CTEQ6 Parton Distributions with Heavy Quark Mass Effects

TL;DR

<3-5 sentence high-level summary> This paper develops and implements a generalized MS-bar (GM-VFNS) framework to include heavy-quark mass effects in parton distributions, addressing limitations of the zero-mass approach when charm and bottom masses are relevant. It adopts the ACOT(χ) scheme with threshold matching to produce CTEQ6HQ parton distributions, and performs a global NLO QCD fit using data including F2^c from HERA and neutrino DIS, ensuring consistency between PDFs and massive hard-scattering cross-sections. The resulting CTEQ6HQ set shows comparable overall data description to CTEQ6M but with improved treatment of heavy flavors, particularly near thresholds, and provides a basis for more precise predictions in charm and bottom production and related observables like ΔxF3 and sin^2θ_W. The work emphasizes the importance of scheme-consistent heavy-quark mass effects for high-precision phenomenology and future analyses of heavy-flavor content in the proton.

Abstract

Previously published CTEQ6 parton distributions adopt the conventional zero-mass parton scheme; these sets are most appropriate for use with massless hard-scattering matrix elements commonly found in most physics applications. For precision observables which are sensitive to charm and bottom quark mass effects, we provide in this paper an additional CTEQ6HQ parton distribution set determined in the more general variable flavor number scheme which incorporates heavy flavor mass effects. The results are obtained by combining these parton distributions with consistently matched DIS structure functions computed in the same scheme. We describe the analysis procedure, examine the predominant features of the new distributions, and compare with previous distributions.

Figures (8)

• Figure 1: Comparisons of the charm production structure function, $F_{2}^{c}$, with the data from H1 (Fig. \ref{['fig:Fc']}-a) H1f2c01 and ZEUS (Fig. \ref{['fig:Fc']}-b). ZeusF2c The $y$-axis is (Experiment-Theory)/Theory; along the x-axis, the data points are ordered using $x$ and $Q$ as the primary and secondary sorting variables respectively. The error bars represent the statistical and uncorrelated systematic errors added in quadrature.
• Figure 2: CCFR $F_2^{\nu N}$ structure function (from the physics model independent analysis, Ref. pmi) compared to CTEQ6HQ fit: (a) low $x$ bins (left panel) show a systematic disagreement; (b) medium to high $x$ bins (right panel) show good agreement.
• Figure 3: Comparison of CTEQ5HQ, CTEQ6M, and CTEQ6HQ parton distributions at $Q=m_c=1.3$ GeV: (a) u-quark; and (b) d-quark. The axes are scaled to highlight the valence components of these distributions.
• Figure 4: Same as Fig. \ref{['fig:UDqkA']}, except the axes are scaled to highlight the sea components of the PDFs: (a) u-quark; and (b) d-quark.
• Figure 5: Comparison of the gluon distributions at $Q_0$= 1.3 GeV.