Heavy-quark mass dependence in global PDF analyses and 3- and 4-flavour parton distributions

TL;DR

This paper investigates how variations in charm and bottom quark masses affect parton distribution functions and related cross sections, providing new PDF sets and analyzing differences between 3-, 4-, and 5-flavour schemes for collider processes.

Contribution

It introduces publicly available PDF sets for different heavy-quark masses and compares 3-, 4-, and 5-flavour schemes, highlighting the importance of resumming large logarithms in Q^2/m_b^2.

Findings

Significant differences in Z cross sections between schemes.

Resummation of large logs is essential in the 5-flavour scheme.

4-flavour scheme remains relevant for processes with massive b-quarks.

Abstract

We study the sensitivity of our recent MSTW 2008 NLO and NNLO PDF analyses to the values of the charm- and bottom-quark masses, and we provide additional public PDF sets for a wide range of these heavy-quark masses. We quantify the impact of varying m_c and m_b on the cross sections for W, Z and Higgs production at the Tevatron and the LHC. We generate 3- and 4-flavour versions of the (5-flavour) MSTW 2008 PDFs by evolving the input PDFs and alpha_S determined from fits in the 5-flavour scheme, including the eigenvector PDF sets necessary for calculation of PDF uncertainties. As an example of their use, we study the difference in the Z total cross sections at the Tevatron and LHC in the 4- and 5-flavour schemes. Significant differences are found, illustrating the need to resum large logarithms in Q^2/m_b^2 by using the 5-flavour scheme. The 4-flavour scheme is still necessary, however, if cuts are imposed on associated (massive) b-quarks, as is the case for the experimental measurement of Z b bbar production and similar processes.