Dijet Invariant Mass Studies in the Higgs boson H→bb- resonance search in association with a W/Z boson using the ATLAS detector
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Date
01/07/2015Author
Proissl, Manuel Daniel
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Abstract
The Standard Model of Particle Physics describes the fundamental
building blocks of matter and phenomena up to the highest particle
interaction energies. The theory demands the existence of a scalar
particle: the Higgs boson. The Higgs boson was discovered by the
ATLAS and CMS collaborations at CERN using bosonic final states and
is measured to have a mass of around 125 GeV. This particle is predicted
to decay predominantly into pairs of b-quarks at this mass, but suffers
from overwhelming backgrounds from the multijet production expected
from QCD interactions. Therefore, H→bb- production in association
with a leptonically decaying W or Z boson is considered, with Z → vv-,
W → lv and Z → ll, where ` denotes electrons and muons.
This thesis presents a search for the Higgs boson decaying into bb-
pairs in association with a W or Z boson using the ATLAS detector
at the Large Hadron Collider (LHC) at CERN. The analysis uses the
full dataset recorded during pp collisions at the LHC in Run-1, corresponding
to 4.7 fb-1 at √s = 7 TeV and 20.3 fb-1 at √s = 8 TeV. A
multivariate technique and a kinematic cut-based approach have been
used to maximize the signal over background ratio, where a particular
emphasis on the latter approach is made in this thesis.
Final state radiation and reconstruction effects may decrease the
bb- resonance resolution significantly, while comparably decreasing the
probability of observing the decay over the background. The b quark
pairs from the Higgs boson are reconstructed as topological clusters
formed to jets in the ATLAS calorimeter. Thus, the reconstruction and
calibration of these jets are crucial for the final Higgs mass resolution
and paramount for the search and for future precision measurements
of V H, H→bb- production. This thesis presents the development and
evaluation of advanced techniques to improve the invariant dijet mass
reconstruction of the H→bb- candidate. Sequential jet calibrations,
semileptonic corrections and pT corrections to account for the interplay
between jet resolution/scale and the underlying signal pT spectrum
obtained from Monte Carlo simulations have been studied. A major
focus has been made on the development and evaluation of an event-level
kinematic likelihood fitting framework to exploit the full kinematic
potential of V H topologies within the detector uncertainties of the
reconstructed final state signatures in order to improve the measurement
of the b-tagged jet kinematics.
The jet energy calibrations of the H→bb- signal candidates yield
an overall improvement of the dijet invariant mass resolution of up
to ~30%, and of the expected statistical significance of ~12%. The
analysis procedure is validated using the resonant V Z(bb-) production
in the same final states as for the Higgs boson search, and is observed,
compatible with the Standard Model expectation, with a significance of
4.9 standard deviations and a signal strength of μ^V Z = 0:74+0:17
-0:16. For
a Higgs boson mass of 125.36 GeV, the observed (expected) deviation
from the background-only hypothesis is found with a significance of
1.4 (2.6) standard deviations and a signal strength is determined to be
μ^V H = 0:52±0:32(stat.)±0:24(syst.).
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