Search for neutral supersymmetric Higgs bosons in multijet events at s**(1/2) = 1.96-TeV Page: 5 of 7
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where S (B) refers to the number of signal (background)
events. Jets containing b quarks are identified using a
secondary vertex (SV) tagging algorithm. A jet is tagged
as a b-jet if it has at least one SV within AR < 0.5 of the
jet axis and a transverse displacement from the primary
vertex that exceeds five times the displacement uncer-
tainty. Jets are b tagged up to Iyn < 2.5, although the
b tagging is about twice as efficient in the central region
(Hnj < 1.1) because of the CFT coverage. The b tagging
efficiency is 55% for central b-jets of PT > 35 GeV,
with a light quark (or gluon) tag rate of about 1%.
Signal events were simulated using the PYTHIA [15]
event generator followed by the full DO detector sim-
ulation and reconstruction chain. PYTHIA minimum-bias
events were added to all generated events, using a Pois-
son probability with a mean of 0.4 events to match the
instantaneous luminosities at which the data were taken
(1 - 6 x 1031cm-2s-1). The bh events, with h->bb, were
generated for Higgs boson masses from 90 to 150 GeV.
Reconstructed jets in simulated events were corrected to
match the jet reconstruction and identification efficien-
cies in data. The energy of simulated jets was smeared
to match the measured jet energy resolution. The PT and
rapidity spectra of the Higgs bosons from PYTHIA were
compared to those from the NLO calculation [5]. The
shapes were similar, indicating that the PYTHIA kine-
matics are approximately correct. The simulated events
were weighted to match the PT spectrum of the Higgs
boson given by NLO, resulting in a 10% reduction of the
overall signal efficiency.
Of all SM processes, multijet production is the major
source of background. This background is determined
from data by normalizing distributions outside of the
signal region. As a cross-check, we also compare data
with simulations. ALPGEN [16] is used to generate three
samples of events for bbj and bbjj with j correspond-
ing to up, down, strange or charm quarks, or gluons,
and bbbb final states with generator-level requirements:
p > 25 GeV, pT > 15 GeV, Irj < 3.0, and AR > 0.4
between any two final-state partons. These selections
do not introduce significant bias because the final sam-
ple contains much harder jets, after the application of
trigger and b-tagging requirements. Samples of bbj and
bbjj are added together, but the bbjj sample is weighted
by 0.85 to match the jet multiplicity observed in doubly
b-tagged data. The cross sections obtained from ALP-
GEN are 8.9 nb, 3.9 nb, and 60 pb, for the respective
three states. All other backgrounds are expected to be
small and are simulated with PYTHIA: pp->Z(->bb)+jets,
pp->Zb, and pp-tt. Cross sections of 1.2 nb, 40 pb [17],
and 7 pb are assumed, respectively.
There are two main categories of multijet background.
One contains genuine heavy-flavor (HF) jets, while the
other has only light-quark or gluon jets that are mistak-
enly tagged as b-quark jets, or correspond to gluons that
branch into nearly collinear bb pairs. Using the selecteddata sample, before the application of b-tagging require-
ments, the probability to b-tag a jet is measured as a
function of its PT in three Iyn regions. These functions
are called "mis-tag" functions. They are corrected for
the contamination from true HF events by subtracting
the estimated fraction of bbj(j) events in the multijet
data sample (1.2%), obtained from an initial fit to the
doubly b-tagged data. These corrected mis-tag functions
are then used to estimate the mis-tagged background, by
applying them to every jet reconstructed in the full data
sample.
In order to test the modeling of the mis-tag back-
ground, the high statistics doubly b-tagged data is com-
pared to simulations first, before extrapolating to the
triply b-tagged background. The expected signal con-
tribution to the doubly b-tagged data is negligible. The
comparison in invariant mass spectrum of the two jets
of highest PT (not necessarily the two b-tagged jets) in
the doubly b-tagged data with the expected background
is shown in Fig. 2. The b-tagging in this analysis does
not distinguish between contributions from bottom and
charm events. However, the efficiency for tagging a c-jet
is known from simulations to be about 1/4 of that for
tagging a b-jet. Therefore, when two b-tags are required,
the fraction of ccj(j) events relative to bbj(j) events will
be a factor of 16 lower after tagging. We have es-
timated the fractions of ccjj to bbjj prior to b-tagging
using the MADGRAPH Monte Carlo generator [18]. The
cEjj cross section is 22% higher than bbjj for the same
generator-level selections. Therefore, the contribution of
ccj(j) in the doubly b-tagged data sample is expected to
represent about 8% of the events. Thus, when we refer
to the bbj(j) normalization, it should be understood that
approximately 8% of the events are from the ccj(j) pro-
cess. After these corrections for ccj(j) events, the HF
multijet processes are only a factor of 1.08 higher in data
than predicted by ALPGEN. The shape of the estimated
background agrees well with the data over the entire in-
variant mass region.
To estimate the background for triply b-tagged events,
the mis-tag function is applied to the non-b-tagged jets
in the doubly b-tagged events. This provides the shape of
the multijet background distribution with at least three
b-tagged jets. This neglects any contributions from pro-
cesses with more than two true b-jets, such as from bbbb
and Z(->bb)bb production. However, the shapes of these
backgrounds from simulations are similar to those of the
doubly b-tagged spectra, and their rates are small. The
overall background normalization is therefore determined
by fitting the leading two jets invariant mass spectrum in
triply b-tagged events outside of the hypothesized signal
region to the estimated shape for triply b-tagged back-
ground. The systematic effect on the normalization of
the background from any signal contributing outside the
search window was studied and found to be small relative
to other uncertainties, as described below.
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Abazov, V. M.; Abbott, B.; Abolins, M.; Acharya, B. S.; Adams, M.; Adams, T. et al. Search for neutral supersymmetric Higgs bosons in multijet events at s**(1/2) = 1.96-TeV, article, April 1, 2005; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1405041/m1/5/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.