Department of Physics

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    Constraints on a seesaw model leading to quasidegenerate neutrinos and signatures at the LHC
    (APS, 2015-05) Mondal, Tanmoy
    We consider a variant of TeV-scale seesaw models in which three additional heavy right-handed neutrinos are added to the standard model to generate the quasidegenerate light neutrinos. This model is theoretically interesting since it can be fully rebuilt from the experimental data of neutrino oscillations except for an unknown factor in the Dirac-Yukawa coupling. We study the constraints on this coupling coming from metastability of electroweak vacuum. An even stronger bound comes from the lepton flavor violating decays on this model, especially in a heavy neutrino mass scenario which is within the collider’s reach. Bestowed with these constrained parameters, we explore the production and discovery potential coming from these heavy neutrinos at the 14 TeV run of the Large Hadron Collider. Signatures with trilepton final state together with backgrounds are considered in a realistic simulation.
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    Heavy stable charged tracks as signatures of non-thermal dark matter at the LHC: a study in some non-supersymmetric scenarios
    (Springer, 2017) Mondal, Tanmoy
    We consider two theoretical scenarios, each including a ℤ2-odd sector and leading to an elementary dark matter candidate. The first one is a variant of the Type-III seesaw model where one lepton triplet is ℤ2-odd, together with a heavy sterile neutrino. It leads to a fermionic dark matter, together with the charged component of the triplet being a quasi-stable particle which decays only via a higher-dimensional operator suppressed by a high scale. The second model consists of an inert scalar doublet together with a ℤ2-odd right-handed Majorana neutrino dark matter. A tiny Yukawa coupling delays the decay of the charged component of the inert doublet into the dark matter candidate, making the former long-lived on the scale of collider detectors. The parameter space of each model has been constrained by big-bang nucleosynthesis constraints, and also by estimating the contribution to the relic density through freeze-out of the long-lived charged particle as well the freeze-in production of the dark matter candidate. We consider two kinds of signals at the Large Hadron Collider for each case. For the first kind of models, namely two charged tracks and single track and for the second kind, the characteristic signals are opposite as well as same-sign charged track pairs. We perform a detailed analysis using event selection criteria consistent with the current experimental programmes. It is found that the scenario with a lepton triplet can be probed upto 960 (1190) GeV with an integrated luminosity of 300 (3000) fb−1, while the corresponding numbers for the inert doublet scenario are 630 (800) GeV. Furthermore, the second kind of signal mentioned in each case allows us to differentiate different dark matter scenarios from each other.