arXiv subjects

Vadim Guzey

Publications and source records attributed to Vadim Guzey.

At least 19 recordsLinked to original sources

arXiv2026

New parton distribution functions of the real photon

Precise determination of the partonic structure of real photons has attracted renewed interest in view of ongoing studies of high-energy photon-induced processes in ultraperipheral collisions (UPCs) at the Large Hadron Collider (LHC) and the future Electron-Ion Collider (EIC). Despite being fundamental in their own right and essential for QCD phenomenology of hard processes initiated by resolved photons, the quark and gluon distributions of the photon are poorly known and merit a new analysis employing modern tools of statistical data analysis. We determine new sets of leading-order (LO) and next-to-leading-order (NLO) parton distributions (PDFs) for the real photon, dubbed VALO1.0, by performing a global QCD analysis of the world data on the photon structure function $F_2^γ$ measured in deep-inelastic scattering (DIS) processes on a real photon target in electron-positron collisions. Our analysis improves on the results available in the literature by providing uncertainties in the form of Monte Carlo replicas and the photon PDFs in the LHAPDF6 format. We observe that while the electron-positron data allow us to determine the singlet quark distribution very well at both LO and NLO, the gluon distribution is constrained to a much lesser degree, especially at LO. For this analysis, we have developed an open-source framework, which extends the pineline framework, developed for the proton, to the photon case and includes the program solving the inhomogeneous scale evolution of photon PDFs, $γ$EKO.

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arXiv2026

Nuclear parton distributions and nuclear shadowing

In this contribution, we review the physics and phenomenology of nuclear parton distribution functions (PDFs), with a particular focus on the small-x region of nuclear shadowing. We summarise experimental evidence for nuclear modifications of PDFs and discuss their theoretical explanations; in particular, we contrast different models of nuclear shadowing. We also overview model-agnostic extractions of nuclear PDFs from global data on hard processes with nuclei, emphasizing the validity of collinear factorization and the dominance of leading-twist nuclear PDFs. Finally, we discuss perspectives for present and future precision studies of nuclear PDFs and small-x QCD dynamics, including photonuclear reactions in heavy-ion ultraperipheral collisions at the Large Hadron Collider and lepton-nucleus deep inelastic scattering at the future Electron-Ion Collider.

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arXiv2026

VALO1.0: New real-photon parton distributions with Monte Carlo uncertainties

Performing a global QCD analysis of data on the photon structure function $F_2^γ$ in $e^{+} e^{-}$ scattering, we determine new leading order (LO) and next-to-leading (NLO) parton distributions functions (PDFs) of the real photon. The resulting photon PDFs, referred to as VALO1.0, are obtained in the form of Monte Carlo (MC) replicas which assess the propagation of experimental uncertainties to the PDFs. To achieve well-converging fits, we employ a five-parameter hadron-like ansatz for the boundary conditions with simplifying assumptions on the flavor structure of the quark distributions and the large-$x$ behavior of the gluon distribution. This results in robust quark distributions at both LO and NLO and the gluon distribution at NLO with modest uncertainties, while leaving LO gluons still largely unconstrained. The resulting photon PDFs broadly agree with the parameterizations available in the literature and set the stage for future analyses including additional photoproduction data, which could help to increase the flexibility of our input PDFs. The LO and NLO VALO1.0 photon-PDF replicas, both in the DIS$_γ$ and $\overline{\rm MS}$ factorization schemes as well as the open-source $γ\texttt{EKO}$ code for solving the scale dependence of photon PDFs and the analysis framework $\texttt{VALOfitter}$ are made publicly available.

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arXiv2026

Diffractive open charm photoproduction in ultraperipheral lead-lead and proton-lead collisions at the LHC

We calculate diffractive $D^{0}$ photoproduction in ultraperipheral lead--lead (Pb--Pb) collisions at the Large Hadron Collider (LHC) within the recently developed G$γ$A--FONLL framework, where photon--lead diffraction is modeled using nuclear diffractive parton distributions obtained in the leading twist shadowing approach and the photon fluxes include corrections for independent electromagnetic dissociation accompanying the hard photoproduction process. We then use the predicted diffractive cross section to quantify the coherent diffractive contribution rejected by the $Xn0n$ neutron-tagged event selection adopted in the first measurement of $D^0$ photoproduction in Pb-Pb collisions at the LHC, which requires neutron emission from only one of the two lead nuclei. In this work, we also extend the G$γ$A--FONLL framework to proton--lead ($p$--Pb) UPCs and present predictions for inclusive and diffractive $D^{0}$ photoproduction at the LHC. In this case, the dominant configuration is photon emission from the lead ion followed by photon--proton scattering, and the diffractive contribution is evaluated using proton diffractive parton distributions constrained by HERA data.

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arXiv2024

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

We present next-to-leading order perturbative QCD predictions for inclusive dijet photoproduction in ultra-peripheral nucleus-nucleus collisions (UPCs) within the impact-parameter dependent equivalent photon approximation. Taking into account the finite size of both the photon-emitting 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 sizeable, 20-40% effect for large values of the $z_γ$ 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 cut-off 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-$z_γ$ 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.

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arXiv2024

Dijet photoproduction and transverse-plane geometry in ultra-peripheral nucleus-nucleus collisions

We present new NLO pQCD predictions for the inclusive photoproduction of dijets in ultra-peripheral (UPC) lead-lead collisions at 5.02 TeV with a realistic impact-parameter dependent effective photon flux obtained through the Woods-Saxon nuclear profile. For the first time in NLO inclusive UPC dijet predictions, we take into account also the modelling of the forward-neutron event class required in the experimental measurements. We show that since the dijet photoproduction at forward rapidities requires an energetic photon in the initial state, this biases the cross section to be dominated by events with relatively small impact parameters between the nuclei, of the order of a few nuclear radii. This leads to a sensitivity to the transverse-plane collision geometry, which we take properly into account by including effects from the finite extent of both the photon-emitting and the target nucleus. We also test the potential sensitivity to the spatial dependence of nuclear parton distribution functions in connection with this finding.

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arXiv2024

Global fits of proton PDFs with non-linear corrections from gluon recombination

We present numerical studies of the leading non-linear corrections to the DGLAP evolution equations of parton distribution functions (PDFs) resulting from gluon recombination, which reduce the pace of evolution at small momentum fractions $x$. The non-linear evolution is implemented in the \textsc{HOPPET} and \textsc{xFitter} toolkits and used to carry out fits of proton PDFs using lepton-proton deep inelastic scattering data from HERA, BCDMS and NMC. While we do not find evidence for non-linear effects, we are able to set upper limits for their strength. We also quantify the potential impact of longitudinal structure function measurements at the Electron-Ion Collider and the Large Hadron Electron Collider on future fits.

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arXiv2024

Collision geometry in UPC dijet production

We present a comprehensive NLO pQCD study on inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs). Our analysis takes into account the finite size of both the photon-emitting and the target nucleus, estimated using the Wood-Saxon nuclear density profile. We show that a significant part of the measured dijets at large $z_γ$ in UPC Pb+Pb collisions at 5.02 TeV come from events with relatively small impact parameters of the order of a few nuclear radii, and the cross section predictions thus become sensitive to the modelling of the collision geometry and photon flux near the source nucleus. In addition, we include the modelling of electromagnetic breakup survival factor needed for a direct comparison with the experimental data and study the resolution power of this process in terms of the impact-parameter dependent nuclear parton distribution functions.

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arXiv2023

Proton PDFs with non-linear corrections from gluon recombination

We present numerical studies of the leading non-linear corrections to the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution equations of parton distribution functions (PDFs) resulting from gluon recombination. The effect of these corrections is to reduce the pace of evolution at small momentum fractions $x$, while slightly increasing it at intermediate $x$. By implementing the non-linear evolution in the xFitter framework, we have carried out fits of proton PDFs using data on lepton-proton deep inelastic scattering from HERA, BCDMS and NMC. While we find no evidence for the presence of non-linearities, they cannot be entirely excluded either and we are able to set limits for their strength. In terms of the recombination scale $Q_r$, which plays a similar role as the saturation scale in the dipole picture of the proton, we find that $Q_r \lesssim 2.5\,{\rm GeV}$. We also quantify the impact of the non-linear terms for the longitudinal structure function at the Electron-Ion Collider and the Large Hadron Electron Collider and find that these measurements could provide stronger constraints on the projected effects.

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arXiv2023

Next-to-leading order perturbative QCD predictions for exclusive $J/ψ$ photoproduction in oxygen-oxygen and lead-lead collisions at the LHC

We present predictions for the cross sections of coherent $J/ψ$ photoproduction in lead-lead and oxygen-oxygen ultraperipheral collisions (UPCs) as a function of the $J/ψ$ rapidity at the LHC in the framework of collinear factorization at next-to-leading order (NLO) in perturbative QCD. Taking generalized parton distribution functions in their forward limit and using the EPPS21, nNNPDF3.0, and nCTEQ15WZSIH nuclear parton distribution functions, we update our recent results for Pb-Pb collisions, make detailed predictions for O-O collisions for several beam energy configurations, and examine the ratio of O-O and Pb-Pb UPC cross sections. We show that the latter observable allows one to significantly reduce the scale uncertainty of NLO predictions for this process.

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arXiv2023

Illuminating the impact-parameter dependence of UPC dijet photoproduction

We present new NLO pQCD predictions for photoproduction of dijets in ultraperipheral PbPb collisions at 5.02 TeV with a realistic photon flux and up-to-date nuclear PDFs. Our calculation of the impact parameter dependence of the photon flux includes the effects of the nuclear form factor in the photon-emitting nucleus and the spatial dependence of nuclear PDFs of the target nucleus, which are estimated using the Wood-Saxon nuclear density profile. We show that a significant portion of the measured dijets at large $z_γ$ originate from events with impact parameters of the order of a few nuclear radii, and that the cross section predictions therefore become sensitive to the modelling of the nuclear geometry and photon flux close to the source nucleus.

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arXiv2023

Exclusive quarkonium photoproduction in $A$+$A$ UPCs at the LHC in NLO pQCD

We present the first study of coherent exclusive quarkonium ($J/ψ$, $Υ$) photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) at the LHC in the framework of collinear factorization and next-to-leading order (NLO) perturbative QCD (pQCD). We make NLO predictions for the $J/ψ$ and $Υ$ rapidity distributions for lead (Pb) and oxygen (O) beams, and quantify their dependence on the factorization/renormalization scale, nuclear parton distribution functions (PDFs) and their uncertainties, and on differences between nuclear PDFs and generalized parton distribution functions (GPDs). We show that within the PDF-originating uncertainties our approach provides a good description of the available $J/ψ$ photoproduction data in Pb+Pb UPCs at the LHC but that the scale uncertainty is significant. We demonstrate that at NLO pQCD the quark contributions are important in the $J/ψ$ case but that gluons clearly dominate the $Υ$ cross sections. We also study how the scale dependence could be tamed by considering O+O/Pb+Pb ratios of the exclusive $J/ψ$ UPC cross sections, and how HERA and p+p/Pb LHC data can help in obtaining better-controlled NLO predictions in the $Υ$ case.

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arXiv2023

Predictions for exclusive $Υ$ photoproduction in ultraperipheral ${\rm Pb}+{\rm Pb}$ collisions at the LHC at next-to-leading order in perturbative QCD

We present predictions for the rapidity-differential cross sections of exclusive $Υ$ 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). While the effects due to the GPD modeling turn out to be small, the direct NLO predictions still carry significant nPDF-originating uncertainties and depend strongly on the choices of the factorization and renormalization scales. 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 ${\rm Pb}+{\rm Pb}$ collisions at the LHC.

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arXiv2022

Next-to-leading order QCD predictions for exclusive $J/ψ$ photoproduction in ultraperipheral Pb+Pb collisions at the LHC

We present the first study of rapidity-differential cross sections of coherent exclusive $J/ψ$ photoproduction in heavy-ion ultraperipheral collisions (UPCs) at the LHC, $\text{d}σ(\text{Pb}+\text{Pb}\rightarrow \text{Pb}+J/ψ+\text{Pb})/\text{d}y$, to NLO in collinear factorisation. Taking the generalised parton distributions as their forward limit counterparts, the NLO contributions in the cross sections are quantified and we show that the real part of the amplitude and quark-PDF contributions must not be neglected. We further evaluate the uncertainties arising from the scale-choice at both LO and NLO, and compare our results with exclusive $J/ψ$ photoproduction data in Pb+Pb UPCs from LHC. The scale dependence in $\text{d}σ(\text{Pb}+\text{Pb}\rightarrow \text{Pb}+J/ψ+\text{Pb})/\text{d}y$ is sizable, but we can find a scale-choice at NLO that reproduces the Pb+Pb UPC data at both Run 1 and Run 2 collision energies. This process has traditionally been suggested to be a direct probe of nuclear gluon distributions. We show that the situation changes rather dramatically from LO to NLO, where the interplay of both gluon and quark contributions, and in particular their relative signs, play a significant role. We end with some concluding remarks about the outlook of this observable within a tamed collinear factorisation.

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arXiv2022

Exclusive $J/ψ$ photoproduction in ultraperipheral Pb+Pb collisions at the LHC to next-to-leading order perturbative QCD

We present the first next-to-leading-order (NLO) perturbative QCD (pQCD) study of rapidity-differential cross sections of coherent exclusive photoproduction of $J/ψ$ mesons in heavy-ion ultraperipheral collisions (UPCs) at the LHC, $dσ/dy(\text{Pb} + \text{Pb} \rightarrow \text{Pb} + J/ψ+ \text{Pb})$. For this, we account for the photon-nucleon NLO cross sections at the forward limit, the $t$ dependence using a standard nuclear form factor, and the photon fluxes of the colliding nuclei. Approximating the generalized parton distributions with their forward-limit parton distribution functions (PDFs), we quantify the NLO contributions in the cross sections, show that the real part of the amplitude and quark-PDF contributions must not be neglected, quantify the uncertainties arising from the scale-choice and PDFs, and compare our results with ALICE, CMS and LHCb $J/ψ$ photoproduction data in Pb+Pb UPCs, exclusive $J/ψ$ photoproduction data from HERA, and LHCb data in p+p. The scale dependence in $dσ/dy(\text{Pb} + \text{Pb} \rightarrow \text{Pb} + J/ψ+ \text{Pb})$ is significant, but we can find a scale-choice that reproduces the Pb+Pb UPC data both at 2.76 and 5.02 TeV collision energies. This process has traditionally been suggested to be a direct probe of nuclear gluon distributions. We show that the situation changes rather dramatically from LO to NLO: the NLO cross sections reflect the nuclear effects of both gluons and quarks in a complicated manner where the relative signs of the LO and NLO terms in the amplitude play a significant role.

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arXiv2021

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

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 $x$ and intermediate-to-low $Q^2$. Our analysis is based on a publicly available open-source tool {\tt 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 {\tt FONLL} general-mass variable-flavor-number scheme. The most recent {\tt 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.

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arXiv2020

New opportunities at the photon energy frontier

Ultra-peripheral collisions (UPCs) involving heavy ions and protons are the energy frontier for photon-mediated interactions. UPC photons can be used for many purposes, including probing low-$x$ gluons via photoproduction of dijets and vector mesons, probes of beyond-standard-model processes, such as those enabled by light-by-light scattering, and studies of two-photon production of the Higgs.

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arXiv2020

Probing Nucleons and Nuclei in High Energy Collisions

This volume is a collection of contributions for the 7-week program "Probing Nucleons and Nuclei in High Energy Collisions" that was held at the Institute for Nuclear Theory in Seattle, WA, USA, from October 1 until November 16, 2018. The program was dedicated to the physics of the Electron Ion Collider (EIC), the world's first polarized electron-nucleon (ep) and electron-nucleus (eA) collider to be constructed in the USA. These proceedings are organized by chapters, corresponding to the weeks of the program: Week I, Generalized parton distributions; Week II, Transverse spin and TMDs; Week III, Longitudinal spin; Week IV, Symposium week; Weeks V & VI, eA collisions; Week VII, pA and AA collisions. We hope these proceedings will be useful to readers as a compilation of EIC-related science at the end of the second decade of the XXI century.

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