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=
Ã , where the two Z=
Ã 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=
Ã as ZZ throughout
and indicate the dilepton invariant mass range(s), where
applicable. The NLO ZZ cross section for pp collisions at
s ¼ 1:96 TeV is 1:4 Æ 0:1 pb in the zero-width Z boson
approximation (Campbell and Ellis, 1999).
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 Ã ¼ 1:2 TeV. The 95% C.L. Z
1D limits are À0:28 < f40 < 0:28, À0:26 < f40 < 0:26, Z
À0:30 < f50 < 0:28.
À0:31 < f50 < 0:29, Z , f vs f , fZ vs f Z ,
95% C.L. 2D contours f40 vs f40 40
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; 106 GeV=c2 , while the requirement for the other pair
is extended to ½40; 140 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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