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

Abstract

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 parametrisations 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 normalisation of the γ 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 .One of the basic ingredients to estimate the hadronic cross sections are the parton distributions functions (PDFs). Theoretically, at large energies the hadrons are dominated by gluons, with its behaviour at small x being determined by the QCD dynamics at high parton densities. Consequently, a precise determination of the gluon distribution is fundamental to probe a possible transition between the linear and nonlinear regimes of the QCD dynamics [1]. Experimentally, our understanding about the partonic structure of the proton has been significantly improved by the results obtained in ep collisions at HERA, which have obtained very precise data in a broad range in photon virtualities Q 2 and Bjorken-x values, imposing the tightest constraints on the existing PDFs (for recent reviews see, e.g. Refs. [2][3][4]). However, the behaviour of the gluon distribution at small x is still poorly known as can be observed in Fig. 1, where we compare the predictions for the gluon distribution obtained by different groups a e-mail: barros@ufpel.edu.br [5][6][7][8][9] that perform the global analysis of the existing experimental data using the DGLAP evolution equations [10][11][12] in order to determine the parton distributions. Consequently, additional measurements are necessary to pin down the gluon distribution. An alternative is the analysis of the experimental results for the heavy quark production at forward rapidities in pp collisions at the LHC energies. Recent results [13][14][15] have analysed the impact of the LHCb data in the determination of the gluon distribution. Another promising observable is the cross section for the diffractive production of vector mesons, which in the leading logarithmic approximation is proportional to the gluon density squared [16,17]. This process was analysed at HERA and is currently been studied in photon-induced interactions at hadronic colliders. In recent years a series of experimental results at RHIC [18,19], Tevatron [20] and LHC [21][22][23][24][25][26][27][28][29] demonstrated that the study of ph...

Mapping the dominant regions of the phase space associated with cc¯ production relevant for the prompt atmospheric neutrino flux

“…For instance, the charm production at the LHC (LHCb detector) is sensitive to x 2 > 10 −5 , while the HERA data lead to constraints on the gluon distributions for x 2 > 10 −4 . The smallest values of x 2 ∼ 10 −6 can be obtained from the inclusive production of χ c mesons [30,31] in pp collisions and in the exclusive J/Ψ photoproduction in hadronic collisions [36]. However, these possible constraints were not used so far to extract the gluon distributions.…”

Dynamical parton distributions from DGLAP equations with nonlinear corrections

“…Finally, PDFs in a wide kinematic range are given with the obtained optimized nonperturbative input. Although a lot of progress has been made, the gluon distribution at small x is still poorly estimated, and has large uncertainties [19,20]. Even worse, the gluon distributions from different collaborations exhibit large differences.…”

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

“…In particular, we expect a larger contribution for the J/Ψ than for the Υ production. On the other hand, the recent LHCb data have been used to extract the gluon distribution (xg) assuming the validity of the linear DGLAP evolution equation [33] in the kinematical range considered [34,35], with the resulting gluon distribution being used as input to predict the cross sections for larger energies. It is important to emphasise that the xg obtained through this procedure differs from that obtained in usual global analysis and also that the contribution of the next-to-leading order corrections for the exclusive vector meson photoproduction in collinear factorisation still are subjects of intense debate [36].…”