الفهرس 
The SMofparticlephysicsandbeyondOnly 14 pages are availabe for public view
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Abstract
Although the existence of Dark Matter (DM) has been established through various observations, its true nature remains a mystery. Among the leading candidates for cold, non-baryonic DM are Weakly Interacting Massive Particles (WIMPs), that are well motivated from particle physics and naturally explain the observed relic density by their thermal production mechanism. The abundance of DM in the present universe is strongly influenced by its annihilation rate. However, if DM interacts through a light mediator, this annihilation rate can be significantly enhanced by a phenomenon known as the Sommerfeld Enhancement (SE).
In this thesis, our focus is on investigating the viability of the lightest right-handed sneutrino (RH-sneutrino), as a candidate for DM in the supersymmetric B − L extension of the Standard Model (BLSSM) with type-I seesaw and inverse seesaw(BLSSM-IS), where the SE is relevant. Without considering the SE, the relic abundance of the right-handed sneutrino in the BLSSM model is generally too large and exceeds the observational limits across most of the parameter space. However, the RH sneutrino can form a quasibound state through the exchange of a light B − L Higgs boson, resulting in a significant
non-perturbative Sommerfeld effect. This effect can bring the relic abundance of the lightest right-handed sneutrino within the observational limits: Ωh2 = 0.12 ± 0.001. As a result, the lightest right-handed sneutrino becomes a viable candidate for DM up to a mass of 1.5 TeV in the BLSSM with type-I seesaw and up to 2.5 TeV with the IS mechanism.