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Register a new userNucleon dissociation and incoherent $J/psi$ photoproduction on nuclei in ion ultraperipheral collisions at the CERN Large Hadron Collider
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Using the general notion of cross section fluctuations in hadron--nucleus scattering at high energies, we derive an expression for the cross section of incoherent $J/psi$ photoproduction on heavy nuclei $dsigma_{gamma A to J/psi Y}/dt$, which includes both elastic $dsigma_{gamma p to J/psi p}/dt$ and proton-dissociation $dsigma_{gamma p to J/psi Y}/dt$ photoproduction on target nucleons. We find that with good accuracy, $dsigma_{gamma A to J/psi Y}/dt$ can be expressed as a product of the sum of the $dsigma_{gamma p to J/psi p}/dt$ and $dsigma_{gamma p to J/psi Y}/dt$ cross sections, which have been measured at HERA, and the common nuclear shadowing factor, which is calculated using the leading twist nuclear shadowing model. Our prediction for the cross section of incoherent $J/psi$ photoproduction in Pb-Pb UPCs at $sqrt{s_{NN}}=2.76$ TeV and $y=0$, $dsigma_{AA to J/psi AY}(y=0)/dy=0.59-1.24$ mb, agrees within significant theoretical uncertainties with the data of the ALICE collaboration.
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We consider $J/psi$ photoproduction in ion--ion ultraperipheral collisions (UPCs) at the LHC and RHIC in the coherent and incoherent quasielastic channels with and without accompanying forward neutron emission and analyze the role of nuclear gluon shadowing at small $x$, $x=10^{-4}-10^{-2}$, in these processes. We find that despite the good agreement between large nuclear gluon shadowing and the ALICE data in the coherent channel, in the incoherent channel, the leading twist approximation predicts the amount of nuclear suppression which is by approximately a factor of $1.5$ exceeds that seen in the data. We hypothesize that part of the discrepancy can be accounted for by the incoherent inelastic process of $J/psi$ photoproduction with nucleon dissociation. To separate the high-photon-energy and low-photon-energy contributions to the $d sigma_{AAto AAJ/psi}(y)/dy$ cross section, we consider ion--ion UPCs accompanied by neutron emission due to electromagnetic excitation of one or both colliding nuclei. We describe the corresponding PHENIX data and make predictions for the LHC kinematics. In addition, in the incoherent quasielastic case, we show that the separation between the low-photon-energy and high-photon-energy contributions can be efficiently performed by measuring the correlation between the directions of $J/psi$ and the emitted neutrons.
Using the data on coherent $J/psi$ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) obtained in Runs 1 and 2 at the Large Hadron Collider (LHC), we determined with a good accuracy the nuclear suppression factor of $S_{Pb}(x)$ in a wide range of the momentum fraction $x$, $10^{-5} leq x leq 0.04$. In the small-$x$ region $x < 10^{-3}$, our $chi^2$ fit favors a flat form of $S_{Pb}(x) approx 0.6$ with approximately a 5% accuracy for $x=6 times 10^{-4} - 10^{-3} $ and a 25% error at $x=10^{-4}$. At the same time, uncertainties of the fit do not exclude a slow decrease of $S_{Pb}(x)$ in the small-$x$ limit. At large $x$, $S_{Pb}(x)$ is constrained to better than 10% precision up to $x=0.04$ and is also consistent with the value of $S_{Pb}(x)$ at $langle x rangle =0.042$, which we extract from the Fermilab data on the $A$ dependence of the cross section of coherent $J/psi$ photoproduction on fixed nuclear targets. The resulting uncertainties on $S_{Pb}(x)$ are small, which indicates the potential of the LHC data on coherent charmonium photoproduction in Pb-Pb UPCs to provide additional constraints on small-$x$ nPDFs. We explicitly demonstrate this using as an example the EPPS16 and nCTEQ16 nuclear parton distribution functions, whose uncertainties decrease severalfold after the Bayesian reweighting of the discussed UPC data.
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.
The exclusive photoproduction of $Upsilon (nS)$ states were calculated in ultra-peripheral collisions for coherent and incoherent process in PbPb at $sqrt{s_{NN}}$= 5.5 TeV. Different dipole models were compared in the theoretical framework of light-cone color dipole formalism. Moreover, it was calculated the differential cross section for the Upsilon states and their total cross section for two intervals of rapidity: $|y|leq 4$ 4 and $ 2 le$ y $le 4.5$. A systematic study is done on the theoretical uncertainties associated to the production and predictions are presented for the first time for the incoherent cross section of the radially excited states.
Tagged jet measurements provide a promising experimental channel to quantify the similarities and differences in the mechanisms of jet production in proton-proton and nucleus-nucleus collisions. We present the first calculation of the transverse momentum asymmetry of Z^0/gamma^*-tagged jet events in sqrt{s}=2.76$ TeV reactions at the LHC. Our results combine the O(G_Falpha_s^2) perturbative cross sections with the radiative and collisional processes that modify parton showers in the presence of dense QCD matter. We find that a strong asymmetry is generated in central lead-lead reactions that has little sensitivity to the fluctuations of the underlying soft hadronic background. We present theoretical model predictions for its shape and magnitude.
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