RevModPhys.84.1477】Tests of the standard electroweak model at the energy frontier

Mod phys vol 84 no 4 octoberdecember 2012 hobbs

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Unformatted text preview: with the unobserved neutrino from the leptonic decay of a W boson. R Using Ldt ¼ 1:1 fbÀ1 , 16 events with an expected background of 2:7 Æ 0:4 events are observed in the CDF analysis, as shown in Table XII. This corresponds to a 6:0 observation of WZ production when ET shape information is 6 included. The measured cross section is " ðpp ! WZ þ X Þ ¼ 5:0þ1::8 ðstat þ systÞ pb; À1 6 consistent with the NLO expectation. Figure 23 shows some important kinematic distributions for the 16 WZ candidates, which are in good agreement with the SM. In the D0 analysis (Abazov et al., 2007b), events with three reconstructed, isolated charged leptons (electrons or muons) and ET > 20 GeV were used to measure the WZ 6 production cross section and search for anomalous WWZ couplings using the observed Z boson pT (pZ ) distribution. T Each WZ candidate event must contain a like-flavor lepton 180 DØ, 1 fb Candidates Missing ET (GeV) 140 WZ Monte Carlo Background 120 100 80 60 40 20 60 1 10 8 6 4 2 0 0 20 40 60 80 100 120 140 160 180 200 Z boson pT (GeV/c) FIG. 25. The reconstructed Z boson pT of the WZ candidate events for the D0 WZ ! ‘‘‘ analysis (Abazov et al., 2007b). The solid histogram is the expected sum of signal and background for the case of the WWZ coupling parameters set to their SM values. The dotted and double-dotted histograms are the expected sums of signal and background for two different cases of anomalous WWZ coupling parameter values. pair with invariant mass close to the Z boson mass. For eee and  decay channels, the lepton pair with invariant mass closest to that of the Z boson mass is chosen to define the Z R boson daughter particles. Using Ldt ¼ 1:0 fbÀ1 , a total of 13 WZ candidates are observed with 4:5 Æ 0:6 expected background events and 9:2 Æ 1:0 expected WZ signal events, corresponding to a 3:0 signal significance. The breakdown by trilepton flavor classification for both the CDF and D0 analyses is shown in Table XIII. Figure 24 shows ET versus 6 the dilepton invariant mass for the background, the expected WZ signal, and the data, including the candidates for the D0 analysis. The WZ production cross section measured by D0 is where the Æ1 uncertainties are the 68% C.L. limits from the minimum of the negative log likelihood. The uncertainty is dominated by the statistics of the number of observed events. The three CP-conserving WWZ coupling parameters Z , ÁgZ , and ÁZ are constrained in the D0 analysis by compar1 ing the measured cross section and pZ distribution to models T with anomalous couplings. A comparison of the observed Z boson pT distribution with predictions from Monte Carlo simulation is shown in Fig. 25. Table XIV presents the onedimensional 95% C.L. limits on Z , ÁgZ , and ÁZ . Figure 26 1 presents the two-dimensional 95% C.L. limits under the assumption ÁgZ ¼ ÁZ for à ¼ 2 TeV. 1 Rejected Data 40 12 " ðpp ! WZ þ X Þ ¼ 2:7þ1::7 ðstat þ systÞ pb; À1 3 -1 160 0 20 WZ Can...
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