Scalar leptoquarks and Higgs pair production at the LHC

The presence of colored particles can affect both the single and the pair Higgs productions substantially. For scalar particles, this happens if their portal couplings to the Standard Model Higgs are large and their masses are not too high. In the present work these processes are studied in the case of several leptoquarks which may appear in many beyond Standard Model theories. It is found that the constraints on the portal couplings from the single Higgs production and the decays to various channels measured by the LHC experiments still allow increased Higgs pair production rate. For the masses in the range from 180 GeV to 300 GeV, depending on the strength of such portal couplings, the Higgs pair production may reach an order to several hundred in magnitude larger rate than the Standard Model case for the 8 TeV run. Therefore, combined with the on going searches for leptoquarks by both the CMS and ATLAS, this is one of the possible scenarios to be probed directly by the current data. The current study demonstrates that if colored scalars modify scalar potentials through portal couplings, which has been studied for variety of motivations such as playing a potentially important role in electroweak phase transition, composite models or radiative neutrino masses, this fact may appear as the modified Higgs pair production.Comment: 19 pages, 4 figures, references added and typos correcte

Supersymmetry, grand unification and flavor symmetry

In this thesis I have presented the findings of my research pursued during my Ph.D. study. The purpose of this thesis was to study different theoretical ideas in high energy physics model building addressed primarily towards understanding the fermion mass problem and the gauge hierarchy problem. These include: Anomalous flavor U/(1) symmetry and its experimental implications, finite GUT models with discrete family symmetry, and a product GUT model in a 2D deconstructed theory space. The second and third chapters of the thesis describe our study of lepton flavor violation (LFV) and electric dipole moments (EDM) induced by a flavor-dependent anomalous U/(1) gauge symmetry of string origin. The models considered also address the fermion mass hierarchy problem successfully. We have shown that the U/(1) sector induces significant LFV and EDMs through the SUSY breaking parameters. These effects arise via renormalization group evolution of the parameters in the momentum regime between the string and the anomalous U/gauge symmetry of string origin. The models considered also address the fermion mass hierarchy problem successfully. We have shown that the U(1) sector induces significant LFV and EDMs through the SUSY breaking parameters. These effects arise via renormalization group evolution of the parameters in the momentum regime between the string and the anomalous U(1) breaking scale. The fourth chapter of the thesis contains our work on a concrete realization of SUSY breaking using interference between the anomalous U(1) flavor gauge symmetry and a strongly coupled SU(Nc), leading to the so called Split SUSY spectrum where the sfermions and the gravitino acquire masses of order 10(^5)/10(^8) GeV while the gauginos and the Higgsinos have masses of order 10(^2)/10(^3) GeV. We have cal- culated the leading order supergravity corrections and have presented a class of explicit models of Split SUSY which are phenomenologically consistent. In the fifth chapter I have presented models for realistic quark masses and mixings in the context of finite SU(5) GUT where in the B functions for the gauge and the Yukawa couplings vanish to all orders in perturbation theory. The models pre-sented are based on non-Abelian discrete symmetries. In the case of (Z4)3xP and A4 symmetries we have found models finite to all order of perturbation theory while in the case of an S4 symmetry we have found a model which is two-loop finite. In the sixth chapter I have presented a model wherein the observed fermion masses and mixing angles emerge from a deconstructed U(1) theory space with a disk structure in SU(5)'xSU(5)'' product GUT. Below the B - L breaking scale, the effective Yukawa couplings and mixing matrices of the fermions are correctly reproduced through non-renormalizable operators. In our model, both the fermion mass matrix structures, and supersymmetry breaking (as a global twist of RP2) can be addressed in the same theory space consistent with phenomenology and anomaly cancelation

Constraints on dark energy models from the Horndeski theory

In light of the cosmological observations, we investigate dark energy models from the Horndeski theory of gravity. In particular, we consider cosmological models with the derivative self-interaction of the scalar field and the derivative coupling between the scalar field and gravity. We choose the self-interaction term to have an exponential function of the scalar field with both positive and negative exponents. For the function that has a positive exponent, our result shows that the derivative self-interaction term plays an important role in the late-time universe. On the other hand, to reproduce the right cosmic history, the derivative coupling between the scalar field and gravity must dominate during the radiation-dominated phase. However, the importance of such a coupling in the present universe found to be negligible due to its drastic decrease over time. Moreover, the propagation speed of gravitational waves estimated for our model is within the observational bounds, and our model satisfies the observational constraints on the dark energy equation of state.Comment: 16pages, 4 figure

Doubly resonant WW plus jet signatures at the LHC

We present search prospects and phenomenology of doubly resonant signals that come from the decay of a neutral weak-singlet color-octet vector state \omega_8 into a lighter weak-triplet color-octet scalar \pi_8, which can arise in several theories beyond the Standard Model. Taking m_{\omega_8}-m_{\pi_8}>m_W, we demonstrate an analysis of the signals pp \to \omega_8 \to \pi^\pm_8 W^\mp (\pi^0_8 Z) \to g W^\pm W^\mp (g Z Z). The present 8 TeV LHC run is found to have the potential to exclude or discover the signal for a range of masses and parameters. The preferred search channel has a boosted W-tagged jet forming a resonance with a second hard jet, in association with a lepton and missing energy.Comment: 15 pages, 8 figures; references update

MSSM in view of PAMELA and Fermi-LAT

We take the MSSM as a complete theory of low energy phenomena, including neutrino masses and mixings. This immediately implies that the gravitino is the only possible dark matter candidate. We study the implications of the astrophysical experiments such as PAMELA and Fermi-LAT, on this scenario. The theory can account for both the realistic neutrino masses and mixings, and the PAMELA data as long as the slepton masses lie in the $500-10^6$TeV range. The squarks can be either light or heavy, depending on their contribution to radiative neutrino masses. On the other hand, the Fermi-LAT data imply heavy superpartners, all out of LHC reach, simply on the grounds of the energy scale involved, for the gravitino must weigh more than 2 TeV. The perturbativity of the theory also implies an upper bound on its mass, approximately $6-7$TeV.Comment: Published version, figures update