arXiv subjects

S. Nandi

Publications and source records attributed to S. Nandi.

At least 163 records · Page 9Linked to original sources

arXiv2002

Implications of a Massless Neutralino for Neutrino Physics

We consider the phenomenological implications of a soft SUSY breaking term BN at the TeV scale (here B is the U(1)_Y gaugino and N is the right-handed neutrino field). In models with a massless (or nearly massless) neutralino, such a term will give rise through the see-saw mechanism to new contributions to the mass matrix of the light neutrinos. We treat the massless neutralino as an (almost) sterile neutrino and find that its mass depends on the square of the soft SUSY breaking scale, with interesting consequences for neutrino physics. We also show that, although it requires fine-tuning, a massless neutralino in the MSSM or NMSSM is not experimentally excluded. The implications of this scenario for neutrino physics are discussed.

hep-ph↗

arXiv2002

Orbifold Breaking of 3-3-1 Model

We study the five dimensional SU(3)_c \times SU(3)_L \times U(1)_X gauge theory (3-3-1 Model) in which the gauge symmetry is broken down through orbifold compactification to four dimensions. The new model has several distinguishing features compared to the usual four dimensional model. We develop the formalism, and give expressions for the gauge boson masses, mixings and their couplings to the fermions. Phenomenology of the model is briefly studied.

hep-ph↗

arXiv2002

New Signal for Universal Extra Dimensions

In the universal extra dimensions (UED) scenario, the tree level masses of the first level Kaluza-Klein (KK) excitations of Standard Model particles are essentially degenerate. Radiative corrections will, however, lift this degeneracy, allowing the first level excitations to decay to the lightest KK particle (LKP), which is the gamma^*. KK number conservation implies that the LKP is stable. Then, since the SM particles radiated during these decays are rather soft, the observation of KK excitations production and decay in collider experiments will be quite difficult. We propose to add to this model KK number violating interactions mediated by gravity, which allow the gamma^* to decay to a photon and a KK graviton. For a variety a models and a large range of parameters, these decays will occur within the detector. Thus, pair production of KK excitations will give rise to a striking collider signal, consisting of two hard photons plus large missing energy (due to escaping gravitons). We evaluate the cross-section for these signals at the Tevatron and LHC, and derive the reach of these colliders in the search for universal extra dimensions.

hep-ph↗

arXiv2002

Enhancement of the gluonic couplings of the Standard Model Higgs Boson through mixing

The mixing of the Standard Model Higgs boson with a ttbar scalar bound state (arising from a new stronger-than-QCD interaction) can enhance the Higgs boson's coupling to two gluons. It is possible that the two-gluon decay mode of a light Higgs boson may even dominate. This alters the Higgs boson search strategy at Tevatron Run 2 in a significant way. We present a simple model in which such a scenario may be realized. The modifications to the Higgs search signal, as well as further experimental tests of this scenario are also discussed.

hep-ph↗

arXiv2002

Orbifold Breaking of Left-Right Gauge Symmetry

We study the five dimensional left-right symmetric gauge theory in which the gauge symmetry is broken down through orbifold compactification to four dimensions. The right handed charged gauge boson, W_R is a Kaluza-Klein excited state in this model, and unlike the usual left-right symmetric model in four dimension, there is no mixing in the charged sector. We develop the formalism and give expressions for the gauge boson masses and mixings and their couplings to the fermions. Phenomenology of the model is briefly studied to put constraints on the right-handed gauge boson masses and the orbifold compactification scale.

hep-ph↗

arXiv2002

Magnetic Monopoles with Wilson loops on a 5D Orbifold

We discuss magnetic monopoles in gauge theories with Wilson loops on orbifolds. We present a simple example in 5 dimensions with the fifth dimension compactified on an S^1/Z_2 orbifold. The Wilson loop in this SO(3) example replaces the adjoint Higgs scalar (needed to break SO(3) to U(1)) in the well-known 't Hooft - Polyakov construction. Our solution is a magnetic monopole string with finite energy, and length equal to the size of the extra dimension.

hep-th↗

arXiv2001

Collider Implications of Kaluza-Klein Excitations of the Gluons

We consider an asymmetric string compactification scenario in which the SM gauge bosons can propagate into one TeV$^{-1}$-size extra compact dimension. These gauge bosons have associated KK excitations that present additional contributions to the SM processes. We calculate the effects that the KK excitations of the gluons, $g^{\star}$'s, have on multijet final state production in proton-proton collisions at the Large Hadron Collider energy. In the case of dijet final states with very high $p_{{}_T}$, the KK signal due to the exchanges of the $g^{\star}$'s is several factors greater than the SM background for compactification scales as high as about 7 TeV. The high-$p_{{}_T}$ effect is not as dramatic for the direct production of a single on-shell $g^{\star}$, which subsequently decays into $q$-$\bar{q}$ pairs, where the KK signal significantly exceeds the SM three-jet background for compactification scales up to about 3 TeV. We also present our results for the four-jet final state signal from the direct production of two on-shell $g^{\star}$'s.

hep-ph↗

arXiv2001

Collider Implications of Kaluza-Klein Excitations of the Electroweak Gauge Bosons

It is possible for the SM fields to propagate into one or more large extra compact dimensions. Such fields have associated KK excitations that produce additional contributions to the SM processes. We calculate the effects that these KK excitations have on cross sections for $e^{+}e^{-}$ collider processes in a model in which the SM gauge bosons, and perhaps the Higgs, can propagate into one TeV$^{-1}$-size extra compact dimension. Specifically, we consider muon pair production, Bhabha scattering, dijet production, Higgs production, and single-photon production. At prospective high energy $e^{+}e^{-}$ colliders, we find significant deviations from the SM cross sections: For example, a 600 GeV collider produces a 20% reduction in the cross section for muon pair production as compared to the SM for a compactification scale of 3.5 TeV. The effect is much smaller at LEP2 energies, where the compactification scale must be lower than 2 TeV in order to produce a 6% effect.

hep-ph↗

arXiv2000

Low-Scale and Gauge-Mediated Supersymmetry Breaking at the Fermilab Tevatron Run II

The prospects for discovering and studying signals of low-scale supersymmetry breaking models at the Tevatron Run II and beyond are explored. These models include gauge-mediated supersymmetry breaking as the most compelling and concrete realization, but more generally are distinguished by the presence of a nearly massless Goldstino as the lightest supersymmetric particle. The next-lightest supersymmetric particle(s) (NLSP) decays to its partner and the Goldstino. Depending on the supersymmetry breaking scale, these decays can occur promptly or on a scale comparable to or larger than the size of a detector. A systematic analysis based on a classification in terms of the identity of the NLSP and its decay length is presented. The various scenarios are discussed in terms of signatures and possible event selection criteria. The Run II and beyond discovery and exclusion reaches, including the effects of background, are detailed for the most compelling cases. In addition to standard event selection criteria based on missing energy and photons, leptons, jets, taus, tagged b-jets, or reconstructed Z-bosons, more exotic signals of metastable NLSPs such as displaced photons, large negative impact parameter tracks, kink tracks, both opposite and same-sign highly ionizing tracks, time of flight measurements, charge-changing tracks, charge-exchange tracks, and same-sign di-top events are investigated. The interesting possibility of observing a Higgs boson signal in events that are efficiently "tagged" by the unique signatures of low-scale supersymmetry breaking is also considered.

hep-ph↗

arXiv1999

Neutrino Masses and Oscillations in Models with Large Extra Dimensions

We discuss the profile of neutrino masses and mixings in models with large extra dimensions when right handed neutrinos are present in the branes along with the usual standard model particles. In these models, string scale must be bigger than $10^{8}$ GeV to have desired properties for the neutrinos at low energies. The lightest neutrino mass is zero and there is oscillations to sterile neutrinos that are different from other models with the bulk neutrino.

hep-ph↗

arXiv1999

Natural fermion mass hierarchy and new signals for the Higgs boson

We suggest a novel approach towards resolving the fermion mass hierarchy problem within the framework of the Standard Model. It is shown that the observed masses and mixings can be explained with order one couplings using successive higher dimensional operators involving the SM Higgs doublet field. This scenario predicts flavor-dependent enhancement in the the Higgs boson coupling to the fermions (by a factor of 3 to the b-quark and τand by a factor of 5 to μrelative to the SM). It also predicts flavor changing \bar{t}ch^0 interaction with a strength comparable to that of \bar{b}bh^0. This opens up a new discovery channel for the Higgs boson at the upgraded Tevatron and the LHC through t -> ch^0 or h^0 -> \bar{t}c + \bar{c}t. Additional tests of the framework include D^0-\bar{D^0} mixing which is predicted to be near the current experimental limit and a host of new phenomena associated with flavor physics at the TeV scale.

hep-ph↗

arXiv1999

The Higgs Boson Mass in Gauge-Mediated Supersymmetry-Breaking Models with Generalized Messenger Sectors

The lighter neutral scalar Higgs mass is examined in gauge-mediated supersymmetry-breaking models in which the messenger sector responsible for SUSY breaking is allowed to involve more general sets of $SU(3) \otimes SU(2) \otimes U(1)$ multiplets than those contained in the SU(5) $5 + \bar{5}$ or $10 + \bar{10}$ multiplets. The largest mass for the lighter scalar Higgs is found to be 132 GeV when the breaking parameter $Λ$ is taken to be 100 TeV\@. Thus the predictions for the lightest Higgs mass can be tested at the upgraded Tevatron runs for these general classes of GMSB models.

hep-ph↗

arXiv1998

Tau Signals at the Tevatron for Gauge Mediated Supersymmetry Breaking

We consider the phenomenology of GMSB models where the lighter stau is the next to lightest supersymmetric particle. In this situation, the dominant signals for supersymmetry are events with two or three high p_T tau leptons accompanied by large missing transverse energy. We find that the inclusive two tau jets signature could be observable at the Tevatron's Run II, while the inclusive three tau jets signature could be important at Run III.

hep-ph↗

arXiv1998

Gauge Mediated Supersymmetry Signals at the Tevatron Involving Tau Leptons

We consider the phenomenology of GMSB models where the lighter stau is the next to lightest supersymmetric particle. In this situation the dominant signals for supersymmetry at the Tevatron are events where two or three high p_T tau leptons accompanied by large missing transverse energy are produced. This leads to signatures that are very different from the photonic signals for GMSB (where the lightest neutralino is the NLSP) and the dilepton and trilepton signals in the usual supergravity models (involving e and/or mu only) that have been investigated extensively. We find that the inclusive 2 tau-jet signature could be observable at the Tevatron Run II, while the inclusive 3 tau-jet signature could be important at Run III.

hep-ph↗

arXiv1998

Collider Production of Spin 3/2 Particles

We consider the production of spin 3/2 quarks in hadron-hadron and photon-photon colliders. The cross sections at LHC energy is large enough to observe such exotic quarks up to a mass of a few TeV.

hep-ph↗

arXiv1998

Multilepton signatures of gauge mediated SUSY breaking at LEPII

In the framework of gauge-mediated supersymmetry breaking models pair production of the lightest neutralinos, scalar leptons, or charginos at LEPII gives rise to interesting signals involving multilepton final states and missing energy. In the parameter space where the scalar tau, $\tildeτ_1$, is the next-to-lightest supersymmetric particle, we identify three interesting regions, which give rise to distinctly different final states: (i) 2 $τ$-leptons plus missing energy, (ii) 4 charged leptons plus missing energy where in some regions all four are $τ$-leptons, or (iii) six charged leptons, of which four are $τ$-leptons and the other two are electrons or muons, plus missing energy. We study in detail the size of these regions in the parameter space of gauge-mediated models and give cross section contours in these regions for various LEPII energies. We also discuss the possibility of chargino-pair production at LEPII.

hep-ph↗

arXiv1998

A four-neutrino mixing scheme for observed neutrino data

It has been observed that simultaneous explanation of the solar and atmospheric neutrino deficits and the reported evidence for $ν_μ\to ν_{e}$ oscillation from the Los Alamos Liquid Scintillator Detector (LSND) requires at least one extra neutrino species in addition to the three known ones. The extra neutrino must be sterile with respect to the known weak interactions. We present a new mass matrix for these four neutrinos in which the LSND effect and the atmospheric neutrino deficit are governed by only one parameter. We investigate the phenomenological implications of such a mass matrix ansatz and suggest possible ways to understand it in gauge theories.

hep-ph↗

arXiv1997

Sparticle Spectroscopy and Phenomenology in a New Class of Gauge Mediated Supersymmetry Breaking Models

Recently, a proposal (by R.N.M. and S.N.) was made for a new class of gauge mediated supersymmetry breaking (GMSB) models where the standard model gauge group is embedded into the gauge group $SU(2)_L\times U(1)_{I_{3R}}\times U(1)_{B-L}$ (or $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$) at the supersymmetry breaking scale $Λ$. Supersymmetry breaking is transmitted to the visible sector via the same fields that are responsible for gauge symmetry breaking rather than by vector-like quarks and leptons as in the conventional GMSB models. These models have a number of attractive properties such as exact R-parity conservation, non-vanishing neutrino masses and a solution to the SUSYCP (and strong CP) problem. In this paper, we present the detailed sparticle spectroscopy and phenomenological implications of the various models of this class that embody the general spirit of our previous work but use a larger variety of messenger fields. A distinct characteristic of this class of models is that unlike the conventional GMSB ones, the lightest neutralino is always the NLSP leading to photonic events in the colliders.

hep-ph↗