Beyond Standard Model Physics under the ground and in the sky

Cosmology and particle physics are in an exciting data-rich era, with several collider and astronomical searches underway. In this dissertation, we have explored some problems which are not addressed by the standard models of particle physics and cosmology. The implications of the Higgs discovery and lack of new physics results are far reaching. To better understand the nature of Higgs and its connections to electroweak symmetry breaking, we have performed a model independent study of spin-1 contributions in gauge extensions of Standard model. The null results of all low energy supersymmetric searches has lead to the development of Split SUSY models which are based only on gauge unification and dark matter as guiding principles. We study in detail the cosmic probes of Split SUSY using indirect dark matter detection constraints and hints of small tensor to scalar ratio. We also investigate the phenomenological viability of models with light dilatons that ameliorate the cosmological constant problem by studying conformal phase transitions using holography. Finally, we have also checked the robustness of soft-wall geometry by including higher curvature terms in the five dimensional bulk action

Heavy Gravitino and Split SUSY in the Light of BICEP2

High-scale supersymmetry (SUSY) with a split spectrum has become increasingly interesting given the current experimental results. A SUSY scale above the weak scale could be naturally associated with a heavy unstable gravitino, whose decays populate the dark matter (DM) particles. In the mini-split scenario with gravitino at about the PeV scale and the lightest TeV scale neutralino being (a component of) DM, the requirement that the DM relic abundance resulting from gravitino decays does not overclose the Universe and satisfies the indirect detection constraints demand the reheating temperature to be below 10^9 - 10^{10} GeV. On the other hand, the BICEP2 result prefers a heavy inflaton with mass at around 10^{13} GeV and a reheating temperature at or above 10^9 GeV with some general assumptions. The mild tension could be alleviated if SUSY scale is even higher with the gravitino mass above the PeV scale. Intriguingly, in no-scale supergravity, gravitinos could be very heavy at about 10^{13} GeV, the inflaton mass scale, while gauginos could still be light at the TeV scale.Comment: 20 pages, 2 figures, references added, to appear in JHE

A Perturbative RS I Cosmological Phase Transition

We identify a class of Randall-Sundrum type models with a successful first order cosmological phase transition during which a 5D dual of approximate conformal symmetry is spontaneously broken. Our focus is on soft-wall models that naturally realize a light radion/dilaton and suppressed dynamical contribution to the cosmological constant. We discuss phenomenology of the phase transition after developing a theoretical and numerical analysis of these models both at zero and finite temperature. We demonstrate a model with a TeV-Planck hierarchy and with a successful cosmological phase transition where the UV value of the curvature corresponds, via AdS/CFT, to an $N$ of $20$, where 5D gravity is expected to be firmly in the perturbative regime.Comment: 34pp, 12 figure

LHC Dark Matter Signals from Vector Resonances and Top Partners

Extensions of the Standard Model which address the hierarchy problem and dark matter (DM) often contain top partners and additional resonances at the TeV scale. We explore the phenomenology of a simplified effective model with a vector resonance $Z'$, a fermionic vector-like coloured partner of the top quark $T'$ as well as a scalar DM candidate $\phi$ and provide publicly available implementations in CalcHEP and MadGraph. We study the $pp \to Z' \to T'\overline{T'} \to t\bar{t}\,\phi\phi$ process at the LHC and find that it plays an important role in addition to the $T'\overline{T'}$ production via strong interactions. It turns out that the presence of the $Z'$ can provide a dominant contribution to the $t\bar{t}+E_T^{\text{miss}}$ signature without conflicting with existing bounds from $Z'$ searches in di-jet and di-lepton final states. We find that through this process, the LHC is already probing DM masses up to about 900 GeV and top partner masses up to about 1.5 TeV, thus exceeding the current bounds from QCD production alone almost by a factor of two for both particles.Comment: 32 pages, 15 figures, 3 table

LHC vector resonance searches in the tt¯ Z final state

LHC searches for BSM resonances in l+l−,jj,tt¯ , γγ and VV final states have so far not resulted in discovery of new physics. Current results set lower limits on mass scales of new physics resonances well into the O(1) TeV range, assuming that the new resonance decays dominantly to a pair of Standard Model particles. While the SM pair searches are a vital probe of possible new physics, it is important to re-examine the scope of new physics scenarios probed with such final states. Scenarios where new resonances decay dominantly to final states other than SM pairs, even though well theoretically motivated, lie beyond the scope of SM pair searches. In this paper we argue that LHC searches for (vector) resonances beyond two particle final states would be useful complementary probes of new physics scenarios. As an example, we consider a class of composite Higgs models, and identify specific model parameter points where the color singlet, electrically neutral vector resonance ρ0 decays dominantly not to a pair of SM particles, but to a fermionic top partner Tf1 and a top quark, with Tf1 → tZ. We show that dominant decays of ρ0 → Tf1t in the context of Composite Higgs models are possible even when the decay channel to a pair of Tf1 is kinematically open. Our analysis deals with scenarios where both mρ and mTf1 are of O(1) TeV, leading to highly boosted tt¯ Z final state topologies. We show that the particular composite Higgs scenario we consider is discoverable at the LHC13 with as little as 30 fb−1, while being allowed by other existing experimental constraints. © 2017, The Author(s)011Nsciescopu

Criterion for dynamical chiral symmetry breaking

The Bethe-Salpeter equation is related to a generalized quantum-mechanical Hamiltonian. Instability of the presumed vacuum, indicated by a tachyon, is related to a negative energy eigenstate of this Hamiltonian. The variational method shows that an arbitrarily weak long-range attraction leads to chiral symmetry breaking, except in the scale-invariant case when the instability occurs at a critical value of the coupling. In the case of short-range attraction, an upper bound for the critical coupling is obtained.Comment: 10 pages, 2 figures; made minor changes, published versio