Narrow Results

Unlike all the previous studies on proton decay which worked only in the Minimal Supersymmetric Standard Model, we study the effects of the dimension-5 operators in the Supersymmetric Unification Models with left–right symmetry at low energy. The full chargino sector is used for the analysis. Dressing of the RRRR operators by the SU(2)_R wino makes important or even dominant contributions to the proton decay modes into charged leptons.

The excess events reported by the ATLAS collaboration in the WZ-final state, and by the CMS collaboration in the e${}^{+}$e${}^{-}$jj, Wh and jj-final states, may be induced by the decays of a heavy boson W${}^{\prime }$ in the 1.8–2 TeV mass range, here modeled via the larger local group SU(2)_L × SU(2)_R × U(1)${}_{B-L}$ in a nonlinear dynamical Higgs scenario. The W${}^{\prime }$-production cross-section at the 13 TeV LHC is around 700–1200 fb. This framework also predicts a heavy Z${}^{\prime }$ boson with a mass of 2.5–4 TeV, and some decay channels testable in the LHC Run II. We determine the cross-section times branching fractions for the dijet, dilepton and top-pair Z${}^{\prime }$-decay channels at the 13 TeV LHC around 2.3, 7.1, 70.2 fb, respectively, for M${}_{Z^{\prime }}$ = 2.5 TeV, while one/two orders of magnitude smaller for the dijet/dilepton and top-pair modes at M${}_{Z^{\prime }}$ = 4 TeV. Nonzero contributions from the effective operators, and the underlying Higgs sector of the model, will induce sizeable enhancement in the W${}^{+}$W${}^{-}$ and Zh-final states that could be probed in the future LHC Run II.

In collaboration with Jogesh Pati, Abdus Salam challenged the chiral gauge nature of the Standard Model by paving the road towards the left-right symmetric electro-weak theory. I describe here the logical and historical construction of this theory, by emphasising the pioneering and key role it played for neutrino mass. I show that it is a self-contained and predictive model with the Higgs origin of Majorana neutrino mass, in complete analogy with the SM situation regarding charged fermions.

I reflect on some of the basic aspects of present day Beyond the Standard Model particle physics, focusing mostly on the issues of naturalness, in particular on the so-called hierarchy problem. To all of us, physics as natural science emerged with Galileo and Newton, and led to centuries of unparalleled success in explaining and often predicting new phenomena of nature. I argue here that the long-standing obsession with the hierarchy problem as a guiding principle for the future of our field has had the tragic consequence of deviating high energy physics from its origins as natural philosophy, and turning it into a philosophy of naturalness.

In this paper, we give a brief account of the history of neutrino, and how that most aloof of all particles shaped our search for a theory of fundamental interactions ever since it was theoretically proposed. We introduce the necessary concepts and phenomena in a nontechnical language aimed at a physicist with some basic knowledge of quantum mechanics. In showing that neutrino mass could be the door to new physics beyond the Standard Model, we emphasize the need to frame the issue in the context of a complete theory, with testable predictions accessible to present and near future experiments. We argue in favor of the minimal Left–Right symmetric theory as the strongest candidate for such theory, connecting neutrino mass with parity breakdown in nature. This is the theory that led originally to neutrino mass and the seesaw mechanism behind its smallness, but even more important, the theory that sheds light on a fundamental question that touches us all: the symmetry between left and right.

Connes's ideeas provide a framework in which the Higgs fields appear as components of a generalized gauge connection. This leads to theories with predictive power in the Higgs sector (for example, one can show that spontaneous symmetry breaking arises in a natural way). We describe a realization of this formalism where the underlying manifold is a two-sheeted space-time. We show how our particular formulation works for the case of the Standard Model and the Left-Right Symmetric Model.

A four-dimensional heterotic string model of free fermionic construction is presented wherein mirror symmetry breaking between observable and hidden sector gauge groups occurs in spite of mirror symmetry between observable and hidden sector worldsheet fermion boundary conditions. The differentiation is invoked by an asymmetry in GSO projections necessarily resulting from the symmetry of the free fermionic boundary conditions. In the specific examples shown, an expected nonchiral Pati–Salam mirror universe model is transformed into a chiral model with enhanced hidden sector gauge symmetry and reduced observable sector gauge symmetry: [SU(4)_C ⊗ SU(2)_L ⊗ SU(2)_R]^O ⊗ [SU(4)_C ⊗ SU(2)_L ⊗ SU(2)_R]^H, is necessarily transformed into a chiral [SU(4)_C ⊗ SU(2)_L]^O ⊗ [SO(10) ⊗ SU(2)_R]^H model because of an unavoidable asymmetry in GSO projections.

We present a general formalism based on the framework of noncommutative geometry, suitable to the study of the standard model of electroweak interactions, as well as that of more general gauge theories. Left- and right-handed chiral fields are assigned to two different sheets of space–time (a discretized version of Kaluza–Klein theory). Scalar Higgs fields find themselves treated on the same footing as the gauge fields, resulting in spontaneous symmetry breaking in a natural and predictable way. As a first test, we apply the formalism to the Standard Model, where one can predict the Higgs mass and the top Yukawa coupling. The results obtained for this case are similar with results obtained in alternate formulations. We then study the left–right symmetric model, where we show that this framework imposes interesting constraints on the type and coefficients of terms appearing in the Higgs potential.

We study various possible Supersymmetric Left–Right (SUSYLR) models with Higgs doublets carrying B-L charge ±1: with single step symmetry breaking down to the Minimal Supersymmetric Standard Model (MSSM) as well as multistep symmetry breaking. Single step symmetry breaking can be achieved with the minimal field content of just Higgs doublet and bidoublets whereas multistep symmetry breaking can be realized only at the cost of including additional Higgs superfields. However, going beyond the minimal field content comes up with the exciting possibility of TeV scale intermediate symmetry which can have important implications in the ongoing collider experiments. We show that spontaneous parity violation can be achieved naturally in all these models and R-parity is spontaneously broken by the vacuum expectation value of B-L odd Higgs doublets. The tiny neutrino mass can arise from a double seesaw mechanism in the presence of additional singlet or triplet fermions. We show that gauge coupling unification can be achieved in these models with the possibility of TeV scale intermediate symmetry in some specific nonminimal versions.

I argue that LHC may shed light on the nature of neutrino mass through the probe of the seesaw mechanism. The smoking gun signature is lepton number violation through the production of same sign lepton pairs, a collider analogy of the neutrinoless double beta decay. I discuss this in the context of left–right symmetric theories, which led originally to neutrino mass and the seesaw mechanism. A W_R gauge boson with a mass in a few TeV region could easily dominate neutrinoless double beta decay, and its discovery at LHC would have spectacular signatures of parity restoration and lepton number violation. Moreover, LHC can measure the masses of the right-handed neutrinos and the right-handed leptonic mixing matrix, which could in turn be used to predict the rates for neutrinoless double decay and lepton flavor violating violating processes. The LR scale at the LHC energies offers great hope of observing these low energy processes in the present and upcoming experiments.

This paper provides a brief overview for nonexperts of some of the highlights in the development of the theory of weak interactions during the past century.

The Baryon–Lepton difference (B-L) is increasingly emerging as a possible new symmetry of the weak interactions of quarks and leptons as a way to understand the small neutrino masses. There is the possibility that current and future searches at colliders and in low energy rare processes may provide evidence for this symmetry. This paper provides a brief overview of the early developments that led to B-L as a possible symmetry beyond the standard model, and also discusses some recent developments.

We study the possibility of explaining the recently reported 750 GeV di-photon excess at LHC within the framework of a left–right symmetric model. The 750 GeV neutral scalar in the model is dominantly an admixture of neutral components of scalar bidoublets with a tiny fraction of neutral scalar triplet. Incorporating $SU(2)$ septuplet scalar pairs into the model, we enhance the partial decay width of the 750 GeV neutral scalar into di-photons through charged septuplet components in loop while keeping the neutral septuplet components as subdominant dark matter candidates. The model also predicts the decay width of the 750 GeV scalar to be around 36 GeV to be either confirmed or ruled out by future LHC data. The requirement of producing the correct di-photon signal automatically keeps the septuplet dark matter abundance subdominant in agreement with bounds from direct and indirect detection experiments. We then briefly discuss different possibilities to account for the remaining dark matter component of the Universe in terms of other particle candidates whose stability arises either due to remnant discrete symmetry after spontaneous breaking of $U{(1)}_{B-L}$ or due to high $SU(2)$-dimension forbidding their decay into lighter particles.

We present a detailed discussion on neutrinoless double beta decay $(0\nu \beta \beta )$ within left–right symmetric models based on the gauge symmetry of type $SU{(2)}_L\times SU{(2)}_R\times U{(1)}_{B-L}$ as well as $SU{(3)}_L\times SU{(3)}_R\times U{(1)}_X$ where fermion masses including that of neutrinos are generated through a universal seesaw mechanism. We find that one or more of the right-handed neutrinos could be as light as a few keV if left–right symmetry breaking occurs in the range of a few TeV to 100 TeV. With such light right-handed neutrinos, we perform a detailed study of new physics contributions to $0\nu \beta \beta$ and constrain the model parameters from the latest experimental bound on such a rare decay process. We find that the new physics contribution to $0\nu \beta \beta$ in such a scenario, particularly the heavy–light neutrino mixing diagrams, can individually saturate the existing experimental bounds, but their contributions to total $0\nu \beta \beta$ half-life cancel each other due to unitarity of the total $6\times 6$ mass matrix. The effective contribution to half-life therefore, arises from the purely left and purely right neutrino and gauge boson mediated diagrams. We find that the parameter space saturating the $0\nu \beta \beta$ bounds remains allowed from the latest experimental bounds on charged lepton flavor violating decays like $\mu \to e\gamma$. We finally include the bounds from cosmology and supernova to constrain the parameter space of the model.

I present a personal vision of what is essential in the field of neutrino mass, both from the point of view of what has been achieved and what could lie ahead. In the process, I offer a logical, theoretical and phenomenological rationale behind my opinions. It is however neither a summary of what was discussed in the conference nor a party-line viewpoint, rather an attempt to dig through the enormous body of material in our field in order to uncover a common unifying thread. The main focus is on the search for a predictive and self-contained theory of the origin and nature of neutrino mass, with the conclusion that the left–right symmetric model plays a special role in this aspect.

We explore an $A_4$ flavor-based left–right symmetric model where neutrino masses and mixing are explained through linear seesaw mechanism. Taking $A_4$ flavor symmetry and left–right gauge symmetry together and adding a low-scale seesaw mechanism gives the advantage of (a) getting diagonal structures for both charged lepton mass matrix and Dirac neutrino mass matrix, (b) simpler relations among neutrino observables in the model and (c) large light-heavy neutrino mixing which gives dominant contributions to various lepton flavor violating decays like $\mu \to e\gamma$, $\mu \to eee$, $\mu \to e$. By saturating the experimental bounds on these decays we derive constraints on input model parameters and study analytically as well as numerically the correlation among model parameters and their dependence on experimentally determined neutrino parameters like ${𝜃}_{12}$, ${𝜃}_{23}$, ${𝜃}_{13}$, $\mathrm{\Delta }{m}_{\mathrm{\text{atm}}}^2$, $\mathrm{\Delta }{m}_{\mathrm{\text{sol}}}^2$. We also study CP-violation via Jarlskog invariants and show extra contributions to CP-violating effects that the model generates due to unitarity violation in the neutrino sector.

We readdress the issue of strong CP violation both in the Standard Model (SM) and in the Minimal Left–Right (LR) Symmetric Model and try to clear the confusion that seems to still pervade the field. We argue that the smallness of strong CP violation, while harmless and basically decoupled from the rest of the physics in the SM, in the context of LR symmetry provides a blessing by helping to narrow down the parameter space of the theory and connecting apparently the uncorrelated physical quantities. In particular, in the context of left–right symmetry being parity, it either points to the suppression of leptonic CP violation noticed before, or leads to relatively light right-handed neutrinos, potentially accessible at the next hadron collider. The latter, more natural in view of complex quark Yukawa couplings, goes hand in hand with the smallness of lepton flavor violation and enhances the possibility of observing neutrinoless double-beta decay.

Abdus Salam challenged the chiral gauge nature of the Standard Model by paving the road towards the Left-Right symmetric electro-weak theory. I describe here the logical and historical construction of this theory, by emphasising the pioneering and key role it played for neutrino mass. I show that it is a self-contained and predictive model with the Higgs origin of neutrino mass, in complete analogy with the SM situation regarding charged fermions.