BFKL evolution and the growth with energy of exclusiveJ/Ψandϒphotoproduction cross sections

Abstract

We investigate whether the Balitsky-Fadin-Kuraev-Lipatov (BFKL) low x evolution equation is capable to describe the energy dependence of the exclusive photo-production cross-section of vector mesons J/Ψ and Υ on protons. Such cross-sections have been measured by both HERA experiments H1 and ZEUS in electron-proton collisions and by LHC experiments ALICE, CMS and LHCb in ultra-peripheral proton-proton and ultraperipheral proton-lead collisions. Our approach provides a perturbative description of the rise with energy and relies only on a fit of the initial transverse momentum profile of the proton impact factor, which can be extracted from BFKL fits to inclusive HERA data. We find that BFKL evolution is capable to provide a very good description of the energy dependence of the current data set, while the available fits of the proton impact factor require an adjustment in the overall normalization.

Signs for the onset of gluon saturation in exclusive photo-production of vector mesons

“…A breakdown of such an description and the simultaneous description of data by a framework which incorporates high density effects, constitutes then a sign for the presence of effects related to gluon saturation. For a detailed discussion we refer the interested reader to [6], see also [7].…”

Leptoproduction of $\rho $ Mesons as Discriminator for the Unintegrated Gluon Distribution in the Proton

“…Hentschinski-Salas-Sabio Vera' (HSS) model The application of this model occurs, originally, in the study of DIS structure functions [12]. Subsequently, it has been used in the description of single-bottom quark production at LHC in [13], in the investigation of the photoproduction of J/Ψ and Υ in [14] and in forward Drell-Yan dilepton production [15]. This UGD takes the form of a convolution between the BFKL gluon Green's function and a LO proton impact factor:…”

Recent ALICE results on ultra-peripheral collisions

“…STARlight [15] uses a power-law fit of fixed-target and HERA data. The last two models describe data using colour-dipole model and the colour glass condensate [16] and the NLO BFKL [17] evolution of HERA data.…”