Measurement of single W Boson Production in ep scattering

A key process in understanding the dynamics of the electroweak interactions in the Standard Model (SM) is single W boson production. One of the most striking signatures of this process at HERA is the observation of events with isolated leptons (electrons or muons) and missing transverse momentum, or ‘ℓ + / PT ’ events. In a data sample collected with the H1 detector corresponding to an integrated luminosity of 481 pb−1, 58 such events are observed for a SM prediction of 57.4 ± 7.3. The purity of the H1 analysis in the signal, dominated within the SM by the W boson production, is at the 75% level. This allowed for the determination of the model-independend production cross section for ℓ+/ PT events σℓ+/ PT = 0.24 ± 0.05 (stat) ± 0.04 (sys) pb, (1.1) which is in agreement with the SM expectation of 0.26 ± 0.04 pb. The measurement of ℓ + / PT events is also used to determine the cross section for single W boson production at HERA σW = 1.23 ± 0.25 (stat) + 0.13 (sys) pb, (1.2) which is in good agreement with the SM expectation of 1.31 ± 0.20 pb. This is the most precise measurement of the single W boson production cross section to date in ep scattering. The single W boson production cross section is sensitive to anomalous values of the coupling parameters dκ and λ that govern the WWγ coupling. The number of observed ℓ + / PT events are used to extract limits on the possible anomalous values of dκ and λ, which have SM expectation values dκ = λ = 0. The following 95% CL single parameter limits are obtained −4.9 < dκ < −2.7 and − 0.8 < dκ < 1.5, (1.3) and −2.9 < λ < 2.9. (1.4) No deviations from the SM are observed. Due to the limited statistics, these limits are not competitive with the corresponding results obtained at LEP and the Tevatron. The W boson is further studied using the charged lepton decay angle in the W boson rest frame. The W boson polarisation fractions F− and F0, for left handed and longitudinally polarised W bosons, respectively, were measured for the first time at HERA. The two-dimensional 68% and 95% CL regions were extracted. In addition, F− and F0 were measured in single parameter fits F− = 0.56 ± 0.16 (stat) + 0.02 − 0.02 (sys), (1.5) F0 = 0.14 ± 0.23 (stat) + 0.01 − 0.01 (sys), (1.6) which is consistent with the SM predictions F−= 0.61 ± 0.01 and F0= 0.17 ± 0.01. These measurements provide a unique test of the SM. The main uncertainty is due to statistics and can be further improved in a future H1/ZEUS combined analysis.