[Paper Review] NNLO PDFs driven by top-quark data

S. Alekhin, M. V. Garzelli+2 more|arXiv (Cornell University)|Jan 1, 2024

Scientific Computing and Data Management2 citations

TL;DR

This paper presents the ABMPtt global fit, a next-to-next-to-leading order (NNLO) determination of proton parton distribution functions (PDFs), the strong coupling constant αs(MZ), and the top-quark mass mt(mt), simultaneously constrained by high-precision top-quark data from the LHC. It achieves significantly reduced uncertainties—up to a factor of two smaller—compared to ABMP16, particularly for the gluon PDF at large x and the top-quark mass, with αs(MZ) = 0.1150 ± 0.0009 and mt(mt) = 160.6 ± 0.6 GeV (mpole_t = 170.2 ± 0.7 GeV).

ABSTRACT

We study the impact of state-of-the-art top-quark data collected at the Large Hadron Collider on parton distribution functions (PDFs). Following the ABMP methodology, the fit extracts simultaneously proton PDFs, the strong coupling $\alpha_s(M_Z)$ and heavy-quark masses at next-to-next-to-leading order (NNLO) accuracy in QCD. It includes recent high-statistics data on absolute total inclusive cross sections for $t\bar{t}+X$, the sum of $(t + X)$ and $(\bar{t} + X)$ hadroproduction, and normalized inclusive data double-differential in the invariant mass and rapidity of the $t\bar{t}$ pair at $\sqrt{S}=13$ TeV. The gluon PDF at large $x$ and the top-quark mass value derived from these data are well compatible with the previous ABMP16 results, but with significantly smaller uncertainties, reduced by up to a factor of two. At NNLO in QCD we obtain for the strong coupling the value $\alpha_s^{(n_f=5)}(M_Z)= 0.1150 \pm 0.0009$ and for the top-quark mass in the ${\overline{\mbox{MS}}}$-scheme $m_t(m_t) = 160.6 \pm 0.6$ GeV, corresponding to $m_t^{ m pole} = 170.2 \pm 0.7$ GeV in the on-shell scheme. The new fit, dubbed ABMPtt, is publicly released in grids in LHAPDF format.

Motivation & Objective

• . The primary objective is to perform a simultaneous, global fit of proton PDFs, αs(MZ), and mt(mt) at NNLO accuracy using state-of-the-art top-quark data.
• To improve the precision of the gluon PDF at large x by incorporating high-statistics double-differential t¯t + X cross-section data from Run 2 and Run 3 of the LHC.
• To reduce uncertainties in the top-quark mass and strong coupling constant by leveraging multi-differential data sensitive to these parameters.
• To provide a publicly available, high-precision PDF set (ABMPtt) in LHAPDF format for use in precision HEP phenomenology.

Proposed method

• . The fit employs the ABMP16 methodology, extending it to include NNLO QCD calculations for top-quark production processes.
• It uses double-differential cross-section data in the invariant mass and rapidity of t¯t pairs at √s = 13 TeV, along with total inclusive cross sections for t¯t + X and (t + X) + (¯t + X) from Run 2 and Run 3.
• Theoretical predictions are computed at NNLO with full QCD radiative corrections, avoiding approximate K-factor schemes.
• The fit simultaneously extracts PDFs, αs(MZ), and mt(mt), with uncertainties evaluated via a Hessian-based statistical analysis.
• The methodology ensures consistency with existing DIS, Drell-Yan, and single-top production data, using the same input as ABMP16 for non-top processes.
• The final PDF set is released in LHAPDF format for public use, enabling broad application in LHC and future collider physics.

Experimental results

Research questions

• RQ1 . To what extent do high-precision double-differential t¯t + X cross-section data from the LHC constrain the gluon PDF at large x?
• RQ2 How do NNLO QCD calculations and multi-differential data improve the precision of the top-quark mass and αs(MZ) compared to previous fits?
• RQ3 What is the impact of including Run 3 data and high-statistics Run 2 data on the uncertainties of PDFs and fundamental parameters?
• RQ4 How do the extracted values of mt(mt) and αs(MZ) compare to PDG averages and other global PDF fits?
• RQ5 To what extent do the new data reduce correlations and uncertainties in the PDFs, especially in the gluon distribution?

Key findings

• . The ABMPtt fit reduces uncertainties in the gluon PDF at large x by up to a factor of two compared to ABMP16, due to the inclusion of high-precision double-differential t¯t + X data.
• The extracted value of the strong coupling constant is αs(MZ) = 0.1150 ± 0.0009 at NNLO, consistent with the PDG average but with significantly reduced uncertainty.
• The top-quark mass in the MS-scheme is determined as mt(mt) = 160.6 ± 0.6 GeV, corresponding to mpole_t = 170.2 ± 0.7 GeV in the on-shell scheme.
• The fit shows excellent agreement with measured double-differential t¯t + X cross sections, validating the theoretical predictions at NNLO.
• The ABMPtt PDF set is publicly released in LHAPDF format, enabling widespread use in precision HEP phenomenology.
• The results demonstrate that top-quark data at NNLO provide a powerful constraint on PDFs, αs, and mt, with measurable improvements in precision over previous global fits.