Parton Distributions and the Strong Coupling Strength: CTEQ6AB PDFs

TL;DR

This paper introduces the CTEQ6AB PDFs, a series spanning $0.110 \le \alpha_s(m_Z) \le 0.128$, to study how the strong coupling affects parton distributions and collider predictions. It highlights a strong correlation between $\alpha_s$ and the gluon distribution, and demonstrates that global fits do not yet yield a precise independent measurement of $\alpha_s$ due to this coupling. The authors quantify how predictions for $W$, $Z$, inclusive jets, and Higgs production shift with $\alpha_s$, showing that jet and gluon-dominated Higgs processes are notably sensitive to $\alpha_s$ while other channels remain more PDF-driven. These alpha-series PDFs enable robust estimation of $\alpha_s$-related uncertainties in collider phenomenology and provide a resource for exploring $\alpha_s$ constraints from global data.

Abstract

We study the global analysis for parton distributions as a function of the QCD strong coupling strength alpha_s, and present a new series of distributions that span the range 0.110 < alpha_s(m_Z) < 0.128. We use these distributions to explore the correlation between alpha_s and the gluon distribution; the viability of global analysis as a method to measure alpha_s; and the dependence on alpha_s of predictions for W, Z, inclusive jet, and Higgs boson production ($b \bar{b} \to H$ and $gg \to H$) cross sections at the Tevatron and the LHC. We find that the uncertainty in alpha_s is the dominant source of uncertainty for inclusive jet production at moderately small p_T and for Higgs production at intermediate masses in the standard model.

Figures (11)

• Figure 1: The overall goodness-of-fit measure $\chi^2$ for global fits vs. $\alpha_s(m_Z)$, using Def. A (solid curve) and Def. B (dashed curve).
• Figure 2: (a) Gluon distributions for CTEQ6A110 (short dash), …, CTEQ6A126 (long dash). These have $\alpha_s(m_Z) = 0.110$ (short dash), …, $0.126$ (long dash) using Def. A for $\alpha_s$; (b) Gluon distributions with $\alpha_s(m_Z) = 0.118$: CTEQ6A118 (dashed), CTEQ6.1M (solid), and uncertainty band from CTEQ6.1 eigenvector sets (shaded); (c) Both plots combined in ratio form.
• Figure 3: Uncertainties at scale $\mu = 100 \, \mathrm{GeV}$ for Gluon (a), and $u$-quark (b).
• Figure 4: The $\chi^2$ values per data point for the individual experiments that are included in the global analysis, as a function of $\alpha_{s}(m_{Z})$. The number of data points is indicated on each figure.
• Figure 5: (a) Cross section $d\sigma/dy_{W}$ for $W^-$ production at the Tevatron. (b) Same cross section with the CTEQ6.1 prediction subtracted. The curves are for $\alpha_s(m_Z) = 0.110$ (short dash), …, $0.126$ (long dash) as in Fig. \ref{['fig:figquark10c12']}.