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Subhaditya Bhattacharya

Publications and source records attributed to Subhaditya Bhattacharya.

At least 73 records · Page 4Linked to original sources
arXiv2016

Dimension Seven Operators in Standard Model with Right handed Neutrinos

In this article we consider the Standard Model extended by a number of (light) right-handed neutrinos, and assume the presence of some heavy physics that cannot be directly produced, but can be probed by its low-energy effective interactions. Within this scenario, we obtain all the gauge-invariant dimension-seven effective operators, and determine whether each of the operators can be generated at tree-level by the heavy physics, or whether it is necessarily loop generated. We then use the tree-generated operators, including those containing right-handed neutrinos, to put limits on the scale of new physics $ Λ$ using low-energy measurements. We also study the production of same-sign dileptons at the Large Hadron Collider (LHC) and determine the constraints on the heavy physics that can be derived form existing data, as well as the reach in probing $ Λ$ expected from future runs of this collider.

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arXiv2016

Minimal Vector-like leptonic Dark Matter and Signatures at the LHC

We propose a minimal vector-like leptonic dark matter (DM) with renormalisable interaction in a beyond the Standard Model (SM) scenario, where the SM is augmented with a vector-like doublet and a singlet leptons. The additional fermions are odd under a discrete $Z_2$ symmetry, while the rest of the SM particles are singlets, and thus providing stability to the DM. In this scenario, we show that, the DM emerges as an admixture of the neutral component of the vector-like doublet and the singlet leptons. The singlet-doublet mixing ($\sin θ$) plays a crucial role in yielding the correct relic density as well as in obtaining null direct DM search results through an interplay of interactions via $Z$ and Higgs mediation. The mixing is also strongly constrained from the invisible Z and Higgs decay width. We found that the correct relic abundance of DM can be obtained in a large region of parameter space for DM-mass larger than $ M_Z/2$ and $\sin θ\le 0.1$. The details of model phenomenology with collider signatures at the Large hadron Collider (LHC) are discussed. In particular, we show that for $\sin θ\le 0.01$, the charged companion of the DM can give rise to an observable displaced vertex signature, marking a significant departure from other fermionic DM scenarios, while keeping the relic abundance intact.

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arXiv2016

Unifying the flavor origin of dark matter with leptonic nonzero $θ_{13}$

We propose a flavor symmetric approach to unify the origin of dark matter (DM) with the non-zero $θ_{13}$ in the lepton sector. In this framework, breaking of a $U(1)$ flavor symmetry to a remnant $Z_2$ ensures the stability of the DM and gives rise to a modification to the existing $A_4$-based tri-bimaximal neutrino mixing to attain the required non-zero $\sin θ_{13}$. This results in a range of Higgs portal coupling of the DM which can be potentially accessible at various ongoing and future direct and collider search experiments.

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arXiv2016

750 GeV Di-photon excess at CERN LHC from a dark sector assisted scalar decay

We present a simple extension of the standard model (SM) to explain the diphoton excess, reported by CMS and ATLAS at CERN LHC. The SM is extended by a dark sector including a vector-like lepton doublet and a singlet of zero electromagnetic charge, which are odd under a $Z_2$ symmetry. These vector-like leptons assist the additional scalar, different from SM Higgs, to decay to di-photons of invariant mass around 750 GeV and thus explaining the excess observed at LHC. The admixture of neutral component of the vector-like lepton doublet and singlet constitute the dark matter component of the Universe. We show the relevant parameter space for correct relic density and direct detection of dark matter.

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arXiv2014

Constraints on Two-Component Dark Matter

We study 'Higgs Portal' 2-component Dark Matter scenario with two interacting cold Dark Matter (DM) candidates: a neutral scalar singlet ($φ$) and a neutral Majorana fermion ($ν$). The relic abundance of $φ$ and $ν$ is found assuming thermal DM production and solving the Boltzmann equations. We scan over the parameter space of the model to determine regions consistent with the WMAP data for DM relic abundance and the XENON100 direct detection limits for the DM-nucleus cross section.

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arXiv2014

Two-Component Dark Matter

We study an extension of the Standard Model (SM) with two interacting cold Dark Matter (DM) candidates: a neutral Majorana fermion ($ν$) and a neutral scalar singlet ($φ$). The scalar $φ$ interacts with the SM through the "Higgs portal" coupling while $ν$ at the tree level interacts only with $φ$ through Yukawa interactions. The relic abundance of $ν$ and $φ$ is found by solving the Boltzmann equations numerically; for the case $m_ν> m_φ$ we also derive a reliable approximate analytical solution. Effects of the interaction between the two DM components are discussed. A scan over the parameter space is performed to determine the regions consistent with the WMAP data for DM relic abundance, and with the XENON100 direct detection limits for the DM-nucleus cross section. We find that although a large region of the parameter space is allowed by the WMAP constraints, the XENON100 data severely restricts the parameter space. Taking into account only amplitudes generated at the tree level one finds three allowed regions for the scalar mass: $m_φ\sim 62.5$ GeV (corresponding to the vicinity of the Higgs boson resonance responsible for $φφ$ annihilation into SM particles), $m_φ\simeq 130-140$ GeV and $m_φ\gesim 3 $TeV. 1-loop induced $ν$-nucleon scattering has been also calculated and discussed. A possibility of DM direct detection by the CREST-II experiment was considered.

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arXiv2013

Non-universal SUGRA at LHC: Prospects and Discovery Potential

We explore supersymmetry (SUSY) parameter space with non-universal high scale parameters in gravity mediated SUSY breaking (SUGRA) scenario that accommodates a Higgs mass of (125$\pm$2) GeV while satisfying cold dark matter relic density and other low energy constraints. We indicate a few benchmark points consistent with different dark matter annihilation processes where third family squarks are lighter than the first two as a requirement to keep the Higgs mass within the limit. We show that bottom rich and leptonic final states have better reach in such parameter space points and is the most likely scenario to discover SUSY at the upcoming run of LHC with center-of-mass energy 14 TeV.

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arXiv2013

Supersymmetric Left-Right Model of Radiative Neutrino Mass with Multipartite Dark Matter

The unifiable supersymmetric left-right model where the neutral fermion $n$ in the $SU(2)_R$ doublet $(n,e)_R$ is a dark-matter candidate, is shown to have the requisite particle content for the neutrino $ν$ in the $SU(2)_L$ doublet $(ν,e)_L$ to acquire a small radiative Majorana mass from dark matter, i.e. scotogenic from the Greek "scotos" meaning darkness. As a result, there are at least three coexisting stable dark-matter particles with different interactions. We study their possible phenomenological impact on present and future experiments.

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arXiv2013

Pionic Dark Matter

We study a phenomenological model where the lightest dark matter (DM) particles are the pseudo-Goldstone excitations associated with a spontaneously broken symmetry, and transforming linearly with respect to an unbroken group H. For definiteness we take H = SU(N) and assume the Goldstone particles are bosons; in parallel with QCD, we refer to these particles as dark-matter pions. This scenario is in contrast to the common assumption that DM fields transform linearly under the full symmetry of the model. We illustrate the formalism by treating in detail the case of H = SU(2), in particular we calculate all the interactions relevant for the Boltzmann equations, which we solve numerically; we also derive approximate analytic solutions and show their consistency with the numerical results. We then compare the results with the constraints derived from the cold DM and direct detection experiments and derive the corresponding restrictions on the model parameters.

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arXiv2013

Radiative Scaling Neutrino Mass with $A_4$ Symmetry

A new idea for neutrino mass was proposed recently, where its smallness is not due to the seesaw mechanism, i.e. not inversely proportional to some large mass scale. It comes from a one-loop mechanism with dark matter in the loop consisting of singlet Majorana fermions $N_i$ with masses of order 10 keV and neutrino masses are scaled down from them by factors of about $10^{-5}$. We discuss how this model may be implemented with the non-Abelian discrete symmetry $A_4$ for neutrino mixing, and consider the phenomenology of $N_i$ as well as the extra scalar doublet $(η^+,η^0)$.

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arXiv2012

Effective Lagrangian approach to neutrinoless double beta decay and neutrino masses

Neutrinoless double beta ($0νββ$) decay can in general produce electrons of either chirality, in contrast with the minimal Standard Model (SM) extension with only the addition of the Weinberg operator, which predicts two left-handed electrons in the final state. We classify the lepton number violating (LNV) effective operators with two leptons of either chirality but no quarks, ordered according to the magnitude of their contribution to \znbb decay. We point out that, for each of the three chirality assignments, $e_Le_L, e_Le_R$ and $e_Re_R$, there is only one LNV operator of the corresponding type to lowest order, and these have dimensions 5, 7 and 9, respectively. Neutrino masses are always induced by these extra operators but can be delayed to one or two loops, depending on the number of RH leptons entering in the operator. Then, the comparison of the $0νββ$ decay rate and neutrino masses should indicate the effective scenario at work, which confronted with the LHC searches should also eventually decide on the specific model elected by nature. We also list the SM additions generating these operators upon integration of the heavy modes, and discuss simple realistic examples of renormalizable theories for each case.

hep-ph↗
arXiv2012

A realistic model of neutrino masses with a large neutrinoless double beta decay rate

The minimal Standard Model extension with the Weinberg operator does accommodate the observed neutrino masses and mixing, but predicts a neutrinoless double beta ($0νββ$) decay rate proportional to the effective electron neutrino mass, which can be then arbitrarily small within present experimental limits. However, in general $0νββ$ decay can have an independent origin and be near its present experimental bound; whereas neutrino masses are generated radiatively, contributing negligibly to $0νββ$ decay. We provide a realization of this scenario in a simple, well defined and testable model, with potential LHC effects and calculable neutrino masses, whose two-loop expression we derive exactly. We also discuss the connection of this model to others that have appeared in the literature, and remark on the significant differences that result from various choices of quantum number assignments and symmetry assumptions. In this type of models lepton flavor violating rates are also preferred to be relatively large, at the reach of foreseen experiments. Interestingly enough, in our model this stands for a large third mixing angle, $\sin^2θ_{13} \gtrsim 0.008$, when $μ\rightarrow eee$ is required to lie below its present experimental limit.

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arXiv2011

Dark Vector-Gauge-Boson Model

A model based on SU(3)_C X SU(2)_L X U(1)_Y X SU(2)_N has recently been proposed, where the SU(2)_N vector gauge bosons are neutral, so that a vector dark-matter candidate is possible and constrained by data to be less than about 1 TeV. We explore further implications of this model, including a detailed study of its Higgs sector. We improve on its dark-matter phenomenology, as well as its discovery reach at the LHC (Large Hadron Collider).

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arXiv2011

Signatures of supersymmetry with non-universal Higgs mass at the Large Hadron Collider

We discuss large non-universality in the Higgs sector at high scale in supersymmetric theories, in the context of the Large Hadron Collider (LHC). In particular, we note that if ${m_{H_u}}^2-{m_{H_d}}^2$ is large and negative ($\simeq 10^6 {\rm ~GeV^2}$) at high scale, the lighter slepton mass eigenstates at the electroweak scale are mostly left chiral, in contrast to a minimal supergravity (mSUGRA) scenario. We use this feature to distinguish between non-universal Higgs masses (NUHM) and mSUGRA by two methods. First, we study final states with same-sign ditaus. We find that an asymmetry parameter reflecting the polarization of the taus provides a notable distinction. In addition, we study a charge asymmetry in the jet-lepton invariant mass distribution, arising from decay chains of left-chiral squarks leading to leptons of the first two families, which sets apart an NUHM scenario of the above kind.

hep-ph↗
arXiv2011

Supersymmetry Signals at the LHC under the most favorable SUGRA scenario

Given that it will take quite some time for the Large Hadron Collider (LHC) to reach its desired luminosity, it is important to investigate the most favorable scenario in which supersymmetry (SUSY) may be discovered at the early runs at the LHC. Our aim in this work is to find such a scenario within the gravity mediated SUSY breaking (SUGRA) framework and select a class of final states that warrant a discovery at the very early runs of the LHC. It turns out that such a situation can be associated with a scenario where gluinos are sufficiently light and so are the third generation scalars while the first two family scalars are heavy. We find that this can be achieved from a high-scale set-up with scalar mass non-universality in the third family and gaugino mass non-universality with $M_3 < M_1, M_2$. We show that the final state channels which are most favorable in such a region of parameter space are $4b+{E_{T}}\!\!\!\!/$, $4b+\ell+{E_{T}}\!\!\!\!/$ and $2b+2\ell+{E_{T}}\!\!\!\!/$. We also justify our claim by comparing the results with a minimal supergravity (mSUGRA) scenario with similar gluino mass.

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arXiv2010

Non-universal scalar mass scenario with Higgs funnel region of SUSY dark matter: a signal-based analysis for the Large Hadron Collider

We perform a multilepton channel analysis in the context of the Large Hadron Collider (LHC) for Wilkinson Microwave Anisotropy Probe (WMAP) compatible points in a model with non-universal scalar masses, which admits a Higgs funnel region of supersymmetry dark matter even for a small $\tanβ$. In addition to two and three-lepton final states, four-lepton events, too, are shown to be useful for this purpose. We also compare the collider signatures in similar channels for WMAP compatible points in the minimal supergravity (mSUGRA) framework with similar gluino masses. Some definite features of such non-universal scenario emerge from the analysis.

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arXiv2010

Gaugino mass non-universality in an SO(10) supersymmetric Grand Unified Theory: low-energy spectra and collider signals

We derive the non-universal gaugino mass ratios in a supergravity (SUGRA) framework where the Higgs superfields belong to the non-singlet representations {\bf 54} and {\bf 770} in a SO(10) Grand Unified Theory (GUT). We evaluate the ratios for the phenomenologically viable intermediate breaking chain $SU(4)_C \times SU(2)_L \times SU(2)_R (G_{422})$. After a full calculation of the gaugino mass ratios, noting some errors in the earlier calculation for {\bf 54}, we obtain, using the renormalisation group equations (RGE), interesting low scale phenomenology of such breaking patterns. Here, we assume the breaking of the SO(10) GUT group to the intermediate gauge group and that to the Standard Model (SM) take place at the GUT scale itself. We also study the collider signatures in multilepton channels at the Large Hadron Collider (LHC) for some selected benchmark points allowed by the cold dark matter relic density constraint provided by the WMAP data and compare these results with the minimal supergravity (mSUGRA) framework with similar gluino masses indicating their distinguishability in this regard.

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