arXiv subjects

Rukmani Mohanta

Publications and source records attributed to Rukmani Mohanta.

At least 73 records · Page 4Linked to original sources

arXiv2020

Unified explanation of flavor anomalies, radiative neutrino mass and ANITA anomalous events in a vector leptoquark model

Driven by the recent experimental hints of lepton-flavor-universality violation in the bottom-quark sector, we consider a simple extension of the Standard Model (SM) with an additional vector leptoquark $V_{\rm LQ}({\bf 3},{\bf 1},2/3)$ and a scalar diquark $S_{\rm DQ}({\bf 6},{\bf 1},4/3)$ under the SM gauge group $SU(3)_c\times SU(2)_L\times U(1)_Y$, in order to simultaneously explain the $b \to s \ell^+ \ell^-$ (with $\ell=e,μ$) and $b \to c l^- \bar ν_l$ (with $l=e,μ,τ$) flavor anomalies, as well as to generate small neutrino masses through a two-loop radiative mechanism. We perform a global fit to all the relevant and up-to-date $b \to s \ell^+ \ell^-$ and $b \to c l^- \bar ν_l$ data under the assumption that the leptoquark couples predominantly to second and third-generation SM fermions. We then look over the implications of the allowed parameter space on lepton-flavor-violating $B$ and $τ$ decay modes, such as $B_s \to l^+_i l^-_j, \ B \to K^{(*)} l^+_i l^-_j, \ B_s \to ϕl^+_i l^-_j$, $Υ(nS) \to μτ$ and $τ\to μγ$, $τ\to μϕ(η^{(\prime)})$, respectively. Minimally extending this model by adding a fermion singlet $χ({\bf 1},{\bf 1},0)$ also explains the ANITA anomalous upgoing events. Furthermore, we provide complementary constraints on leptoquark and diquark couplings from high-energy collider and other low-energy experiments to test this model.

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arXiv2020

A modular $A_4$ symmetric Scotogenic model for Neutrino mass and Dark Matter

Modular symmetries have been impeccable in neutrino and quark sectors. This motivated us, therefore, to propose a variant of scotogenic model based on modular $A_4$ symmetry to realize the neutrino mass generation at one-loop level through radiative mechanism. Alongside, the lepton flavour violating process $μ\to e γ$ and the muon $g-2$ anomaly are also addressed. The lightest Majorana fermions turn out to be potential dark matter candidates, made stable by suitable assignment of modular weights. The relic density of the same has been computed with annihilations mediated by inert scalars and new $U(1)$ gauge boson.

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arXiv2020

Investigating the effect of $U_1$ vector leptoquark on $b\to u τ\barν$ mediated $B$ decays

The recent measurements of lepton flavor university (LFU) violating observables in semileptonic $b \to c \ell \barν $ and $b\to s\ell^+ \ell^-$ transitions by various experiments exhibit $(2-3)σ$ deviations from their corresponding Standard Model (SM) predictions. These tantalizing signals hint towards the possible role of new physics (NP) in $b \to c τ\barν$ and $b\to sμ^+μ^-$ decay channels. This in turn indicates that the same class of NP as appeared in $b \to c τ\barν$, might also show up in other tree level processes involving $b \to u τ\barν$ transition. Since these charged current transitions are doubly Cabibbo suppressed, the NP contributions could be significant enough leading to sizeable effects in some of the observables. In this paper, we study the implications of the vector leptoquark $U_1(3,1,2/3)$, which is one of the few scenarios that can simultaneously explain the LFU violation signals both in the charged-current as well as neutral-current sectors, on the semileptonic decays $B \to (π, ρ, ω)τ\barν$ and $B_s \to (K, K^*) τ\barν$. In particular, we pay our attention to the branching fraction, lepton flavor non-universality (LNU) observable, forward-backward asymmetry and the polarization asymmetry parameters of these modes. We find substantial deviations in the branching fractions as well as LNU observables of these decay modes due to the $U_1$ contributions, which can be probed by the currently running experiments LHCb and Belle-II.

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arXiv2020

Light sterile neutrinos and their implications on currently running long-baseline and neutrinoless double beta decay experiments

Recent $ν_e$ appearance data from the Mini Booster Neutrino Experiment (MiniBooNE) are in support of the excess of events reported by the Liquid Scintillator Neutrino Detector (LSND), which provides an indirect hint for the existence of eV-scale sterile neutrino. As these sterile neutrinos can mix with the standard active neutrinos, in this paper we explore the effect of such active-sterile mixing on the determination of various oscillation parameters by the currently running long-baseline neutrino experiments T2K and NO$ν$A. We find that the existence of sterile neutrino can lead to new kind of degeneracies among these parameters which would substantially deteriorate the mass hierarchy sensitivity of NO$ν$A experiment. We further notice that the inclusion of data from T2K experiment helps in resolving the degeneracies. The impact of new CP violating phases $δ_{14}$ and $δ_{34}$ on the maximal CP-violation exclusion sensitivity for NO$ν$A experiment has also been illustrated. Finally, we discuss the implication of such light sterile neutrinos on neutrinoless double beta decay processes in line with recent experimental results, as well as on the sensitivity reach of future experiments.

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arXiv2020

Neutrino Phenomenology and Dark matter in an $A_4$ flavour extended B-L model

We present an $A_4$ flavor extended $\rm B-L$ model for realization of eV scale sterile neutrinos, motivated by the recent experimental hints from both particle physics and cosmology. The framework considered here is a gauged $\rm B-L$ extension of standard model without the introduction of right-handed neutrinos, where the gauge triangle anomalies are canceled with the inclusion of three exotic neutral fermions $N_{i}$ ($i=1,2,3$) with $\rm B-L$ charges $-4,-4$ and $5$. The usual Dirac Yukawa couplings between the SM neutrinos and the exotic fermions are absent and thus, the model allows natural realization of eV scale sterile-like neutrino and its mixing with standard model neutrinos by invoking $\rm A_4$ flavor symmetry. We demonstrate how the exact tri-bimaximal mixing pattern is perturbed due to active-sterile mixing by analyzing $1+3$ case in detail. We also show the implication of eV scale sterile-like neutrino on various observables in neutrino oscillation experiments and the effective mass in neutrinoless double beta decay. Another interesting feature of the model is that one of three exotic fermions is required to explain eV scale phenomena, while other two fermions form stable dark matter candidates and their total relic density satisfy the observed $3σ$ limit of Planck data. We constrain the gauge parameters associated with $U(1)$ gauge extension, using relic density and collider bounds.

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arXiv2020

Exploring the effect of Lorentz invariance violation with the currently running long-baseline experiments

Neutrinos are the fundamental particles, blind to all kind of interactions except the weak and gravitational. Hence, they can propagate very long distances without any deviation. This characteristic property can thus provide an ideal platform to investigate Planck suppressed physics through their long distance propagation. In this work, we intend to investigate CPT violation through Lorentz invariance violation (LIV) in the long-baseline accelerator based neutrino experiments. Considering the simplest four-dimensional Lorentz violating parameters, for the first time, we obtain the sensitivity limits on the LIV parameters from the currently running long-baseline experiments T2K and NO$ν$A. In addition to this, we show their effects on mass hierarchy and CP violation sensitivities by considering NO$ν$A as a case study. We find that the sensitivity limits on LIV parameters obtained from T2K are much weaker than that of NO$ν$A and the synergy of T2K and NO$ν$A can improve these sensitivities. All these limits are slightly weaker ($2 σ$ level) compared to the values extracted from Super-Kamiokande experiment with atmospheric neutrinos. Moreover, we observe that the mass hierarchy and CPV sensitivities are either enhanced or deteriorated significantly in the presence of LIV as these sensitivities crucially depend on the new CP-violating phases. We also present the correlation between $\sin^2 θ_{23}$ and the LIV parameter $|a_{αβ}|$, as well as $δ_{CP}$ and $|a_{αβ}|$.

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arXiv2019

Exploring Dark Matter, Neutrino mass and flavour anomalies in L_μ-L_τ model

We investigate Majorana dark matter in a new variant of $U(1)_{L_μ-L_τ}$ gauge extension of Standard Model, containing three additional neutral fermions $N_{e}, N_μ, N_τ$, along with a $(\bar{3},1,1/3)$ scalar Leptoquark (SLQ) and an inert doublet, to study the phenomenology of dark matter, neutrino mass generation and flavour anomalies on a single platform. The lightest mass eigenstate of the $N_μ, N_τ$ neutral fermions plays the role of dark matter. We compute the WIMP-nucleon cross section in leptoquark portal and the relic density mediated by inert doublet components, leptoquark and the new $Z^{\prime}$ boson. We constrain the parameter space consistent with Planck limit on relic density, PICO-60 and LUX bounds on spin-dependent direct detection cross section. We also discuss about the neutrino mass generation at one-loop level and the viable parameter region to explain current neutrino oscillation data. The $Z^\prime$ gauge boson of extended $U(1)$ symmetry and the SLQ play an important role in settling the known issues of flavor sector.

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arXiv2019

Investigating the role of new physics in $b \to c τ\bar ν_τ$ transitions

In recent times, the charged-current mediated semileptonic $b \to c τ\bar ν_τ$ processes have attracted a lot of attention after the observation of lepton non-universality ratios, $R_{D^{(*)}}$, $R_{J/ψ}$ and the measurements on $D^*$ and $τ$ longitudinal polarization fractions in $\bar B\to D^* τ\bar ν_τ$ processes. We present a model-independent analysis of $ \bar B\to D^{(*)} τ\bar ν_τ$, $ B_s\to D_s^{(*)} τ\bar ν_τ$, $ B_c^+ \to (η_c, J/ψ) τ^+ ν_τ$, $Λ_b \to Λ_c τ\bar ν_τ$ and $\bar B \to D^{**} τ\bar ν_τ$ (where $D^{**} = \{D^*_0, D_1^*, D_1, D_2^*\}$ are the four lightest excited charm mesons) processes involving $b \to c τ\bar ν$ quark level transitions by considering the most general effective Lagrangian in the presence of new physics. We perform a global fit to various set of new coefficients, including the measurements on $R_{D^{(*)}}$, $R_{J/ψ}$ and the upper limit on Br($B_c^+ \to τ^+ \bar ν_τ$). We then show the implications of constrained new couplings on the branching fractions, lepton non-universality ratios and various angular observables of these decay modes in four different bins of $q^2$.

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arXiv2019

Model independent analysis of $ B^* \to P \ell \barν_\ell$ decay processes

Very compelling deviations in the recently observed lepton nonuniversality observables $\big (R_{D^{(*)}}, R_{K^{(*)}}, R_{J/ψ} \big )$ of semileptonic $B$ meson decays from their Standard Model predictions hint towards the presence of some kind of new physics beyond it. In this regard, we investigate the effect of new physics in the semileptonic $\bar B_{d(s)}^* \to P \ell \barν_\ell$ decay processes, where $P=D,π(D_s,K$), in a model independent way. We consider the presence of additional vector and scalar type interactions and constrain the corresponding new couplings by fitting ${\rm Br(B_{u}^+ \to τ^+ ν_τ)}$, ${\rm Br(B \to πτ\bar ν_τ)}$, ${\rm Br(B_{c}^+ \to τ^+ ν_τ)}$, $R_π^l$, $R_{D^{(*)}}$ and $R_{J/ψ}$ data. Using the constrained new parameters, we estimate the branching ratios, forward-backward asymmetry, lepton-spin asymmetry and lepton non-universality observables of $\bar B_{d,s}^{*} \to P τ\bar ν_τ$ processes. We find that the branching ratios of these decay modes are sizeable and deviate significantly (for vector-type couplings) from their corresponding standard model values, which are expected to be within the reach of Run III of Large Hadron Collider experiment.

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arXiv2019

Exploring dark matter, neutrino mass and $R_{K^{(*)},ϕ}$ anomalies in $L_μ-L_τ$ model

We investigate Majorana dark matter in a new variant of $U(1)_{L_μ-L_τ}$ gauge extension of Standard Model, where the scalar sector is enriched with an inert doublet and a $(\bar{3},1,1/3)$ scalar leptoquark. We compute the WIMP-nucleon cross section in leptoquark portal and the relic density mediated by inert doublet components, leptoquark and the new $Z^{\prime}$ boson. We constrain the parameter space consistent with Planck limit on relic density, PICO-60 and LUX bounds on spin-dependent direct detection cross section. Furthermore, we constrain the new couplings from the present experimental data on ${\rm Br}(τ\to μν_τ\bar ν_μ)$, ${\rm Br}( B \to X_s γ)$, ${\rm Br}( B^0 \to K^0 μ^+ μ^-)$, ${\rm Br}(B^+ \to K^+ τ^+ τ^-)$ and $B_s-\bar{B_s}$ mixing, which occur at one-loop level in the presence of $Z^\prime$ and leptoquark. Using the allowed parameter space, we estimate the form factor independent $P_{4,5}^\prime$ observables and the lepton non-universality parameters $R_{K}$, $R_{K^*}$ and $R_ϕ$. We also briefly discuss about the neutrino mass generation at one-loop level and the viable parameter region to explain current neutrino oscillation data.

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arXiv2018

Probing new physics in semileptonic $Λ_b$ decays

In recent times, several hints of lepton non-universality have been observed in semileptonic $B$ meson decays, both in the charged-current ($b \to c l \bar ν_l$) and neutral-current ($b \to s ll $) transitions. Motivated by these intriguing results, we perform a model independent analysis of the semileptonic $Λ_b$ decays involving the quark level transitions $b \to (u,c) l ν_l$, in order to scrutinize the nature of new physics. We constrain the new parameter space by using the measured branching ratios of $B_{c, u}^+ \to τ^+ ν_τ$, $B \to πτν_τ$ processes and the existing experimental results on $R_{D^{(*)}}$, $R_{J/ψ}$ and $R_π^l$ parameters. Using the constrained parameters, we estimate the branching ratios, forward backward asymmetries, hadron and lepton polarization asymmetries of the $Λ_b \to (Λ_c, p) l ν_l$ processes. Moreover, we also examine whether there could be any lepton universality violation in these decay modes.

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arXiv2018

Majorana Dark Matter in a new B-L model

We present a comprehensive study of Majorana dark matter in a $U(1)_{B-L}$ gauge extension of the standard model, where three exotic fermions with $B-L$ charges as $-4, -4, +5$ are added to make the model free from the triangle gauge anomalies. The enriched scalar sector and the new heavy gauge boson $Z^{\prime},$ associated with the $U(1)_{B-L}$ symmetry make the model advantageous to be explored in dual portal scenarios for the search of dark matter signal. Diagonalizing the exotic fermion mass matrix, we obtain the Majorana mass eigenstates, of which the lightest one plays the role of dark matter. Analyzing the effect of two mediators separately, the scalar portal channels give a viable parameter space consistent with relic density from PLANCK data and the direct detection limits from various experiments such as LUX, XENON1T and PandaX. While the $Z^{\prime}$ mediated channels are constrained from relic abundance and LHC searches for $Z^{\prime}$ in the dilepton channel. A massless physical Goldstone boson plays a key role in the scalar portal relic density. Finally, we briefly discuss the neutrino mass generation at one-loop level.

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arXiv2018

Singlet scalar Dark matter in $U(1)_{B-L}$ models without right-handed neutrinos

We investigate the phenomenology of singlet scalar dark matter in a simple $B-L$ gauge extension of the Standard Model where the dark matter particle is charged under the $U(1)_{B-L}$ symmetry. The non-trivial gauge anomalies are cancelled with the introduction of three exotic fermions with $B-L$ charges as $-4,-4,5$, instead of right-handed neutrinos $ν_{Ri}~(i=1,2,3)$ with $B-L=-1$ in conventional $U(1)_{B-L}$ model. Without the need of any ad-hoc discrete symmetry, the $B-L$ charge plays a crucial role in stabilizing the dark matter. We make a comprehensive study of dark matter phenomenology in the scalar and gauge portals separately. In the gauge-mediated regime, we invoke the LEP-II constraints and dilepton limits of ATLAS on the gauge parameters. A massless physical Goldstone plays a vital role in the scalar-portal dark matter observables, becomes a unique feature of the model. We show the mechanism of generating the light neutrino mass at one-loop level where the dark matter singlet runs in the loop. We shed light on the semi-annihilation and finally, we comment on indirect signals in this framework.

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arXiv2018

Non-unitary lepton mixing in an inverse seesaw and its impact on the physics potential of long-baseline experiments

In this paper, we consider the low-energy scale inverse seesaw mechanism in which the observed neutrino mass and lepton mixing are explained by introducing right handed neutrinos and the gauge-singlet fermions with experimentally testable energy scale. Moreover, the presence of such new fermions leads to unitarity violation in lepton mixing due to significantly large mixing between active neutrinos and the heavy fermions. In addition to this, such large lepton mixing also gives rise to potentially large lepton flavor violation, which allows to constrain the non-unitarity parameters via lepton flavor violating decays ($l_i \to l_j γ$). We make use of these constraints on non-unitarity parameters and investigate their effects on the determination of current unknown oscillation parameters at long-baseline experiments. We find that non-unitarity parameters are sensitive to NO$ν$A experiment. However, it is observed that NO$ν$A experiment is not expected to improve the current knowledge of non-unitarity parameter $η_{21}$. We also find that the sensitivities to current unknowns are deteriorated significantly in presence of non-unitary lepton mixing and these sensitivities crucially depend upon the new CP-violating phase in the non-unitary mixing. Further, we find that the degeneracy resolution capability of NO$ν$A experiment is reduced in the presence of non-unitarity parameters. However, the synergy between the currently running experiments T2K and NO$ν$A can improve the parameter degeneracy resolution and hence there is enhancement in the sensitivities of unknowns.

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arXiv2018

Impact of vector leptoquark on $\bar B \to \bar K^* l^+ l^-$ anomalies

Motivated by the recent measurement of the lepton flavour nonuniversality ratio $R_{K^*}$ by the LHCb Collaboration, we study the implications of vector leptoquarks on the observed anomalies associated with the $\bar B \to \bar K^* l^+ l^-$ decay processes. The leptoquark couplings are constrained from the measured branching ratios of $B_s \to l^+ l^-$, $K_L \to l^+ l^-$ and $B_s \to μ^\mp e^\pm$ processes. Using these constrained couplings, we estimate the branching ratios, forward-backward and lepton polarization asymmetries and also the form factor independent optimized observables ($P_{i}^{(\prime)}$) for $\bar B \to \bar K^* l^+ l^-$ modes in the high recoil limit. We also study the other lepton flavour universality violating observables, such as $Q_{F_{L, T}}$, $Q_{i}$ and $B_{5, 6s}$, where $i=1,2,\cdots6, 8$. Furthermore, we investigate the lepton flavour violating $K_L \to μ^\mp e^\pm$ decay process in this model.

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arXiv2017

Model independent investigation of rare semileptonic b \to u l \barν_l decay processes

Motivated by the recent observation of lepton universality violation in the flavour changing charged current transitions $b \to c l \barν_l$, we intend to scrutinize the lepton non-universality effects in rare semileptonic $B$ meson decays involving the quark level transitions $b \to u l \barν_l$. In this regard, we envisage the model-independent approach and consider the generalized effective Lagrangian in the presence of new physics and constrain the new parameters by using the experimental branching fractions of $B_u^+ \to l^+ ν_l$ and $B^- \to π^0 μ^- \bar ν_μ$ processes, where $l=e, μ, τ$. We then estimate the branching ratios and forward-backward asymmetries of $B_{(s)} \to P (V)l \barν_l$ processes, where $P(=K, π, η^{(\prime)})$ denotes the pseudoscalar meson and $V(=K^*, ρ)$ is the vector meson. We also find out various lepton non-universality parameters in these processes in the presence of new physics.

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arXiv2017

$A_4$ realization of Linear Seesaw and Neutrino Phenomenology

Motivated by the crucial role played by the discrete flavour symmetry groups in explaining the observed neutrino oscillation data, we consider the $A_4$ realization of linear seesaw by extending the standard model (SM) particle content with two types of right-handed neutrinos along with the flavon fields, and the SM symmetry with $A_4\times Z_4\times Z_2$ and a global symmetry $U(1)_X$ which is broken explicitly by the Higgs potential. We scrutinize whether this model can explain the recent results from neutrino oscillation experiments by searching for parameter space that can accommodate the observables such as the reactor mixing angle $θ_{13}$, the CP violating phase $δ_{CP}$, sum of active neutrino masses $Σ_{i} m_i$, solar and atmospheric mass squared differences, and the lepton number violating parameter called as effective Majorana mass parameter, in line with recent experimental results. We also discuss the scope of this model to explain the baryon asymmetry of the universe through Leptogenesis. We also investigate the possibility of probing the non-unitarity effect in this scenario, but it is found to be rather small.

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arXiv2017

Study of the rare decays $B_{s,d}^* \to μ^+ μ^-$

We study rare leptonic decays of vector $B$ mesons in the scalar leptoquark and family non-universal $Z^\prime$ models. We constrain these new couplings by using the measured branching fractions of $B_{s,d} \to μ^+ μ^-$ processes and the existing $B_{s,d}^0-\bar B_{s,d}^0$ mixing data. We estimate the branching fractions of $B_{s,d}^* \to μ^+ μ^-$ in both the models, which are found to be reasonably enhanced from their corresponding standard model values and within the reach of Run 2-3 of LHC.

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