We present next-to-leading order perturbative QCD predictions for inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) within the impact-parameter dependent equivalent photon approximation. Taking into account the finite size of both the photon-emitting nucleus and the target nucleus, we show that this process is sensitive to the transverse-plane geometry of the UPC events. We show that this leads to a sizable, 20–40% effect for large values of the <math><msub><mi>z</mi><mi>γ</mi></msub></math> variable in the dijet photoproduction cross section in lead-lead UPCs at 5.02 TeV compared to the widely used pointlike approximation where the nuclear radius is accounted for only as a sharp cutoff in the photon flux calculation. This resolution of the spatial degrees of freedom is a result of having high-transverse-momentum jets in the final state, which at the large-<math><msub><mi>z</mi><mi>γ</mi></msub></math> kinematics requires a highly energetic photon in the initial state, thus biasing the collisions to small impact-parameter “near-encounter” configurations. We further discuss the role of the forward-neutron event-class selection in isolating the photonuclear cross section in the nucleus-nucleus collisions, and employ the needed electromagnetic breakup survival factor in our predictions.

Finland

Kari J. Eskola

Vadim Guzey

Ilkka Helenius

Petja Paakkinen

Hannu Paukkunen

Spatial resolution of dijet photoproduction in near-encounter ultraperipheral nuclear collisions

We present predictions for the rapidity-differential cross sections of exclusive  $$\varUpsilon $$  <math> <mi>Υ</mi> </math>   photoproduction in ultraperipheral collisions (UPCs) of lead ions at the Large Hadron Collider (LHC). We work in the framework of collinear factorization at next-to-leading order (NLO) in perturbative QCD, modeling the generalized parton distributions (GPDs) through the Shuvaev transform of nuclear parton distribution functions (nPDFs). Our direct NLO predictions depend significantly on nPDF uncertainties and on the choices of the factorization and renormalization scales, but to a much lesser degree on GPD modeling. To tame the scale dependence and to account for the fact that the NLO calculations generally underpredict the photoproduction measurements on protons, we also present alternative, data-driven predictions. In this approach the underlying photoproduction cross sections on lead are found by combining their nuclear modifications calculated at NLO with the measured photoproduction cross sections on protons. The data-driven strategy reduces the uncertainties associated with the scale choices, and essentially eliminates the effects of GPD modeling thereby leaving the cross sections sensitive mainly to the input nPDFs. Our estimates indicate that the process is measurable in  $${\textrm{Pb}}+{\textrm{Pb}}$$  <math> <mrow> <mtext>Pb</mtext> <mo>+</mo> <mtext>Pb</mtext> </mrow> </math>   collisions at the LHC.

Kari Eskola

France

Christopher Flett

Topi Löytäinen

Predictions for exclusive  $$\varUpsilon $$  <math> <mi>Υ</mi> </math>   photoproduction in ultraperipheral  $${\textrm{Pb}}+{\textrm{Pb}}$$  <math> <mrow> <mtext>Pb</mtext> <mo>+</mo> <mtext>Pb</mtext> </mrow> </math>   collisions at the LHC at next-to-leading order in perturbative QCD

While the phenomenon of gluon nuclear shadowing at small <math><mi>x</mi></math> has been getting confirmation in QCD analyses of various LHC measurements involving heavy nuclei, it has not been possible so far to establish experimentally the number of target nucleons responsible for nuclear shadowing in a given process. To address this issue, we study coherent <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> electroproduction on <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>3</mn></mrow></mmultiscripts></mrow></math> in the kinematics of a future electron-ion collider and show that this process has the power to disentangle the contributions of the interaction with a specific number of nucleons <math><mi>k</mi></math>, in particular, with two nucleons at the momentum transfer <math><mi>t</mi><mo>≠</mo><mn>0</mn></math>. We predict a dramatic shift of the <math><mi>t</mi></math> dependence of the differential cross section toward smaller values of <math><mo>|</mo><mi>t</mi><mo>|</mo></math> due to a nontrivial correlation between <math><mi>x</mi></math> and <math><mi>k</mi></math>. This calculation, which makes use for the first time of realistic wave functions, provides a stringent test of models of nuclear shadowing and a novel probe of the 3D imaging of gluons in light nuclei. In addition, thanks to this analysis, unique information on the real part of the corresponding scattering amplitude could be accessed.

Russia

Italy

Matteo Rinaldi

Sergio Scopetta

United States

Mark Strikman

Michele Viviani

Coherent <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> Electroproduction on <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>3</mn></mrow></mmultiscripts></mrow></math> at the Electron-Ion Collider: Probing Nuclear Shadowing One Nucleon at a Time

Using next-to-leading order perturbative QCD, we calculate the diffractive contribution to inclusive dijet photoproduction in Pb-Pb ultraperipheral collisions (UPCs) at the LHC and find that it does not exceed 5–10% in small-<math><msub><mi>x</mi><mi>A</mi></msub></math> bins in the ATLAS kinematics at <math><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>5.02</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math>. Its smallness is a result of the restricted kinematics (<math><msub><mi>p</mi><mrow><mi>T</mi><mn>1</mn></mrow></msub><mo>&gt;</mo><mn>20</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> and <math><msub><mi>x</mi><mi>A</mi></msub><mo>&gt;</mo><mn>0.001</mn></math>), the large nuclear suppression of nuclear diffractive parton distribution functions predicted in the leading twist model of nuclear shadowing, and additional suppression due to QCD factorization breaking in diffraction. At the same time, using looser <math><msub><mi>p</mi><mi>T</mi></msub></math> cuts, e.g., <math><msub><mi>p</mi><mrow><mi>T</mi><mn>1</mn></mrow></msub><mo>&gt;</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> and <math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi><mi>i</mi><mo>≠</mo><mn>1</mn></mrow></msub><mo>&gt;</mo><mn>5</mn><mi>–</mi><mn>7</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, we find that <math><mo>(</mo><mi>d</mi><msub><mi>σ</mi><mrow><mi>diff</mi></mrow></msub><mo>/</mo><mi>d</mi><msub><mi>x</mi><mi>A</mi></msub><mo>)</mo><mo>/</mo><mo>(</mo><mi>d</mi><msub><mi>σ</mi><mrow><mi>inc</mi></mrow></msub><mo>/</mo><mi>d</mi><msub><mi>x</mi><mi>A</mi></msub><mo>)</mo></math> can reach 10–20% at <math><mi>x</mi><mo>≈</mo><mn>5</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup></math>. Also, applying our framework to proton-proton UPCs at <math><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math>, we find that the ratio of the diffractive and inclusive cross sections of dijet photoproduction <math><mo>(</mo><mi>d</mi><msub><mi>σ</mi><mrow><mi>diff</mi></mrow></msub><mo>/</mo><mi>d</mi><msub><mi>x</mi><mi>p</mi></msub><mo>)</mo><mo>/</mo><mo>(</mo><mi>d</mi><msub><mi>σ</mi><mrow><mi>inc</mi></mrow></msub><mo>/</mo><mi>d</mi><msub><mi>x</mi><mi>p</mi></msub><mo>)</mo></math> is as large as 10–15% for <math><msub><mi>x</mi><mi>p</mi></msub><mo>∼</mo><mn>5</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup></math>. An account of the contribution of Pomeron-Pomeron scattering, which is not included in our analysis, should make this ratio somewhat larger.

Germany

Michael Klasen

How large is the diffractive contribution to inclusive dijet photoproduction in ultraperipheral collisions at the LHC?

In this article we obtain a new set of nuclear parton distribution functions (nuclear PDFs) at next-to-leading order and next-to-next-to-leading order accuracy in perturbative QCD. The common nuclear deep-inelastic scattering (DIS) data analyzed in our study are complemented by the available charged-current neutrino DIS data with nuclear targets and data from Drell-Yan cross section measurements for several nuclear targets. In addition, the most recent DIS data from the Jefferson Lab CLAS and Hall C experiments are also added to our data sample. For these specific datasets, we consider the impact of target mass corrections and higher twist effects which are expected to be important in the region of large <math><mi>x</mi></math> and intermediate-to-low <math><msup><mi>Q</mi><mn>2</mn></msup></math>. Our analysis is based on a publicly available open-source tool, APFEL, which has been modified to be applicable for our analysis of nuclear PDFs. Heavy-quark contributions to nuclear DIS are considered within the framework of the FONLL general-mass variable-flavor-number scheme. The most recent CT18 PDFs are used as baseline proton PDFs. The uncertainties of nuclear PDFs are determined using the standard Hessian approach. The results of our global QCD analysis are compared with existing nuclear PDF sets and with the fitted cross sections, for which our set of nuclear PDFs provides a very good description.

Iran

Ireland

Hamzeh Khanpour

Maryam Soleymaninia

S. Atashbar Tehrani

Hubert Spiesberger

Nuclear parton distribution functions with uncertainties in a general mass variable flavor number scheme

We present a first, detailed study of diffractive dijet photoproduction at the recently approved electron-ion collider (EIC) at BNL. Apart from establishing the kinematic reaches for various beam types, energies and kinematic cuts, we make precise predictions at next-to-leading order (NLO) of QCD in the most important kinematic variables. We show that the EIC will provide new and more precise information on the diffractive parton density functions (PDFs) in the pomeron than previously obtained at HERA, illuminate the still disputed mechanism of global vs. only resolved-photon factorization breaking, and provide access to a completely new quantity, i.e. nuclear diffractive PDFs.

Diffractive dijet photoproduction at the EIC

We present GKG18-DPDFs, a next-to-leading order (NLO) QCD analysis of diffractive parton distribution functions (diffractive PDFs) and their uncertainties. This is the first global set of diffractive PDFs determined within the xFitter framework. This analysis is motivated by all available and most up-to-date data on inclusive diffractive deep inelastic scattering (diffractive DIS). Heavy quark contributions are considered within the framework of the Thorne–Roberts (TR) general mass variable flavor number scheme (GM-VFNS). We form a mutually consistent set of diffractive PDFs due to the inclusion of high-precision data from H1/ZEUS combined inclusive diffractive cross sections measurements. We study the impact of the H1/ZEUS combined data by producing a variety of determinations based on reduced data sets. We find that these data sets have a significant impact on the diffractive PDFs with some substantial reductions in uncertainties. The predictions based on the extracted diffractive PDFs are compared to the analyzed diffractive DIS data and with other determinations of the diffractive PDFs.

Muhammad Goharipour

First global next-to-leading order determination of diffractive parton distribution functions and their uncertainties within the xFitter framework

We studied the effects of NLO Q2 evolution of generalized parton distributions (GPDs) using the aligned-jet model for the singlet quark and gluon GPDs at an initial evolution scale. We found that the skewness ratio for quarks is a slow logarithmic function of Q2 , reaching rS=1.5-2 at Q2=100 GeV 2 and rg≈1 for gluons in a wide range of Q2 . Using the resulting GPDs, we calculated the DVCS cross section on the proton in NLO pQCD and found that this model in conjunction with modern parameterizations of proton PDFs (CJ15 and CT14) provides a good description of the available H1 and ZEUS data in a wide kinematic range.

Effects of next-to-leading order DGLAP evolution on generalized parton distributions of the proton and deeply virtual Compton scattering at high energy