Thesis title: Search for high-mass resonances in final states with a boosted-dijet resonance in proton-proton collisions at √s = 13 TeV with the CMS detector
The Standard Model (SM) of particle physics provides the best description
available of the fundamental constituents of the universe and their interactions. It is
a robust theory, tested by many experiments with a high level of precision. However,
there are several reasons to believe that the SM is not a fundamental theory, for
example it does not describe the gravitational interaction between particles, or the
presence of the dark matter in the universe. This led to the conception of several
theories Beyond the Standard Model (BSM), which often foresee the existence of
new resonances that could be detected in experiments at particle colliders, such as
the Large Hadron Collider (LHC) at CERN.
This thesis reports the first search, performed by an LHC experiment, for high-mass
hadronic resonances that decays to a parton and a Lorentz-boosted resonance,
which in turn decays into a pair of partons. Such resonances are predicted, for
example, by BSM theories that foresee the existence of more than three spatial
dimensions. The search is based on data collected with the CMS detector in
proton-proton collisions produced at the LHC at √s = 13 TeV, corresponding to
an integrated luminosity of 138 fb^{-1}. The boosted resonance is reconstructed as a
single wide jet with substructure consistent with a two-body decay. The high-mass
resonance is thus considered as a dijet system. The jet substructure information and
the kinematic properties of cascade resonance decays are exploited to disentangle
the signal from the large quantum chromodynamics multijet background. The dijet
mass spectrum is analyzed for the presence of new high-mass resonances, and is
found to be consistent with the standard model background predictions. Results are
interpreted in a warped extra dimension model where the high-mass resonance is a
Kaluza-Klein gluon, the boosted resonance is a radion, and the final state partons
are all gluons. Limits on the production cross section are set as a function of the
Kaluza-Klein gluon and radion masses. These limits exclude at 95% confidence
level models with Kaluza-Klein gluon masses in the range from 2.0 to 4.3 TeV and
radion masses in the range from 0.20 to 0.74 TeV. By exploring a novel experimental
signature, the observed limits on the Kaluza–Klein gluon mass are extended by up
to about 1 TeV compared to previous searches.