Exclusive Vector Meson Photoproduction Research Articles

Entanglement-enabled spin interference in exclusive J/ψ photoproduction through ultra-peripheral collisions at STAR

In ultra-peripheral collisions (UPC), exclusive vector meson photoproduction, such as ρ0 and J/ψ, serves as a sensitive probe for studying the gluon structure in heavy nuclei. The linear polarization of the photons involved in these processes help to image the nucleus through the so-called entanglement-enabled spin interference in vector meson photoproductions. The photoproduced J/ψ has longer lifetime (2160 fm/c) and non-localized wave function which provides unique opportunity to study the entanglement between the photon and the Pomeron phases emitted from each nucleus. We present the first measurement of the interference pattern for the photoproduced J/ψ in Au+Au UPC at √sNN = 200 with the STAR experiment.

Exclusive Photo-production of J/Ψ and Ψ(2s) as a tool to explore the transition to high and saturated gluon densities at the LHC.


 
 
 We study the energy dependence of the cross-section for exclusive photo-production of vector mesons J/Ψ and Ψ(2s) and search for signs for the onset of non-linear QCD dynamics in this observable. Our study is based on two dipole models: the Golec-Biernat, Wuesthoff Model (GBW) and the Bartels, Golec-Biernat, Kowal- ski Model (BGK). We find that differences between linear and non-linear implementations of these models are relatively small in the kinematic region probed by both HERA and LHC experiments if the energy dependence of the photo-production cross-section is being considered. We however find that the ratio of both cross-sections grows with energy in the presence of non-linear effects, while it remains approximately constant in the linearized case. The ratio of photo-production cross-sections might therefore provide a suitable tool to characterize the size of non-linear QCD effects at current collider energies.
 
 

Measurement of exclusive vector meson photoproduction in pPb collisions with the CMS experiment

The exclusive photoproduction of vector mesons provides a unique opportunity to constrain the gluon distribution function within protons and nuclei. Measuring vector mesons of various masses over a wide range of rapidity and as a function of transverse momentum provides important information on the evolution of the gluon distribution within nuclei. A variety of measurements, including the exclusive J/ψ, ρ, and Υ meson production in pPb (at nucleonnucleon center of mass energies of 5.02 and 8.16 TeV) and PbPb (5.02 TeV) collisions, are presented as a function of squared transverse momentum and the photon-proton center of mass energy. Finally, compilations of these data and previous measurements are compared to various theoretical predictions.

Exclusive photoproduction of vector meson at next-to-leading order from color glass condensate * * Supported by the National Natural Science Foundation of China (11765005, 11947119, 11305040, 11847152, 11775097), the Fund of Science and Technology Department of Guizhou Province ([2018]1023, [2019]5653), the Education Department of Guizhou Province (KY[2017]004), Qian Kehe Platform Talents ([2017]5736-027), the National key research and development program of

The exclusive photoproduction of vector mesons ( and ) is investigated by considering the next-to-leading order corrections in the framework of the color glass condensate. We compare the next-to-leading order modified dipole amplitude with the HERA data, finding a good agreement. Our studies show that the from the leading order, running coupling, and collinearly improved next-to-leading order dipole amplitudes are 2.159, 1.097, and 0.932 for the elastic cross-section, and 2.056, 1.449, and 1.357 for the differential cross-section, respectively. The results indicate that the higher-order corrections contribute significantly to the vector meson productions, and the description of the experimental data is dramatically improved once the higher order corrections are included. We extend the next-to-leading order exclusive vector meson production model to LHC energies using the same parameters obtained from HERA. We find that our model provides a rather good description of the and data in proton-proton collisions at 7 TeV and 13 TeV in LHCb experiments.

Exclusive vector meson photoproduction at high energies using the discrete BFKL approach

A phenomenological study of the exclusive vector meson photoproduction within the discrete BFKL approach is performed. We estimate the BFKL amplitude in terms of a discrete set of eigenfunctions and use the HERA [Formula: see text] data to constrain the free parameters of the model. As a consequence, we derive a parameter free prediction of the energy dependence of the exclusive vector meson cross section. We demonstrate that the HERA and LHCb data for the exclusive [Formula: see text] and [Formula: see text] photoproduction are quite well describe in a large energy range. However, the model overestimates the precise LHCb data for the smallest values of [Formula: see text], which can be interpreted as an indication of the onset of nonlinear (saturation) effects in the QCD dynamics.

QCD evolution based evidence for the onset of gluon saturation in exclusive photo-production of vector mesons

We investigate photo-production of vector mesons J/Ψ and ϒ measured both at HERA and LHC, using 2 particular fits of inclusive unintegrated gluon distributions, based on non-linear Balitsky-Kovchegov evolution (Kutak-Sapeta gluon; KS) and next-to-leading order Balitsky-Fadin-Kuraev-Lipatov evolution (Hentschinski-Sabio Vera-Salas gluon; HSS). We find that linear next-to-leading order evolution can only describe production at highest energies, if perturbative corrections are increased to unnaturally large values; rendering these corrections to a perturbative size, the growth with energy is too strong and the description fails. At the same time, the KS gluon, which we explore both with and without non-linear corrections, requires the latter to achieve an accurate description of the energy dependence of data. We interpret this observation as a clear signal for the presence of high gluon densities in the proton, characteristic for the onset of gluon saturation.

Exclusive ρ and J/Ψ photoproduction in ultraperipheral pA collisions: Predictions of the gluon saturation models for the momentum transfer distributions

In this letter we complement previous studies on exclusive vector meson photoproduction in hadronic collisions presenting a comprehensive analysis of the t-spectrum measured in exclusive ρ and J/Ψ photoproduction in pA collisions at the LHC. We compute the differential cross sections considering two phenomenological models for the gluon saturation effects and present predictions for pPb and pCa collisions. Moreover, we compare our predictions with the recent preliminary CMS data for the exclusive ρ photoproduction. We demonstrate that the gluon saturation models are able to describe the CMS data at small – t. On the other hand, the models underestimate the few data point at large – t. Our results indicate that future measurements of the large – t region can be useful to probe the presence or absence of a dip in the t-spectrum and discriminate between the different approaches to the gluon saturation effects.

Exclusive vector meson photoproduction in fixed-target collisions at the LHC

The exclusive rho , omega and J/Psi photoproduction in fixed-target collisions at the LHC is investigated. We estimate, for the first time, the rapidity and transverse momentum distributions of the vector meson photoproduction in pHe, pAr, PbHe and PbAr fixed-target collisions at the LHC using the STARlight Monte Carlo and present our results for the total cross sections. Predictions for the kinematical range probed by the LHCb detector are also presented. Our results indicate that the experimental analysis of this process in fixed-target collisions at the LHC is feasible. Such future analysis will probe the QCD dynamics in a kinematical range complementary to that studied in the collider mode.

Coherent and incoherent production of vector mesons in ultraperipheral collisions of xenon ions within the QCD parton saturation approach

In this paper, we analyze the exclusive vector meson photoproduction in the recent run using xenon ions at energy of 5.44 TeV performed by the Large Hadron Collider. We focus on the ultraperipheral collisions and provide theoretical predictions for coherent and incoherent cross sections within the color dipole approach and gluon saturation framework. The rapidity distribution is investigated in both cases and comparison to other approaches available in the literature is completed. We show that the expected yields are enough to perform reliable cross section measurements for light mesons as ${\ensuremath{\rho}}^{0}$ and $\ensuremath{\phi}$.

Heavy meson photoproduction in peripheral AA collisions

The exclusive photoproduction of the heavy vector mesons $\psi$'s and $Y$'s is investigated in peripheral lead-lead collisions for the energies available at the LHC, $\sqrt{s}=2.76$ TeV and $\sqrt{s}=5.02$ TeV. In order to evaluate the robustness of the light-cone color dipole formalism, previously tested in the ultraperipheral regime. It was calculated the rapidity distribution as well as the nuclear modification factor ($R_{AA}$) for the three centrality classes: 30%-50%, 50%-70% and 70%-90%. The transition from ultraperipheral to peripheral regime was carried out sophisticating the photon flux description and the photonuclear cross section, taking into account the effective interaction area. In our calculations, three scenarios were considered: (scenario 1) the direct application of the usual photon flux and of the photonuclear cross section without any relevant change in relation to the UPC's, (scenario 2) the application of an effective photon flux keeping the photonuclear cross section unchanged and (scenario 3) where it was also considered an effective photonuclear cross section. The results obtained from the three scenarios were compared with the ALICE measurements (only $J/\psi$ at the moment), showing a better agreement with the data in the more complete approach (scenario 3), mainly in the more central regions (30%-50% and 50%-70%) where the incertainty is smaller.

Exclusive vector meson photoproduction with a leading baryon in photon-hadron interactions at hadronic colliders

Exclusive vector meson photoproduction associated with a leading baryon ($B = n, \Delta^+, \Delta^0$) in $pp$ and $pA$ collisions at RHIC and LHC energies is investigated using the color dipole formalism and taking into account nonlinear effects in the QCD dynamics. In particular, we compute the cross sections for $\rho$, $\phi$ and $J/\Psi$ production together with a $\Delta$ and compare the predictions with those obtained for a leading neutron. Our results show that the $V + \Delta$ cross section is almost 30 % of the $V + n$ one. Our results also show that a future experimental analysis of these processes is, in principle, feasible and can be useful to study leading particle production.

Exclusive photoproduction of vector mesons in proton–lead ultraperipheral collisions at the LHC

Rapidity distributions of vector mesons are computed in dipole model proton–lead ultraperipheral collisions (UPCs) at the CERN Larger Hadron Collider (LHC). The dipole model framework is implemented in the calculations of cross sections in the photon–hadron interaction. The bCGC model and Boosted Gaussian wave functions are employed in the scattering amplitude. We obtain predictions of rapidity distributions of J/ψ meson proton–lead ultraperipheral collisions. The predictions give a good description to the experimental data of ALICE. The rapidity distributions of ϕ, ω and ψ(2s) mesons in proton–lead ultraperipheral collisions are also presented in this paper.

Color dipole predictions for the exclusive vector meson photoproduction in pp , pPb , and PbPb collisions at run 2 LHC energies

In this paper we present a comprehensive analysis of exclusive vector meson photoproduction in $pp$, $pPb$ and $PbPb$ collisions at Run 2 LHC energies using the Color Dipole formalism. The rapidity distributions and total cross sections for the $\rho$, $\phi$, $J/\Psi$, $\Psi (2S)$ and $\Upsilon$ production are estimated considering the more recent phenomenological models for the dipole - proton scattering amplitude, which are based on the Color Glass Condensate formalism and are able to describe the inclusive and exclusive $ep$ HERA data. Moreover, we also discuss the impact of the modelling of the vector meson wave functions on the predictions. The current theoretical uncertainty in the Color Dipole predictions is estimated and a comparison with the experimental results is performed.

J/Ψ photoproduction in peripheral AA collisions

The exclusive photoproduction of the heavy vector mesons $J/\Psi$ is investigated in the context of peripheral lead-lead collisions for the energies available at the LHC, $\sqrt{s}=2.76$ TeV and $\sqrt{s}=5.02$ TeV. Using the light-cone color dipole formalism, it was calculated the rapidity distribution in two centrality bins 50\%-70\% and 70\%-90\% in order to evaluate its robustness in extrapolating down to smaller impact parameter. It is introduced a modified photon flux, without change in the photonuclear cross section in relation to the ultraperipheral (UPC) case. Results were obtained for the two regions analized, which presented a maximum difference of 27\% in frontal rapidity for the two regions. Comparing the results for $\sqrt{s}=2.76$ TeV and $\sqrt{s}=5.02$ TeV, it was verified an increase of approximately half the one obtained in ultraperipheral regime in the central rapidity region.

Exclusive heavy vector meson photoproduction in hadronic collisions at the LHC: Predictions of the color glass condensate model for Run 2 energies

In this letter we update our predictions for exclusive $J/\Psi$ and $\Upsilon$ photoproduction in proton-proton and nucleus - nucleus collisions at the Run 2 LHC energies obtained with the color dipole formalism and considering the impact parameter Color Glass Condensate model (bCGC) for the forward dipole - target scattering amplitude. A comparison with the LHCb data on rapidity distributions and photon - hadron cross sections is presented. Our results demonstrate that the current data can be quite well described by the bCGC model, which takes into account nonlinear effects in the QCD dynamics and reproduces the very precise HERA data, without introducing any additional effect or free parameter.

Investigating the impact of the gluon saturation effects on the momentum transfer distributions for the exclusive vector meson photoproduction in hadronic collisions

The exclusive vector meson production cross section is one of the most promising observables to probe the high energy regime of the QCD dynamics. In particular, the squared momentum transfer (t) distributions are an important source of information about the spatial distribution of the gluons in the hadron and about fluctuations of the color fields. In this paper we complement previous studies on exclusive vector meson photoproduction in hadronic collisions presenting a comprehensive analysis of the t-spectrum measured in exclusive ρ, ϕ and J/Ψ photoproduction in pp and PbPb collisions at the LHC. We compute the differential cross sections taking into account gluon saturation effects and compare the predictions with those obtained in the linear regime of the QCD dynamics. Our results show that gluon saturation suppresses the magnitude of the cross sections and shifts the position of the dips towards smaller values of t.

Exclusive vector meson photoproduction at the LHC and a future circular collider: A closer look on the final state

Exclusive vector meson photoproduction at the LHC and a future circular collider: A closer look on the final state

Transverse Momentum Distribution in Ψ(1S,2S) Photoproduction in PP and AA Collisions at the LHC

The exclusive photoproduction of the heavy vector mesons [Formula: see text] is investigated in the context of ultra-peripheral collisions proton-proton and nucleus-nucleus for the energies available at the LHC run 2. Using the light-cone color dipole formalism, it was calculated the transverse momentum distribution in the central rapidity region, in which it is expected major contribution for the process.

Probing the gluon density of the proton in the exclusive photoproduction of vector mesons at the LHC: a phenomenological analysis

The current uncertainty on the gluon density extracted from the global parton analysis is large in the kinematical range of small values of the Bjorken - $x$ variable and low values of the hard scale $Q^2$. An alternative to reduces this uncertainty is the analysis of the exclusive vector meson photoproduction in photon - hadron and hadron - hadron collisions. This process offers a unique opportunity to constrain the gluon density of the proton, since its cross section is proportional to the gluon density squared. In this paper we consider current parametrizations for the gluon distribution and estimate the exclusive vector meson photoproduction cross section at HERA and LHC using the leading logarithmic formalism. We perform a fit of the normalization of the $\gamma h$ cross section and the value of the hard scale for the process and demonstrate that the current LHCb experimental data are better described by models that assume a slow increasing of the gluon distribution at small - $x$ and low $Q^2$.

Exclusive J/ψ and ϒ photoproduction and the low x gluon

We study exclusive vector meson photoproduction, with or ϒ, at next-to-leading order (NLO) in collinear factorisation, in order to examine what may be learnt about the gluon distribution at very low x. We examine the factorisation scale dependence of the predictions. We argue that, using knowledge of the NLO corrections, terms enhanced by a large can be reabsorbed in the LO part by a choice of the factorisation scale. (In these exclusive processes ξ takes the role of Bjorken-x.) Then, the scale dependence coming from the remaining NLO contributions has no enhancements. As a result, we find that predictions for the amplitude of ϒ production are stable to within about ±15%. This will allow data for the exclusive process at the Large Hadron Collider (LHC), particularly from LHCb, to be included in global parton analyses to constrain the gluon parton distribution function (PDF) down to Moreover, the study of exclusive photoproduction indicates that the gluon density found in the recent global PDF analyses is too small at low x and low scales.