arXiv subjects

Lopamudra Mukherjee

Publications and source records attributed to Lopamudra Mukherjee.

17 recordsLinked to original sources

arXiv2026

Massive right-handed neutrinos in $\bar{B} \to D^* τ\bar X$ decay

We explore signatures of a massive right-handed neutrino (RHN) in angular distributions of $\bar{B} \to D^* (\to D π) τ(\to πν_τ) \bar X$ decays, where $X$ is an invisible state. We assume the new physics is described by the standard model effective field theory extended with an RHN in the MeV-GeV mass range. We calculate for the first time the full differential distributions in terms of the visible final states, including the decay of the $τ$ lepton. We evaluate the sensitivity of various distributions to the new physics operators.

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arXiv2026

Probing anomalous quartic gauge couplings via vector boson scattering at the same-sign muon collider

The measurement of quartic gauge couplings (QGCs) provides a crucial test of the non-Abelian gauge structure of the Standard Model and offers sensitivity to new physics effects. In this work, we explore the potential of the proposed multi-TeV same-sign muon collider, $μ$TRISTAN, to probe anomalous quartic gauge couplings (aQGCs) through vector boson scattering (VBS) processes. Owing to the same-sign initial state, s-channel contributions are absent, rendering VBS as the dominant production mode and thereby significantly enhancing the sensitivity to aQGCs. Using dimension-8 Standard Model Effective Field Theory (SMEFT) operators, we classify the relevant operator sets contributing to charged and neutral QGCs, and confront them with existing bounds from the LHC. A detailed collider analysis is performed across multiple final states: $2V2ν$, $Vγ\ell ν$, $2V\ell ν$, $2γ2\ell$, and $2V2\ell$ ($V=$ reconstructed $W$ and $Z$ boson), applying optimized selection strategies. We present the projected sensitivities at the $μ$TRISTAN with center-of-mass energies 2 TeV and 6 TeV, with integrated luminosities of 1 ab$^{-1}$ and 10 ab$^{-1}$, and demonstrate significant improvements over current experimental limits from the LHC. Our results establish $μ$TRISTAN as a powerful probe of electroweak symmetry breaking dynamics and aQGCs in a model-independent framework.

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arXiv2026

New physics searches via angular distributions of $ \bar{B} \to D^* (\to D π) τ(\to \ell ν_τ\barν_\ell) \barν_τ$ decays

The study of $\bar{B} \to D^* τ{\barν}_τ$ angular distribution can be used to obtain information about new physics (or beyond the Standard Model) couplings, which are motivated by various $B$ anomalies. However, the inability to measure precisely the three-momentum of the $τ$ lepton hinders such measurements, as the tau decay contains one or more undetected neutrinos. Here, we present a measurable angular distribution of $\bar{B} \to D^* τ{\barν}_τ$ by considering the additional decay $τ\to \ell ν_τ\barν_\ell$, where $\ell \in \{ e , μ\}$. The full process used is $\bar{B} \to D^* (\to D π) τ(\to \ell ν_τ\barν_\ell) \barν_τ$, in which only the $\ell$ and $D^*$ are reconstructed. A fit to the experimental angular distribution of this process can be used to extract information on new physics parameters. To demonstrate the feasibility of this approach, we generate simulated data for this process and perform a sensitivity study to obtain the expected statistical errors on new physics parameters from experiments in the near future. We obtain a sensitivity of the order of 5% for the right-handed current and around 6% for the tensor current. In addition, we use the recent lattice QCD data on $B \to D^*$ form factors and obtain correlations between form factors and new physics parameters.

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arXiv2025

Constraining anomalous $W tb$ and related SMEFT couplings using low-energy and electroweak precision observables

We investigate constraints on couplings of Standard Model effective field theory (SMEFT) operators contributing to $Wtb$ effective vertex at tree level. We study the one-loop level impact of these couplings on the low-energy flavour changing charged and neutral current processes and on the electroweak precision observables. We use the available data on these relevant processes to constrain the associated SMEFT/$Wtb$ couplings. Solving the renormalisation group equations, we connect the SMEFT couplings at different scales and use the bounds at low energy to obtain the relevant bounds at the large scale $Λ$. Our findings indicate significantly improved constraints on the couplings compared to existing constraints on $Wtb$ couplings by ATLAS and CMS. Additionally, we predict branching ratios for various top-FCNC processes, which exceed SM expectations by several orders of magnitude but remain within the reach of future colliders. These SMEFT couplings, or anomalous couplings of the effective $Wtb$ vertex, can further constrain different UV-complete and simplified models that generate such interactions at the tree or loop level.

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arXiv2025

Is Dark Matter the origin of the $B\to K ν\barν$ excess at Belle II?

We present two models of dark matter (DM) that can provide a natural explanation of the excess of $B^+\to K^+ +\,\text{invisible}$ events with respect to the Standard Model (SM) prediction for $B^+\to K^+ ν\barν$, which has been reported by the Belle II collaboration. Interactions between the dark and the visible sector are mediated by an axion-like particle (ALP) in one case, by the kinetic mixing between a dark photon and the SM photon in the second case. Both models encompass a light fermion singlet as the DM candidate and can account for the observed DM relic abundance through, respectively, the freeze-in and the freeze-out production mechanism, while simultaneously explaining the Belle II excess.

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arXiv2025

Neutrino Nonstandard Interactions and Lepton Flavor Universality violation at SND@LHC via charm production

In this work, we explore the effect of neutrino nonstandard interactions (NSI) involving the charm quark at SND@LHC. Using an effective description of new physics in terms of four-fermion operators involving a charm quark, we constrain the Wilson coefficients of the effective interaction from two and three-body charmed meson decays. In our fit, we include charmed meson decays not only to pseudoscalar final states but also to vector final states and include decays to the $η$ and $η^\prime$ final states. We also consider constraints from charmed baryon decays. We then study the effect of new physics in neutrino scattering processes, involving charm production at SND@LHC, for various benchmark new physics couplings obtained from the low energy fits. Finally, we also study the effects of lepton universality violation (LUV) assuming that the new physics coupling is not lepton universal.

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arXiv2024

Probing ALP Lepton Flavour Violation at $μ$TRISTAN

Axion-like particles (ALPs) with lepton flavour violating (LFV) interactions are predicted within a wide range of flavoured ALP models. The proposed $μ$TRISTAN high-energy $e^-μ^+$ and $μ^+μ^+$ collider will provide a good opportunity to explore flavour physics in the charged lepton sector. In this work, based on a model-independent effective Lagrangian describing the ALP leptonic interactions, we investigate the potential of $μ$TRISTAN to probe ALP LFV couplings. We analyse the testability of selected ALP production channels with potential sensitivity at $μ$TRISTAN, considering different beams and collision energies, including $e^- μ^+ \to a γ$, $e^- μ^+ \to e^- τ^+ a$, $μ^+ μ^+ \to μ^+ τ^+ a$, and $e^- μ^+ \to τ^- μ^+ a$. The produced ALP $a$ is either long-lived or can promptly decay to flavour violating or conserving charged lepton final states. In particular, combining the above LFV ALP production modes with a suitable LFV decay mode, one can identify signatures that are virtually free of Standard Model background. We show the resulting sensitivity of $μ$TRISTAN to LFV ALP couplings and compare it with multiple low-energy leptonic constraints and the future improvements thereof. We find that $μ$TRISTAN can be generally complementary to searches for low-energy LFV processes and measurements of the leptonic magnetic dipole moments and has the capability to explore unconstrained parameter space for ALP masses in the $\mathcal{O}(1)$ to $\mathcal{O}(100)$~GeV range. In the light ALP regime, however, the parameter space that $μ$TRISTAN is sensitive to, has been already excluded by low-energy searches for LFV decays.

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arXiv2024

Hint of a new scalar interaction in LHCb data?

We explain recent LHCb measurements of the lepton universality ratios, $R_{D^{(*)}}^{τ/\ell}\equiv \frac{\mathcal{B}(\bar B \to D^{(*)+} τ^- \barν_τ)} {\mathcal{B}(\bar B \to D^{(*)+}\ell^- \barν_\ell)}$ and ${R(Λ_c^+)}^{τ/\ell} \equiv \frac{\mathcal{B}(Λ_b \to Λ_c^+ τ^- \barν_τ)}{\mathcal{B}(Λ_b \to Λ_c^+ \ell^- \barν_{\ell})}$ with $\ell=μ$, via new physics that affects $R_D^{τ/\ell}$ and $R(Λ_c^+)^{τ/\ell}$ but not $R_{D^*}^{τ/\ell}$. The scalar operator in the effective theory for new physics is indicated. We find that the forward-backward asymmetry and $τ$ polarization in $\bar{B} \to D^+ τ^{-} \barν_τ$ and $Λ_b \to Λ_c^+ τ^- \barν_τ$ decays are significantly affected by the scalar interaction. We construct a simple two Higgs doublet model as a realization of our scenario and consider lepton universality in semileptonic charm and top decays, radiative $B$ decay, $B$-mixing, and $Z \to b \bar b$.

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arXiv2024

$B \to K ν\barν$, MiniBooNE and muon $g-2$ anomalies from a dark sector

Belle II has reported the first evidence for $B^+ \to K^+ν\barν$ with a branching ratio $2.7 σ$ higher than the standard model expectation. We explain this, and the MiniBooNE and muon anomalous magnetic moment anomalies in a model with a dark scalar that couples to a slightly heavier sterile Dirac neutrino and that communicates with the visible sector via a Higgs portal. We make predictions for rare kaon and other $B$ meson decays.

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arXiv2023

Constraining New Physics with Possible Dark Matter Signatures from a Global CKM Fit

We constrain the parameters of a representative new physics model with possible dark matter (DM) signature from a global CKM fit analysis. The model has neutral quark current interactions mediated by a scalar, impacting the semileptonic and purely leptonic meson decays at one-loop. We take this opportunity to update the fit results for the Wolfenstein parameters and the CKM elements with and without a contribution from the new model using several other updated inputs. Alongside, we have analyzed and included in the CKM fit the $B\to D^*\ellν_{\ell}$ decay. The newly available inputs on the relevant form factors from lattice are included, and the possibility of new physics effects in $B\to D^*\ellν_{\ell}$ is considered. We obtain tight constraints on the relevant new physics parameters. We have studied the possible implications of this constraint on DM phenomenology. Apart from DM, the bounds are also applicable in other relevant phenomenological studies.

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arXiv2023

Dark photon and dark $Z$ mediated $B$ meson decays

We study flavor changing neutral current decays of $B$ and $K$ mesons in the dark $U(1)_D$ model, with the dark photon/dark $Z$ mass between 10 MeV and 2 GeV. Although the model provides an improved fit (compared to the standard model) to the differential decay distributions of $B \to K^{(*)} \ell^+ \ell^-$, with $\ell= μ, e$, and $B_s \to ϕμ^+ μ^-$, the allowed parameter space is ruled out by measurements of atomic parity violation, $K^+ \to μ^+ + invisible$ decay, and $B_s - \overline{B}_s$ mixing, among others. To evade constraints from low energy data, we extend the model to allow for (1) additional invisible $Z_D$ decay, (2) a direct vector coupling of $Z_D$ to muons, and (3) a direct coupling of $Z_D$ to both muons and electrons, with the electron coupling fine-tuned to cancel the $Z_D$ coupling to electrons via mixing. We find that only the latter case survives all constraints.

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arXiv2022

Implications for the $ΔA_{FB}$ anomaly in ${\bar B}^0\to D^{*+}\ell^- {\barν}$ using a new Monte Carlo Event Generator

Recent experimental results in $B$ physics from Belle, BaBar and LHCb suggest new physics (NP) in the weak $b\to c$ charged-current and the $b\to s$ neutral-current processes. Here we focus on the charged-current case and specifically on the decay modes $\bar{B}^0\to D^{*+}\ell^- \barν$ with $\ell = e$ and $μ$. The world averages of the ratios $R_D$ and $R_D^{*}$ currently differ from the Standard Model (SM) predictions by $3.4σ$ while recently a new anomaly has been observed in the forward-backward asymmetry measurement, $A_{FB}$, in $ \bar{B}^0\to D^{*+}μ^- \barν$ decay. It is found that $ΔA_{FB} = A_{FB}(B\to D^{*} μν) - A_{FB} (B\to D^{*} e ν)$ is around $4.1σ$ away from the SM prediction in an analysis of 2019 Belle data. In this work we explore possible solutions to the $ΔA_{FB}$ anomaly and point out correlated NP signals in other angular observables. These correlations between angular observables must be present in the case of beyond the Standard Model physics. We stress the importance of $Δ$ type observables that are obtained by taking the difference of the observable for the muon and the electron mode. These quantities cancel form factor uncertainties in the SM and allow for clean tests of NP. These intriguing results also suggest an urgent need for improved simulation and analysis techniques in $\bar{B}^0\to D^{*+}\ell^- \barν$ decays. Here we also describe a new Monte Carlo Event-generator tool based on EVTGEN that we developed to allow simulation of the NP signatures in $\bar{B}^0\to D^{*+}\ell^- ν$, which arise due to the interference between the SM and NP amplitudes. We then discuss prospects for improved observables sensitive to NP couplings with 1, 5, 50, and 250 ab$^{-1}$ of Belle II data, which seem to be ideally suited for this class of measurements.

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arXiv2022

A new tool to search for physics beyond the Standard Model in ${\bar B}\to D^{*+}\ell^- {\barν}$

Recent experimental results in $B$ physics from Belle, BaBar and LHCb suggest new physics (NP) in the weak $b\to c$ charged-current and the $b\to s$ neutral-current processes. Here we focus on the charged-current case and specifically on the decay modes $B\to D^{*+}\ell^- \barν$ with $\ell = e, μ,$ and $τ$. The world averages of the ratios $R_D$ and $R_D^{*}$ currently differ from the Standard Model (SM) by $3.4σ$ while $ΔA_{FB} = A_{FB}(B\to D^{*} μν) - A_{FB} (B\to D^{*} e ν)$ is found to be $4.1σ$ away from the SM prediction in an analysis of 2019 Belle data. These intriguing results suggest an urgent need for improved simulation and analysis techniques in $B\to D^{*+}\ell^- \barν$ decays. Here we describe a Monte Carlo Event-generator tool based on EVTGEN developed to allow simulation of the NP signatures in $B\to D^*\ell^- ν$, which arise due to the interference between the SM and NP amplitudes. As a demonstration of the proposed approach, we exhibit some examples of NP couplings that are consistent with current data and could explain the $ΔA_{FB}$ anomaly in $B\to D^*\ell^- ν$ while remaining consistent with other constraints. We show that the $Δ$-type observables such as $ΔA_{FB}$ and $ΔS_5$ eliminate most QCD uncertainties from form factors and allow for clean measurements of NP. We introduce correlated observables that improve the sensitivity to NP. We discuss prospects for improved observables sensitive to NP couplings with the expected 50 ab$^{-1}$ of Belle II data, which seems to be ideally suited for this class of measurements.

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arXiv2022

FCNC in Concurrent Dark Photon and Dark $Z$ Models

In this work we fit the available binned data of the differential decay distribution of the exclusive $B \to K^{(*)} \ell^+ \ell^-$ and $B_s \to ϕμ^+ μ^-$ decays to the mass and mixing parameters of a light dark vector boson model. Due to an incorrect assessment of the dominant contribution of the dark $Z$ model to the FCNC B-meson decays, a previous work in literature reported that $\mathcal{O}(1)$ mixings were allowed by the data. In this talk we report the correct calculations and constraints on the mixing parameters as well as the mass of the dark vector boson using the marginalization technique. We also study other relevant bounds on the parameter space from low energy experiments such as the atomic parity violation, $K^+ \to μ^+ + invisible$ decay, $B_s - \overline{B}_s$ mixing etc and find that inspite of obtaining a good fit to the experimental $b \to s \ell^+ \ell^-$ data, the entire parameter space gets ruled out from some of the above bounds. In case of a model with tiny mixing and additional interaction of the dark $Z$ to the muon, some bounds are relaxed while some others are violated.

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arXiv2022

Shedding Flavor on Dark via Freeze-in: $U(1)_{B-3L_i}$ Gauged Extensions

We consider a singlet fermionic dark matter (DM) $χ$ in a gauged $U(1)_{B-3L_i}$ extension of the Standard Model (SM), with $i\in e\,,μ\,,τ$, and derive bounds on the allowed parameter space, considering its production via freeze-in mechanism. The DM communicates with the SM only through flavorful vector-portal $Z_\text{B3L}$ due to its non-trivial charge $x$ under $U(1)_{B-3L_{i}}$, which also guarantees the stability of the DM over the age of the Universe for $x\neq\{\pm 3/2,\pm 3\}$. Considering $Z_\text{B3L}$ to lie within the mass range of a few MeV up to a few GeV, we obtain constraints on the gauge coupling $g_\text{B3L}$ from the requirement of producing right relic abundance. Taking limits from various (present and future) experimental facilities, e.g., NuCal, NA64, FASER, SHiP into account, we show that the relic density allowed parameter space for the frozen in DM can be probed with $g_\text{B3L}\gtrsim 10^{-8}$ for both $m_χ<m_\text{ZB3L}/2$ and $m_χ\gtrsim m_\text{ZB3L}$, while $g_\text{B3L}\lesssim 10^{-8}$ remains mostly unconstrained. We also briefly comment on the implications of neutrino mass generation via Type-I seesaw and anomalous $(g-2)_μ$ in context with $B-3L_μ$ gauged symmetry.

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arXiv2020

Low Scale $U(1)_X$ Gauge Symmetry as an Origin of Dark Matter, Neutrino Mass and Flavour Anomalies

We study a generic leptophilic $U(1)_X$ extension of the standard model with a light gauge boson. The $U(1)_X$ charge assignments for the leptons are guided by lepton universality violating (LUV) observables in semileptonic $b \to s\ell\ell$ decays, muon anomalous magnetic moment and the origin of leptonic masses and mixing. Anomaly cancellation conditions require the addition of new chiral fermions in the model, one of which acts as a dark matter (DM) candidate when it is stabilised by an additional $\mathcal{Z}_2$ symmetry. From our analysis, we show two different possible models with similar particle content that lead to quite contrasting neutrino mass origin and other phenomenology. The proposed models also have the potential to address the anomalous results in $b\to c\ellν_{\ell}$ decays like $R(D), R(D^*)$, electron anomalous magnetic moment and the very recent KOTO anomaly in the kaon sector. We also discuss different possible collider signatures of our models which can be tested in future.

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arXiv2019

Correlating the anomalous results in $b \to s$ decays with inert Higgs doublet dark matter and muon $(g-2)$

In this article, we have considered an extension of the inert Higgs doublet model with $SU(2)_L$ singlet vector like fermions. Our model is capable of addressing some interesting anomalous results in $b\to s\ell^+\ell^-$ decays (like $R(K^{(*)})$) and in muon $(g-2)$. Apart from explaining these anomalies, and being consistent with other flavour data, the model satisfies relevant constraints in the dark matter sector, while remaining within the reach of ongoing direct detection experiments. The model also produces signatures at the large hadron collider (LHC) with final states comprised of dilepton, dijet and missing energy, providing signals to be probed at higher luminosity.

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