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

Production however this channel is generally not

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Unformatted text preview: he final state. The ZZ ! ‘‘ process can also contribute sensitivity to the search for ZZ production although it suffers from large continuum WW backgrounds. The full SM process " is pp ! Z= à Z= à , where the two Z= à interfere with one FIG. 27. Leading-order diagram for ZZ production at the Tevatron. The s-channel diagram involving the neutral TGC is absent in the SM. Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 another. For brevity, we denote Z= à Z= à as ZZ throughout and indicate the dilepton invariant mass range(s), where " applicable. The NLO ZZ cross section for pp collisions at pffiffiffi s ¼ 1:96 TeV is 1:4 Æ 0:1 pb in the zero-width Z boson approximation (Campbell and Ellis, 1999). R Using Ldt ¼ 1 fbÀ1 , the D0 Collaboration searched for ZZ ! ‘‘‘‘ production and set a limit on the cross section of ðZZÞ < 4:4 pb at 95% C.L. (Abazov et al., 2008c). They constrain possible ZZ and ZZZ couplings based on the observed four lepton yield with the added requirement that Mð‘‘Þ > 50 (70) for electrons (muons). The value of the cut was chosen based on the dilepton invariant mass resolution. This lower Mð‘‘Þ requirement was included because the Monte Carlo generator (Baur and Rainwater, 2000) used to constrain possible anomalous couplings does not include contributions from off-shell Z bosons. Without the Mð‘‘Þ cut, one ee event is observed in the data. This event is removed by the Mð‘‘Þ cut used to constrain anomalous couplings. 1D and 2D limits on anomalous ZZ and ZZZ couplings are determined using à ¼ 1:2 TeV. The 95% C.L. Z 1D limits are À0:28 < f40 < 0:28, À0:26 < f40 < 0:26, Z and À0:30 < f50 < 0:28. The À0:31 < f50 < 0:29, Z , f vs f , fZ vs f Z , 95% C.L. 2D contours f40 vs f40 40 40 50 50 Z 28. and f50 vs f50 are shown in Fig.R The CDF Collaboration uses Ldt ¼ 1:9 fbÀ1 to search for ZZ production in a combination of the ZZ ! ‘‘‘‘ and ZZ ! ‘‘ channels (Aaltonen et al., 2008b). To maximize the acceptance, lepton candidates are constructed out of all reconstructed tracks and energy clusters in the EM section of the calorimeter. This is done with the same lepton identification criteria used in a previous CDF measurement of WZ production (Abulencia et al., 2007b). The ZZ ! ‘‘‘‘ candidates are selected from events with exactly four chargedlepton candidates and at least two same-flavor, opposite-sign lepton pairs are required for the event to be accepted. As in the WZ analysis, trackless electrons are considered to have either charge, and track-only leptons either flavor. One pair must have invariant mass M‘þ ‘À in the range ½76; 106Š GeV=c2 , while the requirement for the other pair is extended to ½40; 140Š GeV=c2 to increase the acceptance for off-shell Z decays. The ZZ ! ‘‘‘‘ candidate events are separated into two exclusive categories based on whether or not they contain at least one forward...
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