Measurement of the Polarization of W Bosons with Large Transverse Momenta in W+Jets Events at the LHC

TL;DR

This paper reports the first measurement of W boson polarization at high transverse momentum in proton-proton collisions at the LHC, confirming the Standard Model prediction that they are predominantly left-handed.

Contribution

It provides the first experimental determination of W boson polarization fractions at large transverse momenta in pp collisions, using CMS data at 7 TeV.

Findings

W bosons with high transverse momentum are mainly left-handed.

Measured polarization fractions agree with Standard Model predictions.

The muon decay channel yields the most precise polarization measurements.

Abstract

A first measurement of the polarization of W bosons with large transverse momenta in pp collisions is presented. The measurement is based on 36 inverse picobarns of data recorded at sqrt(s) = 7 TeV by the CMS detector at the LHC. The left-handed, right-handed and longitudinal polarization fractions (f_L, f_R, f_0) of W bosons with transverse momenta larger than 50 GeV are determined using decays to both electrons and muons. The muon final state yields the most precise measurement, (f_L - f_R) = 0.240 +/- 0.036 (stat.) +/- 0.031 (syst.) and f_0 = 0.183 +/- 0.087 (stat.) +/- 0.123 (syst.) for negatively charged W bosons, and (f_L - f_R) = 0.310 +/- 0.036 (stat.) +/- 0.017 (syst.) and f_0 = 0.171 +/- 0.085 (stat.) +/- 0.099 (syst.) for positively charged W bosons. This establishes, for the first time, that W bosons produced in pp collisions with large transverse momenta are predominantly left-handed, as expected in the standard model.