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 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.
We examine the role played in double-parton interactions (DPI) by the parton–parton correlations originating from perturbative QCD parton splittings. Also presented are the results of the numerical ...analysis of the integrated DPI cross sections at Tevatron and LHC energies. To obtain the numerical results the knowledge of the single-parton GPDs gained by the HERA experiments was used to construct the non-perturbative input for
generalized double-parton distributions
. The perturbative two-parton correlations induced by three-parton interactions contribute significantly to a resolution of the longstanding puzzle of an excess of multi-jet production events in the back-to-back kinematics observed at the Tevatron.
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.
Color fluctuations in hadron–hadron collisions are responsible for the presence of inelastic diffraction and lead to distinctive differences between the Gribov picture of high energy scattering and ...the low energy Glauber picture. We find that color fluctuations give a larger contribution to the fluctuations of the number of wounded nucleons than the fluctuations of the number of nucleons at a given impact parameter. The two contributions for the impact parameter averaged fluctuations are comparable. As a result, standard procedures for selecting peripheral (central) collisions lead to selection of configurations in the projectile which interact with smaller (larger) than average strength. We suggest that studies of pA collisions with a hard trigger may allow to observe effects of color fluctuations.
We calculate the rate of double parton scattering (DPS) in proton-proton collisions in the framework of the recently proposed hot spot model of the nucleon structure. The resulting rate, especially ...for the case of three hot spots, is compared with the current experimental data on DPS at the LHC.
Using the framework that interpolates between the leading power limit of the color glass condensate and the high energy (or
k
T
) factorization we calculate the direct component of the forward dijet ...production in ultra-peripheral
Pb
–
Pb
collisions at CM energy
5.1
TeV
per nucleon pair. The formalism is applicable when the average transverse momentum of the dijet system
P
T
is much bigger than the saturation scale
Q
s
,
P
T
≫
Q
s
, while the imbalance of the dijet system can be arbitrary. The cross section is uniquely sensitive to the Weizsäcker–Williams (WW) unintegrated gluon distribution, which is far less known from experimental data than the most common dipole gluon distribution appearing in inclusive small-
x
processes. We have calculated cross sections and nuclear modification ratios using WW gluon distribution obtained from the dipole gluon density through the Gaussian approximation. The dipole gluon distribution used to get WW was fitted to the inclusive HERA data with the nonlinear extension of unified BFKL + DGLAP evolution equation. The saturation effects are visible but rather weak for realistic
p
T
cut on the dijet system, reaching about 20% with the cut as low as
6
GeV
. We find that the LO collinear factorization with nuclear leading-twist shadowing predicts quite similar effects.
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.
pQCD physics of multiparton interactions Blok, B.; Dokshitzer, Yu; Frankfurt, L. ...
The European physical journal. C, Particles and fields,
04/2012, Volume:
72, Issue:
4
Journal Article
Peer reviewed
Open access
We study production of two pairs of jets in hadron–hadron collisions in view of extracting contribution of
double hard interactions
of three and four partons (3→4, 4→4). Such interactions, in spite ...of being power suppressed at the level of the total cross section, become comparable with the standard hard collisions of two partons, 2→4, in the
back-to-back kinematics
when the transverse momentum imbalances of two pairing jets are relatively small.
We express differential and total cross sections for two-dijet production in double parton collisions through the generalized two-parton distributions,
2
GPDs (Block et al., Phys. Rev. D 83, 071501,
2011
), and treat them in the leading logarithmic approximation of pQCD that resums collinear logarithms in all orders.
A special emphasis is given to 3→4 double hard interaction processes which, being of the same order in
as the 4→4 process, turn out to be
geometrically enhanced
compared to the latter and should contribute significantly to four-jet production.
The framework developed here takes into systematic consideration perturbative
Q
2
evolution of
2
GPDs. It can be used as a basis for future analysis of NLO corrections to multiparton interactions (MPI) at LHC and Tevatron colliders, in particular for improving evaluation of QCD backgrounds to new physics searches.
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