arXiv subjects

S. Uma Sankar

Publications and source records attributed to S. Uma Sankar.

At least 19 recordsLinked to original sources

arXiv2025

Features of Charged Lepton Flavor Violation in an $A_4$ Symmetric Neutrino Mass Model

Neutrino flavour oscillations imply that there must be charged lepton flavour violation (CLFV) also. Different neutrino mass models predict different patterns of CLFV decays. Neutrino mass generation through standard see-saw mechanisms leads to the prediction that the branching ratios of meson CLFV decays will always be smaller than the corresponding radiative CLFV decays. In this work, we analyse an interesting neutrino mass model, based on $A_4$ symmetry, in which the symmetry and the symmetry-breaking pattern lead the neutrino mixing matrix to be of tri-bimaximal (TBM) form. In this model, we find that the meson CLFV decay amplitudes are not correlated to the corresponding radiative CLFV amplitudes, unlike in the case of see-saw models. The branching ratios of radiative CLFV decays are predicted to be negligibly small in this model, but those of the meson CLFV decays can be large enough to be observable in the near future.

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arXiv2025

Left-Right Symmetric Neutrino Mass Model without Scalar Bi-doublet

We consider a left-right symmetric model with an $SU(2)_L$ and an $SU(2)_R$ scalar doublet but without the scalar bidoublet. The charged fermion masses in this model are generated via a universal seesaw mechanism. We add a set of three gauge singlet neutral fermions with Majorana masses of the order of a TeV. The masses for the left-handed neutrinos are naturally small in this model because they occur only at one-loop and are generated through a see-saw mechanism. By an appropriate choice of the Yukawa couplings of the $SU(2)_L$ doublet and the masses of the gauge singlet fermions, it is possible to implement resonant leptogenesis at TeV scale. The right-handed sector of the model, through appropriate values of the Yukawa couplings of the $SU(2)_R$ doublet, leads to a warm dark matter candidate in the lightest right-handed neutrino with a mass of a few keV and an observable effective electron mass for neutrinoless double beta decay $m_{ββ}$.

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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

Charged Lepton Flavour Violating Meson Decays in Seesaw Models

The occurrence of neutrino oscillations demands the existence of flavour violation in charged lepton sector. The relation between the branching ratios of different charged lepton flavour violating (CLFV) decay modes depends on the details of the neutrino mass model. In this work, we consider the three types of simple seesaw mechanisms of neutrino masses and study the correlation between the radiative CLFV decays and the meson CLFV decays. We find that the meson CLFV decay branching ratios are negligibly small in type-II seesaw mechanism whereas they are constrained to be at least three (two) orders of magnitude smaller than the radiative CLFV decay branching ratios in the case of type-I (type-III) seesaw mechanism. Thus the relationship between these two modes of CLFV decays helps in distinguishing between different types of seesaw mechanism. If, the branching ratios of CLFV decays of mesons are larger than those of radiative CLFV decays, it provides a strong hint that the neutrino mass generating mechanism is more complicated than simple seesaw.

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arXiv2025

Experimental limits on quantum decoherence from $B$ meson systems

Neutral $B$-meson systems serve as critical tests of the Standard Model and play a key role in limiting its extensions. While these systems are typically studied under the assumption of perfect quantum coherence, interactions with the environment can lead to decoherence. Such decoherence effects can obscure the measured values of key parameters such as the oscillation frequency $ Δm $ and $CP$-violating parameter $ \sin 2β$. Using the experimental data, we present the first combined analysis of mixing asymmetry and $CP$-asymmetry measurements for $ B_d $-mesons, which indicates that $ λ_d $ is non-zero at approximately $ 6 \,σ$. We also establish the first experimental constraints on the decoherence parameter $ λ_s $ for $ B_s $-mesons, finding it to be non-zero at $ 3 \,σ$.

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arXiv2024

Probing quantum decoherence at Belle II and LHCb

With the advent of Belle II and the LHCb upgrade, the precision measurements of various B-Physics observables are on cards. This holds significant potential for delving into physics beyond the standard model of electroweak interactions. These measurements can also serve as means to establish limits on phenomena occurring at much finer length scales, such as quantum decoherence, which may arise due to potential discreteness in space-time or non-trivial topological effects. In this work, we set up the formalism to investigate the impact of quantum decoherence on several potential observables in $B$ meson systems. The approach employs the trace-preserving Kraus operator formalism, extending unitary evolution to non-unitary dynamics while maintaining complete positivity. In this formalism, the decoherence effects are parametrized in terms of a single parameter. Through the analysis of purely leptonic, semileptonic, and non-leptonic decays of $B$ mesons, we identify observables that could, in principle, be influenced by decoherence. The theoretical expressions are provided without neglecting the impact of decay width difference ($ΔΓ$) and $CP$ violation in mixing. Considering that many of these observables can be measured with high precision using the abundant data collected by LHCb and Belle II, our formalism can be applied to establish constraints on the decoherence parameter through multiple decay channels. This offers an alternative set-up for such studies, which, at present, are predominantly conducted in the neutrino sector.

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arXiv2023

Constraints on the Doublet Left-Right Symmetric Model from Higgs data

We study the constraints on the doublet left-right symmetric model (DLRSM) arising due to the Higgs data. The $SU(2)_L$ symmetry of this model is broken by three vacuum expectation values, $κ_1$, $κ_2$, and $v_L$. Most studies of this model assume that the ratios $r = κ_2/κ_1$ and $w = v_L/κ_1$ are very small. In this work, we study the constraints imposed on $r$ and $w$ by the Higgs data from LHC. We consider the most general scalar potential and calculate the masses of the CP-even scalars and the couplings of the lightest of these scalars to itself, to $W$ and $Z$ gauge bosons, and to the third generation quarks. We find that there is no lower bound on either $r$ or $w$. Equating the mass of the lightest CP-even scalar to $125$ GeV leads to an upper limit $w < 6.7$. The requirement that the Yukawa coupling of the quarks to the Higgs bidoublet of the model should be perturbative yields the upper bounds $r < 0.8$ and $w < 3.5$. The Yukawa coupling of the bottom quark to the lightest CP-even scalar strongly disfavours value of $r, w < 0.1$ and shows a marked preference for values of $w \sim \mathcal{O}(1)$.

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arXiv2023

$CP$ violation due to a Majorana phase in two flavor neutrino oscillations with decays

We study the conditions under which the Majorana phase of the two flavor neutrino mixing matrix appears in the oscillation probabilities and causes $CP$ violation. We find that the Majorana phase remains in the neutrino evolution equation if the neutrino decay eigenstates are not aligned with the mass eigenstates. We show that, in general, two kinds of $CP$ violation are possible: one due to the Majorana phase and the other due to the phase of the off-diagonal element of the neutrino decay matrix. We find that the $CP$ violating terms in the oscillation probabilities are also sensitive to neutrino mass ordering.

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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

Neutrino Oscillation parameter determination at INO-ICAL using track and hit information from GEANT

We study the capability of INO-ICAL to determine the atmospheric neutrino oscillation parameters $|Δm^2_{31}|$ and $\sin^2 θ_{23}$. We do not use any generator level information. Instead, we process the generated atmospheric neutrino events through GEANT4 simulation of the detector and the event reconstruction framework. Among the outputs of this framework, only the momentum and direction of the longest track were used in a previous study by other authors. In this study, in addition to these variables, we consider a third variable based on additional hits, which arise due to hadrons in the event. We show that the inclusion of the this variable leads to a 30% reduction in the uncertainties of $|Δm^2_{31}|$ for a 5-year run of ICAL. We find that doubling the exposure time leads to a 30% reduction in the uncertainties of both $|Δm^2_{31}|$ and $\sin^2 θ_{23}$.

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arXiv2022

A unique discrimination between new physics scenarios in $b\rightarrow sμ^+μ^-$ anomalies

A number of observables related to the $b \to s \ell^+ \ell^-$ transition show deviations from their standard model predictions. A global fit to the current $b\rightarrow s\ell^+\ell^-$ data suggests several new physics solutions. Considering only one operator at a time and new physics only in the muon sector, it has been shown that the new physics scenarios (I) $C_9^{\rm NP}<0$, (II) $C_{9}^{\rm NP} = -C_{10}^{\rm NP}$, (III) $C_9^{\rm NP} = -C_9^{\prime \rm NP}$ can account for all data. In this work, we develop a procedure to uniquely identify the correct new physics solution. The scenario II predicts a significantly lower value of $\mathcal{B}(B_s\to μ^+μ^-)$ and can be distinguished from the other two scenarios if the experimental uncertainty comes down by a factor of three. On the other hand, a precise measurement of the CP averaged angular observables $S_9$ in high $q^2$ bin of $B\to K^*μ^+μ^-$ decay can uniquely discriminate between the other two scenarios. We propose new methods, in terms of azimuthal angle asymmetries, to measure $S_9$ with the necessary precision.

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arXiv2022

A review of the tension between the T2K and NO$ν$A appearance data and hints to new physics

In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NO$ν$A. The tension arises mostly due to the mismatch in the appearance data of the two experiments. We explain how this tension arises based on $ν_μ\to ν_e$ and $\barν_μ\to \barν_e$ oscillation probabilities. We define the reference point of vacuum oscillation, maximal $θ_{23}$ and $δ_{CP}=0$ and compute the $ν_e/\barν_e$ appearance events for each experiment. We then study the effects of deviating the unknown parameters from the reference point and the compatibility of any given set of values of unknown parameters with the data from T2K and NO$ν$A. T2K observes a large excess in the $ν_e$ appearance event sample compared to the expected $ν_e$ events at the reference point, whereas \nova observes a moderate excess. The large excess in T2K dictates that $δ_{CP}$ be anchored at $-90^\circ$ and that $θ_{23}>π/4$ with a preference for normal hierarchy. The moderate excess at NO$ν$A leads to two degenerate solutions: A) NH, $0<δ_{CP}<180^\circ$, and $θ_{23}>π/4$; B) IH, $-180^\circ<δ_{CP}<0$, and $θ_{23}>π/4$. This is the main cause of the tension between the two experiments. We have reviewed the status of three beyond standard model (BSM) physics scenarios, (a) non-unitary mixing, (b) Lorentz invariance violation and (c) non-standard neutrino interactions, to resolve the tension.

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arXiv2021

Signatures of $A_4$ symmetry in the charged lepton flavour violating decays in a neutrino mass model

We study the charged lepton flavour violation in a popular neutrino mass model with $A_4$ discrete symmetry. This symmetry requires the presence of multiple Higgs doublets in the model and it also dictates the flavour violating Yukawa couplings of the additional neutral scalars of the model. Such couplings lead to the decays of the neutral mesons, the top quark and the $τ$ lepton into charged leptons of different flavours at tree level. The $A_4$ symmetry of the model leads to certain characteristic signatures in these decays. We discuss these signatures and predict the rates for the most favourable charged lepton flavour violating modes.

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arXiv2020

Matter vs Vacuum oscillations in Atmospheric Neutrinos

Atmospheric neutrinos travel very long distances through earth matter. It is expected that the matter effects lead to significant changes in the neutrino survival and oscillation probabilities. Initial analysis of atmospheric neutrino data by the Super- Kamiokande collaboration is done using the vacuum oscillation hypothesis, which provided a good fit to the data. In this work, we did a study to differentiate the effects of vacuum oscillations and matter modified oscillations in the atmospheric neutrino data. We find that magnetized iron detector, ICAL at INO, can make a 3 sigma discrimination between vacuum oscillations and matter oscillations, for both normal and inverted hierarchies, in ten years.

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arXiv2020

Matter vs. vacuum oscillations at long baseline accelerator neutrino experiments

The neutrino oscillation probabilities at the long-baseline accelerator neutrino experiments are expected to be modified by matter effects. We search for evidence of such modification in the data of T2K and NO$ν$A, by fitting the data to the hypothesis of (a) matter modified oscillations and (b) vacuum oscillations. We find that vacuum oscillations provide as good a fit to the data as matter modified oscillations. Even extended runs of T2K and NO$ν$A, with 5 years in neutrino mode $(5 ν)$ and five years in anti-neutrino mode $(5 \barν)$, can {\bf not} make a $3~σ$ distinction between vacuum and matter modified oscillations. The proposed experiment DUNE, with neutrino and anti-neutrino runs of 5 years each $(5 ν+ 5 \barν)$, can rule out vacuum oscillations by itself at $5~σ$ if the hierarchy is normal. If the hierarchy is inverted, a $5~σ$ discrimination against vacuum oscillations requires the combination of $(5 ν+ 5 \barν)$ runs of T2K, \nova and DUNE.

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arXiv2020

Tensions between the appearance data of T2K and NOvA

The long baseline neutrino experiments, T2K and NOvA, have taken significant amount of data in each of the four channels: (a) $ν_μ$ disappearance, (b) $\barν_μ$ disappearance (c) $ν_e$ appearance and (d) $\barν_e$ appearance. There is a mild tension between the disappearance and the appearance data sets of T2K. A more serious tension exists between the $ν_e$ appearance data of T2K and the $ν_e / \barν_e$ appearance data of NOvA. This tension is significant enough that T2K rules out the best-fit point of NOvA at $95\%$ confidence level whereas NOvA rules out T2K best-fit point at $90\%$ confidence level. We explain the reason why these tensions arise. We also do a combined fit of T2K and NOvA data and comment on the results of this fit.

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arXiv2020

Flavoured CP-asymmetry at the effective neutrino mass floor

Both neutrinoless double beta decay and leptogenesis require neutrinos to be Majorana fermions. A relation between these two phenomena can be derived once the mechanism of neutrino mass generation is specified. We first derive the constraints on the Majorana phases by minimising the effective neutrino mass in neutrinoless double beta decay with respect to the smallest mass among the light neutrinos. Given these phases, we derive a lower bound on $M_{1}$ (the mass of the lightest of the heavy neutrinos) in the framework of Type-I seesaw mechanism, subject to the constraint that the CP asymmetry required for adequate leptogenesis is larger than $10^{-8}$. We find that $M_{1} \geq 10^{10}\,(10^{9})$ GeV for the case of Normal (Inverted) hierarchy. We extend our analysis to the case when one of the heavy neutrinos decouples (two right handed neutrino models). In this case we find $M_{1} \geq 10^{10}\,(10^{11})$ GeV for the case of Normal (Inverted) hierarchy.

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arXiv2020

New Physics solutions for $b\rightarrow c\,τ\,\barν$ anomalies after Moriond 2019

At Moriond 2019, Belle collaboration has announced new measurements on the flavour ratios $R_D - R_{D^*}$ which are consistent with their Standard Model predictions within $1.2σ$. After inclusion of these measurements, the global tension in $R_D - R_{D^*}$ has reduced from $4.1σ$ to $3.1σ$ which is still significant. The measurements of these ratios indicate towards the violation of lepton flavor universality in $b\rightarrow c\,l\,\barν$ decay. Assuming new physics in $b\rightarrow c\,τ\,\barν$ transition, we have done a global fit to all available data in this sector to identify the allowed new physics solutions. We find that there are seven allowed new physics solutions which can account for all measurements in $b\rightarrow c\,τ\,\barν$ transition. We show that a simultaneous measurement of the $τ$ polarization fraction and forward-backward asymmetry in $B\rightarrow D\,τ\,\barν$, the zero crossing point of forward backward asymmetry in $B\rightarrow D^*τ\barν$ and the branching ratio of $B_c\rightarrow τ\,\barν$ decay can distinguish these seven new physics solutions if they can be measured with a required precision.

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