We present predictions for the exclusive and dissociative production of vector mesons off protons in an electron-ion collider. The computation is based on the energy-dependent hot spot model that was ...shown to successfully describe the available photoproduction data. We find that the model also describes correctly all available electroproduction data. In addition, we find that the cross section for dissociative production as a function of the center-of-mass energy of the photon-proton system has a maximum, whose position depends on the virtuality of the photon and the mass of the vector meson. We use these maxima to define a geometrical saturation scale and find that it grows linearly with energy as a function of the scale of the process. This phenomenon can be studied at the proposed electron-ion colliders, JLEIC, eRHIC and LHeC.
The coherent photonuclear production of a J/ψ vector meson at the LHC has been computed using two different sets of solutions of the impact-parameter dependent Balitsky-Kovchegov equation. The ...nuclear dipole scattering amplitudes are obtained either from (i) solutions for this process off proton targets coupled with a Glauber-Gribov prescription, or (ii) from solutions obtained with an initial condition representing the nucleus. These approaches predict different cross sections, which are compared with existing data from ultra-peripheral collisions at the LHC. The latter approach seems to better describe current measurements. Future LHC data should be precise enough to select one of the two approaches as the correct one.
We have developed a model in which the quantum fluctuations of the proton structure are characterised by hot spots, whose number grows with decreasing Bjorken-x. Our model reproduces the F2(x,Q2) ...data from HERA at the relevant scale, as well as the exclusive and dissociative J/ψ photoproduction data from H1 and ALICE. Our model predicts that for Wγp≈500 GeV, the dissociative J/ψ cross section reaches a maximum and then decreases steeply with energy, which is in qualitatively good agreement to a recent observation that the dissociative J/ψ background in the exclusive J/ψ sample measured in photoproduction by ALICE decreases as energy increases. Our prediction provides a clear signature for gluon saturation at LHC energies.
Solutions of the target-rapidity Balitsky-Kovchegov (BK) equation are studied considering, for the first time, the complete impact-parameter dependence, including the orientation of the dipole with ...respect to the impact-parameter vector. In our previous work 1 it has been demonstrated that the spurious Coulomb tails could be tamed using the collinearly-improved kernel and an appropriate initial condition in the projectile-rapidity BK equation. Introducing a different interpretation of the evolution variable, the target-rapidity formulation of the BK equation brings non-locality in rapidity and a kernel modification, removing the term that previously helped to suppress the Coulomb tails. To address this newly emerged non-locality, three different prescriptions are explored here to take into account the rapidities preceding the initial condition. Two of these approaches induce mild Coulomb tails, while the other is free from this effect within the studied rapidity range. The range is chosen to correspond to that of interest for existing and future experiments. To demonstrate that this set up can be used for phenomenological studies, the obtained solutions are used to compute the F2 structure function of the proton and the diffractive photo- and electro-production of J/ψ off protons. The predictions agree well with HERA data, confirming that the target-rapidity Balitsky-Kovchegov equation with the full impact-parameter dependence is a viable tool to study the small Bjorken-x limit of perturbative QCD at current facilities like RHIC and LHC as well as in future colliders like the EIC.
We solved the impact-parameter dependent Balitsky-Kovchegov equation with the recently proposed collinearly improved kernel. We find that the solutions do not present the Coulomb tails that have ...affected previous studies. We also show that once choosing an adequate initial condition it is possible to obtain a reasonable description of HERA data on the structure function of the proton, as well as on the cross section for the exclusive production of a J/ψ vector meson off proton targets. As a further application of the solutions, we computed the impact-parameter dependent Weizsäcker-Williams gluon distribution.
The solution to the impact-parameter dependent Balitsky-Kovchegov equation with the collinearly improved kernel is studied in detail. The solution does not present the phenomenon of Coulomb tails at ...large impact parameters that have affected previous studies. The origin of this behavior is explored numerically. It is found to be linked to the fact that this kernel suppresses large daughter dipoles. Solutions based on a physics motivated form of the initial condition are used to compute predictions for structure functions of the proton and the exclusive photoproduction and electroproduction of vector mesons. A reasonable agreement is found when comparing to HERA and LHC data.
One of the more promising observables to probe the high energy regime of the QCD dynamics in the future Electron - Ion Colliders (EIC) is the exclusive vector meson production cross section in ...coherent and incoherent interactions. Such processes measure the average spatial distribution of gluons in the target as well the fluctuations and correlations in the gluon density. In this paper we present a comprehensive analysis of the energy, photon virtuality, atomic number and momentum transfer dependencies of the coherent and incoherent cross sections considering two different models for the nuclear profile function. In particular, we present the predictions of the hot - spot model, which assumes the presence of subnucleonic degrees of freedom and an energy-dependent profile. Our results indicate that the analysis of the ratio between the incoherent and coherent cross sections and the momentum transfer distributions in the future EIC can be useful to constrain the description of the hadronic structure at high energies.
We predict that the onset of gluon saturation can be uniquely identified using incoherent J/ψ production in Pb–Pb collisions at currently accessible energies of the LHC. The diffractive incoherent ...photo-production of a J/ψ vector meson off a hadron provides information on the partonic structure of the hadron. Within the Good-Walker approach it specifically measures the variance over possible target configurations of the hadronic colour field. For this process then, gluon saturation sets in when the cross section reaches a maximum, as a function of the centre-of-mass energy of the photon-hadron system (W), and then decreases. We benchmark the energy-dependent hot-spot model against data from HERA and the LHC and demonstrate a good description of the available data. We show that the study of the energy dependence of the incoherent production of J/ψ allows us to pinpoint the onset of saturation effects by selecting the region of Mandelstam-t around 1 GeV2 where the contribution of hot spots is dominant. We predict the onset of saturation in a Pb target to occur for W around a few hundred GeV. This can be measured with current data in ultra-peripheral Pb–Pb collisions at the LHC.
The impact of nonlinear effects in the deeply virtual Compton scattering (DVCS) process that will be measured in future electron-hadron collisions is investigated. We present, for the first time, the ...predictions derived using the solution to the Balitsky–Kovchegov equation with the collinearly-improved kernel and including the impact-parameter dependence. We estimate the total cross section and
t
-distribution of the DVCS process in
ep
and
eA
collisions and demonstrate that
d
σ
/
d
t
is strongly dependent on the assumption for the impact-parameter dependence of the dipole-hadron scattering amplitude. Our results indicate that a future experimental analysis of this process will be useful to discriminate among different models for the saturation physics and, consequently, will allow us to constrain the description of QCD dynamics in parton densities.
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