TOPICS IN STRING PHENOMENOLOGY

Abstract

The first part of the thesis is devoted to investigation of important phenomenological and particle-cosmology-related issues in the context of Type IIB string compactifications. After undertaking a brief review of (split) supersymmetry in the context of Beyond Standard Model Physics, we discuss the possibility of realizing “μ-split SUSY” scenario from a phenomenological model, which we show could be realizable locally as the large volume limit of a type IIB Swiss-Cheese Calabi-Yau orientifold involving a mobile space-time filling D3-brane localized at a nearly special Lagrangian three-cycle embedded in the ”big” divisor (hence the local nature of the model’s realization) and multiple fluxed stacks of space-time filling D7-branes wrapping the same “big” divisor. Naturally realizing split-SUSY scenario of N. Arkani-Hamed and S. Dimopoulos in our model, we show that the mass of one of the Higgs formed by a linear combination of two Higgs doublets (related to the D3-brane position moduli), can be produced to be of the order of 125 GeV whereas other Higgs as well as higgsino mass parameter to be very heavy- the “μ-split SUSY” scenario. The squarks’/sleptons’ (the Wilson line moduli on D7-branes’ world volume) masses also turn out to be very heavy. Motivated by the fact that the gravitino appears as the Lightest Supersymmetric Particle (LSP) in our model, we explore the possibility of the gravitino as a viable cold dark matter candidate by showing its life time to be of the order of or greater than the age of the universe whereas lifetimes of decays of the co-NLSPs (the first generation squark/slepton and the lightest neutralino) to the LSP (the gravitino) turns out to be too short to disturb predictions of Big Bang Nucleosynthesis (BBN). i ii Assuming non-thermal gravitino production mechanism, we estimate the gravitino relic abundance to be around 0.1 by evaluating the neutralino/slepton annihilation cross sections and hence show that the former satisfies the requirement for a dark matter candidate. As another testing ground for split-SUSY scenarios, we estimate Electric Dipole Moment (EDM) of electron/neutron up to two-loops in our model. By explaining distinct O(1) CP violating phases associated with Wilson line moduli and position moduli, we show that it is possible to obtain dominant contribution of EDM of electron/neutron to be around 10−29 esu-cm at two-loop level by including heavy sfermions and a light Higgs, around 10−27 esu-cm for a two-loop Barr-Zee diagram involving W boson and SM interaction vertices, eEDM to be around 10−33 esu-cm at one-loop by including heavy chargino and a light Higgs, and nEDM to be around 10−32 esu-cm at one-loop by including SM fermions and a light Higgs. In second part of the thesis, we obtain local M-theory uplift of Type IIB background involving non-compact (resolved) warped deformed conifold and (N) space time filling D3-branes placed on the singularity of the conifold, (M) D5-branes wrapped around compact S2, and (Nf ) D7-branes wrapped around non-compact four-cycle via Strominger-Yau-Zaslow’s mirror symmetry prescription [1]. This work is mainly concerned with the investigation of hydro/thermodynamical aspects relevant to explain the behavior of thermal QCD with fundamental quarks, as well as to demonstrate the thermodynamical stability of the M-theory uplift. The uplift gives a black M3-brane solution whose near-horizon geometry near !1,2 = 0,"-branches, preserves 1/8 SUSY. We propose a new MQGP limit corresponding to finite-string-coupling (gs ! 1)- large-t’Hooft-coupling regime of M-theory in addition to the one discussed in [2]. Interestingly, we obtain #/s = 1/4" for the uplift and the diffusion constant D ! 1/T for types IIB/IIA backgrounds in the both limits. The thermodynamical stability of M-theory uplift is checked by evaluating the D = 11 Euclideanized supergravity action upto O(R4, |G4|2) term in the two limits, and thereafter showing the positive sign of specific heat from the finite part of the action.