arXiv subjects

Kirtiman Ghosh

Publications and source records attributed to Kirtiman Ghosh.

At least 37 records · Page 2Linked to original sources

arXiv2021

Type-III see-saw: Search for triplet fermions in final states with multiple leptons and fat-jets at 13 TeV LHC

The type-III see-saw model holding out a riveting rationale for the minuscule neutrino masses caters for a well-to-do phenomenology at the Large Hadron Collider (LHC). Several searches targetting the triplet fermions have been performed at the LHC. Not only are the signals for the leptonic final states considered in these searches suppressed by the branching fractions of the Standard Model (SM) bosons, but they are also beset with considerably large SM backgrounds. Thus, these searches are deemed not to be sensitive enough in probing the triplet fermions much heavier than 1 TeV. To this end, we perform a search for the triplet fermions in final states with multiple leptons and fat-jets that are cleaner than the usual LHC searches and allow kinematic reconstruction of the triplets. After performing a systematic and comprehensive analysis with seven distinct final states, we project the required luminosities for both $3σ$ and $5σ$ discoveries of the triplet fermions as a function of their mass. The triplet fermions with mass as large as 1265 (1380) and 1480 (1600) GeV could be discovered with $5σ$ ($3σ$) significance at 500 and 3000 fb$^{-1}$ luminosities, respectively.

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arXiv2021

Revisiting Type-II see-saw: Present Limits and Future Prospects at LHC

The type-II see-saw mechanism based on the annexation of the Standard Model by weak gauge triplet scalar field proffers a natural explanation for the very minuteness of neutrino masses. Noting that the phenomenology for the non-degenerate triplet Higgs spectrum is substantially contrasting than that for the degenerate one, we perform a comprehensive study for an extensive model parameter space parametrised by the triplet scalar vacuum expectation value (VEV), the mass-splitting between the triplet-like doubly and singly charged scalars and the mass of the doubly charged scalar. Considering all Drell-Yan production mechanisms for the triplet-like scalars and taking into account the all-encompassing complexity of their decays, we derive the most stringent 95% CL lower limits on the mass of the doubly charged scalar for a vast model parameter space by implementing already existing direct collider searches by CMS and ATLAS. These estimated limits are beyond those from the existing LHC searches by approximately 50-230 GeV. However, we also find that a specific region of the parameter space is not constrained by the LHC searches. Then, we forecast future limits by extending an ATLAS search at high-luminosity, and we propose a search strategy that yields improved limits for a part of the parameter space.

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arXiv2021

Type-III Seesaw: Phenomenological Implications of the Information Lost in Decoupling from High-Energy to Low-Energy

The type-III seesaw seems to explain the very minuteness of neutrino masses readily and naturally. The high-energy see-saw theories usually involve a larger number of effective parametres than the physical and measurable parametres appearing in the low-energy neutrino phenomenology. Casas-Ibarra parametrisation facilitates to encode the information lost in integrating the heavy fermions out in an arbitrary complex orthogonal matrix. The CMS collaboration has already searched for triplet fermions in the type-III seesaw model with only one generation of triplet fermion flavour democratically decaying into SM leptons. We reinterpret this CMS search in the context of a realistic type-III seesaw model with two or three generations of triplet fermions, and endeavour to comprehend the implications of the foregoing matrix on the $95\%$ CL lower limit on the mass of the triplet fermions. We also discuss the phenomenological implication of the aforesaid matrix in view of charged lepton flavour violating observables and displaced decays of the triplet fermions at colliders.

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arXiv2020

The Genuine Type-V Seesaw Model: Phenomenological Introduction

We study a model which generates Majorana neutrino masses at tree-level via low-energy effective operator with mass-dimension-9. Introduction of such a higher dimensional operator brings down the lepton number violating mass scale to TeV making such model potentially testable at present or near future colliders. This model possesses several new $SU(2)_L$ fermionic multiplets, in particular, three generations of triplets, quadruplets and quintuplets, and thus a rich phenomenology at the LHC. As the lepton flavour violation arises very naturally in such setup, we put constraints on the Yukawa couplings and heavy fermion masses from the current experimental bounds on lepton flavour violating processes. We also obtain 95% CL lower bounds on the masses of the triplets, quadruplets and quintuplets using a recent CMS search for multilepton final states with 137 inverse femtobarn integrated luminosity data at 13 TeV center of mass energy. The possibility that the heavy fermions could be long-lived leaving disappearing charge track signatures or displaced vertex at the future colliders like LHeC, FCC-he, MATHUSLA, etc. is also discussed.

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arXiv2020

Minimal and non-minimal Universal Extra Dimension models in the light of LHC data at 13 TeV

Universal Extra Dimension (UED) is a well-motivated and well-studied scenario. One of the main motivations is the presence of a dark matter (DM) candidate namely, the lightest level-1 Kaluza-Klein (KK) particle (LKP), in the particle spectrum of UED. The minimal version of UED (mUED) scenario is highly predictive with only two parameters namely, the radius of compactification and cut-off scale, to determine the phenomenology. Therefore, stringent constraint results from the WMAP/PLANCK measurement of DM relic density (RD) of the universe. The production and decays of level-1 quarks and gluons in UED scenarios give rise to multijet final states at the Large Hadron Collider (LHC) experiment. We study the ATLAS search for multijet plus missing transverse energy signatures at the LHC with 13 TeV center of mass energy and 139 inverse femtobarn integrated luminosity. In view of the fact that the DM RD allowed part of mUED parameter-space has already been ruled out by the ATLAS multijet search, we move on to a less restricted version of UED namely, the non-minimal UED (nmUED), with non-vanishing boundary-localized terms (BLTs). The presence of BLTs significantly alters the dark matter as well as the collider phenomenology of nmUED. We obtain stringent bounds on the BLT parameters from the ATLAS multijet plus missing transverse energy search.

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arXiv2020

Multi-charged TeV scale scalars and fermions in the framework of a radiative seesaw model

Explaining the tiny neutrino masses and non-zero mixings have been one of the key motivations for going beyond the framework of the Standard Model (SM). We discuss a collider testable model for generating neutrino masses and mixings via radiative seesaw mechanism. That the model does not require any additional symmetry to forbid tree-level seesaws makes its collider phenomenology interesting. The model includes multi-charged fermions/scalars at the TeV scale to realize the Weinberg operator at 1-loop level. After deriving the constraints on the model parameters resulting from the neutrino oscillation data as well as from the upper bound on the absolute neutrino mass scale, we discuss the production, decay and resulting collider signatures of these TeV scale fermions/scalars at the Large Hadron Collider (LHC). We consider both Drell-Yan and photoproduction. The bounds from the neutrino data indicate the possible presence of a long-lived multi-charged particle (MCP) in this model. We obtain bounds on these long-lived MCP masses from the ATLAS search for abnormally large ionization signature. When the TeV scale fermions/scalars undergo prompt decay, we focus on the 4-lepton final states and obtain bounds from different ATLAS 4-lepton searches. We also propose a 4-lepton event selection criteria designed to enhance the signal to background ratio in the context of this model.

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arXiv2019

Same-sign Multilepton Signatures of an $SU(2)_R$ Quintuplet at the LHC

We study in detail the collider signatures of an $SU(2)_R$ fermionic quintuplet in the framework of left-right symmetric model in the context of the 13 TeV LHC. Apart from giving a viable dark matter candidate ($χ^0$), this model provides unique collider imprints in the form of same-sign multileptons through the decays of multi-charged components of the quintuplet. In particular, we consider the scenario where the quintuplet carries $(B - L) = 4$ charge, allowing for the presence of high charge-multiplicity particles with relatively larger mass differences among them compared to $(B - L)$ = 0 or 2. In this paper, we mainly focus on the same-sign n-lepton signatures (nSSL). We show that with an integrated luminosity of 500 $fb^{-1}$, the mass of the neutral component, $M_{χ^0} \leq 480~(800)$ GeV can be excluded at 95\% CL in the 2SSL (3SSL) channel after imposing several selection criteria. We also show that a $5σ$ discovery is also achievable if $M_{χ^0} \leq 390~(750)$ GeV in the 2SSL (3SSL) channel with 1000 $fb^{-1}$ integrated luminosity.

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arXiv2018

Universal Extra Dimension models with gravity mediated decays after LHC Run II data

In the 'fat-brane' realization of Universal Extra Dimension (UED) models, the gravity mediated decays of Kaluza-Klein (KK) excitations of the Standard Model (SM) particles offer interesting collider signals. Colored level-1 KK-particles (quarks $q^1$ and/or gluons $g^{1}$) are pair-produced at the colliders due to conserved KK-parity. These particles, then, cascade decay into lighter level-1 KK-particle in association with one or more SM particles until producing lightest KK particle (LKP). The gravity mediation allows LKP to decay into photon or $Z$-boson plus gravity excitation, hence resulting in di-photon/$ZZ$/$Zγ$ plus missing transverse energy signatures at collider experiments. Alternatively, pair-produced level-1 KK quarks/gluons may directly decay into the corresponding SM quark/gluon and a gravity excitation resulting in di-jet plus missing transverse energy signal. The ATLAS Collaboration has recently communicated the results for di-photon and multi-jet plus missing transverse energy searches with $36.1$ inverse-femtobarn of integrated luminosity at $13$ TeV center-of-mass energy. No significant excess of events above the SM expectation was observed in both searches. We constrain the 'fat-brane' UED model parameters, namely the fundamental Planck mass $M_{D}$ and the size of small extra dimensions $R$, in the light of above-mentioned ATLAS searches.

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arXiv2018

Sub-TeV Quintuplet Minimal Dark Matter with Left-Right Symmetry

A detailed study of a fermionic quintuplet dark matter in a left-right symmetric scenario is performed in this article. The minimal quintuplet dark matter model is highly constrained from the WMAP dark matter relic density (RD) data. To elevate this constraint, an extra singlet scalar is introduced. It introduces a host of new annihilation and co-annihilation channels for the dark matter, allowing even sub-TeV masses. The phenomenology of this singlet scalar is studied in detail in the context of the Large Hadron Collider (LHC) experiment. The production and decay of this singlet scalar at the LHC give rise to interesting resonant di-Higgs or diphoton final states. We also constrain the RD allowed parameter space of this model in light of the ATLAS bounds on the resonant di-Higgs and diphoton cross-sections.

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arXiv2018

Neutrino Mass Generation at TeV Scale and New Physics Signatures from Charged Higgs at the LHC for Photon Initiated Processes

We consider the collider phenomenology of a simple extension of the Standard Model (SM), which consists of an EW isospin $3/2$ scalar, $Δ$ and a pair of EW isospin $1$ vector like fermions, $Σ$ and $\barΣ$, responsible for generating tiny neutrino mass via the effective dimension seven operator. This scalar quadruplet with hypercharge Y = 3 has a plethora of implications at the collider experiments. Its signatures at TeV scale colliders are expected to be seen, if the quadruplet masses are not too far above the electroweak symmetry breaking scale. In this article, we study the phenomenology of multi-charged quadruplet scalars. In particular, we study the multi-lepton signatures at the Large Hadron Collider (LHC) experiment, arising from the production and decays of triply and doubly charged scalars. We studied Drell-Yan (DY) pair production as well as pair production of the charged scalars via photon-photon fusion. For doubly and triply charged scalars, photon fusion contributes significantly for large scalar masses. We also studied LHC constraints on the masses of doubly charged scalars in this model. We derive a lower mass limit of 725 GeV on doubly charged quadruplet scalar.

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arXiv2018

Probing sterile neutrinos in the framework of inverse seesaw mechanism through leptoquark productions

We consider an extension of the Standard Model (SM) augmented by two neutral singlet fermions per generation and a leptoquark. In order to generate the light neutrino masses and mixing, we incorporate inverse seesaw mechanism. The right handed neutrino production in this model is significantly larger than the conventional inverse seesaw scenario. We analyze the different collider signatures of this model and find that the final states associated with three or more leptons, multi jet and at least one b-tagged and (or) $τ$-tagged jet can probe larger RH neutrino mass scale. We have also proposed a same-sign dilepton signal region associated with multiple jets and missing energy that can be used to distinguish the the present scenario from the usual inverse seesaw extended SM.

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arXiv2018

Non-Standard Neutrino Interactions : Obviating Oscillation Experiments

Searching for non-standard neutrino interactions, as a means for discovering physics beyond the Standard Model, has one of the key goals of dedicated neutrino experiments, current and future. We demonstrate here that much of the parameter space accessible to such experiments is already ruled out by the RUN II data of the Large Hadron Collider experiment.

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arXiv2016

LHC diphoton excess in a left-right symmetric model with minimal dark matter

We construct a model containing a viable dark matter candidate, in the framework of left-right symmetry, which can explain both width and cross-section of the observed 750 GeV diphoton excess at the LHC. We introduce a fermion quintuplet whose neutral component can be a possible dark matter candidate, whereas the charged components enhance the loop induced coupling of a 750 GeV singlet scalar with a pair of photons. We study the photo-production of the singlet scalar and its various decay modes, successfully addressing both the ATLAS and the CMS diphoton excess.

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arXiv2016

Neutrino Mass Generation and 750 GeV Diphoton excess via photon-photon fusion at the Large Hadron Collider

We propose a new model for explaining the diphoton excess observed at the LHC. The uniqueness of our model is that this excess is being produced by photon-photon induced sub-process. The model includes an isospin $3/2$ scalar multiplet (having triply, doubly and singly charged scalars) at the electroweak (EW) scale to explain the observed neutrino masses via the effective dimension $7$ operator. The model also include a singlet scalar which is the di-photon resonance. The loop induced coupling of the singlet scalar with a pair of photons is enhanced due to contributions from multi-charged EW-scale scalars (component of isospin $3/2$ multiplet) in the loop. We examine the production of the singlet scalar at the LHC via photon-photon fusion sub-process and its subsequent decay to di-photon. We found that a large part of parameter space of the present model can explain the observed excess of di-photon events at the LHC. We also study the other decay modes of the singlet scalar and the ensuing predictions can be tested in the upcoming runs at the LHC.

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arXiv2016

Bounds on Universal Extra Dimension from LHC Run I and II data

We discuss the collider bounds on minimal Universal Extra Dimension (mUED) model from LHC Run-I and II data. The phenomenology of mUED is determined by only two parameters namely, the compactification scale ($R^{-1}$) of the extra dimension and cutoff scale ($Λ$) of the theory. The characteristic feature of mUED is the occurrence of nearly degenerate mass spectrum for the Kaluza-Klein (KK) particles and hence, soft leptons, soft jets at the collider experiments. The degree of degeneracy of KK-mass spectrum crucially depends on $Λ$. The strongest direct bound on $R^{-1}$ ($\sim $950 GeV for large $Λ$) arises from a search for a pair of soft dimuons at the Large Hadron Collider (LHC) experiment with 8 TeV center-of-mass energy and $20~{\rm fb}^{-1}$ integrated luminosity. However, for small $Λ$ and hence, small splitting within the first KK-level, the bounds from the dimuon channel is rather weak. On the other hand, the discovery of 126 GeV Higgs boson demands small $Λ$ to prevent the scalar potential form being unbounded from below. We discuss LHC monojet searches as a probe of low $Λ$ region of mUED parameter space. We also compute bounds on the mUED parameter space from 13 TeV multijets results.

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arXiv2016

The LHC Diphoton excess at 750 GeV in the framework of the Constrained Minimal Supersymmetric Standard Model

We examine the observed diphoton excess at 750 GeV in the framework of constrained Minimal Supersymmetric Standard Model (cMSSM). The consistency of cMSSM with the discovery of a 125 GeV Higgs boson and PLANCK/WMAP data for dark matter relic density (RD) demands large mixing in the scalar top (stop) sector and consequently, the existence of a light stop quasi-degenerate with the lightest neutralino. Small decay width due to kinematic and loop suppression of such a stop allows for the existence of stop-antistop bound states (stoponia). Identifying the particular part of cMSSM parameter space consistent with Higgs boson mass as well as RD, we study stoponia as the source of the diphoton excess.

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arXiv2015

Superworld without Supersymmetry

It is a possibility that the superworld (supersymmetric partners of our world) does exist without supersymmetry. The two worlds are being distinguished by an unbroken discrete $Z_2$ symmetry (similar to R-parity in supersymmetry). We lose the solution to the hierarchy problem. However, such a scenario has several motivations. For example, the lightest neutral superworld particle will be a candidate for dark matter. The other being, as in supersymmetry, it is possible to achieve gauge coupling unification. One major difference with the supersymmetric theory is that such a theory is much more general since it is not constrained by supersymmetry. For example, some of the gauge couplings connecting the Standard Model particles with the superpartners now become free Yukawa couplings. As a result, the final state signals as well as the limits on the superworld particles can be modified both qualitatively and quantitatively. The reach for these superworld particles at the Large Hadron Collider (LHC) can be much higher than the superpartners, leading to the increased possibility of discovering new physics at the LHC.

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arXiv2015

A model for Dirac neutrino mass matrix with only four parameters

The exchange symmetry between the muon neutrino and the tau neutrino for the neutrino mass matrix has been very useful in understanding the near maximal atmospheric neutrino mixing angle. However, this symmetry can not be imposed at the Lagrangian level, since the charged lepton partners, muon and tau do not satisfy this symmetry. We extend the Standard model to include three right handed singlet neutrinos, and impose the most general symmetry between $ν_{μR} $ and $ν_{τR}$ sectors followed by a CP transformation of the leptonic sector at the Lagrangian level. This symmetry does not affect the charged leptons. With the additional assumption of the hermiticity of the ensuing Dirac neutrino mass matrix, we get a 4 parameter neutrino mass matrix in good agreement with the available neutrino data for the inverted neutrino mass hierarchy. The model also predicts the values of the three neutrino masses, and the leptonic CP violating phase which can be tested in the upcoming neutrino experiments.

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