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Register a new userLarge t diffractive J/psi photoproduction with proton dissociation in ultraperipheral pA collisions at LHC
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Leonid Frankfurt
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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.
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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.
We calculate the cross section of inclusive dijet photoproduction in ultraperipheral collisions (UPCs) of heavy ions at the CERN Large Hadron Collider using next-to-leading order perturbative QCD and demonstrate that it provides a good description of the ATLAS data. We study the role of this data in constraining nuclear parton distribution functions (nPDFs) using the Bayesian reweighting technique and find that it can reduce current uncertainties of nPDFs at small $x$ by a factor of 2. We also make predictions for diffractive dijet photoproduction in UPCs and examine its potential to shed light on the disputed mechanism of QCD factorization breaking in diffraction.
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.
We make predictions for the cross sections of diffractive dijet photoproduction in $pp$, $pA$ and $AA$ ultraperipheral collisions (UPCs) at the LHC during Runs 1 and 2 using next-to-leading perturbative QCD. We find that the resulting cross sections are sufficiently large and, compared to lepton-proton scattering at HERA, have an enhanced sensitivity to small observed momentum fractions in the diffractive exchange, commonly denoted $z_{P}^{rm jets}$, and an unprecedented reach in the invariant mass of the photon-nucleon system $W$. We examine two competing schemes of diffractive QCD factorization breaking, which assume either a global suppression factor or a suppression for resolved photons only and demonstrate that the two scenarios can be distinguished by the nuclear dependence of the distributions in the observed parton momentum fraction in the photon $x_{gamma}^{rm jets}$.
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.
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