Cross Section and Parity Violating Spin Asymmetries of W^+/- Boson Production in Polarized p+p Collisions at sqrt(s)=500 GeV

TL;DR

This paper reports the observation of large parity-violating spin asymmetries in W boson production from polarized proton collisions at 500 GeV, confirming parity violation in W couplings to light quarks.

Contribution

It provides the first measurements of parity-violating asymmetries in W boson production in polarized proton-proton collisions at RHIC, directly demonstrating parity violation in W couplings.

Findings

Large asymmetries observed for electrons and positrons from W decays.

Estimated W boson production cross sections with uncertainties.

Confirmation of parity violation in W couplings.

Abstract

Large parity violating longitudinal single spin asymmetries A^{e^-}_L= -0.86^{+0.14}_{-0.30} and A^{e^+}_L= 0.88^{+0.12}_{-0.71} are observed for inclusive high transverse momentum electrons and positrons in polarized pp collisions at a center of mass energy of \sqrt{s}=500\ GeV with the PHENIX detector at RHIC. These e^{+/-} come mainly from the decay of W^{+/-} and Z^0 bosons, and the asymmetries directly demonstrate parity violation in the couplings of the W^{\pm} to the light quarks. The observed electron and positron yields were used to estimate W^\pm boson production cross sections equal to \sigma(pp \to W^+ X) \times BR(W^ \to \nu_e)= 144.1+/-21.2(stat)^{+3.4}_{-10.3}(syst) +/- 15%(norm) pb, and \sigma(pp \to W^{-}X) \times BR(W^\to e^-\bar{\nu_e}) = 31.7+/-12.1(stat)^{+10.1}_{-8.2}(syst)+/-15%(norm) pb.