arXiv subjects

W. Melnitchouk

Publications and source records attributed to W. Melnitchouk.

At least 19 recordsLinked to original sources

arXiv2026

QED radiative effects in semi-inclusive deep-inelastic scattering: traditional and factorized approaches

Semi-inclusive deep-inelastic scattering (SIDIS) of leptons is a vital tool for probing the three-dimensional momentum space partonic structure of the nucleon. Reliable extraction of the intrinsic collinear or transverse momentum dependent parton distributions from SIDIS data requires careful treatment of QED radiative effects beyond the Born level. In this work we perform a detailed comparative analysis of QED effects in SIDIS using the traditional Bardin-Shumeiko approach, augmented by the electron structure function method, and a more recent factorized approach that treats QED and QCD radiation on equal footing. We compare analytical and numerical results of these approaches for the unpolarized $\pi^+$ electroproduction cross sections off the proton and the Collins and Sivers transverse single-spin asymmetries, and identify the sources of discrepancies between the calculations, which for the cross sections can reach up to $\approx 2\%$, $\approx 7\%$, and $\approx 5\%$ at Jefferson Lab, HERMES, and EIC kinematics, respectively. To provide a unified approach for reliably extracting partonic information from future SIDIS measurements, we propose a hybrid framework that employs the salient features of both frameworks in their respective regions of applicability.

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arXiv2026

Impact of parity-violating deep-inelastic scattering on the weak mixing angle and high-$x$ parton distributions

We discuss the impact of neutral current parity-violating deep-inelastic scattering (PVDIS) of electrons from protons and deuterons on the determination of the weak mixing angle, $\sin^{2}{\theta_{\rm W}}$, and parton distribution functions (PDFs) at large parton momentum fractions $x$. Using the JAM global QCD analysis framework, we study the effect of incorporating pseudodata simulated for 11 GeV and 22 GeV Jefferson Lab kinematics, accounting for radiative corrections in a factorized QED+QCD approach and uncertainties from higher twist corrections in $\gamma Z$ exchange. We find that including future PVDIS pseudodata could yield important constraints on the value of $\sin^{2}{\theta_{\rm W}}$ at low $Q^2$ and on the high-$x$ behavior of the strange quark and $d/u$ PDF ratio. The strong correlation between $\sin^{2}{\theta_{\rm W}}$ and the $x$ dependence of the PDFs demonstrates the necessity for simultaneous analysis of QCD and electroweak quantities to ensure an unbiased determination of the weak mixing angle from PVDIS data.

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arXiv2026

Stability of parton distributions at high $x$: impact of nuclear and power corrections

We present a comprehensive new global QCD analysis of unpolarized parton distribution functions (PDFs) based upon proton, deuteron and $A\!=\!3$ data, including the latest inclusive deep-inelastic scattering (DIS) measurements from Jefferson Lab at high Bjorken-$x$. Using the JAM Bayesian Monte Carlo framework, we systematically explore the stability of the PDFs with respect to variations in the cuts on the invariant mass $W$ of the DIS final state, the implementation of target mass and higher twist corrections, as well as on the nuclear wave functions for the $A\!=\!2$ and 3 data. We find the $u$ and $d$ quark PDFs (and the $d/u$ ratio) are relatively stable up to $x \approx 0.8$, and able to describe DIS data down to $W^2=3.5$ GeV$^2$ and $Q^2=m_c^2$. Within the collinear factorization framework, the fitted higher twist corrections to DIS are found to be positive, and largely isospin independent. The description of the nuclear data also requires nonzero isoscalar and isovector nucleon off-shell PDF contributions, which gives specific predictions for the ratio, $R_D$, of deuteron to isoscalar nucleon structure functions.

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arXiv2026

Precision QCD with the Electron-Ion Collider

This document summarizes the discussions at the program "Precision QCD with the Electron Ion Collider", held from May to June 2025 at the Institute for Nuclear Theory (INT) at the University of Washington. The program was co-sponsored by the INT and by the Center for Frontiers in Nuclear Science (CFNS, Stony Brook University). Over its five-week duration it brought together about 70 theorists, experimentalists and computer scientists all interested in the physics program at the future Electron Ion Collider in preparation at Brookhaven National Laboratory. Key topics at the program were: higher-order perturbative-QCD calculations and techniques; nuclear structure and tomography; comparisons of phenomenological and lattice determinations of parton distribution functions; identification of signature observables for saturated gluons; assessment of the importance of AI techniques for EIC studies and detector development.

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arXiv2026

Isospin dependence of nuclear EMC effect from global QCD analysis

We perform a new global QCD analysis of unpolarized parton distribution functions (PDFs) in the nucleon from proton, deuteron and $A=3$ data, including recent measurements of $^3$He/$D$ and $^3$H/$D$ cross section ratios from the MARATHON experiment at Jefferson Lab. Simultaneously inferring the PDFs and nucleon off-shell corrections allows both to be determined consistently, without theoretical assumptions about the isospin dependence of nuclear effects. The analysis provides strong evidence for the need of nucleon off-shell corrections to describe the $A=3$ data, with large isoscalar and a suggestion of nonzero isovector contributions in $A \leq 3$ nuclei. We find that the extracted EMC ratios of nuclear to nucleon structure functions for $A=2$ and 3 differ from those naively extrapolated from heavy nuclei down to low $A$.

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arXiv2025

First simultaneous analysis of transverse momentum dependent and collinear parton distributions in the proton

We present the first simultaneous global QCD analysis of unpolarized transverse momentum dependent (TMD) and collinear parton distribution functions (PDFs) in the proton. Our study incorporates data from deep-inelastic scattering, Drell-Yan, inclusive weak boson, $W$+\,charm, and jet production involving PDFs, as well as TMD Drell-Yan and $Z$-boson production data from fixed target and collider experiments sensitive to both TMD and collinear distributions. The analysis is performed at next-to-next-to-leading logarithmic accuracy for QCD resummation in TMD observables and next-to-leading order for observables described in collinear factorization. The combined analysis improves knowledge of both TMD and collinear PDFs, particularly in the sea-quark sector, providing a consistent simultaneous description of the aforementioned observables.

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arXiv2025

First simultaneous global QCD analysis of kaon and pion parton distributions with lattice QCD constraints

We perform the first simultaneous global QCD analysis of pion and kaon parton distribution functions (PDFs), constrained by pion- and kaon-induced Drell-Yan (DY) and leading neutron electroproduction data, together with lattice QCD data on pion and kaon PDF moments. The analysis indicates a softer valence $\bar u$ distribution in the $K^-$ than in the $\pi^-$, and a significantly more peaked valence $s$-quark density in $K^-$ compared with the $\bar u$. The effective exponent governing the high-$x$ behavior of the PDF is found to be larger for $\bar u$ in the kaon, $\beta_{\bar u}^{K^-}\!= 1.6(2)$, than in the pion, $\beta_{\bar u}^{\pi^-}\!= 1.16(4)$, in the range $0.7 \leq x \leq 0.95$. From the gluon momentum fractions we find the pion's gluon content accounts for $\approx 1/3$ of the mass budget of the pion at $\mu=2~{\rm GeV}$, but only $\approx 1/4$ for the kaon.

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arXiv2025

Nonlocal effective field theory and its applications

We review recent applications of nonlocal effective field theory, focusing in particular on nonlocal chiral effective theory and nonlocal quantum electrodynamics (QED), as well as an extension of nonlocal effective theory to curved spacetime. For the chiral effective theory, we discuss the calculation of generalized parton distributions (GPDs) of the nucleon at nonzero skewness, along with the corresponding gravitational (or mechanical) form factors, within the convolution framework. In the QED application, we extend the nonlocal formulation to construct the most general nonlocal QED interaction, in which both the propagator and fundamental QED vertex are modified due to the nonlocal Lagrangian, while preserving the Ward-Green-Takahashi identities. For consistency with the modified propagator, a solid quantization is proposed, and the nonlocal QED is applied to explain the lepton $g-2$ anomalies without the introduction of new particles or interactions. Finally, with an extension of the chiral effective action to curved spacetime, we investigate the nonlocal energy-momentum tensor and gravitational form factors of the nucleon with a nonlocal pion-nucleon interaction.

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arXiv2025

Pionic gluons from global QCD analysis of experimental and lattice data

We perform the first global QCD analysis of parton distribution functions (PDFs) in the pion, with lattice-QCD data on gluonic pseudo--Ioffe-time distributions fitted simultaneously with experimental Drell-Yan and leading neutron electroproduction data. Inclusion of the lattice results with parametrized systematic corrections significantly reduces the uncertainties on the gluon PDF at parton momentum fractions $x \gtrsim 0.2$, revealing a higher gluon density in the pion at large $x$ than in the proton. The similar gluon momentum fractions in the pion and proton further suggests a relative suppression of the pion gluon density at small $x$.

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arXiv2025

Global QCD analysis of spin PDFs in the proton with high-$x$ and lattice constraints

We perform a comprehensive global QCD analysis of spin-dependent parton distribution functions (PDFs), combining all available data on inclusive and semi-inclusive deep-inelastic scattering (DIS), as well as inclusive weak boson and jet production in polarized $pp$ collisions, simultaneously extracting spin-averaged PDFs and fragmentation functions. Including recent Jefferson Lab DIS data at high $x$, together with subleading power corrections to the leading twist framework, allows us to verify the stability of the PDFs for $W^2 \geq 4$ GeV$^2$ and quantify the uncertainties on the spin structure functions more reliably. We explore the use of new lattice QCD data on gluonic pseudo Ioffe-time distributions, which, together with jet production and high-$x$ DIS data, improve the constraints on the polarized gluon PDF. The expanded kinematic reach afforded by the data into the high-$x$ region allows us to refine the bounds on higher twist contributions to the spin structure functions, and test the validity of the Bjorken sum rule.

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arXiv2025

First study of polarized proton-proton scattering with small-$x$ helicity evolution

We perform a phenomenological study of helicity-dependent parton distribution functions (PDFs) using small-$x$ helicity evolution equations, incorporating for the first time single-inclusive jet production data in polarized proton-proton ($pp$) scattering at parton momentum fractions $x < 0.1$. We also simultaneously include double-longitudinal spin asymmetries in inclusive and semi-inclusive deep-inelastic scattering probing $x < 0.1$. Employing the polarized small-$x$ pure-glue calculation of $pp\to gX$ for the jet production cross section, we modify the large-$N_c\&N_f$ KPS-CTT evolution equations by setting $N_f = 0$ to replicate the large-$N_c$ (pure-glue) limit, while retaining external quark flavors for the spinor field operators. We find that the $pp$ data have a considerable impact on the helicity PDFs at small $x$, reducing their uncertainties and leading to a total quark and gluon helicity in the proton for $x < 0.1$ of $-0.04 \pm 0.23$. Combining our analysis with the a recent JAM helicity PDF analysis of the world polarized data, which includes $x > 0.1$, we find a total quark and gluon helicity contribution for $x > 10^{-7}$ of between 0.02 and 0.51.

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arXiv2024

Strangeness in the proton from W+charm production and SIDIS data

We perform a global QCD analysis of unpolarized parton distribution functions (PDFs) in the proton, including new $W +$\,charm production data from $pp$ collisions at the LHC and semi-inclusive pion and kaon production data in lepton-nucleon deep-inelastic scattering, both of which have been suggested for constraining the strange quark PDF. Compared with a baseline global fit that does not include these datasets, the new analysis reduces the uncertainty on the strange quark distribution over the range $0.01 < x < 0.3$, and provides a consistent description of processes sensitive to strangeness in the proton. Including the new datasets, the ratio of strange to nonstrange sea quark distributions is $R_s = (s+\bar s)/(\bar u+\bar d) = \{0.72^{+0.52}_{-0.34},\, 0.46^{+0.30}_{-0.20},\, 0.32^{+0.23}_{-0.15}\}$ for $x = \{ 0.01, 0.04, 0.1 \}$ at $Q^2 = 4$~GeV$^2$. The data place more stringent constraints on the strange asymmetry $s-\bar s$, which is found to be consistent with zero in this range.

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arXiv2024

Kernel methods for evolution of generalized parton distributions

Generalized parton distributions (GPDs) characterize the 3-dimensional structure of hadrons, combining information about their internal quark and gluon longitudinal momentum distributions and transverse position within the hadron. The dependence of GPDs on the factorization scale $Q^2$ allows one to connect hard exclusive processes involving GPDs at disparate energy and momentum scales, which is needed in global analyses of experimental data. In this work we explore how finite element methods can be used to construct fast and differentiable $Q^2$ evolution codes for GPDs in momentum space, which can be used in a machine learning framework. We show numerical benchmarks of the methods' accuracy, including a comparison to an existing evolution code from PARTONS/APFEL++, and provide a repository where the code can be accessed.

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arXiv2024

Constraints on the $U(1)_{B-L}$ model from global QCD analysis

We perform the first global QCD analysis of electron-nucleon deep-inelastic scattering and related high-energy data including the beyond the Standard Model $U(1)_{B-L}$ gauge boson, $Z'$. Contrary to the dark photon case, we find no improvement in the $\chi^2$ relative to the baseline result. The finding allows us to place exclusion limits on the coupling constant of the $Z'$ with mass in the range $M_{Z'} = 2$ to 160 GeV.

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arXiv2024

Nonlocal chiral contributions to generalized parton distributions of the proton at nonzero skewness

We compute the one-loop contributions to spin-averaged generalized parton distributions (GPDs) in the proton from pseudoscalar mesons with intermediate octet and decuplet baryon states at nonzero skewness. Our framework is based on nonlocal covariant chiral effective theory, with ultraviolet divergences regularized by introducing a relativistic regulator derived consistently from the nonlocal Lagrangian. Using the splitting functions calculated from the nonlocal Lagrangian, we find the nonzero skewness GPDs from meson loops by convoluting with the phenomenological pion GPD and the generalized distribution amplitude, and verify that these satisfy the correct polynomiality properties. We also compute the lowest two moments of GPDs to quantify the meson loop effects on the Dirac, Pauli and gravitational form factors of the proton.

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arXiv2024

On the resolution of the sign of gluon polarization in the proton

Recently the possible existence of negative gluon helicity, $Δg$, has been observed to be compatible with existing empirical constraints, including from jet production in polarized proton-proton collisions at RHIC, and lattice QCD data on polarized gluon Ioffe time distributions. We perform a new global analysis of polarized parton distributions in the proton with new constraints from the high-$x$ region of deep-inelastic scattering (DIS). A dramatic reduction in the quality of the fit for the negative $Δg$ replicas compared to those with positive $Δg$ suggest that the negative $Δg$ solution cannot simultaneously account for high-$x$ polarized DIS data along with lattice and polarized jet data.

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arXiv2023

Global analysis of polarized DIS & SIDIS data with improved small-$x$ helicity evolution

We analyze the world polarized deep-inelastic scattering (DIS) and semi-inclusive DIS (SIDIS) data at low values of $x < 0.1$, using small-$x$ evolution equations for the flavor singlet and nonsinglet helicity parton distribution functions (hPDFs). The hPDFs for quarks, antiquarks, and gluons are extracted and evolved to lower values of $x$ to make predictions for the future Electron-Ion Collider (EIC). We improve on our earlier work by employing the more realistic large-$N_c\, \& N_f$ limit of the revised small-$x$ helicity evolution, and incorporating running coupling corrections along with SIDIS data into the fit. We find an anti-correlation between the signs of the gluon and $C$-even quark hPDFs as well as the $g_1$ structure function. While the existing low-$x$ polarized DIS and SIDIS data are insufficient to constrain the initial conditions for the polarized dipole amplitudes in the helicity evolution equations, future EIC data will allow more precise predictions for hPDFs and the $g_1$ structure function for $x$ values beyond those probed at the EIC. Using the obtained hPDFs, we discuss the contributions to the proton spin from quark and gluon spins at small $x$.

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arXiv2023

Gluon helicity from global analysis of experimental data and lattice QCD Ioffe time distributions

We perform a new global analysis of spin-dependent parton distribution functions with the inclusion of Ioffe time pseudo-distributions computed in lattice QCD (LQCD), which are directly sensitive to the gluon helicity distribution, $Δg$. These lattice data have an analogous relationship to parton distributions as do experimental cross sections, and can be readily included in global analyses. We focus in particular on the constraining capability of current LQCD data on the sign of $Δg$ at intermediate parton momentum fractions $x$, which was recently brought into question by analysis of data in the absence of parton positivity constraints. We find that present LQCD data cannot discriminate between positive and negative $Δg$ solutions, although significant changes in the solutions for both the gluon and quark sectors are observed.

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