arXiv subjects

Sourov Roy

Publications and source records attributed to Sourov Roy.

At least 37 records · Page 2Linked to original sources

arXiv2022

Non-adiabatic evolution of dark sector in the presence of $U(1)_{L_μ-L_τ}$ gauge symmetry

In secluded dark sector scenario, the connection between the visible and the dark sector can be established through a portal coupling and its presence opens up the possibility of non-adiabatic evolution of the dark sector. To study the non-adiabatic evolution of the dark sector, we have considered a $U(1)_{L_μ- L_τ} \otimes U(1)_X$ extension of the standard model (SM). Here the dark sector is charged only under $U(1)_X$ gauge symmetry whereas the SM fields are singlet under this symmetry. Due to the presence of tree-level kinetic mixing between $U(1)_X$ and $U(1)_{L_μ- L_τ}$ gauge bosons, the dark sector evolves non-adiabatically and thermal equilibrium between the visible and dark sector is governed by the portal coupling. Depending on the values of the portal coupling ($ε$), dark sector gauge coupling ($g_X$), mass of the dark matter ($m_χ$) and mass of the dark vector boson ($m_{Z^\prime}$), we study the temperature evolution of the dark sector as well as the various non-equilibrium stages of the dark sector in detail. Furthermore we have also investigated the constraints on the model parameters from various laboratory and astrophysical searches. We have found that the parameter space for the non-adiabatic evolution of dark sector is significantly constrained for $m_{Z^\prime}$ $\lesssim 100 \, {\rm MeV}$ from the observations of beam dump experiments, stellar cooling etc. The relic density satisfied region of our parameter space is consistent with the bounds from direct detection, and self interaction of dark matter (SIDM) for the mass ratio $r \equiv m_{Z^\prime}/m_χ= 10^{-3}$ and these bounds will be more relaxed for larger values of $r$. However the constraints from measurement of diffuse $γ$-ray background flux and cosmic microwave background (CMB) anisotropy are strongest for $r = 10^{-1}$ and for smaller values of $r$, they are not significant.

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arXiv2021

Probing $L_μ-L_τ$ models with CE$ν$NS: A new look at the combined COHERENT CsI and Ar data

The minimal gauged $U(1)_{L_μ-L_τ}$ model has long been known to be able to explain the tension between the theoretical and experimental values of the muon magnetic moment. It has been explored and tested extensively, pushing the viable parameter space into a very tight corner. Further, embedding the $U(1)_{L_μ-L_τ}$ model in a supersymmetric (SUSY) framework has been shown to relax some of these constraints and has recently been shown to explain the electron anomalous magnetic moment as well. In this model, the logarithm of the mass ratio of third to second generation (s)leptons control the non-negligible kinetic mixing and may crucially alter many of the constraints. We confront both the non-SUSY and SUSY versions of this class of models with the CsI(2017), the recently released CENNS10 data from the liquid Argon detector as well as the updated CsI(2020) data of the COHERENT experiment. We use the recoil energy and timing binned data from CsI(2017) and the energy, time, and Pulse Shape Discriminator binned data from CENNS10 to find estimates for the model parameters in a likelihood maximization test. We also show updated exclusions using all of the above data from the COHERENT Collaboration, as well as projected exclusions from the ongoing Coherent CAPTAIN-Mills experiment. The $(g-2)_μ$ favored values of the $U(1)_{L_μ-L_τ}$ gauge coupling that are still unconstrained overlap with the estimates from COHERENT data within $1σ$. The combined COHERENT data is found to prefer the presence of the $U(1)_{L_μ-L_τ}$ gauge boson over the Standard Model at $\sim1.4σ$. The global minima of a chi-square deviation function using CsI(2020) as well as CENNS10 total counts has significant overlap with the $(g-2)_μ$ favored parameter space in the context of the SUSY and non-SUSY $L_μ-L_τ$ models with a mediator mass in the $20-100$ MeV range.

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arXiv2021

When Freeze-out occurs due to a non-Boltzmann suppression: A study of degenerate dark sector

Exponential suppression or commonly known as the Boltzmann suppression in the number density of dark matter is the key ingredient for creating chemical imbalance prior to the usual thermal freeze-out. A degenerate/quasi-degenerate dark sector can experience a different exponential suppression in the number density analogous to the radioactive decay law leading to a delayed freeze-out mechanism of dark matter known as the co-decaying dark matter. In this work, we study the dynamics of a multicomponent dark matter from thermally decoupled degenerate dark sector in a hidden U$(1)_{X}$ extension of the Standard Model. We compute the relic density of dark matter frozen-out through the co-decaying mechanism by solving four coupled Boltzmann equations. We demonstrate how temperature $T^\prime $ of the dark sector changes due to all types of $3\rightarrow 2$ and $2\rightarrow 2$ interactions along with the eternal expansion of the Universe. We find that $3\rightarrow 2$ interactions enhance $T^\prime$ by producing energetic particles in the dark sector while the excess heat is transferred by $2\rightarrow 2$ interactions to the entire dark sector. As the direct detection is possible only through the feeble portal couplings, we investigate the neutrino and $γ$-ray signals from dark matter annihilation via one step cascade processes and compare our results with the measured fluxes of atmospheric neutrinos by Super-Kamiokande and diffuse $γ$-rays by Fermi-LAT, EGRET, INTEGRAL collaborations. We find that the present scenario easily evades all the existing bounds from atmospheric neutrino and diffuse $γ$-ray observations for degenerate dark sector. However, the constraints are significant for quasi degenerate scenario.

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arXiv2021

Supersymmetric gauged U(1)$_{L_μ-L_τ}$ model for electron and muon $(g-2)$ anomaly

Minimal gauged U(1)$_{L_μ-L_τ}$ models can provide for an additional source for the muon anomalous magnetic moment however it is difficult to accommodate the discrepancy in the electron magnetic moment in tandem. Owing to the relative sign between the discrepancies in these quantities, it seems unlikely that they arise from the same source. We show that a supersymmetric (SUSY) gauged U(1)$_{L_μ-L_τ}$ model can accommodate both the muon and electron anomalous magnetic moments in a very simple and intuitive scenario, without utilizing lepton flavor violation. The currently allowed parameter space in this kind of a scenario is constrained from the latest LHC and various low energy experimental data,e.g., recent COHERENT data, CCFR, Borexino, BaBaR, supernova etc. These constraints, in conjunction with the requirement to explain both lepton magnetic moments, lead to an upper bound on the first generation slepton mass, a lower bound on the second generation slepton mass and constricts the allowed range for the new gauge boson mass and coupling. The scheme can be probed at the ongoing COHERENT and Coherent CAPTAIN-Mills experiments and at future experiments, e.g., DUNE, BELLE-II etc.

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arXiv2020

Fermionic dark matter via UV and IR freeze-in and its possible X-ray signature

Non-observation of any dark matter signature at various direct detection experiments over the last decade keeps indicating that immensely popular WIMP paradigm may not be the actual theory of particle dark matter. Non-thermal dark matter produced through freeze-in is an attractive proposal, naturally explaining null results by virtue of its feeble couplings with the Standard Model (SM) particles. We consider a minimal extension of the SM by two gauge singlet fields namely, a $\mathbb{Z}_2$-odd fermion $χ$ and a pseudo scalar $\tildeϕ$, where the former has interactions with the SM particles only at dimension five level and beyond. This introduces natural suppression in the interactions of $χ$ by a heavy new physics scale $Λ$ and forces $χ$ to be a non-thermal dark matter candidate. We have studied production of $χ$ in detail taking into account both ultra-violate (UV), infra-red (IR) as well as mixed UV-IR freeze-in and found that for $10^{10}{\rm GeV}\leqΛ\leq 10^{15}{\rm GeV}$, $χ$ is dominantly produced via UV and mixed UV-IR freeze-in when reheat temperature $T_{RH}\gtrsim 10^4$ GeV and below which the production is dominated by IR and mixed freeze-in. Furthermore, we have considered the cascade annihilation $χ\barχ \rightarrow \tildeϕ\tildeϕ\rightarrow 4γ$ to address the longstanding $\sim3.5$ keV X-ray line observed from various galaxies and galaxy clusters. We have found that the long-lived intermediate state $\tildeϕ$ modifies dark matter density around the galactic centre to an effective density $ρ_{eff}$ which strongly depends on the decay length of $\tildeϕ$. Finally, the allowed parameter space in $Λ-g$ plane ($g$ is the coupling between $χ\barχ$ and $\tildeϕ$) is obtained by comparing our result with the XMM Newton observed X-ray flux from the centre of Milky Way galaxy in $2σ$ range.

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arXiv2019

Signatures of supersymmetry and a $L_μ- L_τ$ gauge boson at Belle-II

We propose that the $γ+ E \hspace{-.5em}/\;\:$ signal at the Belle-II will be a smoking gun for supersymmetry (SUSY) in the presence of a gauged $U(1)_{L_μ-L_τ}$ symmetry. A striking consequence of breaking the enhanced symmetry in the limit of degenerate (s)leptons is the nondecoupling of the radiative contribution of heavy charged sleptons, appearing in SUSY, to the $γ- Z^\prime$ kinetic mixing. The signal process, $e^+ e^- \rightarrow γZ^\prime \rightarrow γ+ E \hspace{-.5em}/\;\:$, is an outcome of this ubiquitous feature. We take into account the severe constraints on gauged $U(1)_{L_μ-L_τ}$ models by several low-energy observables and show that any significant excess in all but the highest photon energy bin would be an undeniable signature of such heavy scalar fields in SUSY coupling to $Z^\prime$. The number of signal events depends crucially on the logarithm of the ratio of stau to smuon mass in the presence of SUSY. In addition, the number is also inversely proportional to the $e^+-e^-$ collision energy, making a low-energy, high-luminosity collider like Belle-II an ideal testing ground for this channel. This process can probe large swathes of the slepton mass ratio vs the additional gauge coupling ($g_X$) parameter space. More importantly, it can explore the narrow slice of $M_{Z^{\prime}}-g_X$ parameter space still allowed in gauged $U(1)_{L_μ-L_τ}$ models for superheavy sparticles.

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arXiv2018

Supersymmetric gauged U(1)$_{L_μ-L_τ}$ model for neutrinos and the muon $(g-2)$ anomaly

The gauged $U(1)_{L_μ-L_τ}$ model can provide for additional contributions to the muon anomalous magnetic moment by means of a loop involving the $Z^{\prime}$ gauge boson. However, the parameter space of such models is severely constrained if one combines the latest muon $(g-2)$ data with various neutrino experiments, such as neutrino trident production, $ν-e$ and $ν-q$ elastic scattering, etc. In a supersymmetric $U(1)_{L_μ-L_τ}$ model, a larger region of parameter space opens up, thus enabling one to explore otherwise forbidden regions of parameter space in nonsupersymmetric models involving the new gauge coupling ($g_X$) and the mass of the $Z^\prime$ gauge boson ($M_{Z^{\prime}}$) . We show that the minimal model with the minimal supersymmetric Standard Model (MSSM) field content is strongly disfavored from $Z$-boson decay and neutrino data. We also show that the nonminimal model with two extra singlet superfields can lead to correct neutrino masses and mixing involving both tree-level and one-loop contributions. We find that, in this model, both muon $(g-2)$ and neutrino data may be simultaneously explained in a parameter region consistent with experimental observations. In addition, we observe that the muon $(g-2)$ anomaly can be accommodated even with higher values of electroweak sparticle masses compared to the MSSM. Charged lepton-flavor-violating processes (like $μ\rightarrow eγ$, $τ\rightarrow μγ$, etc.) may have potentially large branching ratios in this scenario. Depending on the magnitude of the supersymmetry contribution to these processes, they may constrain hitherto unconstrained regions of the $M_{Z^{\prime}}-g_X$ parameter space. However, we find that these branching fractions never exceed their upper bounds in a region where both muon $(g-2)$ and neutrino oscillation data can be simultaneously accommodated.

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arXiv2017

Generation of radiative neutrino mass in the linear seesaw framework, charged lepton flavor violation and dark matter

We investigate a model with local U(1)$_{B-L}$ and discrete $Z_2$ symmetries where two types of weak isospin singlet neutrinos, vector-like charged lepton and exotic scalar fields are introduced. The linear seesaw mechanism is induced at one-loop level through Yukawa interactions associated with the standard model leptons and exotic fields. We also discuss lepton flavor violation and muon anomalous dipole magnetic moment induced by the new Yukawa interaction. In addition, our model has dark matter candidate which is the lightest $Z_2$ odd neutral particle. We calculate the relic density and constraints from direct detection.

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arXiv2016

$h \rightarrow γγ$ in $U(1)_{R}-$ lepton number model with a right-handed neutrino

We perform a detailed study of the signal rate of the lightest Higgs boson in the diphoton channel ($μ_{γγ}$), recently analyzed by both the ATLAS and CMS collaborations at the Large Hadron Collider, in the framework of $U(1)_R-$ lepton number model with a right handed neutrino superfield. The corresponding neutrino Yukawa coupling, `$f$', plays a very important role in the phenomenology of this model. A large value of $f\sim\mathcal O(1)$ provides an additional tree level contribution to the lightest Higgs boson mass along with a very light (mass $\sim$ a few hundred MeV) bino like neutralino and a small tree level mass of one of the active neutrinos that is compatible with various experimental results. In the presence of this light neutralino, the invisible decay width of the Higgs boson can become important. We studied this scenario in conjunction with the recent LHC results. The signal rate $μ_{γγ}$ obtained in this scenario is compatible with the recent results from both the ATLAS and the CMS collaborations at 1$σ$ level. A small value of `$f$', on the other hand, is compatible with a sterile neutrino acting as a 7 keV dark matter that can explain the observation of a mono-energetic X-ray photon line by the XMM-Newton X-ray observatory. We also study the impact of $μ_{γγ}$ in this case.

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arXiv2016

Light top squarks in $U(1)_{R}$-lepton number model with a right handed neutrino and the LHC

We investigate the phenomenology of top squarks at the Large Hadron Collider (LHC) in a supersymmetric model where lepton number is identified with an approximate $U(1)_R$ symmetry in such a way that one of the left chiral sneutrinos can acquire a large vacuum expectation value ($vev$) and can play the role of the down-type Higgs. This $R$-symmetry allows a subset of trilinear $R$-parity violating interactions, which determine the collider phenomenology of this model in a significant way. The gauginos are Dirac particles and gluinos are relatively heavy in this class of models. The model contains a right handed neutrino superfield, which gives a tree level mass to one of the active neutrinos. An order one neutrino Yukawa coupling also helps enhance the Higgs boson mass at the tree level and results in a very light bino-like neutralino ($\widetilde χ_2^0$) with mass around a few hundred MeV, which is a carrier of missing (transverse) energy (\met). The model can accommodate two rather light top squarks, compatible with the observed mass of the Higgs boson. The lighter top squark (${\widetilde t}_1$) can decay into $t\widetildeχ_2^0$, and thus the signal would be similar to the signal of top quark pair production at the LHC. In addition, fully visible decays such as ${\widetilde t}_2 \rightarrow b e^+$ can give rise to interesting final states. Such signals at the LHC combined with other features like a heavy gluino could provide a strong evidence for this kind of a model. Our analysis shows that $m_{\widetilde t_1}\lsim 575~(750)$ GeV and $m_{\widetilde t_2}\lsim 1.2~(1.4)$ TeV can be probed with 5$σ$ statistical significance at the 13 TeV LHC with 300~(3000) fb$^{-1}$ of integrated luminosity. Finally, we observe that in the presence of super-light carriers of \met, the so-called `stealth' top squark scenario may naturally appear in our model.

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arXiv2015

Stability constraints in triplet extension of MSSM

We study the stability constraints on the parameter space of a triplet extension of MSSM. Existence of unbounded from below directions in the potential can spoil successful Electroweak (EW) symmetry breaking by making the corresponding minimum unstable, and hence the model should be free from those directions. Avoiding those directions restricts the parameter space of the model. We derive four stability constraints, of which only three independent from each other. After scanning the model's parameter space for phenomenologically viable data points, we impose the stability constraints and find that only about a quarter of the data points features a stable EW minimum. At those data points featuring stability, $μ$ and the up Higgs soft mass turn out to be smaller than about a TeV in absolute value, which make the mass of the lightest chargino and neutralino smaller than about 700 GeV. Two relevant phenomenological consequences of lifting the unbounded from below directions are that the lightest Higgs boson decay rate to diphoton predicted by the triplet extension of MSSM generally features larger deviations from MSSM and fine tuning is actually higher, that what each of the two would be without imposing stability constraints.

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arXiv2014

7 keV Sterile neutrino dark matter in $U(1)_R-$ lepton number model

We study the phenomenology of a keV sterile neutrino in a supersymmetric model with $U(1)_R-$ lepton number in the light of a very recent observation of an X-ray line signal at around 3.5 keV, detected in the X-ray spectra of Andromeda galaxy and various galaxy clusters including the Perseus galaxy cluster. This model not only provides a small tree level mass to one of the active neutrinos but also renders a suitable warm dark matter candidate in the form of a sterile neutrino with negligible active-sterile mixing. Light neutrino masses and mixing can be explained once one-loop radiative corrections are taken into account. The scalar sector of this model can accommodate a Higgs boson with a mass of $\sim$ 125 GeV. In this model gravitino is the lightest supersymmetric particle (LSP) and we also study the cosmological implications of this light gravitino with mass $\sim \mathcal O$(GeV).

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arXiv2014

Vacuum Stability Constraints on the Minimal Singlet TeV Seesaw Model

We consider the minimal seesaw model in which two gauge singlet right handed neutrinos with opposite lepton numbers are added to the Standard Model. In this model, the smallness of the neutrino mass is explained by the tiny lepton number violating coupling between one of the singlets with the standard left-handed neutrinos. This allows one to have the right handed neutrino mass at the TeV scale as well as appreciable mixing between the light and heavy states. This model is fully reconstructible in terms of the neutrino oscillation parameters apart from the overall coupling strengths. We show that the overall coupling strength $y_ν$ for the Dirac type coupling between the left handed neutrino and one of the singlets can be restricted by consideration of the (meta)stability bounds on the electroweak vacuum. In this scenario the lepton flavor violating decays of charged leptons can be appreciable which can put further constraint on $y_ν$, for right-handed neutrinos at TeV scale. We discuss the combined constraints on $y_ν$ for this scenario from the process $μ\rightarrow e γ$ and from the consideration of vacuum (meta)stability constraints on the Higgs self coupling. We also briefly discuss the implications for neutrinoless double beta decay and possible signatures of the model that can be expected at colliders.

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arXiv2014

Higgs boson mass, neutrino masses and mixing and keV dark matter in an $U(1)_R-$ lepton number model

We discuss neutrino masses and mixing in the framework of a supersymmetric model with an $U(1)_{R}$ symmetry, consisting of a single right handed neutrino superfield with an appropriate R charge. The lepton number ($L$) of the standard model fermions are identified with the negative of their R-charges. As a result, a subset of leptonic R-parity violating operators can be present and are consistent with the $U(1)_R$ symmetry. This model can produce one light Dirac neutrino mass at the tree level without the need of introducing a very small neutrino Yukawa coupling. We analyze the scalar sector of this model in detail paying special attention to the mass of the lightest Higgs boson. One of the sneutrinos might acquire a substantial vacuum expectation value leading to interesting phenomenological consequences. Different sum rules involving the physical scalar masses are obtained and we show that the lightest Higgs boson mass receives a contribution proportional to the square of the neutrino Yukawa coupling $f$. This allows for a 125 GeV Higgs boson at the tree level for $f \sim {\cal O} (1)$ and still having a small tree level mass for the active neutrino. In order to fit the experimental results involving neutrino masses and mixing angles we introduce a small breaking of $U(1)_R$ symmetry, in the context of anomaly mediated supersymmetry breaking. In the presence of this small R-symmetry breaking, light neutrino masses receive contributions at the one-loop level involving the R-parity violating interactions. We also identify the right handed sterile neutrino as a warm dark matter candidate in our model. In the case of R-symmetry breaking, the large $f$ case is characterized by a few hundred MeV lightest neutralino as an unstable lightest supersymmetric particle (LSP) and we briefly discuss the cosmological implications of such a scenario.

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arXiv2013

Invisible Higgs Decay in a Supersymmetric Inverse Seesaw Model with Light Sneutrino Dark Matter

Within the framework of a constrained Minimal Supersymmetric Standard Model (cMSSM) augmented by an MSSM singlet-pair sector to account for the non-zero neutrino masses by inverse seesaw mechanism, the lightest supersymmetric particle (LSP) can be a mixed sneutrino with mass as small as 50 GeV, satisfying all existing constraints, thus qualifying as a light dark matter candidate. We study the possibility of the lightest neutral Higgs boson in this model decaying invisibly into a pair of sneutrino LSPs, thereby giving rise to novel missing energy signatures at the LHC. We perform a two-parameter global analysis of the LHC Higgs data available till date to determine the optimal invisible Higgs branching fraction in this scenario, and obtain a $2σ(1σ)$ upper limit of 0.23 (0.12). A detailed cut-based analysis is carried out thereafter, demonstrating the viability of our proposed signal vis-a-vis backgrounds at the LHC.

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arXiv2012

Phenomenology of Light Sneutrino Dark Matter in cMSSM/mSUGRA with Inverse Seesaw

We study the possibility of a light Dark Matter (DM) within a constrained Minimal Supersymmetric Standard Model (cMSSM) framework augmented by a SM singlet-pair sector to account for the non-zero neutrino masses by inverse seesaw mechanism. Working within a 'hybrid' scenario with the MSSM sector fixed at high scale and the singlet neutrino sector at low scale, we find that, contrary to the case of the usual cMSSM where the neutralino DM cannot be very light, we can have a light sneutrino DM with mass below 100 GeV satisfying all the current experimental constraints from cosmology, collider as well as low-energy experiments. We also note that the supersymmetric inverse seesaw mechanism with sneutrino as the lightest supersymmetric partner can have enhanced same-sign dilepton final states with large missing transverse energy (mET) coming from the gluino- and squark-pair as well as the squark-gluino associated productions and their cascade decay through charginos. We present a collider study for the same-sign dilepton+jets+mET signal in this scenario and propose some distinctions with the usual cMSSM. We also comment on the implications of such a light DM scenario on the invisible decay width of an 125 GeV Higgs boson.

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arXiv2012

Muon (g-2) from the bulk neutrino field in a warped extra dimensional model

In the Randall-Sundrum model, a bulk neutrino field in the 5-dimensional space-time can give rise to tiny Dirac masses to neutrinos. In such a scenario, we have computed the contribution of the bulk neutrino field to the anomalous magnetic moment $(g-2)_μ$ of muon. We have computed this contribution in the 't Hooft-Feynman gauge and have found that the contribution has the right sign to fit the current discrepancy between the experiment and the standard model value of $(g-2)_μ$. We have also studied possible constraints on the model parameters by including contributions to $(g-2)_μ$ from other sources such as bulk gravitons.

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arXiv2012

Non-standard interaction in neutrino oscillations and recent Daya Bay, T2K experiments

We study the possible constraints on non-standard interaction(NSIs) in a model independent way by considering the recent results from T2K and Daya Bay neutrino oscillations experiments. Using perturbation method we present generic formulas (suitable for T2K baseline and for large $θ_{13}$ as evident from Daya Bay) for the probability of oscillation for $ν_μ\rightarrow ν_e$, taking into account NSIs at source ($ε^s$), detector ($ε^d$) and during propagation ($ε^m$) of neutrinos through matter. Two separate cases of perturbation with small (slightly large) NSI ($ε_{αβ}^m \sim 0.03(0.18)$) are discussed in detail. Using various possible presently allowed NSI values we reanalyze numerically the $θ_{13} - δ$ allowed region given by recent T2K experimental data. We obtain model independent constraints on NSIs in the $δ-ε_{αβ}^m$ plane using the $θ_{13}$ value as measured by Daya Bay, where $δ$ is the CP violating phase. Depending on $δ$ values significant constraints on $ε_{eτ}$ and $ε_{ττ}$, in particular, are possible for both hierarchies of neutrino masses. Corresponding to T2K's 66% confidence level result, the constraints on $ε_{ττ}$ is shown to be independent of any $δ$ value.

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