Exclusive J/ψ photoproduction off deuteron in d+Au ultra-peripheral collisions at STAR
EExclusive J / ψ photoproduction off deuteron in d+Auultra-peripheral collisions at STAR Zhoudunming Tu, for the STAR Collaboration a , ∗ a Department of Physics, Brookhaven National Laboratory, Upton, NY 11973, USA
E-mail: [email protected]
Gluon density and its distributions inside nuclei and the parton modification of bounded nucleonsinside a nucleus, are some of the main standing problems in nuclear and particle physics. In recentyears, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing thegluon density, which is based on coherent diffractive vector-meson productions, e.g., J / ψ meson.For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC J / ψ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis,we aim to look at a unique set of data taken by the STAR experiment, where J / ψ mesons arephotoproduced off the deuteron target with no other particle produced, except for the deuteron or itsbreakup products. The Zero Degree Calorimeter response with respect to the deuteron dissociationby detecting a beam-rapidity neutron is also investigated and provides additional information aboutthe underlying physics process. The cross section of J / ψ photoproduction in the photon-deuteronsystem is measured at the photon-nucleon center-of-mass energy W ∼
25 GeV, as well as themomentum transfer t dependence cross section, d σ / dt . Data suggests a wider gluon densitydistribution than the Hulthen charge density distribution in deuteron. HardProbes20201-6 June 2020Austin, Texas ∗ Speaker © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p xclusive J / ψ photoproduction off deuteron in d+Au ultra-peripheral collisions at STAR ZhoudunmingTu, for the STAR Collaboration
1. Introduction
One of the outstanding puzzles in nuclear physics is the EMC effect [1]. Initially discoveredby the European Muon Collaboration, the bounded nucleons inside nucleus showed a significantsuppression of their parton distributions in the valance quark regions. The measurement was donewith respect to a deuterium target - assuming the deuteron is free of nuclear effect. How much thesuppression is for the same kinematics depends on the Atomic mass (A) number, where heaviernucleus were found to be more suppressed than medium or light nucleus. In recent years, newexperimental evidences suggest that the origin of EMC effect is deeply connected to the Short-Range Nucleon Correlations (SRC), i.e., significant off-shell nucleon pairs, which might exhibithuge nuclear suppression on parton density in medium and heavy nucleus [2]. However, alternativeinterpretations of the data are not excluded [2], and this problem remains as a puzzle that is notdefinitively answered to-date.Besides the EMC effect in the region of valance quarks, the gluon density at low Bjorken-xregion in bounded nucleons, has also been found to be different than free nucleons. In recentLHC experiments, photoproduction of vector-meson, e.g., J / ψ meson, has been measured inultra-peripheral collisions (UPC) of heavy ions, and its cross section is found to be significantlysuppressed with respect to free nucleons [3]. The modification of gluon density in heavy nucleus andits underlying physics mechanism are a subject of interest for a wide range of physics communities,going from nuclear and particle physics to studies of high density neutron stars in astrophysics [4, 5].Hereby we propose to investigate the gluon density in the simplest two-body nuclear system -deuteron, with coherent photoproduction of J / ψ meson off deuteron in UPC. In addition, themomentum transfer distribution − t , which can be approximated by the transverse momentumsquared ( p ) of the J / ψ particle in photoproduction, is measured in order to probe the gluonspatial distribution inside the deuteron. Similar measurement has been proposed [6] to performthe gluon imaging of bounded nucleons with and without novel short range correlations at thefuture Electron-Ion Collider. This analysis will be the first measurement of gluon imagining for thedeuteron target.
2. Result
The online trigger for the enhanced UPC J / ψ dataset was mainly based on the Barrel Electro-magnetic Calorimeter response above roughly 0.5 GeV energy threshold for a back-to-back pair oftowers and no other activity in the main detector. In Fig. 1 left, the invariant mass distribution isshown for reconstructed the J / ψ candidates. The total fit includes a signal template based on theGEANT simulation of J / ψ signal and a background function to describe the data. The backgroundfunction is ( x − A ) e B ( x − A )( x − C ) + Cx , where A , B , and C are fit parameters. The signal yield is ex-tracted from the fit, which is used for calculation of the differential cross section. The definition ofthe differential cross section is the same as in Ref. [7]. In Fig. 1 right, the Zero Degree Calorimeter(ZDC) ADC distribution is shown for both the east (Au-going) and the west (d-going) side of STARdetector. The Au-going side has no neutron peak, while the deuteron-going side is shown with aprominent neutron peak, indicating the deuteron has dissociated at least one neutron. All eventsshown are coincidence with a J / ψ candidate, which is reconstructed via the di-electron channel at2 xclusive J / ψ photoproduction off deuteron in d+Au ultra-peripheral collisions at STAR ZhoudunmingTu, for the STAR Collaborationmid rapidity. In Fig. 2 left, the differential cross section of photoproduction J / ψ off the deuteron as ) mass (GeV/c - ) d N / d m ( G e V / c /ndf = 1.84 c – signal yield = 305 unlike sign like sign Total fit Signal Background = 200 GeV NN sd+Au STAR
Preliminary + X y J/ fi *d g · ZDC ADC · c oun t s = 200 GeV NN sd+Au STAR
Preliminary
Neutron peak d-going ZDC Au-going ZDC
Figure 1:
Left: Invariant mass peak distribution of d+Au UPC event around J / ψ mass. Right: Zero DegreeCalorimeter energy distribution for east and west side of STAR a function of p T of J / ψ is shown for both with and without vetoing a neutron in the deuteron-goingdirection ZDC. The cross section is corrected by the transversely polarized photon flux such thatthe cross section is at the level of photon-deuteron system. The transversely polarized photon fluxis calculated based on STARlight event generator [8]. The total differential cross section includesthree components, the coherent diffractive production, incoherent diffractive but without breakingthe nucleon, and the nucleon dissociations. The latter two components are almost equivalent to theelastic and nucleon dissociation of J / ψ photoproduction off protons measured at HERA [9]. In thisanalysis, the H1 data is used for a template fit, where the coherent diffractive component is assumedto be an exponential function and the slope is a fit parameter. The shape of incoherent and nucleondissociation are fixed while leaving the absolute cross section a fit parameter. The extracted slopeof the coherent diffractive component is − . ± . ( stat . ) ± . ( sys . ) GeV − , which is consistentwith a naive expectation that the slope parameter is closely related to the deuteron size. Finally, byusing this slope parameter, a Fourier transformation has been performed and the source distributionof gluon density F ( b ) as a function of the impact parameter b is shown in the Fig. 2 right. The datasuggests a wider gluon density distribution than its charge density distribution.
3. Summary
The photoproduction of J / ψ vector meson off the deuteron has been measured for the first timeby using the d+Au ultra-peripheral collisions at STAR. The differential cross section as a functionof the J / ψ transverse momentum squared is presented and the coherent diffractive component hasbeen extracted based on template fit method. The incoherent and nucleon dissociation contributionsare based on the H1 data of electron proton scattering. The gluon density distribution is found to bewider than the charge density distribution of the deuteron, which provides further insight into thegluon structure of light nuclei, e.g., deuteron, at high energy.3 xclusive J / ψ photoproduction off deuteron in d+Au ultra-peripheral collisions at STAR ZhoudunmingTu, for the STAR Collaboration ) (GeV y T, J/2 p » -t -
10 110 ) / d t d y ( nb / G e V s d = 200 GeV NN sd+Au STAR
Preliminary + X y J/ fi *d g
25 GeV » æ *p g W Æ | < 1 y J/ |y Total data Total fit Coherent Elastic nucleon Nucleon disso. n-tagged data - b (fm) F ( b ) Gluon density Charge density
Deuteron
STAR
Preliminary
Figure 2:
Left: Differential cross section of J / ψ photoproduction as a function p is shown in d + Au UPCat STAR. Right: Fourier transformation from coherent J / ψ photoproduction t distribution to the spatialdistribution in terms of impact parameter b for the deuteron. References [1] J. Aubert et al. , The ratio of the nucleon structure functions F n for iron and deuterium, Phys.Lett. B , 275 (1983).[2] D. W. Higinbotham, Short-Range Nucleon-Nucleon Correlations, AIP Conf. Proc. , 85(2011), .[3] V. Khachatryan et al. , Coherent J / ψ photoproduction in ultra-peripheral PbPb collisions at √ s N N = , 489 (2017), .[4] E. Ferreiro, F. Fleuret, J. Lansberg, and A. Rakotozafindrabe, Impact of the Nuclear Modificationof the Gluon Densities on J / ψ production in p Pb collisions at √ s N N = ,047901 (2013), .[5] E. Annala, T. Gorda, A. Kurkela, J. Nättilä, and A. Vuorinen, Evidence for quark-matter coresin massive neutron stars, Nature Phys. (2020), .[6] Z. Tu, A. Jentsch, M. Baker, L. Zheng, J.-H. Lee, R. Venugopalan, O. Hen, D. Higinbotham, E.-C. Aschenauer, and T. Ullrich, Probing short-range correlations in the deuteron via incoherentdiffractive J / ψ production with spectator tagging at the EIC (2020), .[7] S. Acharya et al. , Coherent J/ ψ photoproduction at forward rapidity in ultra-peripheral Pb-Pbcollisions at √ s NN = .
02 TeV, Phys. Lett. B , 134926 (2019), .[8] S. R. Klein, J. Nystrand, J. Seger, Y. Gorbunov, and J. Butterworth, STARlight: A Monte Carlosimulation program for ultra-peripheral collisions of relativistic ions, Comput. Phys. Commun. , 258 (2017), .[9] C. Alexa et al. , Elastic and Proton-Dissociative Photoproduction of J/psi Mesons at HERA,Eur. Phys. J. C , 2466 (2013),1304.5162