Abstract
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.
Original language | English |
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Awarding Institution |
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Supervisors/Advisors |
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Award date | 10 Jan 2007 |
Place of Publication | Enschede |
Publisher | |
Print ISBNs | 978-90-365-2577-0 |
Publication status | Published - 10 Jan 2008 |
Keywords
- IR-58510