2P3) for financial support.1. T. Matsui and H. Satz, Phys. Lett. B 178 (1986) 416.2. A. Adare et al.,Phys. Rev. Lett. 98, 232301 (2007).3. A. Adare et al., Phys. Rev. C 77 (2008) 024912.4. E. G. Ferreiro, F. Fleuret, J. P. Lansberg and A. Rakotozafindrabe, Eur. Phys. J. C DOI10.1140/epjc/s10052-008-0843-8, arXiv:0809.4684 [hep-ph].5. J. P. Lansberg, Int. J. Mod. Phys. A 21 (2006) 3857; J. P. Lansberg, Eur. Phys. J. C DOI10.1140/epjc/s10052-008-0826-9, arXiv:0811.4005 [hep-ph].6. E. G. Ferreiro, F. Fleuret and A. Rakotozafindrabe, arXiv:0801.4949 [hep-ph].7. A. Adare et al., Phys. Rev. Lett. 98 (2007) 232002.8. H. Haberzettl and J. P. Lansberg, Phys. Rev. Lett. 100 (2008) 032006.9. J. P. Lansberg, J. R. Cudell and Yu. L. Kalinovsky, Phys. Lett. B 633 (2006) 301.10. K. J. Eskola, V. J. Kolhinen and C. A. Salgado, Eur. Phys. J. C 9 (1999) 61.11. K. J. Eskola, H. Paukkunen and C. A. Salgado, JHEP 0807 (2008) 102.12. D. de Florian and R. Sassot, Phys. Rev. D 69 (2004) 074028.13. S. R. Klein and R. Vogt, Phys. Rev. Lett. 91 (2003) 142301. [arXiv:nucl-th/0305046].14. A. Adare et al., Phys. Rev. C 77 (2008) 064907.

Cold Nuclear Matter effects on J/psi production at RHIC: comparing shadowing models

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Cold Nuclear Matter effects on J/psi production at RHIC: comparing shadowing models

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