REVIEW
nCTEQ26 nuclear parton distribution functions
High Energy Physics - Phenomenology · Submitted 9 Oct 2026 · 298 references
Pith's verdict: nCTEQ26 is a solid new benchmark for nuclear PDFs, but the data-driven heavy-quark treatment carries a load-bearing assumption at low x that deserves scrutiny.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
nCTEQ26 provides the most precise nuclear PDFs using 3598 data points.
The reading
What carries the argument
The central machinery is the nPDF parametrization Eq. (2)-(6) and (10): each parton flavor f_k at the initial scale Q0 = 1.3 GeV is parametrized as x $f^{{p/A}}$_k(x, Q0) = c_{k0} $x^{{c_{k1}}$} (1-x)^{c_{k2}} (1 + c_{k3} sqrt{x} + c_{k4} x) $e^{{c_{k5}}$ sqrt{x}}, with the coefficients c_j(A) = p_j + a_j ln A + b_j $ln^{2}$ A for A>1. This links all nuclei through a common analytic form, pooling constraining power across different nuclear targets while allowing non-monotonic A-dependence. Sum rules fix normalization parameters, and the gluon and strange normalizations are fitted. Hessian error propagation with a tolerance T=50 (based on Wilks theorem) provides uncertainties.
Extended reading notes
Core claim
The nCTEQ26 nPDF set significantly advances the state of the art by simultaneously fitting 3598 data points from 21 different nuclei with a total chi2/Ndof = 0.943. The new parametrization combines a flexible x-dependence (extended from the proton baseline CJ15) with a novel quadratic-logarithmic A-dependence: c_j(A) = p_j + a_j ln A + b_j $ln^{2}$ A. This reduces parameter correlations and allows the uncertainties to reflect data constraints more faithfully, avoiding artificially smooth A-dependence. Target-mass corrections and deuteron corrections are included, enabling the reliable inclusion of high-x JLab data. Heavy-quark(onium) production is treated via a data-driven Crystal-Ball parametrization fitted to pp data, and the consistency of this approach is verified by an alternative fit using perturbative predictions for open heavy-flavor alone. The resulting nPDFs show good agreement with other global sets (EPPS21, nNNPDF3.0) within uncertainties, with notable improvements in the gluon and strange distributions.
Load-bearing premise
Heavy-quark(onium) production in proton-lead collisions can be described by a Crystal-Ball parametrization with the same five parameters (lambda, kappa, <pT>, n, a) fitted to proton-proton data, implying that all nuclear effects beyond leading-twist collinear factorization (e.g., energy loss, saturation) are negligible in the fitted region, especially at low x and forward rapidity where the fit is weakest.
Editorial extensions
If this is right
- nCTEQ26 will serve as the standard baseline for distinguishing cold nuclear matter effects from hot QCD matter (e.g., quark-gluon plasma) in pA and AA collisions at the LHC.
- The improved constraints on the gluon at small x (down to about 10^-5) directly impact predictions for heavy-quark production, dijet, and direct-photon processes at current and future colliders.
- The strange-quark distribution remains the most uncertain, but the inclusion of CC DIS and dimuon data provides tighter constraints around x~0.01-0.1 for iron and lead; the fitted strange-to-light-sea ratio varies with A and scale, which can be tested by future neutrino experiments (e.g., DUNE).
- The A-dependence results (e.g., uncertainties that peak at medium A for some flavors) provide quantitative tests for models of nuclear structure, such as those based on short-range correlated pairs.
Reading between the lines
- The poor fit (chi2/Ndata = 2.5) of the LHCb Run-II forward D0 data in pPb collisions at 8.16 TeV, despite good description of Run-I data, may hint at missing nuclear effects (e.g., saturation) at very low x, which could be clarified by future EIC measurements in the same kinematic region.
- The Crystal-Ball parametrization assumption that the same 5 parameters fit to pp data transfer directly to pPb without any final-state nuclear modifications is most vulnerable at forward rapidities and low pT; a dedicated study of direct-photon pPb data in this region would provide an independent cross-check.
- The observation that the symmetric uncertainty definition gives much larger lower bounds for the strange distribution at small x than the asymmetric definition suggests that the low-x strange PDF is genuinely unconstrained by current data, not just asymmetric. Future dimuon measurements with extended coverage could dramatically reduce this uncertainty.
Original abstract
We present the new release of nuclear parton distribution functions (nPDFs) from the nCTEQ collaboration: nCTEQ26. In this global analysis, we combine our recent, more selective studies of different experimental measurements and extend them to determine a modern set of nPDFs based on the largest selection of data considered to date. This selection includes neutral- and charged-current deep-inelastic scattering (DIS), fixed-target Drell-Yan, and a large variety of collider data from RHIC and LHC Runs I and II. On the theoretical side, we match this considerably extended kinematic range with a new nPDF parametrization that combines a flexible functional form of the x-dependence with a quadratic logarithmic nuclear A-dependence as well as target-mass and deuteron corrections for the reliable inclusion of high-x DIS data. Following our previous successful work, we partially employ a data-driven approach that allows us to not only include open heavy-quark data, but also quarkonium data. In addition, we establish the consistency of this approach by comparing it to an alternative fit based on perturbative predictions of open heavy-quark data alone. We determine the uncertainties of our nCTEQ26 nPDFs with a Hessian error analysis, test their reliability by comparing our nPDFs to other, similar recent analyses, and make predictions for current and future experimental measurements that have not been included in the analysis.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We present the new release of nuclear parton distribution functions (nPDFs) from the nCTEQ collaboration: nCTEQ26. In this global analysis, we combine our recent, more selective studies of different experimental measurements and extend them to determine a modern set of nPDFs based on the largest selection of data considered to date. This selection includes neutral- and charged-current deep-inelastic scattering (DIS), fixed-target Drell-Yan, and a large variety of collider data from RHIC and LHC Runs I and II. On the theoretical side, we match this considerably extended kinematic range with a new nPDF parametrization that combines a flexible functional form of the x-dependence with a quadratic logarithmic nuclear A-dependence as well as target-mass and deuteron corrections for the reliable inclusion of high-x DIS data. Following our previous successful work, we partially employ a data-driven approach that allows us to not only include open heavy-quark data, but also quarkonium data. In addition, we establish the consistency of this approach by comparing it to an alternative fit based on perturbative predictions of open heavy-quark data alone. We determine the uncertainties of our nCTEQ26 nPDFs with a Hessian error analysis, test their reliability by comparing our nPDFs to other, similar recent analyses, and make predictions for current and future experimental measurements that have not been included in the analysis.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We present the new release of nuclear parton distribution functions (nPDFs) from the nCTEQ collaboration: nCTEQ26. | contribution 1.00 | 0.89 | 0.28 | asserted 0.99 |
| s1 | In this global analysis, we combine our recent, more selective studies of different experimental measurements and extend them to determine a modern set of nPDFs based on the largest selection of data considered to date. | contribution 0.56 | 0.93 | 0.15 | asserted 1.00 |
| s2 | This selection includes neutral- and charged-current deep-inelastic scattering (DIS), fixed-target Drell-Yan, and a large variety of collider data from RHIC and LHC Runs I and II. | method 0.93 | 0.53 | 0.05 | asserted 1.00 |
| s3 | On the theoretical side, we match this considerably extended kinematic range with a new nPDF parametrization that combines a flexible functional form of the x-dependence with a quadratic logarithmic nuclear A-dependence as well as target-mass and deuteron corrections for the reliable inclusion of high-x DIS data. | method 0.96 | 0.93 | 0.08 | asserted 1.00 |
| s4 | Following our previous successful work, we partially employ a data-driven approach that allows us to not only include open heavy-quark data, but also quarkonium data. | method 0.86 | 0.90 | 0.05 | asserted 0.99 |
| s5 | In addition, we establish the consistency of this approach by comparing it to an alternative fit based on perturbative predictions of open heavy-quark data alone. | method 0.89 | 0.95 | 0.05 | asserted 0.99 |
| s6 | We determine the uncertainties of our nCTEQ26 nPDFs with a Hessian error analysis, test their reliability by comparing our nPDFs to other, similar recent analyses, and make predictions for current and future experimental measurements that have not been included in the analysis. | method 0.60 | 0.97 | 0.19 | asserted 1.00 |
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Discussion
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