We model effects of color fluctuations (CFs) in the light-cone photon wave function and for the first time make predictions for the distribution over the number of wounded nucleons ν in the inelastic ...photon–nucleus scattering. We show that CFs lead to a dramatic enhancement of this distribution at ν=1 and large ν>10. We also study the implications of different scales and CFs in the photon wave function on the total transverse energy ΣET and other observables in inelastic γA scattering with different triggers. Our predictions can be tested in proton–nucleus and nucleus–nucleus ultraperipheral collisions at the LHC and will help to map CFs, whose first indications have already been observed at the LHC.
We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon–nucleon (nucleus) interactions. The current LHC ...detector configurations can explore hard phenomena at small
x
with nuclei and nucleons at photon–nucleon center-of-mass energies above 1 TeV, extending the
x
range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic
J
/
ψ
and
Υ
production as well as in high
t
ρ
0
production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than those at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used as a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.
We argue that study of total cross section of photoabsorption and coherent photoproduction of
ρ,
ρ′-mesons in ultraperipheral heavy ion collisions (UPC) is effective method to probe onset of black ...body limit (BBL) in the soft and hard QCD interactions. We illustrate the expected features of the onset of BBL using generalized vector dominance model. We show that this model describes very well
ρ-meson coherent photoproduction at 6⩽
E
γ
⩽10 GeV. In the case of
ρ-meson production we find a UPC cross section which is a factor ∼1.5 larger than the one found by Klein and Nystrand. The advantages of the process of coherent dijet production to probe onset of BBL in hard scattering regime where decomposition over the twists becomes inapplicable are explained and relative importance of the
γ+pomeron and
γ+
γ mechanisms is estimated.
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/ψ photoproduction on heavy nuclei ...dσγA → J/ψY /dt, which includes both elastic dσγ → Jψp / dt and proton-dissociation dσγp → J/ψY /dt photoproduction on target nucleons. We find that, with good accuracy, dσγA → J/ψY /dt can be expressed as a product of the sum of the dσγp → J/ψp /dt and dσγp → J/ψY /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/ψ photoproduction in Pb-Pb ultraperipheral collisions at √sNN = 2.76 TeV and y = 0, dσAA → J/ψAY (y = 0) / dy = 0.59 – 1.24 mb agrees within significant theoretical uncertainties with the data of the ALICE Collaboration.
We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisions in kinematics of Relativistic Heavy Ion Collider (RHIC) at sNN=200 GeV. Our improved estimate of ...the total coherent cross section is a factor of two smaller than the earlier ones. We find that the counting rate of quasielastic J/ψ photoproduction tagged by neutrons emitted due to cascading of the recoiled nucleon within the residual nucleus exceeds the rate of the coherent events. We argue that measurements of this process can be used to learn about the dynamics of color dipole–nucleon interactions in nuclei in the wide effective range of energies of γA interactions.
We show that the recent ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions at 2.76 TeV provide the first direct experimental evidence for the strong nuclear gluon ...shadowing in lead at x∼10−3. The evidence is based on the comparison of the nuclear suppression factor S(x≈0.001)=0.61−0.04+0.05 found in the analysis of the coherent J/ψ photoproduction cross sections measured by ALICE with the nuclear gluon shadowing predicted by the global fits of nuclear parton distributions and by the leading twist theory of nuclear shadowing.
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