arXiv subjects

M. Jamil Aslam

Publications and source records attributed to M. Jamil Aslam.

At least 19 recordsLinked to original sources

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.

hep-ph↗

arXiv2026

Weak decay of the positronium ion

The positronium ion ($\mathrm{Ps}^-$), a coulombic three-body bound state of two electrons and a positron, predominantly decays via electron-positron annihilation into electromagnetic final states. While its radiative decay channels have been extensively studied, much less attention has been given to weak processes in this system. In this work, we investigate the rare decay $\mathrm{Ps}^- \to e^- ν_μ\barν_μ$, obtained by replacing the photon in $\mathrm{Ps}^- \to e^- γ$ with a virtual $Z$ boson. Treating the three-body process as an effective two-body transition, $\mathrm{Ps}^- \to e^- Z^*\left(\to ν_μ\barν_μ\right)$, we compute the decay rate by explicitly evaluating all spin configurations of the initial bound state and final particles. The result agrees with that obtained using the standard spin-summation formalism of quantum field theory. We find that the branching ratio is comparable to that of the weak decay of ortho-positronium, $\mathrm{o\text{-}Ps} \to γν\barν$.

hep-ph↗

arXiv2026

Landau-Khalatnikov-Fradkin Transformations in Reduced Quantum Electrodynamics: Perturbative and Nonperturbative Dynamics of the Fermion Propagator

We present a comprehensive analysis of the Landau-Khalatnikov-Fradkin transformations for the charged fermion propagator in reduced quantum electrodynamics (RQED). Starting from the propagator in a reference gauge, we perform a gauge transformation to obtain its analytical expression valid to all orders in an arbitrary covariant gauge and also applicable in a nonperturbative context. This work complements and extends previous studies of quantum electrodynamics in various spacetime dimensions, for both massless and massive fermions. At the perturbative level, we expand the resulting expressions up to two-loop order for both massless and massive cases, and compare our results with those available in the literature wherever possible. We argue that the most suitable choice of the reference covariant gauge in RQED is $ξ=1/3$, as in this case the leading logarithmic contribution to the massless wave-function renormalization vanishes at one-loop order. This choice provides a direct connection between perturbation theory and the constraints imposed by multiplicative renormalizability on the massless fermion propagator. We also investigate the implications of the Landau-Khalatnikov-Fradkin transformations for the dynamically generated mass function of the fermion propagator. Finally, through numerical computation, we demonstrate that both the chiral fermion condensate and the fermion pole mass are gauge-invariant quantities.

hep-th↗

arXiv2026

Symmetry-Breaking Effects on Form Factors and Observables in $B \to K_0^*(1430)μ^+μ^-$ Decay

In the heavy-quark and large-energy limits, symmetry relations reduce the number of independent form factors governing heavy-to-light $B$-meson decays. Exploiting these relations, the form factors can be parametrized while systematically incorporating symmetry-breaking corrections from perturbative QCD. Using vertex renormalization together with light-cone distribution amplitudes, we compute the vertex and hard-spectator contributions for the $B \to K_0^*(1430)$ transition. We then analyze the impact of these form factors on physical observables, including the branching ratio and lepton polarization asymmetries $(P_L, P_N)$, in $B \to K_0^*(1430)μ^+μ^-$. Our results indicate that perturbative corrections induce modest shifts of $\sim 3\%$ in both the branching ratio and the normal lepton polarization asymmetry. Consequently, any significant deviation observed experimentally from these predictions would provide a clear signal of potential New Physics effects.

hep-ph↗

arXiv2025

Radiative B to tensor meson decays at NLO in SCET

The radiative $B$ to tensor $\left(K_2^*(1430),\; f_2(1270),\; a_2(1230)\right)$ meson decays are studied at next-to-leading order (NLO) in soft-collinear effective theory (SCET). The SCET allows the systematic treatment of factorizable and non-factorizable contributions along with the resummation of large perturbative logarithms. We performed a two step matching and determined the soft-overlap function $ζ^\perp_{T}$ and branching ratios for these $B$ to tensor meson decays. In the case of $B\to K_2^*(1430)γ$, the numerical value of the branching ratio lies close to its experimental measurements. The estimated values of the branching ratios of CKM suppressed decays $B \to \left(a_2(1230),\; f_2(1270)\right)γ$ are significant small compared to that of $B\to K_2^*(1430)γ$, but still could be measured in some ongoing and future $B$ physics experiments.

hep-ph↗

arXiv2025

Landau-Khalatnikov-Fradkin Transformations in Quantum Electrodynamics: For Perturbation Theory and Dynamical Mass Generation

We carry out a comprehensive analysis of the Landau-Khalatnikov-Fradkin transformations for a charged fermion propagator at the two-loop level in quantum electrodynamics (QED). Starting with an arbitrary covariant gauge $ξ$ and space-time dimension $d$, we provide its explicit expressions in three and four-dimensional QED. We begin with the tree-level fermion propagator in the Landau gauge and gauge-transform it to obtain an analytical expression for an all order result in an arbitrary covariant gauge. We expand it out to two-loops both for the massless and massive propagators in three and four space-time dimensions. In addition to comparing with all earlier results in the literature wherever possible, we also study constraints of multiplicative renormalizabilty of our results in four-dimensional QED which are logarithmically divergent. Finally, we analyze representative solutions of the fermion propagator which correspond to dynamical chiral symmetry breaking and mass generation in QED. We study the gauge dependence of these emergent solutions, that of the Euclidean pole mass and the chiral fermion condensate.

hep-ph↗

arXiv2025

Polarized and un-polarized $\mathcal{R}_{K^*}$ in and beyond the SM

The Standard Model (SM) is lepton flavor universal, and the recent measurements of lepton flavor universality in $B \to (K,K^*)\ell^{+}\ell^{-}$, for $\ell = μ, \; e$, decays now lie close to the SM predictions. However, this is not the case for the $τ$ to $μ$ ratios in these decays, where there is still some window open for the new physics (NP), and to accommodate them various extensions to the SM are proposed. It will be interesting to identify some observables which are not only sensitive on the parametric space of such NP models but also have some discriminatory power. We find that the polarization of the $K^{*}$ may play an important role, therefore, we have computed the unpolarized and polarized lepton flavor universality ratios of $τ$ to $μ$ in $B\to K^{*}\ell^{+}\ell^{+}$, $\ell= μ, τ$ decays. The calculation shows that in most of the cases, the values of the various proposed observables fall within the current experimental sensitivity, and their study at some on going and future experiments will serve as a tool to segregate the variants of the NP models.

hep-ph↗

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.

hep-ph↗

arXiv2023

Radiative $B$ to axial-vector meson decays at NLO in Soft-Collinear Effective Theory

The rare decay $B\rightarrow Aγ$, with $A$ representing axial-vector mesons such as $K_1 (1270),\; K_1 (1400),\; b_1(1300),\; a_1(1260)$, is studied at next-to-leading order (NLO) in soft collinear effective theory (SCET). The large outgoing meson energy encourages the study of the decay with an appropriate factorization scheme that separates the factorizable and non-factorizable parts systematically. We have analyzed the leading-power and $\mathcal{O}(α_s)$ diagrams that contribute to matching to SCET$_I$. The new intermediate theory is matched onto SCET$_{II}$ and the running of SCET$_I$ operators is performed to sum large perturbative logarithms. The values of soft-overlap function $ζ_{\perp}$ for $K_1 (1270,\;1400), a_{1}$ and $b_{1}$ mesons are estimated from the light cone-sum-rules (LCSR), and later using it the corresponding branching fractions for $B \to \left(K_{1}(1270,\; 1400),\; a_{1},\; b_{1}\right)γ$ decays are calculated. We find that in case of $B \to K_{1}(1270,\; 1400)γ$ decays the results are in good agreement with their experimental measurements. Also the estimated values of the branching ratios of the $B \to (b_{1},\; a_1)γ$ decays are potentially large to be measured at the LHCb and future B-factories.

hep-ph↗

arXiv2022

Positronium hydride decay into proton, electron, and one or zero photons

Decay rates of the positronium hydride PsH, a bound state of a proton, a positron, and two electrons, are determined for two rare channels, PsH $\to p^+ e^- γ$ and PsH $\to p^+ e^-$. Previous studies overestimated these rates by factors of about 2 and 700, respectively. We explain the physics underlying these wrong predictions. We confirm a range of static PsH properties, including the non-relativistic ground state energy, expectation values of inter-particle distances and their powers, and the three and four particle coalescence probabilities, using a variational method in the Gaussian basis.

hep-ph↗

arXiv2021

Rare decays of the positronium ion and molecule, $\text{Ps}^{-}\to e^{-}γ$ and $\text{Ps}_{2}\to e^{+}e^{-}γ,~γγ,~e^{+}e^{-}$

Decay rates of the positronium molecule $\text{Ps}_{2}$ into two photons and into an electron-positron pair are determined. Previous studies find that these rates are very different, \[ Γ\left(\text{Ps}_{2}\to e^{+}e^{-}\right)/Γ\left(\text{Ps}_{2}\toγγ\right)\simeq250\text{ (previous studies)}. \] This is puzzling since both processes have two body final states and are of the same order in the fine structure constant. We propose a simple calculational method and test it with the well-established decay of the positronium ion into an electron and a photon. We then employ it to correct predictions for both these $\text{Ps}_{2}$ decays. We find that previous studies overestimated the $e^{+}e^{-}$ and underestimated the $γγ$ channel by factors of about 5.44 and 3.93 respectively. Our results give $Γ\left(\text{Ps}_2\to e^+e^-\right)/Γ\left(\text{Ps}_2 \to γγ\right) \simeq 11.7$.

hep-ph↗

arXiv2020

Interpretation of LHCb Hidden-Charm Pentaquarks within the Compact Diquark Model

The LHCb collaboration have recently updated their analysis of the resonant $J/ψ\, p$ mass spectrum in the decay $Λ_b^0 \to J/ψ\, p\, K^-$, making use of their combined Run~1 and Run~2 data. In the updated analysis, three narrow states, $P_c (4312)^+$, $P_c (4440)^+$, and $P_c (4457)^+$, are observed. The spin-parity assignments of these states are not yet known. We interpret these narrow resonances as compact hidden-charm diquark-diquark-antiquark pentaquarks. Using an effective Hamiltonian, based on constituent quarks and diquarks, we calculate the pentaquark mass spectrum for the complete $SU (3)_F$ lowest $S$- and $P$-wave multiplets, taking into account dominant spin-spin, spin-orbit, orbital and tensor interactions. The resulting spectrum is very rich and we work out the quark flavor compositions, masses, and $J^P$ quantum numbers of the pentaquarks. However, heavy quark symmetry restricts the observable states in $Λ_b$-baryon, as well as in the decays of the other weakly-decaying $b$-baryons, $Ξ_b$ and $Ω_b$. In addition, some of the pentaquark states are estimated to lie below the $J/ψ\, p$ threshold in $Λ_b$-decays (and corresponding thresholds in $Ξ_b$- and $Ω_b$-decays). They decay via $c \bar c$ annihilation into light hadrons or a dilepton pair, and are expected to be narrower than the $P_c$-states observed. We anticipate their discovery, as well as of the other pentaquark states present in the spectrum at the LHC, and in the long-term future at a Tera-$Z$ factory.

hep-ph↗

arXiv2020

Radiative Baryonic Decay $\mathcal{B}^{*}\left(\frac{3}{2}\right)\to\mathcal{B}\left(\frac{1}{2}\right)+γ$ in Constituent Quark Model: A Tutorial

The radiative baryonic decay $\mathcal{B}^{*}\left(\frac{3}{2}\right)\to\mathcal{B}\left(\frac{1}{2}\right)+γ$ is a magnetic dipole $(M1)$ transition. It requires the transition magnetic moment $μ_{\mathcal{B}\left(3/2\right)\to\mathcal{B}\left(1/2\right)}$. The transition magnetic moments for the helicities $1/2$ and $3/2$ are evaluated in the frame work of constituent quark model in which the intrinsic spin and the magnetic moments of quarks $u,d$ and $s$ play a key role. Within this framework, the radiative decays $Δ^{+}\to p +γ$, $Σ^{*0}\to Λ+γ$, $Σ^{*+}\to Σ^{+}+γ$ and $Ξ^{*0}\to Ξ^{0}+γ$ are analyzed in detail. The branching ratio for these decays is found to be in good agreement with the corresponding experimental values.

hep-ph↗

arXiv2020

Probing new physics effects in $Λ_b \to Λ(\to pπ^-)\ell^+\ell^-$ decay via model independent approach

The New Physics (NP) effects are studied in the rare baryonic decay $Λ_b \to Λ(\to pπ^-)\ell^{+}\ell^{-}$, with unpolarized $Λ_b$ using most general model independent approach by introducing new axial(vector), (pseudo)scalar and tensor operators in the weak effective Hamiltonian corresponding to $b\to s$ transitions. Recently, for $Λ_b \to Λ(\to pπ^-)μ^{+}μ^{-}$ decay the LHCb collaboration has measured the branching ratio $(d\mathcal{B}/ds)$, lepton- and hadron-side forward-backward asymmetries, denoted by $A_{FB}^{\ell}$ and $A_{FB}^Λ$, respectively, and the longitudinal polarization fraction $F_L$ both in the low- and high-recoil regions. To see whether the new $VA$, $SP$ and $T$ couplings can accommodate the available experimental data of these observables, first we have examined their influence on these observables and later we have checked the imprints of these new couplings on a number of interesting but yet not measured observables. It is found that compared to the $VA$ the $SP$ couplings favor experimental data for all the four observables but still no individual coupling is able to accommodate all of the available data simultaneously. To achieve this goal, the pairs of new WCs are taken to check their range that simultaneously satisfy constraints of $B$-Physics and available LHCb data on $d\mathcal{B}/ds$, $F_L$, $A_{FB}^{\ell}$ and $A_{FB}^Λ$ in several bins for the decay channel under consideration. We find that most of the available data could be accommodated by the different pairs of $VA$ and $SP$ WCs giving more severe constraints on the parametric space of these WCs that is still satisfied with the $B$-physics data.

hep-ph↗

arXiv2020

Decay of a bound muon into a bound electron

When a muon bound in an atom decays, there is a small probability that the daughter electron remains bound. That probability is evaluated. Surprisingly, a significant part of the rate is contributed by the negative energy component of the wave function, neglected in a previous study. A simple integral representation of the rate is presented. In the limit of close muon and electron masses, an analytic formula is derived.

hep-ph↗

arXiv2019

Mass spectrum of the hidden-charm pentaquarks in the compact diquark model

The LHCb collaboration have recently updated their analysis of the resonant $J/ψp$ mass spectrum in the decay $Λ_b^0 \to J/ψp K^-$, making use of their combined Run 1 and Run 2 data. In the updated analysis, three narrow states, $P_c (4312)^+$, $P_c (4440)^+$,and $P_c (4457)^+$, are observed. The spin-parity assignments of these states are not yet known. We interpret these narrow resonances as compact hidden-charm diquark-diquark-antiquark pentaquarks. Using an effective Hamiltonian, based on constituent quarks and diquarks, we calculate the pentaquark mass spectrum for the complete $SU (3)_F$ lowest $S$- and $P$-wave multiplets, taking into account dominant spin-spin, spin-orbit, orbital and tensor interactions. The resulting spectrum is very rich and we work out the quark flavor compositions, masses, and $J^P$ quantum numbers of the pentaquarks. However, heavy quark symmetry restricts the observable states in $Λ_b$-baryon, as well as in the decays of the other weakly-decaying $b$-baryons, $Ξ_b$ and $Ω_b$. In addition, some of the pentaquark states are estimated to lie below the $J/ψp$ threshold in $Λ_b$-decays (and corresponding thresholds in $Ξ_b$- and $Ω_b$-decays). They decay via $c \bar c$ annihilation into light hadrons or a dilepton pair, and are expected to be narrower than the $P_c$-states observed. We anticipate their discovery, as well as of the other pentaquark states present in the spectrum at the LHC, and in the long-term future at a Tera-$Z$ factory.

hep-ph↗

arXiv2019

Vertex renormalization and hard scattering symmetry breaking corrections to $B$ to axial vector meson form factors at large recoil

The symmetries arise due to heavy quark and large energy limit help us to reduce the number of independent form factors in the heavy-to-light $B$-meson decays. It is expected that these symmetry relations are not exact and are broken by the perturbative effects, namely, the vertex corrections and the hard-spectator scatterings. The former are included in the form factors via vertex renormalization whereas the later are calculated through light-cone distribution amplitudes. We first calculate these symmetry breaking corrections to the form factors involved in semileptonic $B$-meson to an axial-vector $(K_{1})$-meson decay. Later, by using these form factors we see their effect on the physical observables such as the zero-position of the forward-backward $(\mathcal{A}_{FB})$ asymmetry and the longitudinal lepton polarization ($P_L$) asymmetry in $B\rightarrow K_1(1270) μ^+μ^-$ decay. We find that as a result of these corrections to the form factors, the zero-position of the forward-backward asymmetry is shifted by $10\%$ from its SM value while the effects on $P_L$ are rather insignificant.

hep-ph↗

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.

hep-ph↗