Abstract:
The standard model(SM) of particle physics is an experimentally tested successful the-
ory of the modern physics era. However there exist phenomena which the SM does not
describe. Among others, the neutrino mass hierarchy which has been observed by ex-
periments like Super-Kamiokande has no explanation in SM. This requires an extension
or perhaps new physics beyond the SM. One of the extensions for the standard model,
type-III seesaw mechanism introduces a fermionic triplet, which can explain why the
neutrino masses are small and why they indeed have the mass hierarchy. The fermionic
triplet in type-III seesaw mechanism decays to the multileptonic final state via elec-
troweak interactions. The multileptonic final decay has an advantage of being rare,
relative to other standard model backgrounds.
In my thesis, I have undertaken a search for type-III seesaw signal in events with 3 or
more leptons (e, mu, and tau). The data on which this search has been performed cor-
√
responds to 35.9 fb −1 of integrated luminosity in pp collisions at s = 13 TeV collected
by the CMS experiment at the LHC. Since the signal populates channels with at least
three leptons and diverse kinematic properties, the data is binned in exclusive chan-
nels. The primary selection is based on the number of leptons and the invariant mass of
opposite-sign dilepton systems which helps discriminate the signal against the Standard
Model background. The final optimization for the type-III seesaw signal is based on the
sum of leptonic transverse momenta and missing transverse energy. Control samples in
data are used to check the robustness of background evaluation techniques and to mini-
mize the reliance on simulation. The observations are consistent with expectations from
Standard Model processes. No sign of type-III seesaw fermion production is observed.
The results are used to exclude heavy fermions of the type-III seesaw model with masses
below 840 GeV.
