Forward-Backward Drell-Yan Asymmetry and PDF Determination

We investigate the impact of high-statistics Drell-Yan (DY) measurements at theLHC on the study of non-perturbative QCD effects from parton distributionfunctions (PDF). We present the results of a PDF profiling analysis based on theneutral-current DY forward-backward asymmetry, using the open source fitplatform xFitter. The high statistics at the Large Hadron Collider (LHC) Run IIand the forthcoming Run III and high-luminosity HL-LHC open the way to precisionmeasurements at the TeV scale, which will be used both for studies of theStandard Model (SM) and for searches for beyond-Standard-Model (BSM) physics. Inorder to keep up with the increasing statistical precision of experimentalmeasurements, an impressive effort is being made on the theoretical side toprovide higher-order perturbative QCD calculations — see e.g. 1. With improvingperturbative accuracy, nonperturbative QCD contributions such as partondistribution functions (PDF) more and more become a crucial limiting factor inthe theoretical systematics affecting both precision SM studies and BSMsearches. An important part of the physics program to be carried out withcurrent and upcoming collider data is thus to identify which measurements can bemost helpful in placing constraints on the nonperturbative PDF and theiruncertainties. In the Drell-Yan (DY) production channel, measurements ofdifferential distributions in mass and rapidity and of the charged-current (CC)asymmetry have long been used to constrain PDFs (see e.g. 2,3,4,5,6,7 for recentresults), while measurements of the neutral current (NC) forward-backwardasymmetry (henceforth denoted as A FB) have traditionally been used fordeterminations of the weak mixing angle θ W (see e.g. 8,9,10,11,12,13). In 14,15it was observed that A FB measurements in NC processes at the LHC can usefullybe employed for PDF deter-minations. Ref. 16 investigates the impact of A FBdata on PDF extractions by using the open source fit platform xFitter 17 ,considering different scenarios for luminosities (from Runs II, III to the HL-LHC stage 18) and performing PDF profiling to analyze quantitatively the effectof A FB on PDF uncertainties. In this article we report on this study. The five-fold differential DY cross section in the vector boson mass, rapidity,transverse momentum and lepton decay angles may be written in terms of angularcoefficients A k as dσ dM dY dP ⊥ d cos θdφ = dσ (U) dM dY dP ⊥ 3 16π 1 + cos 2θ + 1 2 A 0 (1 − 3 cos 2 θ) (1) + A 1 sin 2θ cos φ + 1 2 A 2 sin 2 θ cos 2φ + A3 sin θ cos φ + A 4 cos θ + A 5 sin 2 θ sin 2φ + A 6 sin 2θ sin φ + A 7 sin θsin φ. a CERN-TH-2019-116