Using QCD calculations of the cross section of inclusive dijet photoproduction in Pb–Pb ultraperipheral collisions in the LHC kinematics as pseudo-data, we study the effect of including these data ...using the Bayesian reweighting technique on nCTEQ15, nCTEQ15np, and EPPS16 nuclear parton distribution functions (nPDFs). We find that, depending on the assumed error of the pseudo-data, it leads to a significant reduction of the nPDF uncertainties at small values of the momentum fraction
x
A
. Taking the error to be 5%, the uncertainty of nCTEQ15 and nCTEQ15np nPDFs reduces approximately by a factor of two at
x
A
=
10
-
3
. At the same time, the reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form.
A
bstract
Using the leading twist approach (LTA) to nuclear shadowing, we calculate the ratios of diffractive and usual parton distributions for a heavy nucleus (Pb) and the proton,
R
A
/
p
=
f
i
/
A
...D
3
/
f
i
/
A
/
f
i
/
p
D
3
/
f
i
/
p
, for coherent and summed (coherent plus quasi-elastic) nuclear deep-inelastic scattering. We find that
R
A/p
≈ 0.5 − 1 for quarks as well as for the ratio of the diffractive and total cross sections
dσ
diff
/
dM
X
2
/
σ
tot
eA
/
dσ
diff
/
dM
X
2
/
σ
tot
ep
and
R
A/p
≈ 0.5 − 1.3 for gluons in a broad range of
x
, including the kinematics of the Electron-Ion Collider, which reaffirms the difference from the nuclear enhancement of
R
A
/
p
predicted in the gluon saturation framework. We demonstrate that the magnitude of
R
A
/
p
is controlled by the cross section of the interaction of hadronic fluctuations of the virtual photon with target nucleons, which explains an enhancement of
R
A
/
p
in the color dipole model and its suppression in LTA. We argue that the black disk limit corresponds to
R
A
/
p
= 1 and
R
A
/
p
coh
= 0.86 for the summed and coherent scattering, respectively. Relying on an intuitive definition of the saturation scale, we show that the ratio of the saturation scales of a heavy nucleus and proton
Q
sA
2
b
/
Q
sp
2
b
≈
1
at small impact parameters
b
due to the strong leading twist nuclear shadowing and diluteness of the nuclear density.
Using the data on coherent J/ψ 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 SPb(x) in a wide range of the momentum fraction x, 10−5≤x≤0.04. In the small-x region x<10−3, our χ2 fit favors a flat form of SPb(x)≈0.6 with approximately a 5% accuracy for x=6×10−4−10−3 and a 25% error at x=10−4. At the same time, uncertainties of the fit do not exclude a slow decrease of SPb(x) in the small-x limit. At large x, SPb(x) is constrained to better than 10% precision up to x=0.04 and is also consistent with the value of SPb(x) at 〈x〉=0.042, which we extract from the Fermilab data on the A dependence of the cross section of coherent J/ψ photoproduction on fixed nuclear targets. The resulting uncertainties on SPb(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.
A
bstract
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/ψ
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 γ
p
→
J/ψp
and γ
A
→
J/ψA
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/ψ
in nucleus-nucleus ultraperipheral collisions accompanied by the electromagnetic excitation of nuclei and the subsequent neutron emission.
An Electron-Ion Collider (EIC) in USA is currently discussed as a next-generation facility for high-energy nuclear physics. The main goal of the EIC is to study fundamental questions of Quantum ...Chromodynamics, which include the origin of the nucleon mass and spin and the three-dimensional structure of the nucleon in terms of quarks and gluons, the emergent properties of dense systems of gluons, and influence of nuclear matter on distributions of quarks and gluons and propagation of color charges through it. The EIC machine designs are aimed at achieving variable center of mass energies of 20 - 100 GeV, upgradable to 150 GeV, high degree of polarization ( 70%) of beams of electrons, protons and light nuclei, high collision luminosity of 1033-34 cm-2s-1, and ion beams from deuteron to heaviest (Lead) nuclei. In this contribution, we present the current status of the EIC project, its physics program, and proposed designs of EIC realization.
We present and discuss the theory and phenomenology of the leading twist theory of nuclear shadowing which is based on the combination of the generalization of the Gribov–Glauber theory, QCD ...factorization theorems, and the HERA QCD analysis of diffraction in lepton–proton deep inelastic scattering (DIS). We apply this technique for the analysis of a wide range of hard processes with nuclei–inclusive DIS on deuterons, medium-range and heavy nuclei, coherent and incoherent diffractive DIS with nuclei, and hard diffraction in proton–nucleus scattering–and make predictions for the effect of nuclear shadowing in the corresponding sea quark and gluon parton distributions. We also analyze the role of the leading twist nuclear shadowing in generalized parton distributions in nuclei and in certain characteristics of final states in nuclear DIS. We discuss the limits of applicability of the leading twist approximation for small x scattering off nuclei and the onset of the black disk regime and methods of detecting it. It will be possible to check many of our predictions in the near future in the studies of the ultraperipheral collisions at the Large Hadron Collider (LHC). Further checks will be possible in pA collisions at the LHC and forward hadron production at the Relativistic Heavy Ion Collider (RHIC). Detailed tests will be possible at an Electron–Ion Collider (EIC) in the USA and at the Large Hadron–Electron Collider (LHeC) at CERN.
A
bstract
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
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
γ
jets
.
We argue that with an increase of the collision energy, elastic photoproduction of ρ 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 σγA→ρA and the reduced effective ρ-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 ρ photoproduction on nuclei in heavy ion UPCs in the 7 GeV<Wγp<46 GeV energy range and to predict the value of the cross section of coherent ρ photoproduction in Pb–Pb UPCs at sNN=5.02 TeV in Run 2 at the LHC, dσPbPb→ρPbPb(y=0)/dy=560±25 mb.
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