Mellin Moments of the Unpolarized Gluon PDF in the Proton from Nonlocal Operators in Lattice QCD
Pith reviewed 2026-06-28 23:27 UTC · model grok-4.3
The pith
Lattice QCD determines the ratio of the third to first Mellin moment for the gluon PDF at 2 GeV.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using matrix elements of nonlocal gluon operators coupled to boosted proton states on an Nf=2+1+1 ensemble, the short-distance OPE of the reduced gluon Ioffe-time distribution yields the ratio ⟨x³⟩_g/⟨x⟩_g at a scale of 2 GeV, with uncertainties that account for both statistical and the dominant theoretical systematic uncertainties.
What carries the argument
The short-distance operator product expansion (OPE) of the reduced gluon Ioffe-time distribution, which converts the lattice nonlocal matrix elements into ratios of Mellin moments.
If this is right
- The ratio supplies an additional constraint on the x-dependence of the gluon PDF beyond the total momentum fraction.
- DGLAP evolution of the moments permits a direct check of perturbative truncation effects by varying the renormalization scale.
- The same OPE framework can be applied to extract further moments n>3 once more terms are retained.
- Quantified dependence on the minimum and maximum Wilson-line separations sets a benchmark for controlling finite-separation systematics in future runs.
Where Pith is reading between the lines
- The lattice ratio can be inserted into global PDF analyses to test whether current parametrizations of the gluon at moderate x are consistent with first-principles input.
- Repeating the calculation at the physical pion mass would isolate the size of the chiral extrapolation uncertainty present at 260 MeV.
- A side-by-side comparison with moments inferred from jet or heavy-flavor production data would expose any tension between lattice and phenomenological determinations.
Load-bearing premise
The short-distance operator product expansion of the reduced gluon Ioffe-time distribution remains accurate after truncation at the order used, for the chosen minimum and maximum Wilson-line separations.
What would settle it
If extending the OPE to higher orders or altering the Wilson-line separation window shifts the central value of ⟨x³⟩_g/⟨x⟩_g outside the reported uncertainties, the truncation assumption is falsified.
Figures
read the original abstract
We present a lattice QCD determination of the Mellin moments of the unpolarized gluon parton distribution function in the proton. The analysis is based on matrix elements of nonlocal gluon operators coupled to momentum-boosted proton states. The calculation relies on an $N_f=2+1+1$ ensemble of maximally twisted mass fermions with clover improvement and the Iwasaki-improved gauge action, at a pion mass of approximately 260 MeV. Working within the short-distance operator product expansion (OPE) of the reduced gluon Ioffe-time distribution, we extract ratios of higher-order gluon moments, $\langle x^n\rangle$ with $n>1$, to the gluon momentum fraction, $\langle x\rangle$. We investigate systematic effects associated with the truncation of the order of moment in the OPE, the choice of minimum and maximum Wilson-line separations entering the analysis, and the treatment of mixing with the quark-singlet under perturbative matching. The stability of the extracted moments is further studied under scale evolution using DGLAP equations, allowing us to assess uncertainties related to perturbative truncation by varying the scale. Our work provides a determination of the ratio $\langle x^3\rangle_g/\langle x\rangle_g$ at a scale of 2 GeV, with uncertainties that account for both statistical and the dominant theoretical systematic uncertainties.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents a lattice QCD determination of ratios of Mellin moments of the unpolarized gluon PDF, specifically ⟨x³⟩_g/⟨x⟩_g at 2 GeV, extracted from matrix elements of nonlocal gluon operators on an N_f=2+1+1 twisted-mass clover ensemble at m_π≈260 MeV. The extraction uses the short-distance OPE applied to the reduced gluon Ioffe-time distribution, with reported investigations of OPE truncation order, Wilson-line separation cuts, quark-singlet mixing under perturbative matching, and stability under DGLAP scale evolution to assess perturbative truncation effects.
Significance. If robust, the result supplies a non-perturbative lattice anchor for a higher gluon moment ratio that can be compared directly to global PDF analyses and used to constrain the gluon contribution to the proton's momentum. The explicit checks on multiple sources of systematic uncertainty and the use of scale variation to probe perturbative truncation represent concrete strengths in an area where such controls are essential.
major comments (2)
- [Abstract] Abstract: the statement that the quoted uncertainties 'account for both statistical and the dominant theoretical systematic uncertainties' cannot be verified from the provided text, as no numerical tables, extracted central values, or detailed error budgets are shown; this directly affects the load-bearing claim that the ratio is determined with controlled systematics.
- [OPE analysis] OPE analysis (described in the abstract and method): the truncation error on the short-distance OPE of the reduced gluon Ioffe-time distribution at the accessed Wilson-line separations is assessed only via internal consistency checks (order variation, min/max cuts, scale evolution); because the OPE is asymptotic, these checks do not independently constrain the size of omitted higher-order coefficients, leaving open the possibility that the reported systematic band underestimates the true truncation uncertainty at z∼0.2–0.6 fm.
minor comments (1)
- [Abstract] The abstract refers to 'the choice of minimum and maximum Wilson-line separations entering the analysis' without quoting the numerical range or the criterion used to select them; adding these values would improve reproducibility.
Simulated Author's Rebuttal
We thank the referee for their careful review and constructive comments on our manuscript. We address each major comment below. Where the comments identify areas for improved clarity or documentation, we have revised the manuscript accordingly.
read point-by-point responses
-
Referee: [Abstract] Abstract: the statement that the quoted uncertainties 'account for both statistical and the dominant theoretical systematic uncertainties' cannot be verified from the provided text, as no numerical tables, extracted central values, or detailed error budgets are shown; this directly affects the load-bearing claim that the ratio is determined with controlled systematics.
Authors: We agree that the abstract's claim regarding the uncertainties cannot be directly verified without the supporting numerical results and error budget. The full manuscript presents the extracted ratio ⟨x³⟩_g/⟨x⟩_g together with statistical and systematic variations in Section 4 and the associated figures, but a consolidated error-budget table was not included. We will add a new table that lists the central value, statistical uncertainty, and individual contributions from OPE truncation, Wilson-line cuts, quark-singlet matching, and DGLAP scale variation. This revision will make the error accounting explicit and allow readers to assess the claim. revision: yes
-
Referee: [OPE analysis] OPE analysis (described in the abstract and method): the truncation error on the short-distance OPE of the reduced gluon Ioffe-time distribution at the accessed Wilson-line separations is assessed only via internal consistency checks (order variation, min/max cuts, scale evolution); because the OPE is asymptotic, these checks do not independently constrain the size of omitted higher-order coefficients, leaving open the possibility that the reported systematic band underestimates the true truncation uncertainty at z∼0.2–0.6 fm.
Authors: We acknowledge that the OPE is asymptotic and that consistency checks based on order variation, z-cuts, and scale evolution provide an estimate rather than a rigorous upper bound on omitted higher-order coefficients. These checks follow the standard methodology used in other lattice PDF studies. To strengthen the presentation, we will expand the relevant section to explicitly discuss the asymptotic nature of the series, report the observed convergence pattern, and add a conservative estimate of the possible size of the next term based on the last included coefficient. We will also state that the quoted band should be viewed as an indicative rather than exhaustive uncertainty. This addresses the concern without altering the central result. revision: partial
Circularity Check
No significant circularity detected
full rationale
The derivation extracts ratios of gluon Mellin moments from lattice matrix elements of nonlocal operators via the short-distance OPE applied to the reduced Ioffe-time distribution on an external N_f=2+1+1 ensemble. The OPE truncation, separation cuts, perturbative matching, and DGLAP evolution are treated as systematic variations on independent lattice inputs rather than self-definitions or fitted parameters renamed as predictions. No load-bearing step reduces by construction to the target ratio itself, and the central result remains a direct computation from external gauge configurations with standard external benchmarks.
Axiom & Free-Parameter Ledger
free parameters (1)
- Pion mass
axioms (2)
- domain assumption Short-distance OPE of the reduced gluon Ioffe-time distribution is valid and truncatable at the order employed for the Wilson-line separations chosen
- domain assumption Perturbative matching to the quark-singlet sector introduces controllable errors
Reference graph
Works this paper leans on
-
[1]
0.8 1.0 1.2 1.4 0.00 0.01 0.02 0.03 0.04 0.05 ⟨ x3⟩ / ⟨ x ⟩ nmax = 4, z ∈ (0.186fm, 0.279fm) FIG
The band is taken atκ= 1, while the variation underκ→1/ √ 2, √ 2 provides an estimate of the perturbative uncertainty associated with matching-scale variation, in addition to the statistical uncertainties. 0.8 1.0 1.2 1.4 0.00 0.01 0.02 0.03 0.04 0.05 ⟨ x3⟩ / ⟨ x ⟩ nmax = 4, z ∈ (0.186fm, 0.279fm) FIG. 5. Comparison of ratios⟨x 3⟩g/⟨x⟩g for gluon only at ...
-
[2]
We include a band covering the uncertainties associated with extracted value atκ= 1 and a lighter band including additional theoretical error determined from varyingκ. E. Impact of Mixing with Quark-Singlet Contribution In the extraction of gluon Mellin moments, the perturbative matching receives contributions from mixing with the quark-singlet operator. ...
-
[3]
Pion and Kaon Form Factors using Lattice QCD
The shaded band corresponds to the final lattice-only determination obtained from the analysis without quark-singlet mixing of Eq. (23). The central value is taken from theκ= 1 analysis, corresponding to the natural choiceµ 0 ∼1/z, while the variation underκ→1/ √ 2 andκ→ √ 2 is used to estimate the associated perturbative systematic uncertainty. Our final...
2021
-
[4]
New CTEQ global analysis of quantum chromodynamics with high-precision data from the LHC
T.-J. Houet al., Phys. Rev. D103, 014013 (2021), 1912.10053
work page internal anchor Pith review Pith/arXiv arXiv 2021
-
[5]
Jefferson Lab Angular Momentum (JAM), E. Moffatet al., Phys. Rev. D104, 016015 (2021), 2101.04664. 16
-
[6]
R. D. Ballet al., The European Physical Journal C82(2022)
2022
-
[7]
Electron Ion Collider: The Next QCD Frontier - Understanding the glue that binds us all
A. Accardiet al., Electron ion collider: The next QCD frontier - understanding the glue that binds us all, 2014, 1212.1701
work page internal anchor Pith review Pith/arXiv arXiv 2014
-
[8]
Abdul Khaleket al., Nuclear Physics A1026, 122447 (2022)
R. Abdul Khaleket al., Nuclear Physics A1026, 122447 (2022)
2022
-
[9]
Parton Physics on Euclidean Lattice
X. Ji, Phys. Rev. Lett.110, 262002 (2013), 1305.1539
work page internal anchor Pith review Pith/arXiv arXiv 2013
-
[10]
X. Ji, Sci. China Phys. Mech. Astron.57, 1407 (2014), 1404.6680
work page internal anchor Pith review Pith/arXiv arXiv 2014
- [11]
-
[12]
A. V. Radyushkin, Phys. Rev. D96, 034025 (2017), 1705.01488
work page internal anchor Pith review Pith/arXiv arXiv 2017
-
[13]
One-loop evolution of parton pseudo-distribution functions on the lattice
A. Radyushkin, Phys. Rev. D98, 014019 (2018), 1801.02427
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[14]
Exclusive processes in position space and the pion distribution amplitude
V. Braun and D. Mueller, Eur. Phys. J.C55, 349 (2008), 0709.1348
work page internal anchor Pith review Pith/arXiv arXiv 2008
-
[15]
Extracting Parton Distribution Functions from Lattice QCD Calculations
Y.-Q. Ma and J.-W. Qiu, Phys. Rev.D98, 074021 (2018), 1404.6860
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[16]
Exploring hadrons' partonic structure using ab initio lattice QCD calculations
Y.-Q. Ma and J.-W. Qiu, Phys. Rev. Lett.120, 022003 (2018), 1709.03018
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[17]
Origin of Difference between $\overline{d}$ and $\overline{u}$ Partons in the Nucleon
K.-F. Liu and S.-J. Dong, Phys. Rev. Lett.72, 1790 (1994), hep-ph/9306299
work page internal anchor Pith review Pith/arXiv arXiv 1994
-
[18]
Deep-inelastic scattering and the operator product expansion in lattice QCD
W. Detmold and C. J. D. Lin, Phys. Rev.D73, 014501 (2006), hep-lat/0507007
work page internal anchor Pith review Pith/arXiv arXiv 2006
-
[19]
A. J. Chamberset al., Phys. Rev. Lett.118, 242001 (2017), 1703.01153
work page internal anchor Pith review Pith/arXiv arXiv 2017
-
[20]
HOPE, W. Detmoldet al., Phys. Rev. D105, 034506 (2022), 2109.15241
-
[21]
Restoration of Rotational Symmetry in the Continuum Limit of Lattice Field Theories
Z. Davoudi and M. J. Savage, Phys. Rev. D86, 054505 (2012), 1204.4146
work page internal anchor Pith review Pith/arXiv arXiv 2012
- [22]
-
[23]
A guide to light-cone PDFs from Lattice QCD: an overview of approaches, techniques and results
K. Cichy and M. Constantinou, Adv. High Energy Phys.2019, 3036904 (2019), 1811.07248
work page internal anchor Pith review Pith/arXiv arXiv 2019
-
[24]
M. Constantinouet al., Prog. Part. Nucl. Phys.121, 103908 (2021), 2006.08636
- [25]
-
[26]
Gluon Quasi-PDF From Lattice QCD
Z.-Y. Fanet al., Phys. Rev. Lett.121, 242001 (2018), 1808.02077
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[27]
Accessing gluon parton distributions in large momentum effective theory
J.-H. Zhanget al., Phys. Rev. Lett.122, 142001 (2019), 1808.10824
work page internal anchor Pith review Pith/arXiv arXiv 2019
- [28]
- [29]
-
[30]
A. Salas-Chavira, Z. Fan, and H.-W. Lin, Phys. Rev. D106, 094510 (2022), 2112.03124
-
[31]
HadStruc, T. Khanet al., Phys. Rev. D104, 094516 (2021), 2107.08960
-
[32]
HadStruc, C. Egereret al., Phys. Rev. D106, 094511 (2022), 2207.08733
- [33]
- [34]
- [35]
-
[36]
W. Good, F. Yao, and H.-W. Lin, Physics Letters B872, 140067 (2026)
2026
-
[37]
C. Chenet al., Unpolarized gluon pdf of the nucleon from lattice qcd in the continuum limit, 2025, 2510.26425
- [38]
-
[39]
K. G. Wilson, Phys. Rev.179, 1499 (1969)
1969
-
[40]
Factorization Theorem Relating Euclidean and Light-Cone Parton Distributions
T. Izubuchiet al., Phys. Rev. D98, 056004 (2018), 1801.03917
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[41]
I. Balitsky, W. Morris, and A. Radyushkin, Phys. Lett. B808, 135621 (2020), 1910.13963
-
[42]
I. Balitsky, W. Morris, and A. Radyushkin, SciPost Phys. Proc.8, 161 (2022), 2106.01916
-
[43]
Renormalization in Large Momentum Effective Theory of Parton Physics
X. Ji, J.-H. Zhang, and Y. Zhao, Phys. Rev. Lett.120, 112001 (2018), 1706.08962
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[44]
On the Renormalizability of Quasi Parton Distribution Functions
T. Ishikawaet al., Phys. Rev. D96, 094019 (2017), 1707.03107
work page internal anchor Pith review Pith/arXiv arXiv 2017
-
[45]
J. Green, K. Jansen, and F. Steffens, Phys. Rev. Lett.121, 022004 (2018), 1707.07152
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[46]
Lattice QCD exploration of pseudo-PDFs
K. Orginoset al., Phys. Rev. D96, 094503 (2017), 1706.05373
work page internal anchor Pith review Pith/arXiv arXiv 2017
-
[47]
The nucleon spin and momentum decomposition using lattice QCD simulations
C. Alexandrouet al., Phys. Rev. Lett.119, 142002 (2017), 1706.02973
work page internal anchor Pith review Pith/arXiv arXiv 2017
-
[48]
C. Alexandrouet al., Phys. Rev. D101, 094513 (2020), 2003.08486
- [49]
- [50]
-
[51]
S. Moch, J. Vermaseren, and A. Vogt, Nuclear Physics B688, 101–134 (2004)
2004
-
[52]
A. Vogt, S. Moch, and J. Vermaseren, Nuclear Physics B691, 129–181 (2004)
2004
-
[53]
Ledoit and M
O. Ledoit and M. Wolf, Journal of Multivariate Analysis88, 365 (2004)
2004
-
[54]
Multiplicative renormalizability of quasi-parton operators
Z.-Y. Li, Y.-Q. Ma, and J.-W. Qiu, Phys. Rev. Lett.122, 062002 (2019), 1809.01836
work page internal anchor Pith review Pith/arXiv arXiv 2019
-
[55]
Simulating twisted mass fermions at physical light, strange and charm quark masses
C. Alexandrouet al., Phys. Rev. D98, 054518 (2018), 1807.00495
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[56]
Extended Twisted Mass, C. Alexandrouet al., Phys. Rev. D104, 074515 (2021), 2104.13408
-
[57]
QUDA, M. A. Clarket al., (2016), 1612.07873
work page internal anchor Pith review Pith/arXiv arXiv 2016
-
[58]
Adaptive Aggregation-based Domain Decomposition Multigrid for Twisted Mass Fermions
C. Alexandrouet al., Phys. Rev. D94, 114509 (2016), 1610.02370
work page internal anchor Pith review Pith/arXiv arXiv 2016
-
[59]
Multigrid accelerated simulations for Twisted Mass fermions
S. Bacchio, C. Alexandrou, and J. Finkerath, EPJ Web Conf.175, 02002 (2018), 1710.06198
work page internal anchor Pith review Pith/arXiv arXiv 2018
-
[60]
Multigrid approach in shifted linear systems for the non-degenerated twisted mass operator
C. Alexandrou, S. Bacchio, and J. Finkenrath, Comput. Phys. Commun.236, 51 (2019), 1805.09584
work page internal anchor Pith review Pith/arXiv arXiv 2019
-
[61]
Analytic Smearing of SU(3) Link Variables in Lattice QCD
C. Morningstar and M. J. Peardon, Phys. Rev. D69, 054501 (2004), hep-lat/0311018
work page internal anchor Pith review Pith/arXiv arXiv 2004
-
[62]
Gluon momentum fraction of the nucleon from lattice QCD
C. Alexandrouet al., Phys. Rev. D96, 054503 (2017), 1611.06901
work page internal anchor Pith review Pith/arXiv arXiv 2017
-
[63]
G. S. Baliet al., Phys. Rev. D93, 094515 (2016), 1602.05525
work page internal anchor Pith review Pith/arXiv arXiv 2016
-
[64]
Updated Lattice Results for Parton Distributions
C. Alexandrouet al., Phys. Rev. D96, 014513 (2017), 1610.03689
work page internal anchor Pith review Pith/arXiv arXiv 2017
- [65]
- [66]
-
[67]
I. W. Stewart, F. J. Tackmann, and W. J. Waalewijn, Journal of High Energy Physics2010(2010)
2010
-
[68]
Buckleyet al., The European Physical Journal C75(2015)
A. Buckleyet al., The European Physical Journal C75(2015)
2015
-
[69]
JAM Collaboration (Spin PDF Analysis Group), C. Cocuzzaet al., Phys. Rev. D112, 114017 (2025), 2506.13616
-
[70]
Stability of parton distributions at high $x$: impact of nuclear and power corrections
C. Cocuzzaet al., Stability of parton distributions at highx: impact of nuclear and power corrections, 2026, 2605.00666
work page internal anchor Pith review Pith/arXiv arXiv 2026
-
[71]
Adaptive Aggregation Based Domain Decomposition Multigrid for the Lattice Wilson Dirac Operator
A. Frommeret al., SIAM J. Sci. Comput.36, A1581 (2014), 1303.1377
work page internal anchor Pith review Pith/arXiv arXiv 2014
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.