2011 QCD and High Energy Interactions - Rencontres de Moriond ...

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Figure 3: Fit results for the LP (µ − ) (left) and LP (µ + ) (right) distributions. The left-handed, right-handed and
longitudinal 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 backgrounds. The solid line represents the
sum of all individual components, and can be directly compared with the data distribution (circles).
of the W boson in the helicity frame, where the polar angle (θ∗ ) of the charged lepton from the
decay in the W rest frame is measured with respect to the boson flight direction in the laboratory
frame. However, the inability to determine the momentum of the neutrino along the beam axis
introduces a two-fold ambiguity in the determination of the momentum of the W boson. To
overcome this ambiguity the lepton projection variable, LP = pT (ℓ) · pT (W)/|pT (W)| 2 , which
exhibits a strong correlation with cos θ∗ , is used. The fractions of left-handed, right-handed,
and longitudinal W bosons (fL, fR and f0, respectively) are measured using a binned maximum
likelihood fit to the LP variable, separately for W + and W− bosons in the electron and muon
final states. The LP distribution for each of the three polarization states of the W boson is
extracted from Monte Carlo samples which are reweighted to the angular distributions expected
from each polarization state in the W boson center-of-mass frame. The LP distributions for
muons are shown in Figs. 3. Also shown are the results of the fit to the individual components
corresponding to the three W polarization states, and to the background. The results from
W → eν and W → µν are consistent. The muon fit result yields the most precise measurement,
(fL − fR) − = 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 (fL − fR) + = 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 measurement
establishes a difference between the left-handed and right-handed polarization parameters with a
significance of 7.8 standard deviations for W + bosons and 5.1 standard deviations for W − bosons.
4 Observation of Z +b
This final state is important at the LHC, both as a benchmark channel to the production of
the Higgs boson in association with b-quarks, and as a Standard Model background to Higgs
and new physics searches in final states with leptons and b-jets. In this analysis both electrons
(muons) of the Z candidate are required to have pT > 25 (20) GeV. Jets with pT > 25 GeV are
searched for the presence of a secondary vertex and a b-tagging discriminant variable is built
from the three-dimensional flight distance from the primary vertex to the chosen secondary
vertex. We used both an high-efficiency (HE) and an high-purity (HP) selection, where at
least two (three) tracks are required to be attached to the secondary vertex, respectively. To
extract the purity P in b-jets, the b-tagging discriminant variable and the mass of the secondary
vertex are fitted using a binned likelihood method. The purity in b-jets is found to be 55 ± 9 %
(88 ± 11 %) for the HE (HP) selection, in good agreement with the MC estimate of 57 ± 3 %
(82 ± 4 %). Figure 4 shows the di-lepton mass and secondary vertex mass of events surviving
P