Abstract
We investigate the parton distribution function (PDF) uncertainty in the measurement of the effective weak mixing angle at the CERN Large Hadron Collider (LHC). The PDF-induced uncertainty is large in proton-proton collisions at the LHC due to the dilution effect. The measurement of the Drell-Yan forward-backward asymmetry ( ) at the LHC can be used to reduce the PDF uncertainty in the measurement. However, when including the full mass range of lepton pairs in the data analysis, the correlation between the PDF updating procedure and the extraction leads to a sizable bias in the obtained value. From our studies, we find that the bias can be significantly reduced by removing Drell-Yan events with invariant mass around the Z-pole region, while most of the sensitivity in reducing the PDF uncertainty remains. Furthermore, the lepton charge asymmetry in the W boson events as a function of the rapidity of the charged leptons, , is known to be another observable which can be used to reduce the PDF uncertainty in the measurement. The constraint from is complementary to that from , and thus no bias affects the extraction. The studies are performed using the error PDF Updating Method Package (ePump), which is based on Hessian updating methods. In this article, the CT14HERA2 PDF set is used as an example.
Highlights
Measurement of the leptonic effective weak mixing angle, θelff, is one of the most important topics in experimental particle physics
We investigate the correlation between the two tasks of further reducing the parton distribution function (PDF) uncertainty and performing the precision determination of sin2 θelff from measuring the same experimental observable AF B
For the DY samples, each lepton flavor channel of electron and muon has 250 million events in the mass range of 60 GeV ≤ Mll ≤ 130 GeV. This sample size corresponds to an integrated luminosity of roughly 130 fb−1, which is the size of the total data collected by the ATLAS detector during the Large Hadron Collider (LHC) Run 2
Summary
Measurement of the leptonic effective weak mixing angle, θelff, is one of the most important topics in experimental particle physics. Compared to the lepton-collider cases, measurements at hadron colliders suffer from additional uncertainties on modeling the directions of the incoming fermions and antifermions in the initial state Such uncertainties will dilute AF B and reduce the sensitivity for the determination of sin θelff. In order to distinguish forward from backward events in pp collisions, the beam pointing to the same hemisphere as the Z boson reconstructed from final state leptons, is assumed to be the one which provides the quark This is motivated by the observation that the valence quarks inside the protons generally carry more energy than the antiquarks (or sea quarks) inside the protons. We investigate the correlation between the two tasks of further reducing the PDF uncertainty and performing the precision determination of sin θelff from measuring the same experimental observable AF B. We instead use the software package ePump (error PDF Updating Method Package), which can update any given set of Hessian PDFs obtained from an earlier global analysis [18]
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