arXiv subjects

Chayan Majumdar

Publications and source records attributed to Chayan Majumdar.

At least 19 recordsLinked to original sources

arXiv2026

Displaced Signals from Long-lived Particles in Neutrinoless Double Beta Decay

Much of the literature on neutrinoless double beta ($0\nu\beta\beta$) decay with light new-physics mediators focuses on invisible missing energy. We investigate the scenario in which a massive Majoron-like particle $\phi$ is produced on-shell during $0\nu\beta\beta$ decay and subsequently decays into visible final states after travelling a macroscopic distance. Specifically, we analyze the displaced energy deposition from $\phi$ decays into a photon pair ($\gamma\gamma$), a photon and a dark photon ($\gamma\gamma_D$) and an electron-positron pair ($e^+e^-$). We demonstrate that the displaced decays modify the expected visible energy spectra and provide novel, distinct experimental signatures at current and upcoming $0\nu\beta\beta$ experiments, with the promise of improving the sensitivity of the standard invisible Majoron searches in $0\nu\beta\beta$ decay by more than two orders of magnitude. The relevant effective couplings can naturally arise in well-motivated ultraviolet-complete scenarios that conventional $0\nu\beta\beta$ decay searches cannot probe.

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arXiv2026

Multiboson Signatures of Doubly Charged Scalars at a Same-Sign Muon Collider

We investigate the sensitivity of a same-sign $\mu^+\mu^+$ collider to doubly charged scalars in the Type-II seesaw framework, focusing on the regime in which the doubly charged scalar decays dominantly into same-sign $W$-boson pairs. Motivated by the $\mu$TRISTAN proposal, for a benchmark we consider a degenerate triplet spectrum at a center-of-mass energy of $2~{\rm TeV}$ and an integrated luminosity of $1~{\rm ab}^{-1}$. The signal process is studied in the fully hadronic $W$-decay mode, leading to a characteristic $\mu^+\mu^+ + 8j$ final state. We develop a cut-based analysis based on high jet multiplicity and global hadronic $W$ reconstruction, and then improve its sensitivity with a multivariate strategy exploiting reconstructed $W$ observables, global event kinematics, multi-boson variables and spectator-muon information. The best-performing setup reaches a $2\sigma$ sensitivity level for doubly charged scalar masses up to $425-430~{\rm GeV}$, extending the cut-based reach by a few tens of GeV. This indicates an improvement over the current LHC coverage, while also providing an independent probe based on a qualitatively different production mode and collider environment.

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arXiv2026

Radiative Dirac neutrino masses and dark matter in a $U(1)_{B-L}$ extended model

We study a $U(1)_{B-L}$ extension of the Standard Model (SM) in which Dirac neutrino masses are generated radiatively at the one-loop level through the exchange of new beyond the SM fields. This framework establishes a direct connection between neutrino mass generation and the dark sector, with the stability of the dark matter ensured by a residual discrete $Z_6$ symmetry arising from the spontaneous breaking of $U(1)_{B-L}$. We investigate the resulting charged lepton flavor violating processes and dark matter phenomenology, saturating relic observations and direct-detection constraints, and analyze the collider signatures of the dark sector at the Large Hadron Collider, its proposed high luminosity extension and at a future muon collider. We have identified excellent prospects for observing the considered dark matter candidates in these colliders, even with lower integrated luminosities than the proposed one.

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arXiv2025

Probing Dark Sector Particles Coupling to Neutrinos with Double Beta Decay

Motivated by the observation of non-zero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta $(0\nu\beta\beta)$ decay. In all of these searches, a substantial amount of data on two-neutrino double beta $(2\nu\beta \beta)$ decay has been collected. In this work, we explore the sensitivity of current and future double beta decay experiments to a massive Majoron-like scalar particle coupled to neutrinos and potentially dark sector fermions, and compare their reach to the relevant cosmological constraints. On- and off-shell production of such a scalar leads to characteristic distortions in the emitted electron spectrum. We investigate how these distortions manifest in current and future double beta decay experiments, deriving the sensitivity to such a scenario. We project the reach of future experiments which can probe scalar-neutrino couplings of $|a_\nu| \approx 2\times 10^{-6}$ for sub-MeV scalar particles and remain sensitive to off-shell production above the Q-value of double beta isotopes.

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arXiv2025

Exploring two component doublet dark matter

We propose a two-component dark matter (DM) scenario by extending the Standard Model with two additional $SU(2)_L$ doublets, one scalar, and another fermion. To ensure the stability of the DM components, we impose a global $Z_2 \times Z_2^\prime$ symmetry. The lightest neutral states for both the scalar and fermion, which are non-trivially transformed under the extended symmetry, behave as stable two-component DM candidates. While single components are under-abundant due to their gauge interactions, in a mass region between $m_W$ and $525$ GeV for the scalar and a mass below $1200$ GeV for the fermion, and the fermion DM conflicts with direct detection limits over the whole parameter space, having two components helps to saturate relic density in the regions with under-abundance. Compliance with direct detection constraints leads to two options, either introducing dim-5 effective operators, or embedding the scenarios into a complete UV theory, which reproduces a type II seesaw model, thus naturally including neutrino masses. We analyze the consequences of this scenario at the LHC.

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arXiv2025

Neutrino Theory in the Precision Era

This document summarises discussions on future directions in theoretical neutrino physics, which are the outcome of a neutrino theory workshop held at CERN in February 2025. The starting point is the realisation that neutrino physics offers unique opportunities to address some of the most fundamental questions in physics. This motivates a vigorous experimental programme which the theory community fully supports. \textbf{A strong effort in theoretical neutrino physics is paramount to optimally take advantage of upcoming neutrino experiments and to explore the synergies with other areas of particle, astroparticle, and nuclear physics, as well as cosmology.} Progress on the theory side has the potential to significantly boost the physics reach of experiments, as well as go well beyond their original scope. Strong collaboration between theory and experiment is essential in the precision era. To foster such collaboration, \textbf{we propose to establish a CERN Neutrino Physics Centre.} Taking inspiration from the highly successful LHC Physics Center at Fermilab, the CERN Neutrino Physics Centre would be the European hub of the neutrino community, covering experimental and theoretical activities.

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arXiv2025

Constraining the SMEFT Extended with Sterile Neutrinos at FCC-ee

We investigate how extensions of the Standard Model (SM) involving heavy neutral leptons (HNLs) can be probed at FCC-ee, the proposed high-energy circular $e^+e^-$ collider. Using the effective field theory (EFT) approach, we determine the impact of new interactions on the production and decay of HNLs at FCC-ee. In particular, we consider $d\leq 7$ $\nu$SMEFT operators which induce vector, scalar and tensor four-fermion and effective charged- and neutral-current interactions of HNLs, that may also mix with the active neutrinos of the SM. We consider sensitivities to the active-sterile mixing and EFT Wilson coefficients from monophoton searches and displaced vertex decay signatures. In both analyses, we consider the scenarios where HNLs are Majorana or Dirac fermions. We translate the upper bounds on the Wilson coefficients to lower limits on the scale of new physics.

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arXiv2024

Relaxing Limits from Big Bang Nucleosynthesis on Heavy Neutral Leptons with Axion-like Particles

Heavy neutral leptons (HNLs) are constrained by requirements of Big Bang Nucleosynthesis (BBN) as their decays significantly impact the formation of the primordial elements. We propose here a model where the primary decay channel for the HNLs is to an axion-like particle (ALP) and a neutrino. Consequently, HNLs can decay earlier and evade the BBN bound for lower masses, provided the ALPs themselves decay considerably later. Further cosmological and astrophysical constraints limit severely the range of validity of the ALP properties. We find that a new parameter region opens up for HNLs with masses between 1 MeV and 1 GeV, and active-sterile neutrino mixing strengths between $10^{-9}$ and $10^{-6}$ that is consistent with constraints and can be probed in future searches. In such a scenario, current bounds as well as sensitivities of future direct HNL searches such as at NA62 and DUNE will be affected.

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arXiv2024

Leptonic probes of Alternative Left-Right Symmetric Models

We explore constraints on the parameter space of the alternative left-right model originating from the leptonic sector. Our analyses focuses on both lepton-flavour-conserving observables, particularly the anomalous magnetic moment of the muon, and lepton-flavour-violating processes like $\mu \to e \gamma $ decay and $\mu-e$ conversions in nuclei. While contributions to the anomalous magnetic moment fall below the measured values at 2$\sigma$, current and future experimental sensitivities to flavour-violating branching rations of the Standard Model leptons are expected to impose lower bounds on the mass of the peculiar $SU(2)_R$ gauge boson of the model. This provides complementary constraints relative to existing limits, which are indirect and derived from collider bounds on the mass of the associated neutral gauge boson $Z^\prime$.

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arXiv2024

Neutrinoless Double Beta Decay from Scalar Leptoquarks: Interplay with Neutrino Mass and Flavor Physics

We perform a comprehensive analysis of neutrinoless double beta decay and its interplay with low-energy flavor observables in a radiative neutrino mass model with scalar leptoquarks $S_1(\bar{3},1,1/3)$ and $\widetilde{R}_2(3,2,1/6)$. We carve out the parameter region consistent with constraints from neutrino mass and mixing, collider searches, as well as measurements of several flavor observables, such as muon and electron anomalous magnetic moments, charged lepton flavor violation and rare (semi)leptonic kaon and $B$-meson decays, including the recent anomalies in $R_{D^{(*)}}$ and $B\to K\nu\bar{\nu}$ observables. We perform a global analysis to all existing constraints and show the (anti)correlations between all relevant Yukawa couplings satisfying these restrictions. We find that the most stringent constraint on the parameter space comes from $\mu \to e$ conversion in nuclei and $K^{+} \rightarrow\, \pi^{+}\nu \bar{\nu}$ decay. We also point out a tension between the muon and electron $(g-2)$ anomalies in this context. Taking benchmark values from the combined allowed regions, we study the implications for neutrinoless double beta decay including both the canonical light neutrino and the leptoquark contributions. We find that for normal ordering of neutrino masses, the leptoquark contribution removes the cancellation region that occurs for the canonical case. The effective mass in presence of leptoquarks can lie in the desert region between the standard normal and inverted ordering cases, and this can be probed in future ton-scale experiments like LEGEND-1000 and nEXO.

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arXiv2024

Collider Signatures of $W_R$ boson in the Alternative Left-Right Model

Alternative Left-Right Models offer an attractive option to left-right models. Emerging from $E_6$ grand unification, these models are consistent with light scalars which do not induce flavour-changing neutral currents due to the presence of exotic quarks. Here we investigate the signature at the LHC collider of the charged $W_R$ boson, which can be lighter than in left-right models. We include constraints from collider data and show that $W_R$ can be produced in pairs, or in conjunction with a light charged Higgs boson. The final decay products involve leptons or jets. We explore all production and decay possibilities and indicate which ones are most promising to be observed at the colliders. Our analysis shows that signals of $W_R$ bosons can be observed at the LHC at 27 TeV, some for lower luminosity, and under most favourable conditions, even at 13 TeV.

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arXiv2022

Dark Matter in the Alternative Left Right Model

The Alternative Left-Right Model is an attractive variation of the usual Left-Right Symmetric Model because it avoids flavour-changing neutral currents, thus allowing the additional Higgs bosons in the model to be light. We show here that the model predicts several dark matter candidates naturally, through introduction of an $R$-parity similar to the one in supersymmetry, under which some of the new particles are odd, while all the SM particles are even. Dark matter candidates can be fermionic or bosonic. We present a comprehensive investigation of all possibilities. We analyze and restrict the parameter space where relic density, direct and indirect detection bounds are satisfied, and investigate the possibility of observing fermionic and bosonic dark matter signals at the LHC. Both the bosonic and fermionic candidates provide promising signals, the first in LHC at 300 fb$^{-1}$, the second at higher luminosity, 3000 fb$^{-1}$. Signals from bosonic candidates are indicative of the presence of exotic $d^\prime$ quarks, while fermionic candidates imply the existence of charged Higgs bosons, all with masses in the TeV region.

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arXiv2022

Neutrino mass and charged lepton flavor violation in an extended left-right symmetric model

We consider an $U(1)_{L_μ-L_τ}$ extended left-right symmetric gauge theory where the neutrino masses are generated through inverse seesaw mechanism. In this model the muon $(g-2)$ anomaly is accounted for by the mediation of $Z_{μτ}$, the gauge boson of $U(1)_{L_μ- L_τ}$ symmetry. The symmetries of the model require the light neutrino mass matrix to have a particular two-zero texture, which leads to non-trivial constraints in the minimum neutrino mass. In addition, the model predicts observable charged lepton flavor violation in $μ-τ$ sector.

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arXiv2022

Vacuum Structure of Alternative Left-Right Model

We investigate an Alternative Left-Right Model (ALRM) with $SU(2)_L$ as well as $SU(2)_R$ gauge groups, but unlike the traditional left-right symmetric models (LRSM) is not symmetric under the exchange of the fermion content. Interestingly, it can be embedded in $E_6$, while its low energy Higgs structure resembles the LRSM, involving Higgs doublets $χ_{L,R}$ and one Higgs bidoublet $Φ$. We analyze the scalar potential and the vacuum structure of the theory analytically to ensure the stability of scalar potential via bounded from below (BFB) and copositivity criteria, accompanied by a numerical study. We establish the necessary criteria for electric charge preserving vacua, yielding constraints on various coupling parameters of the theory. Finally we obtain constraints on the parameters of the model from collider data on the masses of the Higgs scalars.

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arXiv2020

Exploring $0νββ$ and Leptogenesis in the Alternative Left-Right Model

We investigate the possibility of neutrinoless double beta decay ($0νββ$) and leptogenesis within the Alternative Left-Right Model (ALRM). Unlike the usual left-right symmetric model, ALRM features a Majorana right-handed neutrino which does not carry any charge. Further, in this picture the down-type quark and the charged leptons receive mass through the additional left-handed scalar field, rather than the usual doublet. Together, these features conspire to generate significant contributions to the $0νββ$ through vector-scalar ($WH$) mediation. For moderate masses of the relevant charged Higgs boson ($M_{H_1^\pm}\sim 200$ GeV), the half-life of $T_{\frac{1}{2}}^{WH}$ is $\sim 3\times 10^{26}~{\rm yrs}$ for both the case of $^{76}$Ge and $^{136}$Xe, well within the sensitivity expected by future experiments. Invoking the resonant leptogenesis, CP violation arising from the right-handed neutrino decay could be the required order to generate the correct baryogenesis, $ε\sim 10^{-6}$, for small Dirac phases and without any fine tuning.

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arXiv2020

Neutrino mass, mixing and muon $g-2$ explanation in $U(1)_{L_μ-L_τ}$ extension of left-right theory

We consider a gauged $U(1)_{L_μ-L_τ}$ extension of the left-right symmetric theory in order to simultaneously explain neutrino mass, mixing and the muon anomalous magnetic moment. We get sizeable contribution from the interaction of the new light gauge boson $Z_{μτ}$ of the $U(1)_{L_μ-L_τ}$ symmetry with muons which can individually satisfy the current bounds on muon $(g-2)$ anomaly ($Δa_μ$). The other positive contributions to $Δa_μ$ come from the interactions of singly charged gauge bosons $W_L$, $W_R$ with heavy neutral fermions and that of neutral CP-even scalars with muons. The interaction of $W_L$ with heavy neutrino is facilitated by inverse seesaw mechanism which allows large light-heavy neutrino mixing and explains neutrino mass in our model. CP-even scalars with mass around few hundreds GeV can also satisfy the entire current muon anomaly bound. The results show that the model gives a small but non-negligible contribution to $Δa_μ$ thereby eliminating the entire deviation in theoretical prediction and experimental result of muon $(g-2)$ anomaly. We have briefly presented a comparative study for symmetric and asymmetric left-right symmetric model in context of various contribution to $Δa_μ$. We also discuss how the generation of neutrino mass is affected when left-right symmetry breaks down to Standard Model symmetry via various choices of scalars.

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arXiv2020

Observing Left-Right Symmetry in the Cosmic Microwave Background

We consider the possibility of probing left-right symmetric model (LRSM) via cosmic microwave background (CMB). We adopt the minimal LRSM with Higgs doublets, also known as the doublet left-right model (DLRM), where all fermions including the neutrinos acquire masses only via their couplings to the Higgs bidoublet. Due to the Dirac nature of light neutrinos, there exist additional relativistic degrees of freedom which can thermalise in the early universe by virtue of their gauge interactions corresponding to the right sector. We constrain the model from Planck 2018 bound on the effective relativistic degrees of freedom and also estimate the prospects for planned CMB Stage IV experiments to constrain the model further. We find that $W_R$ boson mass below 4.06 TeV can be ruled out from Planck 2018 bound at $2σ$ CL in the exact left-right symmetric limit which is equally competitive as the LHC bounds from dijet resonance searches. On the other hand Planck 2018 bound at $1σ$ CL can rule out a much larger parameter space out of reach of present direct search experiments, even in the presence of additional relativistic degrees of freedom around the TeV corner. We also study the consequence of these constraints on dark matter in DLRM by considering a right handed real fermion quintuplet to be the dominant dark matter component in the universe.

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arXiv2020

A comparative study of $0νββ$ decay in symmetric and asymmetric left-right model

We study the new physics contributions to neutrinoless double beta decay ($0νββ$) in a TeV scale left-right model with spontaneous D-parity breaking mechanism where the values of the $SU(2)_L$ and $SU(2)_R$ gauge couplings, $g_L$ and $g_R$ are unequal. Neutrino mass is generated in the model via gauge extended inverse seesaw mechanism. We embed the model in a non-supersymmetric $SO(10)$ GUT with a purpose of quantifying the results due to the condition $g_{L} \neq g_{R}$. We compare the predicted numerical values of half life of $0νββ$ decay, effective Majorana mass parameter and other lepton number violating parameters for three different cases; (i) for manifest left-right symmetric model ($g_L = g_R$), (ii) for left-right model with spontaneous D parity breaking ($g_L \neq g_R$), (iii) for Pati-Salam symmetry with D parity breaking ($g_L \neq g_R$). We show how different contributions to $0νββ$ decay are suppressed or enhanced depending upon the values of the ratio $\frac{g_R}{g_L}$ that are predicted from successful gauge coupling unification.

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