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

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.

Figures (3)

• Figure 1: Fit results using 36 $\textrm{pb}^{-1}$ of collision data for the $L_P(\mathrm{e}^-)$ (top) and $L_P(\mathrm{e}^+)$ (bottom) distributions. The left-handed, right-handed and longitudinal $\mathrm{W}$ components, with normalization as determined by the fit, are represented by the dashed, dotted, and dash-dotted lines respectively. The shaded distributions show the QCD and EWK backgrounds. The solid line represents the sum of all individual components, and can be directly compared with the data distribution (circles).
• Figure 2: Fit results using 36 $\textrm{pb}^{-1}$ of collision data for the $L_P({\mu^-})$ (top) and $L_P({\mu^+})$ (bottom) distributions. The left-handed, right-handed and longitudinal $\mathrm{W}$ components, with normalization as determined by the fit, are represented by the dashed, dotted, and dash-dotted lines respectively. The shaded distribution shows the EWK backgrounds. The solid line represents the sum of all individual components, and can be directly compared with the data distribution (circles).
• Figure 3: The muon fit result (black dot) in the $((f_{L} - f_{R}), f_0)$ plane for negatively charged (top) and positively charged (bottom) leptons. The 68% confidence level contours for the statistical and total uncertainties are shown by the green shaded region and the black contour respectively. The disallowed region is hatched.