Shrihari Gopalakrishna

Warped five-dimensional models: phenomenological status and experimental prospects

Warped five-dimensional models, based on the original Randall–Sundrum geometry, have been extended beyond their initial purpose of resolving the gauge hierarchy problem. Over the past decade, various ingredients have been added to their basic structure in order to provide natural and predictive models of flavor and also to address existing constraints from precision data. In this paper, we examine the theoretical and experimental status of realistic models that accommodate current data, while addressing the hierarchy and flavor puzzles of the Standard Model. We also discuss the prospects for future discovery of the TeV-scale Kaluza–Klein states that are predicted to emerge in these models, and outline some of the challenges that the detection of such particles pose for experiments at the Large Hadron Collider.

Survey of vector-like fermion extensions of the Standard Model and their phenomenological implications

A bstractWith the renewed interest in vector-like fermion extensions of the Standard Model, we present here a study of multiple vector-like theories and their phenomenological implications. Our focus is mostly on minimal flavor conserving theories that couple the vector-like fermions to the SM gauge fields and mix only weakly with SM fermions so as to avoid flavor problems. We present calculations for precision electroweak and vector-like state decays, which are needed to investigate compatibility with currently known data. We investigate the impact of vector-like fermions on Higgs boson production and decay, including loop contributions, in a wide variety of vector-like extensions and their parameter spaces.

Higgs boson decays to four fermions through an abelian hidden sector

We consider a generic Abelian hidden sector that couples to the standard model only through gauge-invariant renormalizable operators. This allows the exotic Higgs boson to mix with the standard model Higgs boson, and the exotic Abelian gauge boson to mix with the standard model hypercharge gauge boson. One immediate consequence of spontaneous breaking of the hidden sector gauge group is the possible decay of the lightest Higgs boson into four fermions through intermediate exotic gauge bosons. We study the implications of this decay for Higgs boson phenomenology at the Fermilab Tevatron Collider and the CERN Large Hadron Collider. Our emphasis is on the four-lepton final state.

Right-handed Sneutrinos as Nonthermal Dark Matter

When the minimal supersymmetric standard model is augmented by three right-handed neutrino superfields, one generically predicts that the neutrinos acquire Majorana masses. We postulate that all supersymmetry (SUSY) breaking masses as well as the Majorana masses of the right-handed neutrinos are around the electroweak scale and, motivated by the smallness of neutrino masses, assume that the lightest supersymmetric particle (LSP) is an almost-pure right-handed sneutrino. We discuss the conditions under which this LSP is a successful dark matter candidate. In general, such an LSP has to be nonthermal in order not to overclose the universe, and we find the conditions under which this is indeed the case by comparing the Hubble expansion rate with the rates of the relevant thermalizing processes, including self-annihilation and co-annihilation with other SUSY and standard model particles.

LHC signatures of warped-space vectorlike quarks

A bstractWe study the LHC signatures of TeV scale vectorlike quarks b′, t′ and χ with electromagnetic charges −1/3, 2/3 and 5/3 that appear in many beyond the standard model (BSM) extensions. We consider warped extra-dimensional models and analyze the phenomenology of such vectorlike quarks that are the custodial partners of third generation quarks. In addition to the usually studied pair-production channels which depend on the strong coupling, we put equal emphasis on single production channels that depend on electroweak couplings and on electroweak symmetry breaking induced mixing effects between the heavy vectorlike quarks and standard model quarks. We identify new promising gg-initiated pair and single production channels and find the luminosity required for discovering these states at the LHC. For these channels, we propose a cut that allows one to extract the relevant electroweak couplings. Although the motivation is from warped models, we present many of our results model-independently.

Dark matter and Higgs boson collider implications of fermions in an Abelian-gauged hidden sector

Abstract We add fermions to an Abelian-gauged hidden sector. We show that the lightest can be the dark matter with the right thermal relic abundance, and discovery is within reach of upcoming dark matter detectors. We also show that these fermions change Higgs boson phenomenology at the Large Hadron Collider (LHC), and in particular could induce a large invisible width to the lightest Higgs boson state. Such an invisibly decaying Higgs boson can be discovered with good significance in the vector boson fusion channel at the LHC.

Stop decay into right-handed sneutrino LSP at hadron colliders

Right-handed neutrinos offer us the possibility of accommodating neutrino masses. In a supersymmetric model, this implies the existence of right-handed sneutrinos. Right-handed sneutrinos are expected to be as light as other supersymmetric particles if the neutrinos are Dirac fermions or if the lepton-number breaking scale is at (or below) the supersymmetry (SUSY) breaking scale, assumed to be around the electroweak scale. Depending on the mechanism of SUSY breaking, the lightest right-handed sneutrino may be the lightest supersymmetric particle (LSP). We consider the unique hadron collider signatures of a weak scale right-handed sneutrino LSP, assuming R-parity conservation. For concreteness, we concentrate on stop pair-production and decay at the Tevatron and the Large Hadron Collider, and briefly comment on the production and decay of other supersymmetric particles.

Big signals of Little Randall―Sundrum models

Abstract We examine signals at the Large Hadron Collider (LHC) of Kaluza–Klein modes, in volume-truncated “Little Randall–Sundrum” (LRS) models of flavor, characterized by 5D cutoff scales M 5 that are small compared to the 4D Planck mass M P ∼ 10 19 GeV . In particular, for the phenomenologically viable choice M 5 ∼ 10 4 TeV , the discovery of a 2 (3)-TeV “Little” Z ′ at the LHC requires about 1 (4) fb −1 at s = 10 (14) TeV, in the clean di-lepton channel . Our results highlight the possibility of probing interesting values of M 5 , starting with the early LHC data. With M 5 ∼ 10 4 TeV , discovering the second KK mode Z ″ , at about 4.3 TeV, requires O ( 100 ) fb − 1 at s = 14 TeV , providing a probe of the warped nature of the bulk that is encoded in the mass ratio of the first two KK modes, at design luminosity. By comparison, discovering a 3-TeV Z ′ of the Planck-weak hierarchy models (with M 5 ∼ M P ), in any channel , would require upwards of O ( 300 ) fb − 1 at s = 14 TeV . We also point out that discovery prospects markedly improve for Little KK gluons as well, but the challenging reconstruction of their t t ¯ decay products may hamper their early discovery.

Low-Energy Neutrino Majorana Phases and Charged-Lepton Electric Dipole Moments

(Dated: February 2, 2008)If the neutrinos are Majorana fermions, there are at least three new, potentially observable CP-oddphases that parameterize CP-invariance violating phenomena. We currently have no informationregarding any of them and know that two out of the three, the so-called Majorana phases, are veryhard to access experimentally. Here, we discuss the effect of Majorana phases on charged-leptonelectric dipole moments (EDM), and explicitly show that neutrino Majorana phases induce EDMseven in the absence of other sources of CP-invariance violation. We also argue that while theMajorana neutrino contribution to EDMs is tiny, there is one generic ultraviolet completion to thestandard model plus massive Majorana neutrinos — the standard model plus a triplet Higgs boson— that leads to significantly enhanced contributions which are still proportional to the low-energyneutrino Majorana phases. If this particular scenario is realized in nature, it seems possible, at leastin principle, to measure the Majorana phases by precisely measuring charged-lepton EDMs.I. INTRODUCTION

Extra neutral scalars with vectorlike fermions at the LHC

Many theories beyond the standard model (BSM) contain new CP-odd and CP-even neutral scalars