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Register a new userExclusive $rho$ and $J/Psi$ photoproduction in ultraperipheral $pA$ collisions: Predictions of the gluon saturation models for the momentum transfer distributions
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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 $rho$ and $J/Psi$ 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 $rho$ 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.
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Based on accurate calculations of the flux of equivalent photons of the proton and heavy nuclei and the pQCD framework for the gluon distribution in the proton and nuclei, we analyze the rapidity and momentum transfer distributions of coherent $J/psi$ photoproduction in ultraperipheral proton-Pb collisions at the LHC. We demonstrate that unlike the case of proton-proton UPCs marred by certain theoretical uncertainties and experimental limitations, after a cut excluding the region of small momentum transfers, ultraperipheral proton-Pb collisions offer a clean way to study the gluon distribution in the proton down to $x approx 10^{-5}$. Our analysis of the momentum transfer distributions shows that an interplay of $J/psi$ production by low-energy photons on the nucleus and by high-energy photons on the proton in proton-Pb UPCs can result in some excess of events at small $p_t$ in a definite region of the rapidity y.
Using the framework of collinear factorization in perturbative QCD (pQCD), we analyze the recent data of the LHCb and ALICE collaborations on exclusive photoproduction of J/{psi} in ultraperipheral pp and AA collisions, respectively. We demonstrate that the simultaneous analysis of the proton and Pb data allows us to reduce the ambiguity of the pQCD description of the $gamma p to J/psi p$ and $gamma A to J/psi p$ cross sections and, hence, to place additional constraints on the gluon distributions in the nucleon and nuclei at small x. We also make predictions for the cross section of coherent exclusive photoproduction of J/{psi} in nucleus-nucleus ultraperipheral collisions accompanied by the electromagnetic excitation of nuclei and the subsequent neutron emission.
Using the Gribov-Glauber model for photon-nucleus scattering and a generalization of the vector meson dominance model for the hadronic structure of the photon, we make predictions for the cross section of incoherent $rho$ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) in the Large Hadron Collider kinematics. We find that the effect of the inelastic nuclear shadowing is significant and leads to an additional 25% suppression of the incoherent cross section. Comparing our predictions to those of the STARlight Monte Carlo framework, we observe very significant differences.
We evaluate the large momentum transfer $J/psi$ photoproduction with rapidity gaps in ultraperipheral proton-ion collisions at the LHC which provides an effective method of probing dynamics of large t elastic hard QCD Pomeron interactions. It is shown that the experimental studies of this process would allow to investigate the energy dependence of cross section of elastic scattering of a small $cbar c$ dipole off the gluon over a wide range of invariant energies 10^3 < s_{cbar c - gluon} < 10^6 GeV^2. The accessible energy range exceeds the one reached in gamma p at HERA by a factor of 10 and allows the kinematic cuts which improve greatly sensitivity to the Pomeron dynamics as compared to the HERA measurements. The cross section is expected to change by a factor ge 20 throughout this interval and our estimates predict quite reasonable counting rates for this process with the several of the LHC detectors.
We argue that with an increase of the collision energy, elastic photoproduction of $rho$ mesons on nuclei becomes affected by the significant cross section of photon inelastic diffraction into large masses, which results in the sizable inelastic nuclear shadowing correction to $sigma_{gamma A to rho A}$ and the reduced effective $rho$-nucleon cross section. We take these effects into account by combining the vector meson dominance model, which we upgrade to include the contribution of high-mass fluctuations of the photon according to QCD constraints, and the Gribov-Glauber approximation for nuclear shadowing, where the inelastic nuclear shadowing is included by means of cross section fluctuations. The resulting approach allows us to successfully describe the data on elastic $rho$ photoproduction on nuclei in heavy ion UPCs in the $7 {rm GeV} < W_{gamma p} < 46$ GeV energy range and to predict the value of the cross section of coherent $rho$ photoproduction in Pb-Pb UPCs at $sqrt{s_{NN}}=5.02$ TeV in Run 2 at the LHC, $dsigma_{Pb Pb to rho Pb Pb} (y=0)/dy= 560 pm 25$ mb.
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