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

Symmetry at 2 tev for a vs b vs gz

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Unformatted text preview: b) s-channel diagrams for WZ production at the Tevatron. Hobbs, Neubauer, and Willenbrock: Tests of the standard electroweak model at . . . R D0 Collaboration using Ldt ¼ 0:3 fbÀ1 , where three 0  0 ‘‘ candidates were found (Abazov WZ ! ‘ ‘ et al., 2005c). The observed candidates had a probability of 3.5% to be due to background fluctuations, corresponding to ðWZÞ < 13:3 pb at 95% C.L. The D0 Collaboration published an update to Abazov et al. (2005c) with additional data to measure the WZ production cross section and search for anomalous WWZ couplings (Abazov et al., 2007b). As with other diboson processes, the signal for WZ production is most easily measured in fully leptonic decay. The experimental signature of WZ production is three isolated high ET charged leptons, at least two of which have the same flavor and opposite charge with invariant mass consistent with decay of a Z boson, and large ET consistent with a neutrino 6 from W decay. To observe WZ in fully leptonic decay, high acceptance for charged leptons is required since all three must be detected to suppress backgrounds from larger crosssection processes. In the CDF analysis (Abulencia et al., 2007b), a novel lepton identification strategy was developed to maximize charged-lepton acceptance while keeping the backgrounds comparatively low by exploiting the charge and flavor correlation of identified leptons in the events. The standard electron and muon identification was combined with forward electron candidates beyond the tracking acceptance and a ‘‘track-only’’ lepton category consisting of high-quality tracks that neither project to the fiducial regions of the calorimeters nor are identified as muons by the muon chambers. Because of the lack of calorimeter information, electrons and muons cannot be reliably differentiated for this category and are therefore treated as having either flavor in the WZ candidate selection. For forward electrons without a matched track, both charge hypotheses are considered when forming WZ candidates, since the charge is determined from the track curvature. Other SM processes that can lead to three high-pT leptons include dileptons from the DY Z= Ã process, with an additional lepton from a photon conversion (Z ) or a misidentified jet (Z þ jets) in the event; ZZ production where only three leptons are identified and the unobserved lepton results " " in ET ; and a small contribution from tt ! WbW b, where 6 two charged leptons result from the W boson decays and one " or more from decay of the b quarks. Except for tt, these TABLE XII. Expected number of events in the signal region for WZ and the background contributions for the CDF WZ ! ‘‘‘ analysis (Abulencia et al., 2007b). ‘‘Lumi’’ refers to the integrated luminosity uncertainty, which is absent for the Z þ jets because it is determined from the same data set. Source Expectation Æ stat Æ syst Æ lumi Z þ jets ZZ Z " tt 1:21 Æ 0...
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This document was uploaded on 09/28/2013.

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