Unformatted text preview: expected these channels have considerably less
sensitivity than the WH channels because of the signiﬁcantly
lower signal cross section times branching fraction.
3. ZH ! bb and related ﬁnal states The ZH ! bb ﬁnal state has a production rate intermediate between the WH and ZH ! ‘‘bb ﬁnal states. This
ﬁnal state also has a signiﬁcant contribution from the process
WH ! ‘bb in which the charged lepton ‘ escapes detection. This is particularly true for the case ‘ ¼ because the
muon leaves very little energy in the calorimeter and thus
results in event ET similar to that from Z ! decay.
Unlike either of the previously discussed ﬁnal states, the
ZH ! bb ﬁnal state has no charged leptons from vector
boson decay. This implies a signiﬁcantly increased background from SM multijet events in which ET arises from
6 Events/0.05 10 Observed, 1 b-tag
Z+jets w/ ALPGEN
ZH(120 GeV/c ) @ 95% CL upper limit 2 10 110 120 130 140 150 160 Higgs Mass (GeV/c2) FIG. 56. Expected and observed limits for the CDF 1:0 fbÀ1
ZH ! ‘‘bb analysis. mismeasurement. This background is difﬁcult to model from
simulation, and analyses of this ﬁnal state must develop
techniques to measure it using data control samples. Both
CDF (Aaltonen et al., 2008g) and D0 (Abazov et al., 2010a)
published results in this ﬁnal state. The two experiments
developed different methods for controlling and estimating
the multijet background.
a. CDF search The CDF analysis uses a data sample corresponding to
1 fbÀ1 and begins with selection of events passing a ET
trigger with level one ET > 25 GeV, a level two requirement
of two jet clusters having ET > 10 GeV, and a level three
requirement of ET > 35 GeV. At least one of the level two
jets must also satisfy < 1:1. The initial ofﬂine selection
(‘‘pretag’’) requires events to have MET > 50 GeV and exactly two jets with ET > 20 GeV. One of the jets must have
ET > 35 GeV, and the other must have ET > 25 GeV.
Additionally, one of the jets must satisfy jj < 0:9, and the
other jet must satisfy jj < 2:4. The two jets must also have
Á > 1:0 rad, and events with high pT , isolated leptons are
vetoed. Finally, at least one of the jets is required to have a
secondary vertex b tag.
All nonmultijet backgrounds, tt, W þ jets, Z þ jets, and
diboson production are modeled using simulated events. The Events/0.05 10 Observed, 2 b-tags
Z+jets w/ ALPGEN
ZH(120 GeV/c ) @ 95% CL upper limit 2 10
0 0.2 0.4 0.6 0.8 1 NN Projection (Z+jets vs ZH) σ (pp → ZH) × B(H → bb) (pb) 1 10 2 DO, 370-450 pb-1 10
95% C.L. upper limit
( --- expected limit) 1
10 -1 standard model 10 -2
100 110 120 130 140 150 2 Higgs Mass (GeV/c ) FIG. 55. NN distributions for the (a) single-tagged and (b) doubletagged samples from the CDF 1:0 fbÀ1 ZH ! ‘‘bb analysis. The
95% C.L. upper bound cross section is 19Â the SM prediction.
Rev. Mod. Phys., Vol. 84,...
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This document was uploaded on 09/28/2013.
- Fall '13