arXiv subjects

Joydeep Chakrabortty

Publications and source records attributed to Joydeep Chakrabortty.

At least 55 records · Page 3Linked to original sources

arXiv2017

Cut and compute: Quick cascades with multiple amplitudes

In an earlier paper we have proposed a novel method to compute the decay width for a general $1\to n$ cascade decay where the propagators are off-shell and may be of different spins. Here, we extend our algorithm to accommodate those decays that are mediated by more than one such cascades. This generalizes our prescription and widens its applicability. We compute the three- and four-body toy decay chains where identical final states appear through different cascades. Here, we also provide the algorithm to calculate the interference terms. For four-body decays we discuss both symmetric and asymmetric cascades, providing the expressions for the detailed phase space structure in each case. We find that the results obtained with this algorithm have a very impressive agreement with those from standard softwares using a sophisticated Monte Carlo based phase space integration.

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arXiv2016

Di-photon resonance around 750 GeV: shedding light on the theory underneath

Both the ATLAS and CMS collaborations have recently observed an excess in the di-photon invariant mass distribution in the vicinity of 750 GeV with a local significance of $\sim3σ$. In this article we try to investigate this excess in the context of a minimal simplified framework assuming effective interactions of the hinted resonance with photons and gluons. We scrutinise the consistency of this observation with possible accompanying yet hitherto unseen signatures of this resonance. Subsequently, we try to probe the nature of new particles, e.g., spin, electric charge and number of colour, etc., that could remain instrumental to explain this excess through loop-mediation.

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arXiv2016

Reconciling $(g-2)_μ$ and charged lepton flavour violating processes through a doubly charged scalar

In this work, we investigate the phenomenological consequences of a doubly charged scalar which may belong to different uncoloured scalar multiplets. This doubly charged scalar couples to the charged leptons as well as gauge bosons, which we parametrize in a model independent way. Restricting ourselves in the regime of conserved charged-parity (CP), we assume only a few non-zero Yukawa couplings ($y_{μ\ell}$, where $\ell=e,μ,τ$) between the doubly charged scalar and the charged leptons. Our choices allow the doubly charged scalar to impinge low-energy processes like anomalous magnetic moment of muon and a few possible charged lepton flavour violating (CLFV) processes. These same Yukawa couplings are also instrumental in producing same-sign dilepton signatures at the LHC. In this article we examine the impact of individual contributions from the diagonal and off-diagonal Yukawa couplings in the light of muon $(g-2)$ excess. Subsequently, we use the derived information to inquire the possible CLFV processes and finally the collider signals from the decay of a doubly charged scalar. Our simplified analyses, depending on the mass of doubly charged scalar, provide a good estimate for the magnitude of the concerned Yukawa couplings. Our findings would appear resourceful to test the phenomenological significance of a doubly charged scalar by using complementary information from muon $(g-2)$, CLFV and the collider experiments.

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arXiv2016

Novel method to deal with off-shell particles in cascade decays

We propose a novel algorithm to compute the width of any generic $n$-body decay involving multiple off-shell particles having zero and non-zero spins. Starting from a toy example, we show the computations for three different processes that contain spin-0, $\frac{1}{2}$, and 1 off-shell particles. We have checked that our results match with the existing results at the analytical level. This proposal can be automatized and should be useful to compute the phase space for long cascade decays, without any Monte Carlo sampling.

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arXiv2015

Non-universal Gaugino mass models under the lamppost of muon (g-2)

In unified $\mathcal{N}=1$ supergravity scenario the gaugino masses can be non-universal. The patterns of these non-universalities are dictated by the vacuum expectation values of non-singlet chiral super-fields in visible sector. Here, we have analysed the model independent correlations among the gaugino masses with an aim to explain the $[1÷3]σ$ excess of muon (g-2) ($Δa_μ$). We have also encapsulated the interconnections among other low and high scale parameters, compatible with the collider constraints, Higgs mass, relic density and flavour data. We have noted that the existing non-universal models are not capable enough to explain $Δa_μ$ at $[1÷2]σ$ level. In the process, we have also shown the impact of recent limits from the searches for disappearing track and long lived charged particles at the LHC. These are the most stringent limits so far ruling out a large parameter space allowed by other constraints. We have also performed model guided analysis where gaugino masses are linear combination of contributions coming from singlet and non-singlet chiral super-fields. Here, a new mixing parameter has been introduced. Following the earlier methodology, we have been able to constrain this mixing parameter and pin down the promising models on this notion.

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arXiv2014

Higgs vacuum stability and inflationary dynamics after BICEP2 and PLANCK dust polarisation data

If the recent detection of $B-$mode polarization of the Cosmic Microwave Background by BICEP2 observations, withstand the test of time after the release of recent PLANCK dust polarisation data, then it would surprisingly put the inflationary scale near Grand Unification scale if one considers single-field inflationary models. On the other hand, Large Hadron Collider has observed the elusive Higgs particle whose presently observed mass can lead to electroweak vacuum instability at high scale $(\sim{\mathcal O}(10^{10})$ GeV). In this article, we seek for a simple particle physics model which can simultaneously keep the vacuum of the theory stable and yield high-scale inflation successfully. To serve our purpose, we extend the Standard Model of particle physics with a $U(1)_{B-L}$ gauged symmetry which spontaneously breaks down just above the inflationary scale. Such a scenario provides a constrained parameter space where both the issues of vacuum stability and high-scale inflation can be successfully accommodated. The threshold effect on the Higgs quartic coupling due to the presence of the heavy inflaton field plays an important role in keeping the electroweak vacuum stable. Furthermore, this scenario is also capable of reheating the universe at the end of inflation. Though the issues of Dark Matter and Dark Energy, which dominate the late-time evolution of our universe, cannot be addressed within this framework, this model successfully describes the early universe dynamics according to the Big Bang model.

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arXiv2014

Copositive Criteria and Boundedness of the Scalar Potential

To understand physics beyond the Standard Model (SM) it is important to have the precise knowledge of Higgs boson and top quark masses as well as strong coupling. Recently discovered new boson which is likely to be the SM Higgs with mass 123-127 GeV has a submissive impact on the stability of the new physics beyond standard model (BSM). The beyond standard model scenarios that include many scalar fields posses scalar potential with many quartic couplings. Due to the complicated structures of such scalar potentials it is indeed difficult to adjudge the stability of the vacuum. Thus one needs to formulate a proper prescription for computing the vacuum stability criteria. In this paper we have used the idea of copositive matrices to deduce the conditions that guarantee the boundedness of the scalar potential. We have discussed the basic idea behind the copositivity and then used that to determine the vacuum stability criteria for the Left-Right symmetric models with doublet, and triplet scalars and Type-II seesaw. As this idea is based on the strong mathematical arguments it helps to compute simple and unique stability criteria embracing the maximum allowed parameter space.

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arXiv2014

Constraining a class of B-L extended models from vacuum stability and perturbativity

The precise knowledge of the Standard Model Higgs boson and top quark masses and couplings are crucial to understand the physics beyond it. SM like Higgs boson having mass in the range of 123-127 GeV squeezes the parameters for beyond standard model. In recent LHC era many TeV scale neutrino mass models have earned much attention as they pose many interesting phenomenological aspects. We have contemplated B-L extended models which are theoretically well motivated and phenomenologically interesting, and successfully explain neutrino mass generation. In this article we have analysed the detailed structures of the scalar potentials for such models. We have computed the criteria which guarantee that vacuum is bounded from below in all directions. Along with that triviality and perturbativity bounds are also necessitated. Incorporating all such effects we constrain the parameters of such models by performing their renormalization group evolutions.

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arXiv2014

Non-universal gaugino mass GUT models in the light of dark matter and LHC constraints

We perform a comprehensive study of $SU(5)$, $SO(10)$ and $E(6)$ supersymmetric GUT models where the gaugino masses are generated through the F-term breaking vacuum expectation values of the non-singlet scalar fields. In these models the gauginos are non-universal at the GUT scale unlike in the mSUGRA scenario. We discuss the properties of the LSP which is stable and a viable candidate for cold dark matter. We look for the GUT scale parameter space that leads to the the lightest SM like Higgs mass in the range of 122-127 GeV compatible with the observations at ATLAS and CMS, the relic density in the allowed range of WMAP-PLANCK and compatible with other constraints from colliders and direct detection experiments. We scan universal scalar ($m_0^G$), trilinear coupling $A_0$ and $SU(3)_C$ gaugino mass ($M_3^G$) as the independent free parameters for these models. Based on the gaugino mass ratios at the GUT scale, we classify 25 SUSY GUT models and find that of these only 13 models satisfy the dark matter and collider constraints. Out of these 13 models there is only one model where there is a sizeable SUSY contribution to muon $(g-2)$.

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arXiv2013

Generation of Neutrino mass from new physics at TeV scale and Multi-lepton Signatures at the LHC

In this paper we consider generation of naturally small neutrino masses from a dimension-7 operator. Such a term can arise in presence of a scalar quadruplet and a pair of vector-like fermion triplets and enables one to obtain small neutrino masses through TeV scale linear seesaw mechanism. We study the phenomenology of the charged scalars of this model, in particular, the multi-lepton signatures at the Large Hadron Collider. Of special importance are the presence of the same-sign-tri-lepton signatures originating from the triply-charged scalars. The Standard Model background for such processes is small and hence this is considered as smoking gun signal of new physics. We also looked for events with three, four, five and six-leptons which have negligible contamination from the Standard Model. We further point out the spectacular lepton flavour violating four-lepton signal which can be the hallmark for these type of models. We also compute the added contributions in the rate for the Standard Model Higgs decaying to two photons via the charged scalars in this model.

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arXiv2012

Lower Limits on $μ\to e γ$ from new Measurements on $U_{e3}$

New data on the lepton mixing angle $θ_{13}$ imply that the $eμ$ element of the matrix $m_νm_ν^\dagger$, where $m_ν$ is the neutrino Majorana mass matrix, cannot vanish. This implies a lower limit on lepton flavor violating processes in the $eμ$ sector in a variety of frameworks, including Higgs triplet models or the concept of minimal flavor violation in the lepton sector. We illustrate this for the branching ratio of $μ\to e γ$ in the type II seesaw mechanism, in which a Higgs triplet is responsible for neutrino mass and also mediates lepton flavor violation. We also discuss processes like $μ\to e\bar{e}e$ and $μ\to e$ conversion in nuclei. Since these processes have sensitivity on the individual entries of $m_ν$, their rates can still be vanishingly small.

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arXiv2012

Constraints on TeV scale Majorana neutrino phenomenology from the Vacuum Stability of the Higgs

The vacuum stability condition of the Standard Model Higgs potential with mass in the range of 124-127 GeV puts an upper bound on the Dirac mass of the neutrinos. We study this constraint with the right-handed neutrino masses upto TeV scale. The heavy neutrinos contribute to $ΔL=2$ processes like neutrinoless double beta decay and same-sign-dilepton production in the colliders. The vacuum stability criterion also restricts the light-heavy neutrino mixing and constrains the branching ratio of lepton flavour violating process, like $μ\to e γ$ mediated by the heavy neutrinos. We show that neutrinoless double beta decay with a lifetime $\sim 10^{25}$ years can be observed if the the lightest heavy neutrino mass is $<$ 4.5 TeV. We show that the vacuum stability condition and the experimental bound on $μ\rightarrow e γ$ together put a constrain on heavy neutrino mass $M_R >$ 3.3 TeV. Finally we show that the observation of same-sign-dileptons (SSD) associated with jets at the LHC needs much larger luminosity than available at present. We have estimated the possible maximum cross-section for this process at the LHC and show that with an integrated luminosity 100 $fb^{-1}$ it may be possible to observe the SSD signals as long as $M_R <$ 400 GeV.

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arXiv2012

Neutrinoless double-$β$ decay in TeV scale Left-Right symmetric models

In this paper we study in detail the neutrinoless double beta decay in left-right symmetric models with right-handed gauge bosons at TeV scale which is within the presently accessible reach of colliders. We discuss the different diagrams that can contribute to this process and identify the dominant ones for the case where the right-handed neutrino is also at the TeV scale. We calculate the contribution to the effective mass governing neutrinoless double beta decay assuming type-I, and type-II dominance and discuss what are the changes in the effective mass due to the additional contributions. We also discuss the effect of the recent Daya-Bay and RENO measurements on $\sin^2θ_{13}$ on the effective mass in different scenarios.

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arXiv2012

Resonant Leptogenesis with nonholomorphic R-Parity violation and LHC Phenomenology

In R-parity violating supersymmetric models both leptogenesis and the correct neutrino masses are hard to achieve together. The presence of certain soft nonholomorphic R-parity violating terms helps to resolve this problem. We consider a scenario where the lightest and the second-lightest neutralino are nearly degenerate in mass and enough CP-asymmetry can be produced through resonant leptogenesis. In this model, the lighter chargino and the lightest neutralino are highly degenerate. We have relatively lighter gauginos which can be produced at the LHC leading to heavily ionizing charged tracks. At the same time this model can also generate the correct neutrino mass scale. Thus our scenario is phenomenologically rich and testable at colliders.

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arXiv2010

An SO(10) model with adjoint fermions for double seesaw neutrino masses

An $SO(10)$ model where the $10_H$ and $120_H$ representations are used for generating fermion masses is quite predictive, though due to the absence of $SU(2)_{L,R}$ triplet/singlet fields it cannot give rise to neutrino masses through the usual type-I or type-II seesaw mechanisms. In this paper for neutrino masses we propose an extension of such an $SO(10)$ model by adding fermions in the adjoint representation (${45}_F$) and a symmetry breaking scalar $\bar{16}_H$. The $\bar{16}_H$ couples the adjoint fermions to the standard fermions in ${16}_F$ and induces neutrino masses through the `double seesaw' mechanism. In order to enhance the predictivity of the model we impose $μ-τ$ flavour symmetry on the Yukawa matrices for $10_H$ and $\bar{16}_H$ whereas for the $120_H$ it is assumed to be antisymmetric. We discuss the conditions that the mass matrices must obey so that the model can reproduce the tri-bimaximal mixing pattern.

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arXiv2010

Multi-photon signal in supersymmetry comprising non-pointing photon(s) at the LHC

We study a distinct supersymmetric signal of multi-photons in association with jets and missing transverse energy. At least one of these photons has the origin in displaced vertex, thus delayed and non-pointing. We consider a supersymmetric scenario in which the gravitino is the lightest supersymmetric particle (LSP) (with a mass $\sim 1~{keV}$) and the lightest neutralino is the next-to-lightest supersymmetric particle (NLSP). The NLSP decays dominantly into a photon and a gravitino within the detector with a decay length ranging from $cτ_{\tildeχ}\sim$ 50-100 cm. In addition, we assume that the second lightest neutralino and the lightest neutralino are nearly degenerate and this leads to a prompt radiative decay of the next-to-lightest neutralino into a photon and a lightest neutralino with a large branching ratio. Such degenerate neutralinos can be realised in various representations of the $SU(5)$, $SO(10)$, and $E(6)$ Grand Unified Theories (GUTs). The non-pointing photons can be reconstructed at the electromagnetic calorimeter of the ATLAS inner-detector, which have been designed with good timing and directional resolution. We find that with a centre-of-mass energy $E_{cm}=14 ~{TeV}$ at an integrated luminosity of 100 $fb^{-1}$ one may see evidence of hundreds of tri-photon events and a few four-photons events at the LHC, in addition to several thousands di-photon events. We also predict the event rates even at the early phase of LHC run.

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arXiv2010

Dimension-5 operators and the unification condition in SO(10) and E(6)

Effective dimension-5 operators which modify the gauge kinetic term in Grand Unified Theories may arise as a consequence of quantum gravity or string compactification. We exhaustively calculate the modification of the gauge unification condition due to such operators for all viable rank-preserving symmetry breakings of SO(10) and E(6) grand unified models.

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arXiv2010

Type I and $new$ seesaw in left-right symmetric theories

We extend the Type I seesaw and suggest a $new$ seesaw mechanism to generate neutrino masses within the left-right symmetric theories where parity is spontaneously broken. We construct a next to minimal left-right symmetric model where neutrino masses are determined irrespective of the B-L breaking scale and call it the $new$ seesaw mechanism. In this scenario B-L scale can be very low. This makes B-L gauge boson and the quasi-Dirac $heavy$ leptons very light. These TeV scale particles could have large impact on lepton flavor and CP violating processes. We also shed light on the phenomenological aspects of the model within the reach of the LHC.

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