Ratios of $W$ and $Z$ cross sections at large boson $p_T$ as a constraint on PDFs and background to new physics

Sarah Alam Malik; Graeme Watt

Paper

Ratios of $W$ and $Z$ cross sections at large boson $p_T$ as a constraint on PDFs and background to new physics

Abstract

We motivate a measurement of various ratios of

and

cross sections at the Large Hadron Collider (LHC) at large values of the boson transverse momentum (

). We study the dependence of predictions for these cross-section ratios on the multiplicity of associated jets, the boson

and the LHC centre-of-mass energy. We present the flavour decomposition of the initial-state partons and an evaluation of the theoretical uncertainties. We show that the

ratio is sensitive to the up-quark to down-quark ratio of parton distribution functions (PDFs), while other theoretical uncertainties are negligible, meaning that a precise measurement of the

ratio at large boson

values could constrain the PDFs at larger momentum fractions

than the usual inclusive

charge asymmetry. The

ratio is insensitive to PDFs and most other theoretical uncertainties, other than possibly electroweak corrections, and a precise measurement will therefore be useful in validating theoretical predictions needed in data-driven methods, such as using

+jets events to estimate the

+jets background in searches for new physics at the LHC. The differential

and

cross sections themselves,

, have the potential to constrain the gluon distribution, provided that theoretical uncertainties from higher-order QCD and electroweak corrections are brought under control, such as by inclusion of anticipated next-to-next-to-leading order QCD corrections.