arXiv subjects

Soumitra Nandi

Publications and source records attributed to Soumitra Nandi.

At least 37 records · Page 2Linked to original sources

arXiv2023

Test of new physics effects in $\bar{B} \to (D^{(*)}, π) \ell^-\barν_{\ell}$ decays with heavy and light leptons

We study the $\bar{B}\to D(D^*) \ell^-\barν_{\ell}$ decays based on the up-to-date available inputs from experiments and the lattice. First, we review the standard model (SM) predictions of the different observables associated with these decay channels. In the analyses, we consider new physics (NP) effects in the channels with the heavy ($τ$), as well as the light leptons ($μ, e$). We have extracted $|V_{cb}|$ along with the new physics Wilson coefficients (WCs) from the available data on light leptons; the extracted value of $|V_{cb}|$ is $(40.3 \pm 0.5)\times 10^{-3}$. The extracted WCs are zero consistent, but some could be of order $10^{-2}$. Also, we have done the simultaneous analysis of the data in $\bar{B} \to D^{(*)}(μ^-,e^-)\barν$ alongside the inputs on $R(D^{(*)}) = \frac{Γ(\bar{B}\to D^{(*)}τ^-\barν_τ)}{Γ(\bar{B}\to D^{(*)}\ell^-\barν_{\ell})}$ and the $D^*$ longitudinal polarisation fraction $F_L^{D^*}$ in different NP scenarios and extracted $|V_{cb}|$ which is consistent with the number mentioned above. Also, the simultaneous explanation of $R(D^{(*)})$ and $F_L^{D^*}$ is not possible in the one-operator scenarios. However, the two operator scenarios with $\mathcal{O}_{S_2}^τ = (\bar{q}_R b_L)(\barτ_Rν_{τL})$ as one of the operators could explain all these three measurements. Finally, we have given predictions of all the related observables in $\bar{B} \to D^{(*)}(τ^-,μ^-,e^-)\barν$ decays in the NP scenarios, which could be tested in future experiments. We have repeated this exercise for $\bar{B} \to π\ell^-\barν_{\ell}$ decays with the light lepton and extracted $|V_{ub}|$ and the new WCs. Finally, using all these available data for the light and heavy leptons, we have given bounds on the couplings of the relevant SM effective field theory (SMEFT) operators and the probable NP scale $Λ$.

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arXiv2023

Constraining inverse moment of $B$-meson distribution amplitude using Lattice QCD data

We constrain the inverse moment of the $B$-meson light-cone distribution amplitude (LCDA), $λ_B$ in heavy quark effective theory, using form factor estimates from Lattice QCD collaboration. The estimation of the parameter $λ_B$ has, until now, relied solely on QCD sum rule methods and deals with significant uncertainty. In this work, we express the form factors for the $B \to K$ channel, calculated within the light-cone sum rule (LCSR) approach, in terms of the $B$-meson LCDAs. By incorporating recent Lattice results from the HPQCD collaboration for the $B \to K$ form factors at zero momentum transfer ($q^2$ = 0), we impose constraints on this parameter. Consequently, we achieve a twofold reduction in uncertainty compared to the QCD sum rule estimate, yielding $λ_B=338\pm 68$ MeV, when the $B$-meson LCDAs are expressed in the Exponential model. Additionally, we compare the form factor predictions, using the constrained $λ_B$ value, with the earlier analyses for other channels as well, such as $B\to π$ and $B \to D$.

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arXiv2023

Extractions of $|V_{ub}|/|V_{cb}|$ from a combined study of the exclusive $b\to u(c) \ellν_{\ell}$ decays

We have extracted the ratio $|V_{ub}|/|V_{cb}|$ from a combined study of the available inputs on $\bar{B}\to (π,ρ,ω)\ell^- \barν_{\ell}$ and $\bar{B}(\bar{B}_s)\to (D^{(\ast)}, D_s^{(\ast)})\ell^- \barν_{\ell}$ ($\ell = μ$ or $e$) decays. We have done our analysis with and without the inclusion of the available experimental results on the branching fraction $\mathcal{BR}(\bar{B}_s\to K^+μ^-\barν_μ)$ or the ratio of the rates $Γ(\bar{B}_s\to K^+ \ell^-\barν_{\ell})/ Γ(\bar{B}_s \to D_s^+ \ell^- \barν_{\ell})$. We have used all the available experimental data on the branching fractions in different $q^2$-bins, the available theory inputs on the form factors from lattice and the light cone sum rule (LCSR) approach; analysed the data in different possible scenarios and presented the results. From a combined analysis of all the $b\to c\ell^- \barν_{\ell}$ exclusive decays, we have obtained $|V_{cb}|=(40.5 \pm 0.6)\times 10^{-3}$ and from all the $b\to u\ell^- \barν_{\ell}$ modes, we obtain $|V_{ub}|=(3.46\pm 0.12)\times 10^{-3}$ which changes to $|V_{ub}|=(3.51 \pm 0.13)\times 10^{-3}$ after dropping $\mathcal{BR}(\bar{B}_s\to K^+μ^-\barν_μ)$. We hence report the values of the ratio $|V_{ub}|/|V_{cb}| = 0.085 \pm 0.003$ and $0.087 \pm 0.003$ with and without the input from $\bar{B}_s\to K^+μ^-\barν_μ$ modes, respectively. We have also predicted the ratio $Γ(\bar{B}_s\to K^+ \ell^-\barν_{\ell})/ Γ(\bar{B}_s \to D_s^+ \ell^- \barν_{\ell})$ using the fit results. In addition, we provide the predictions of $\mathcal{BR}(\bar{B}_s\to K^+μ^-\barν_μ)$ and $\mathcal{BR}(\bar{B}\to (ρ,ω)\ell^- \barν_{\ell})$ in the Standard Model (SM) in small $q^2$-bins.

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arXiv2023

Study of the $b \to d \ell\ell$ transitions in the Standard Model and test of New Physics sensitivities

After incorporating all the available experimental data and the most up-to-date Lattice and light cone sum rule (LCSR) inputs on the form factors, we analyze the exclusive $b\to u\ellν_{\ell}$ and $b\to d\ell \ell$ decays simultaneously. We have extracted the shape of all the associated form factors using which we provide predictions in the standard model for the branching ratios, direct CP asymmetries and isospin asymmetry for $B\toπ\ell \ell$ and various angular observables for $B\toρ\ell \ell$ transitions. Also, we have tested the sensitivities of these observables towards physics beyond the standard model (BSM).For the $B\toρ\ell \ell$ decays, we have defined tagged and untagged observables and predicted them in the SM and BSM. In the context of BSM, we have found some compelling information. The respective predictions in a few benchmark scenarios are given, which can be tested in the experiments at the LHCb and the Belle.

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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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arXiv2022

Perturbative corrections to power suppressed effects in $\bar B\to X_u\ellν$

We compute the $O(α_s)$ corrections to the Wilson coefficients of the dimension five operators in inclusive semileptonic $B$ decays in the limit of a massless final quark. Our calculation agrees with reparameterization invariance and with previous results for the total width and places constraints on the shape functions that enter those decays.

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arXiv2021

A closer look at observables from exclusive semileptonic $B\to(π,ρ)\ellν_{\ell}$ decays

This article analyses the available inputs in $\btopilnu$ and $\btorholnu$ decays which include the measured values of differential rate in different $q^2$-bins (lepton invariant mass spectrum), lattice, and the newly available inputs on the relevant form-factors from the light-cone sum rules (LCSR) approach. We define different fit scenarios, and in each of these scenarios, we predict a few observables in the standard model (SM). For example, $R(M) =\frac{\mathcal{B}(B \to M\ell_iν_{\ell_i})}{\mathcal{B}(B\to M\ell_jν_{\ell_j})} $, $R^{\ell_i}_{\ell_j}(M) =\frac{\mathcal{B}(B\to \ell_iν_{\ell_i})}{\mathcal{B}(B \to M\ell_jν_{\ell_j})}$ with M = $π$ or $ρ$ and $\ell_{i,j} = e, μ$ or $τ$. We also discuss the new physics (NP) sensitivities of all these observables and obtain bounds on a few NP Wilson coefficients in $b\to u τν_τ$ decays using the available data. We have noted that the data at present allows sizeable NP contributions in this mode. Also, we have predicted a few angular observables relevant to these decay modes.

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arXiv2021

Axion-like particles resolve the $B \to πK$ and g-2 anomalies

We offer a new solution to an old puzzle in the penguin-dominated $B\toπK$ decays. The puzzle is the inconsistency among the measurements of the branching ratios and CP asymmetries of the four $B\toπK$ decays: $B^+ \to π^+ K^0$, $B^+\to π^0 K^+$, $B_d^0\toπ^- K^+$, $B_d^0\toπ^0 K^0$. We solve the $B\toπK$ puzzle by considering the effect of an axion-like particle (ALP) that mixes with the $π^0$ and has mass close to the $π^0$ mass. We show that the ALP can also explain the anomalies in the electron and muon anomalous magnetic moments.

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arXiv2021

New physics in $b\to s \ell\ell$ decays with complex Wilson coefficients

We perform a data-driven analysis of new physics (NP) effects in exclusive $b \to s \ell^+\ell^-$ decays in a model-independent effective theory approach with dimension six operators considering scalar, pseudo-scalar, vector and axial-vector operators with the corresponding Wilson coefficients (WC) taken to be complex. The analysis has been done with the most recent data while comparing the outcome with that from the relatively old data-set. We find that a left-handed quark current with vector muon coupling is the only one-operator $(\mathcal{O}_9)$ scenario that can explain the data in both the cases with real and complex WC with a large non-zero imaginary contribution. We simultaneously apply model selection tools like cross-validation and information-theoretic approach like Akaike Information Criterion (AIC) to find out the operator or sets of operators that can best explain the available data in this channel. The $\mathcal{O}_9$ with complex WC is the only one-operator scenario which survives the test. However, there are a few two and three-operator scenarios (with real or complex WCs) which survive the test, and the operator $\mathcal{O}_9$ is common among them.

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arXiv2021

A closer look at the extraction of $|V_{ub}|$ from $B\toπlν$

To extract the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $|V_{ub}|$, we have re-analyzed all the available inputs (data and theory) on the $B\toπlν$ decays including the newly available inputs on the form-factors from light cone sum rule (LCSR) approach. We have reproduced and compared the results with the procedure taken up by the Heavy Flavor Averaging Group (HFLAV), while commenting on the effect of outliers on the fits. After removing the outliers and creating a comparable group of data-sets, we mention a few scenarios in the extraction of $|V_{ub}|$. In all those scenarios, the extracted values of $|V_{ub}|$ are higher than that obtained by HFLAV. Our best results for $|V_{ub}|^{exc.}$ are $(3.88 \pm 0.13)\times 10^{-3}$ and $(3.87 \pm 0.13)\times 10^{-3}$ in frequentist and Bayesian approaches, respectively, which are consistent with that extracted from inclusive decays $|V_{ub}|^{inc}$ within $1~σ$ confidence interval.

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arXiv2021

Kinetic operator effects in B-> X_c l nu at O(alpha_s)

We compute the O(alpha_s) corrections to the Wilson coefficient of the kinetic operator in inclusive semileptonic B decays. Our analytic calculation agrees with reparameterization invariance and with previous numerical results and paves the way to the calculation of analogous corrections to other power-suppressed coefficients.

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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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arXiv2020

'Deep' Dive into $b \to c$ Anomalies: Standardized and Future-proof Model Selection Using Self-normalizing Neural Networks

Noting the erroneous proclivity of information-theoretic approaches, like the Akaike information criterion (AIC), to select simpler models while performing model selection with a small sample size, we address the problem of new physics model selection in $b\to c τν_τ$ decays in this paper by employing a specific machine learning algorithm (self-normalizing neural networks, a.k.a. SNN) for supervised classification and regression, in a model-independent framework. While the outcomes of the classification with real data-set are compared with AIC, with the SNNs outperforming AIC$_c$ in all aspects of model selection, the regression-outcomes are compared with the results from Bayesian analyses; the obtained parameter spaces differ considerably while keeping maximum posterior (MAP) estimates similar. A few of the two-operator scenarios with a tensor-type interaction are found to be the most probable solution for the data. We also test the effectiveness of our trained networks with the expected, more precise data in Belle-II. The trained networks and associated functionalities are supplied for the use of the community.

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arXiv2020

Updates on SM predictions of $|V_{cb}|$ and $R(D^{})$ in $B\to D^{}\ellν_\ell$ decays

We update the standard model (SM) predictions of $R(D^*)$ using the latest results on the decay distributions in $B \to D^* \ell ν_{\ell}$ ($\ell = μ, e$) by Belle collaboration, while extracting $|V_{cb}|$ at the same time. Depending on the inputs used in the analysis, we define various fit scenarios. Although the central values of the predicted $R(D^*)$ in all the scenarios have reduced from its earlier predictions in 2017, the results are consistent with each other within the uncertainties. In this analysis, our prediction of $R(D^*)$ is consistent with the respective world average at $\sim 3σ$. We have also predicted several angular observables associated with $B \to D^* τν_τ$ decays. We note that the predicted $F_L(D^*)$ is consistent with the corresponding measurement at 2$σ$. Utilizing these new results, we fit the Wilson coefficients appearing beyond the standard model of particle physics (BSM). To see the trend of SM predictions, we have utilized the recently published preliminary results on the form-factors at non-zero recoil by the lattice groups like Fermilab-MILC and JLQCD and predicted the observables in $B \to D^* \ell ν_{\ell}$, and $B \to D^* τν_τ$ decays.

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arXiv2020

Exhaustive Model Selection in $b \to s \ell \ell$ Decays: Pitting Cross-Validation against AIC$_c$

In the light of recent data, we study the new physics effects in the exclusive $b \to s \ell^+\ell^-$ decays from a model independent perspective. Different combinations of the dimension six effective operators along with their respective Wilson coefficients are chosen for the analysis. To find out the operator or sets of operators that can best explain the available data in this channel, we simultaneously apply popular model selection tools like cross-validation and the information theoretic approach like Akaike Information Criterion (AIC). There are one, two, and three-operator scenarios which survive the test and a left-handed quark current with vector muon coupling is common among them. This is also the only surviving one-operator scenario. Best-fit values and correlations of the new Wilson coefficients are supplied for all the selected scenarios. We find that the angular observables play the dominant role in the model selection procedure. We also note that while a left-handed quark current with axial-vector muon coupling is the only one-operator scenario able to explain the ratios $R_{K^{(*)}}$ ($R_{K^*}$ for $q^2\in [ 0.045, 1.1] {\rm GeV}^2$ in particular), there are also a couple of two operator scenarios that can simultaneously explain the measured $R_{K^{(*)}}$.

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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

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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arXiv2018

$b \to c τν_τ$ Decays: A Catalogue to Compare, Constrain, and Correlate New Physics Effects

In this article, we have predicted the standard model (SM) values of the asymmetric and angular observables in $B\to D^{(\ast)}τν_τ$ decays, using the results of the new up-to-date analysis in $B\to D^{(*)}\ellν_{\ell}$. We have also revisited the SM prediction of the inclusive ratio $\mathcal{R}_{X_c}$, and have given its values in different schemes of the charm quark mass. This is the first analysis which includes all the known corrections in the SM. In addition, we have analysed the $b\to cτν_τ$ decay modes in a model-independent framework of effective field theory beyond the standard model. Considering all the possible combinations of the effective operators in $b \to c τν_τ$ decays and using the Akaike Information Criterion, we find out the scenarios which can best explain the available data on these channels. In the selected scenarios, best-fit values and correlations of the new parameters are extracted. Using these results, predictions are made on various observables in the exclusive and inclusive semitaunic $b \to c $ decays. The graphical correlations between these observables are shown, which are found to be useful in discriminating various new physics scenarios.

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