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This thesis address theoretical and phenomenological aspects of active and sterile mixing pattern within minimal extended seesaw frameworks. It consists of six chapters, where chapters one and six are dedicated to introduction and conclusion chapters, respectively. In chapter two, we study active-sterile phenomenology with a single generation of sterile neutrino ($m_Ssim mathcal{O}$(eV)) along with the three active neutrinos. Three independent cases for sterile mass matrices are studied in both normal and inverted hierarchy mass ordering. Chapter three is an extension of the previous chapter. The neutrino mass generation and baryogenesis {it via} thermal leptogenesis are studied in the fermionic sector. On the other hand, an extended multi Higgs doublet model is studied in the scalar sector. Among the three Higgs doublet, one of them does not acquire any vacuum expectation value (VEV); hence, it behaves as an inert Higgs. The lightest component of this behaves as a viable dark matter candidate. Within MES, sterile mass can be stressed up to the $keV$ scale. In the fourth chapter, we studied various phenomenologies considering sterile neutrino mass in the $keV$ range. This $keV$ scaled sterile neutrino also plays the role of dark matter candidate in our study. The fifth chapter is dedicated to the texture-zero neutrino dark matter model. We study active-sterile mixing, baryogenesis $via$ resonant leptogenesis and $0 ubetabeta$ in the fermion sector. While, in the scalar sector, the complex scalar flavon that gives rise to sterile mass takes part in dark matter study. The imaginary component of that scalar flavon is behaving as a viable dark matter candidate.
The Exceptional Supersymmetric Standard Model (E$_6$SSM) provides a low energy alternative to the MSSM, with an extra gauged U(1)$_N$ symmetry, solving the $mu$-problem of the MSSM. Inspired by the possible embedding into an E$_6$ GUT, the matter con
We examine active-sterile neutrino conversion in the late time post-core-bounce supernova environment. By including the effect of feedback on the Mikheyev-Smirnov-Wolfenstein (MSW) conversion potential, we obtain a large range of neutrino mixing para
The charm quark has unique properties that make it a very important probe of many facets of the Standard Model. New experimental information on charm decays is becoming available from dedicated experiments at charm factories, and through charm physic
We address the possible impact of New Physics on neutrino oscillation experiments. This can modify the neutrino production, propagation and/or detection, making the full cross section non-factorizable in general. Thus, for example, the neutrino flux
The recent data indicate that the neutrino mixing angle $theta_{23}$ deviates from the maximal-mixing value of 45$^circ$, showing two nearly degenerate solutions, one in the lower octant (LO) ($theta_{23}<45^circ$) and one in the higher octant (HO) (