Measurement of the Higgs boson o -shell coupling to constrain the total width in the H → ZZ(*) → 4l channel and Level-1 Track muon isolation performance for the HL-LHC with the ATLAS detector
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Date
10/07/2017Author
Olivares, Sebastian Andres
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Abstract
Since the observation of a narrow mass resonance consistent with the Higgs
boson by ATLAS and CMS collaborations in 2012, a number of important
studies have been made in order to understand the properties of the newly
discovered particle. The most fundamental precision measurements include the
Higgs coupling to other particles and itself; properties that have a direct relation
with the total decay width. The theoretical total width of the Standard Model
(SM) Higgs boson is extremely small (4.2 MeV), making its direct measurement
by the LHC experiments non feasible due to the finite experimental detector
resolution. Recent publications have shown a novel way to set an indirect limit
on the total Higgs boson width by using measurements of both off-shell and
on-shell production. This thesis presents a determination of the off-shell Higgs
boson coupling, and a further interpretation of the Higgs total width in the
H → ZZ → 4l channel (l = e; μ). The results are based on pp collision data
collected by the ATLAS experiment at the LHC, corresponding to an integrated
luminosity of 20.3 fb-1 at a collision energy of √s = 8 TeV. Using the CLs
statistical method and assuming the same higher-order QCD corrections applied
for both signal and background processes, the observed 95% confidence level (CL)
upper limit on the off-shell signal strength is 7.3 (with the yields normalized to
the SM expectation). Similarly, the 95% CL upper limit on the Higgs total width
is 24.7 MeV.
The LHC will undergo its last big upgrade in 2021, in preparation for the high-luminosity
LHC Run (HL-LHC), with a luminosity increase of approximately
a factor of 5 beyond its nominal design rate. Raising the muon transverse
momentum threshold becomes a necessity in order to maintain a low online
selection rate with the existing trigger system, at the cost of a reduced efficiency
for the electroweak scale physics. An alternative to this approach is a proposed
design of a first-level hardware trigger that uses tracking information. Being
able to use tracking information at the first level of the ATLAS trigger in
the implementation of a muon isolation algorithm offers an extra handle for
differentiating between signal and background. The second part of this thesis
presents studies on the performance of tracking-based muon isolation designed
for a first-level hardware trigger system. These studies demonstrate the improved
trigger performance of the muon isolation algorithm when compared to an
increase of the transverse momentum threshold of the muon candidates.
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