[Paper Review] Impact of inclusive hadron production data on nuclear gluon PDFs

nCTEQ Collaboration, Duwent\"aster, P.+9 more|arXiv (Cornell University)|May 20, 2021

Particle physics theoretical and experimental studiesPhysics and Astronomy100 references31 citations

TL;DR

This paper extends the nCTEQ15 global fit of nuclear parton distribution functions (nPDFs) by incorporating single inclusive hadron (SIH) production data from RHIC (PHENIX, STAR) and LHC (ALICE), significantly improving constraints on the nuclear gluon PDF at small x. Using a new nCTEQ++ framework with pre-computed grids and multiple fragmentation functions, the study demonstrates that SIH data reduce uncertainties in the nuclear gluon distribution, particularly in the small-x region, and validate the robustness of the results against fragmentation function variations.

ABSTRACT

A precise knowledge of nuclear parton distribution functions (nPDFs) is -- among other things -- important for the unambiguous interpretation of hard process data taken in pA and AA collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The available fixed target data for deep inelastic scattering (DIS) and Drell-Yan (DY) lepton pair production mainly constrain the light quark distributions. It is hence crucial to include more and more collider data in global analyses of nPDFs in order to better pin down the different parton flavors, in particular the gluon distribution at small x. To help constrain the nuclear gluon PDF, we extend the nCTEQ15 analysis by including single inclusive hadron (SIH) production data from RHIC (PHENIX and STAR) and LHC (ALICE). In addition to the DIS, DY and SIH data sets, we will also include LHC W/Z production data. As the SIH calculation is dependent on hadronic fragmentation functions (FFs), we use a variety of FFs available in the literature to properly estimate this source of uncertainty. We study the impact of these data on the PDFs, and compare with both the nCTEQ15 and nCTEQ15WZ sets. The calculations are performed using a new implementation of the nCTEQ code (nCTEQ++) including a modified version of INCNLO which allows faster calculations using pre-computed grids. The extension of the nCTEQ15 analysis to include the SIH data represents an important step toward the next generation of PDFs.

Motivation & Objective

• To improve constraints on the nuclear gluon PDF at small x by including single inclusive hadron (SIH) production data from RHIC and LHC.
• To assess the impact of fragmentation function (FF) uncertainties on the extraction of nuclear gluon PDFs in global nPDF fits.
• To extend the nCTEQ15 global analysis by integrating SIH data alongside existing DIS, Drell-Yan, and W/Z production data.
• To validate the robustness of nPDF fits by testing multiple fragmentation function sets and evaluating scale uncertainties.
• To demonstrate that the observed nuclear effects in SIH data are consistent with cold nuclear matter initial-state modifications, independent of final-state hadron fragmentation.

Proposed method

• The nCTEQ++ framework is used, featuring a modified version of INCNLO with pre-computed grids to accelerate PDF fitting calculations.
• SIH production data from p+Pb collisions at √sNN = 200 GeV (RHIC) and 5.02 TeV (LHC) are included, covering π⁰, η, and charged hadrons.
• Multiple fragmentation functions (DSS, Kretzer, AKK, NNPDF, and others) are used to quantify theoretical uncertainties in the fragmentation channel.
• The analysis includes scale uncertainties in fragmentation functions and evaluates their impact on the final nPDF fits.
• Fits are performed using a Hessian-based uncertainty method, with data normalization parameters fitted in parallel to account for experimental systematics.
• The results are compared with the nCTEQ15 and nCTEQ15WZ global fits to assess improvements in gluon PDF constraints.

Experimental results

Research questions

• RQ1 How do single inclusive hadron production data from RHIC and LHC improve constraints on the nuclear gluon PDF at small x?
• RQ2 To what extent do uncertainties in fragmentation functions affect the extracted nuclear gluon PDFs?
• RQ3 Are the observed nuclear effects in SIH data consistent with cold nuclear matter initial-state modifications, independent of final-state fragmentation?
• RQ4 How do the inclusion of SIH data and fragmentation function variations affect the overall goodness-of-fit and uncertainties in the nPDFs?
• RQ5 What is the relative contribution of gluon-initiated subprocesses to the total cross section in p+Pb → π⁰ + X at different pT and center-of-mass energies?

Key findings

• SIH data from RHIC and LHC significantly reduce uncertainties in the nuclear gluon PDF at small x, particularly in the 0.01 < x < 0.1 range.
• The gluon contribution to the p+Pb → π⁰ + X cross section dominates at low to mid-pT, reaching ~60–70% at √sNN = 200 GeV and exceeding 80% at 5 TeV.
• The inclusion of SIH data leads to a measurable shift in the nuclear gluon PDF, especially in the antishadowing region, improving the global fit quality.
• The impact of fragmentation function uncertainties on the nuclear gluon PDF is found to be small, with variations across different FF sets producing consistent results within uncertainties.
• The analysis shows that the observed nuclear effects in SIH data are robust against fragmentation function variations, supporting the interpretation of these effects as initial-state modifications.
• The new nCTEQ++ framework enables faster and more stable global fits, making the inclusion of SIH data feasible and efficient for future nPDF global analyses.