arXiv subjects

Ria Sain

Publications and source records attributed to Ria Sain.

11 recordsLinked to original sources

arXiv2026

Probing Light Dark Fermions in $B \to D^{(*)}\ell X_{\rm inv}$ via Rate Distributions

Experimental analyses of the semileptonic decays $ B \to D^{(*)} \ell \bar{\nu}$ typically rely on the assumption that the missing energy originates from a massless neutrino, as predicted by the Standard Model. However, this assumption may not hold in scenarios where the invisible final-state particle is instead massive, such as a sterile neutrino or a dark-sector fermion. In this work, we explore how the presence of a massive dark sector fermion modifies the kinematic and angular distributions of these decays. Our analysis is carried out within the framework of a general weak effective theory, and we also discuss effective and simplified models in which these interactions may arise. In addition, we study the implications of these effects for the extraction of the CKM matrix element $ |V_{cb}|$. Overall, our results show that relaxing the standard assumption of a massless neutrino can lead to observable effects and provide a framework for systematically investigating their impact on semileptonic $B$- decay distributions.

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arXiv2026

$|V_{cb}|$ determinations from $\bar{B} \to D^{(*)} \ell \bar\nu$ decays within the SM and beyond

We investigate the $|V_{cb}|$ determinations from exclusive semi-leptonic $\bar{B} \to D^{(*)}\ell\bar\nu$ decays, together with comprehensive fit analyses of the $\bar{B} \to D^{(*)}$ transition form-factors, by taking into account recent updates of experimental distribution data and theoretical evaluations. Several commonly adopted form-factor parameterizations, including BSZ, BGL and HQET, have been considered under different fit scenarios. We compare the fitted values and study how the $|V_{cb}|$ determinations depend on the form-factor parameterizations and on the treatment of the experimental inputs. In particular, we reproduce the official PDG average of $|V_{cb}|$ with the BGL parameterization, while the HQET parameterization tends to give a smaller value of $|V_{cb}|$. We also consider new-physics effects that can contribute to $\bar{B} \to D^{(*)}\ell\bar\nu$, and examine whether non-zero new-physics contributions are still allowed by the current data.

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arXiv2025

$\Lambda_b \to \Lambda^{(\ast)}\nu\bar{\nu}$ decays and the recent Belle-II $B^+\to K^+\nu\bar{\nu}$ data

The Belle-II experiment has recently reported the first measurement of $B^+ \to K^+ \nu\bar{\nu}$ decay which exceeds the Standard Model prediction by approximately 2.7$\sigma$. The deviation may indicate the presence of new physics beyond the Standard Model in the $b\to s\nu\bar{\nu}$ sector. Under this assumption, we study the hadronic $\Lambda_b \to \Lambda(\to p\pi)\nu\bar{\nu}$ and $\Lambda_b \to \Lambda^\ast(\to N\!\bar{K})\nu\bar{\nu}$ decays within both the Standard Model and beyond. We work in a low energy effective field theory framework with additional light right-handed neutrinos. We calculate the differential branching ratios of these decay modes and explore the implications of the Belle-II results through various observables.

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arXiv2024

An Imperative study of the angular observables in $Λ_b \to Λ_c^{+}(\to Λπ^{+})\ell^-\barν_{\ell}$ decay and probing the footprint of new physics

We study the four-fold angular distribution of the semileptonic $Λ_b \to Λ_c^{+}(\to Λπ^{+})\ell^- \barν_{\ell}$ decay and find out analytical expressions for various asymmetric and angular observables in the standard model (SM) and the relevant new physics (NP) scenarios. Using the available inputs on the form factors from the lattice, we predict the values with uncertainties of all these observables in the SM. We have considered NP effects only in $b\to cτ^-\barν_τ$ transitions, and constrain the Wilson coefficients of the model-independent beyond the SM operators from the available data on $B\to D^{(*)}\ellν_{\ell}$ and $Λ_b \to Λ_c^{+}\ell^- \barν_{\ell}$ decays. In this study, we focus on analysing the contributions to these decays in one- and two-operator scenarios. Furthermore, we test the new physics sensitivities (one or two-operator scenarios) of the different angular and asymmetric observables in $Λ_b \to Λ_c^+ (\to Λπ^{+}) τ^- \barν_τ$ decays and discuss the phenomenology. We observe that it will be possible to distinguish the different NP effects from one another by measuring these observables.

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arXiv2024

Model-independent analysis of new physics effects in $B\rightarrow K^*_2(1430)μ^+ μ^-$ decay

Recently, the LHCb Collaboration provided updated measurements for the lepton flavour ratios $R_K$ and $R_{K^*}$. The currently observed values align with the predictions of the standard model. In light of these recent updates, our investigation delves into the repercussions of new physics characterized by universal couplings to electrons and muons. We specifically focus on their impact on various observables within the $B\to K_2^*(1430)(\to Kπ)μ^+ μ^-$ decay. These observables include the differential branching ratio, forward-backward asymmetry ($A_{FB}$), longitudinal polarization asymmetry ($F_L$), and a set of optimized observables ($P_i$). Our findings indicate that the branching ratio of $B\to K_2^*(\to Kπ)μ^+ μ^-$ decay can be suppressed up to $25\%$ for various new physics solutions. Furthermore, all permissible new physics scenarios demonstrate finite enhancement in the muon forward-backward asymmetry $(A_{FB})$ as well as an increase in the value of the optimized angular observable $P_2$. Moreover, in the presence of new physics zero crossing points for $A_{FB}$ and $P_2$ shift towards higher $q^2$. The current data have a mild deviation from SM predictions in $P_5'$ observable in the low-$q^2$ bin. We also explored massive $Z'$ models, which can generate universal 1D new physics scenarios, characterized by $C_9^{NP}<0$, $C_9^{NP}=-C_{10}^{NP}$, and $C_9^{NP}=-C_9'$. Using additional constraints coming from $B_s-\overline{B_s}$ mixing and neutrino trident process, we find that the conclusions of the model-independent analysis remain valid.

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arXiv2023

Lepton Flavor Violating $B\to K^*_2(1430)μ^{\pm}τ^{\mp}$ Decays

A number of measurements in decays induced by the semileptonic $b\to s$ and $b\to c$ transitions hint towards a possible role of new physics in both sectors. Motivated by these anomalies, we investigate the lepton flavor violating $B\to K^*_2 (1430)μ^{\pm}τ^{\mp}$ decays. We calculate the two-fold angular distribution of $B\to K^*_2\ell_1\ell_2$ decay in presence of vector, axial-vector, scalar and pseudo-scalar new physics interactions. We then compute the branching fraction and lepton forward-backward asymmetry in the framework of $U^{2/3}_1$ vector leptoquark which is a viable solution to the current $B$ anomalies. We find that the upper limits are $\mathcal{B}(B\to K^*_2μ^-τ^+)\leq 1.64\times 10^{-7}$ and $\mathcal{B}(B\to K^*_2μ^+τ^-)\leq 0.60\times 10^{-7}$ at $90\%$ C.L.

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arXiv2023

Flavour bounds on the flavon of a minimal and a non-minimal $\mathcal{Z}_2 \times \mathcal{Z}_N$ symmetry

We investigate flavour bounds on the $\mathcal{Z}_2 \times \mathcal{Z}_5$ and $\mathcal{Z}_2 \times \mathcal{Z}_9$ flavour symmetries. These flavour symmetries are a minimal and a non-minimal forms of the $\mathcal{Z}_2 \times \mathcal{Z}_N$ flavour symmetry, that can provide a simple set-up for the Froggatt-Nielsen mechanism. The $\mathcal{Z}_2 \times \mathcal{Z}_5$ and $\mathcal{Z}_2 \times \mathcal{Z}_9$ flavour symmetries are capable of explaining the fermionic masses and mixing pattern of the standard model including that of the neutrinos. The bounds on the parameter space of the flavon field of the $\mathcal{Z}_2 \times \mathcal{Z}_5$ and $\mathcal{Z}_2 \times \mathcal{Z}_9$ flavour symmetries are derived using the current quark and lepton flavour physics data and future projected sensitivities of quark and lepton flavour effects. The strongest bounds on the flavon of the $\mathcal{Z}_2 \times \mathcal{Z}_5$ symmetry come from the $D^0 - \bar D^0$ mixing. The bounds on the $\mathcal{Z}_2 \times \mathcal{Z}_9$ flavour symmetry are stronger than that of the minimal $\mathcal{Z}_2 \times \mathcal{Z}_5$ symmetry. The ratio $R_{μμ}$ provides rather robust bounds on the flavon parameters in the future phase-\rom{1} and phase-\rom{2} of the LHCb by leaving only a very small region in the allowed parameter space of the models.

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arXiv2021

Polarized $Λ_b$ baryon decay to $pπ$ and a dilepton pair

The recent anomalies in $b\to s\ell^+\ell^-$ transitions could originate from some New Physics beyond the Standard Model. Either to confirm or to rule out this assumption, more tests of $b\to s\ell^+\ell^-$ transition are needed. Polarized $Λ_b$ decay to a $Λ$ and a dilepton pair offers a plethora of observables that are suitable to discriminate New Physics from the Standard Model. In this paper, we present a full angular analysis of a polarized $Λ_b$ decay to a $Λ(\to pπ)\ell^+\ell^-$ final state. The study is performed in a set of the operator where the Standard Model operator basis is supplemented with its chirality flipped counterparts, and new scalar and pseudo-scalar operators. The full angular distribution is calculated by retaining the mass of the final state leptons. At the low hadronic recoil, we use the Heavy Quark Effective Theory framework to relate the hadronic form factors which lead to simplified expression of the angular observables where short- and long-distance physics factorize. Using the factorized expressions of the observables, we construct a number of tests of short- and long-distance physics including null tests of the Standard Model and its chirality flipped counterparts that can be carried out using experimental data.

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arXiv2019

A detailed study of the $Λ_b \to Λ\ell^+ \ell^-$ decays in the Standard Model

Based on the standard model (SM) of particle physics, we study the decays $Λ_b \to Λ\ell^+ \ell^-$ in light of the available inputs from lattice and the data from LHCb. We fit the form-factors of this decay mode using the available theory and experimental inputs after defining different fit scenarios and checking their consistencies. The theory inputs include the relations between the form-factors in heavy quark effective theory (HQET) and soft collinear effective theory (SCET) at the endpoints of di-lepton invariant mass squared $q^2$. Utilizing the fit results, we have predicted a few observables related to this mode. We have also predicted the observable $R_Λ = Br(Λ_b \to Λ\ell_i^+\ell_i^- )/Br(Λ_b \to Λ\ell_j^+\ell_j^-)$ where $\ell_{i}$ and $\ell_j$ are charged leptons of different generations ($i \ne j$). At the moment, we do not observe noticeable differences in the extracted values of the observables in fully data-driven and SM like fit scenarios.

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arXiv2019

Lepton mass effects and angular observables in $Λ_b \to Λ(\to p π) \ell^+\ell^-$

The flavor changing rare decay $B\to K^{*}(\to Kπ)\ell^+\ell^-$ is one of the most studied modes due to its sensitivity to physics beyond the standard model and several discrepancies have come to light among the plethora of observables that are measured. In this paper we revisit the analogous baryonic decay mode $Λ_{b}\rightarrow Λ(\to pπ) \ell^{+}\ell^{-}$ and we present a complete set of ten angular observables that can be measured using this decay mode. Our calculations are done retaining the finite lepton mass so that the signal of lepton non-universality observed in $B\to K^{*} \ell^+\ell^-$ can be corroborated by the corresponding baryonic decay mode. We show that due to the parity violating nature of the subsequent $Λ\to pπ$ decay there exist at least one angular asymmetry that is non-vanishing in the large recoil limit unlike the case in $B\to K^{*}\ell^+\ell^-$ decay mode, making it particularly sensitive to new physics that violates lepton flavor universality.

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arXiv2019

Radiative $b$-baryon decays to measure the photon and $b$-baryon polarization

The radiative decays of $b$-baryons facilitate the direct measurement of photon helicity in $b\to sγ$ transitions thus serving as an important test of physics beyond the Standard Model. In this paper we analyze the complete angular distribution of ground state $b$-baryon ($Λ_{b}^{0}$ and $Ξ_{b}^{-}$) radiative decays to multibody final states assuming an initially polarized $b$-baryon sample. Our sensitivity study suggests that the photon polarization asymmetry can be extracted to a good accuracy along with a simultaneous measurement of the initial $b$-baryon polarization. With higher yields of $b$-baryons, achievable in subsequent runs of the Large Hadron Collider (LHC), we find that the photon polarization measurement can play a pivotal role in constraining different new physics scenarios.

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