arXiv subjects

Faisal Munir Bhutta

Publications and source records attributed to Faisal Munir Bhutta.

12 recordsLinked to original sources

arXiv2026

Search for New Physics through the Observables of Semileptonic $B_{c}^+\to D^{\ast+}\ell^{+}\ell^{-}$ Decay

Motivated by the current flavor anomalies and the comparatively less explored nature of the $b\to d$ sector, we present a model-independent study of the rare semileptonic $B_c^{+}\to D^{\ast+}\ell^{+}\ell^{-}$ ($\ell=μ,τ$) decay, to investigate its sensitivity to New Physics effects. The analysis incorporates penguin-box contributions, long-distance resonance effects, and the sizable weak-annihilation amplitudes. Using the $B_c\to D^\ast$ transition form factors calculated in the covariant confined quark model and the weak-annihilation form factors obtained within the Bethe--Salpeter approach, we analyze the differential branching fraction, forward-backward asymmetry, longitudinal helicity fraction, and a comprehensive set of normalized angular observables in several one- and two-dimensional New Physics scenarios and compare our results with the Standard Model predictions. Notably, the branching fraction, forward-backward asymmetry, and the normalized angular coefficients such as, $\langle I_{2c}\rangle$, $\langle I_{3}\rangle$, $\langle I_{5}\rangle$, and $\langle I_{6s} \rangle$, show clear sensitivity to New Physics effects. Our results indicate that the decay $B_c^{+}\to D^{\ast+}\ell^{+}\ell^{-}$ serves as a complementary probe of the flavor structure of New Physics and can therefore be investigated in future measurements at LHCb and other high-luminosity flavor experiments.

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arXiv2026

Lepton polarization dependent angular observables and the polarization asymmetries in the four-fold $Λ_b \rightarrow Λ(\rightarrow N π) \ell^+\ell^-$ decay

The rare decays mediated by flavor-changing neutral current processes, such as $b \to s \ell^{+}\ell^{-}$, provide powerful probes of the Standard Model and potential windows into new physics. Particularly, the angular observables in these exclusive decays are valuable because of their sensitivity to short-distance dynamics and their reduced dependence on hadronic uncertainties, which mainly arise from form factors. In this work, we analyze the $Λ_b \to Λ(\to Nπ)\ell^{+}\ell^{-}$ (with $Nπ=\{pπ^-,nπ^0\}$) decay with polarized final-state lepton and derive the corresponding four-fold differential decay distributions. For the longitudinal, normal, and transverse polarization states, we systematically identify the additional angular coefficients that emerge relative to the unpolarized case. We find that the longitudinal polarization preserves the structure of the unpolarized distribution, while the normal and transverse polarizations introduce some new additional angular coefficients. The analytical expressions of all polarized and unpolarized angular coefficients are explicitly derived in terms of the helicity and transversity amplitudes. To compare the variation in the polarized and unpolarized angular observables, we have plotted them against the square of the momentum transfer $q^2$. Additionally, the Standard Model predictions of the polarization asymmetry observables are provided and their sensitivity to new physics is explored under different new physics scenarios. The obtained results, in the current study, for longitudinal and transverse polarization cases, provide a baseline for the lepton polarization dependent observables, which may serve as sensitive probes to test the Standard Model in these decays.

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arXiv2026

Weak Annihilation Contribution to Angular Observables in $B_{c}^+\to D^{\ast+}\ell^{+}\ell^{-}$ Decays

We analyze the rare semileptonic decays $B_{c}^+ \to D^{\ast+}(\to P_1 P_2)\ell^{+}\ell^{-}$, with $P_1 P_2 = D^+ π^0$ or $D^0 π^+$, and $\ell=μ, τ$. We focus on the impact of weak annihilation contributions alongside penguin, box, and long-distance effects. Using the effective Hamiltonian for $b \to d \ell^+ \ell^-$ transitions and $B_c \to D^{*}$ form factors from covariant confined quark model inputs, we compute the differential branching ratios, forward-backward asymmetry, longitudinal helicity fraction of the $D^{\ast}$, and various normalized angular coefficients. The results of the observables show that weak annihilation effects are sizable, particularly at low $q^2$, significantly modifying several observables and shifting zero-crossings. Resonance effects dominate at high $q^2$, restricting reliable analysis windows. We conclude that the inclusion of weak annihilation is essential for precise Standard Model predictions and for isolating possible New Physics effects in $B_c^+ \to D^{*+} \ell^+ \ell^-$ decays.

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arXiv2025

Investigating New Physics through the Observables of Semileptonic $B_{s}\to K^{\ast}(\to K π)μ^{+}μ^{-}$ Decay

The rare four-fold decay $B_s \to K^*(\to Kπ)μ^+μ^- $, governed by the flavor-changing neutral current transition $b \to dμ^+μ^-$, provides a sensitive probe for testing the Standard Model (SM) and investigating signatures of new physics (NP). This work presents a comprehensive model-independent analysis of the decay using the framework of the weak effective field theory. We compute a set of key physical observables, including the differential branching ratio, forward-backward asymmetry, longitudinal polarization fraction, and several normalized angular coefficients $\langle I_i\rangle$, as a function of the dilepton invariant mass squared $q^2$. The impact of NP is explored via both one-dimensional (1D) and two-dimensional (2D) scenarios involving NP Wilson coefficients $C_7^{\text{NP}}$, $C_9^{(\prime)\text{NP}}$, and $C_{10}^{(\prime)\text{NP}}$. Our findings reveal notable deviations from the SM predictions across multiple observables. Furthermore, we analyze the correlations between different observables and their 2D contour plots which would be useful to further constrain the parametric space of possible NP contributions. This study reinforces the potential of $B_s \to K^* μ^+ μ^-$ decay as a complementary channel in the search for physics beyond the SM.

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arXiv2025

Reinvestigating the semileptonic $B\to D^{(\ast)}τ\barν_τ$ decays in the model independent scenarios and leptoquark models

In this work, we revisit the possible new physics (NP) solutions by analyzing the observables associated with $B\to D^{(\ast)}τ\barν_τ$ decays. To explore the structure of new physics, the form factors of $B\to D^{(\ast)}$ decays play a crucial role. In this study, we utilize the form factor results obtained from a simultaneous fit to Belle data and lattice QCD calculations. Using these form factors, we conduct a global fit of the new physics Wilson coefficients, incorporating the most recent experimental data. Additionally, we use the form factors and Wilson coefficients to make predictions for physical observables, including the lepton flavor universality ratio $R_{D^{(\ast)}}$, polarized observables, and the normalized angular coefficients $\langle I\rangle$'s related to the four-fold decays $B\to D^{\ast}(\to Dπ, Dγ)τ\barν_τ$. These predictions are calculated in both model-independent scenarios and for three different leptoquark models.

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arXiv2024

Angular observables of the four-fold $B \to K_{1}(1270,1400)(\to V P) \ell^{+}\ell^{-}$ decays in and beyond the Standard Model

Recent measurements of the lepton flavor universality ratios $R^{μe}_{K}$ and $R^{μe}_{K^*}$ in $B\to \left(K, K^*\right)μ^{+}μ^{-}\left(e^+e^-\right)$ at LHCb align with the Standard Model predictions, necessitating search for the complementary decay modes. In this context, we derive the angular decay distributions of the four-fold $B \to K_{1}(1270,1400)(\to V P) μ^{+}μ^{-}$ decays, where $K_1 $ is an axial-vector meson, $V=ρ, K^{\ast}$, and $P=K, π$. Considering the weak effective Hamiltonian with vector and axial-vector new physics operators, and employing the helicity formalism, we obtain the four-dimensional differential decay distributions and extract various physical observables. These include differential branching ratios, lepton forward-backward asymmetry, forward-backward asymmetry for transversely polarized $K_1$ meson, longitudinal and transverse polarization fractions of the $K_1$ meson, and the normalized angular coefficients with longitudinally and transversely polarized final state vector meson $V$, in the cascade decay $K_1\to VP$. Based on the latest global fit analysis data for all $b\to s$ transitions, we predict the results of these observables in various bins of the square of the momentum transfer, $q^2$. Our findings indicate that these physical observables are not only sensitive to new physics but can also differentiate between various new physics scenarios in certain kinematical regions. The precise measurements of these observables in $B \to K_{1}(1270,1400)(\to V P) μ^{+}μ^{-}$ decays at current and future experiments will provide opportunities for the complementary searches for physics beyond the Standard Model in $b\to s\ell^+\ell^-$ decays.

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arXiv2024

Effects of Family Non-universal $Z^{\prime}$ Model in the angular observables of $B\to(ρ,a_{1})μ^{+}μ^{-}$ decays

We present the angular distribution of the four-fold $B\toρ(\toππ)μ^{+}μ^{-}$ and $B\to a_{1}(\toρ_{\parallel, \perp}π)μ^{+}μ^{-}$ decays both in the Standard Model and the family non-universal $Z^{\prime}$ model. At the quark level, these decays are governed by the $b\to dμ^{+}μ^{-}$ transition. Along with different angular observables, we also give predictions of differential branching ratios, forward-backward asymmetry, longitudinal polarization fraction of $ρ$, and $a_{1}$ mesons. Our analysis shows that the signatures of family non-universal $Z^{\prime}$ model are more distinct in the observables associated with the $B\toρ(\toππ)μ^{+}μ^{-}$ decay, compared to that of the $B\to a_{1}(\toρ_{\parallel, \perp}π)μ^{+}μ^{-}$ decay. Future measurements of the predicted angular observables, both at current and future high energy colliders, will add to the useful complementary data required to clarify the structure of the family non-universal $Z^{\prime}$ model in $|Δb|$=$|Δd|=1$ processes.

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arXiv2023

Footprints of New Physics in the angular distribution of $B_{c}\to D_{s}^{\ast}(\to D_{s}γ,(D_{s}π))\ell^{+}\ell^{-}$ decays

We investigate the angular decay distribution of the four-fold $B_{c}\to D^{\ast}_{s}(\to D_{s}γ)μ^{+}μ^{-}$, and $B_{c}\to D^{\ast}_{s}(\to D_{s}π)μ^{+}μ^{-}$ decays that proceed through $b\to sμ^{+}μ^{-}$ quark level transition. We use the model independent effective Hamiltonian with vector and axial vector new physics operators to formulate the angular observables and study the implications of different latest new physics scenarios, taken from the global fits to all the $b\to s$ data, on these observables. We also give Standard Model and new physics predictions of several observables such as differential branching ratios, forward backward asymmetry, longitudinal polarization fraction of $D_s^{\ast}$, and the unpolarized and polarized lepton flavor universality violating ratios. Future measurements of the predicted angular observables, both at current and future high energy colliders, will add to the useful complementary data required to clarify the structure of new physics in $b\to s\ell\ell$ neutral current decays.

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arXiv2022

New Physics in $b\to s\ell\ell$ anomalies and its implications for the complementary neutral current decays

We study the Standard Model and the new physics predictions for the lepton-flavour-universality violating (LFUV) ratios in various $b\to s \ell^+\ell^-$ channels with scalar, pseudoscalar, vector, axial-vector, and $Λ$ baryon final states, considering both unpolarized and polarized final state hadrons. In order to formulate physical observables, we use the model independent effective Hamiltonian approach and employ the helicity formalism. We provide the explicit expressions of the helicity amplitudes in terms of the Wilson coefficients and the hadronic form factors by using the same kinematical configuration and polarization conventions for all the decay channels.We perform the numerical analysis with new physics scenarios selected from the recent global fits to $b\to s\ell^+\ell^-$ data, having specific new physics model interpretations. We find that some of the LFUV ratios for these complementary channels in different kinematical regions have high sensitivity to new physics and the future measurements of them in Belle II and LHCb experiments, along with testing new physics/LFUV, can help to distinguish among some of the different new physics possibilities.

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arXiv2019

Search for New Physics Signals via Doubly Weak B Decays

The doubly weak $b\to dd{\bar s}$ and $b\to ss{\bar d}$ processes are highly suppressed in the standard model that offer the unique opportunity to explore new physics signals. The wrong sign decay $\smash{\overline B}^0\to K^+π^-$ mediated by the $b\to dd{\bar s}$ transition can be distinguished from the penguin decay $\smash{B}^0\to K^+π^-$, through time dependent measurement in experiments. We consider a model independent analysis of $\smash{\overline B}^0\to K^+π^-$ decay, within the perturbative QCD approach and explore various effective dimension-6 operators, in which large effects are possible. We also study the doubly weak exclusive process in two example models namely Randall-Sundrum model with custodial protection and the bulk-Higgs Randall-Sundrum model. A large and significant enhancement of the branching ratio, in comparison to the standard model, is observed after satisfying all the relevant constraints on the parameter spaces of these models, which requires to be searched in future experiments.

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arXiv2018

The $Λ_b \to Λ (\to p π^-)μ^+μ^-$ decay in the $\text{RS}_{c}$ model

We study the four body decay $Λ_b \rightarrow Λ( \rightarrow p π^-) μ^{+} μ^{-}$ in the Randall-Sundrum model with custodial protection $(\text{RS}_c)$. By considering the constraints coming from the direct searches of the lightest Kaluza-Klein (KK) excitation of the gluon, electroweak precision tests, the measurements of the Higgs signal strengths at the LHC and from $ΔF=2$ flavor observables, we perform a scan of the parameter space of the $\text{RS}_c$ model and obtain the maximum allowed deviations of the Wilson coefficients $ΔC^{(\prime)}_{7,\; 9,\; 10}$ for different values of the lightest KK gluon mass $M_{g^{(1)}}$. Later, their implications on the observables such as differential branching fraction, longitudinal polarization of the daughter baryon $Λ$, forward-backward asymmetry with respect to leptonic, hadronic and combined lepton-hadron angles are discussed where we present the analysis of these observables in different bins of di-muon invariant mass squared $s\;(= q^2)$. It is observed that with the current constraints the Wilson coefficients in $\text{RS}_c$ model show slight deviations from their Standard Model values and hence can not accommodate the discrepancies between the Standard Model calculations of various observables and the LHCb measurements in $Λ_b$ decays.

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arXiv2018

New Physics Search in the Doubly Weak Decay $\overline B^0 \to K^+ π^- $

The doubly weak transition $b\to dd{\bar s}$ is highly suppressed in the standard model, which makes it a potential channel to explore new physics signals. We present a study of the exclusive two body wrong sign weak decay $\smash{\overline B}^0\to K^+π^-$ belonging to this class within the perturbative QCD approach. We perform a model independent analysis for various effective dimension-6 operators, in which large effects are possible. We further analyze the considered process in example models such as Randall-Sundrum model, including the custodially protected and the bulk-Higgs Randall-Sundrum model. Exploring the experimentally favored parameter spaces of these models leads to a large and significant enhancement of the decay rate, compared to the standard model result, which might be accessible in future experiments. We propose to look for the wrong sign decay $\smash{\overline B}^0\to K^+π^-$ via flavour-tagged time-dependent analyses, which can be performed at LHCb and Belle-II.

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