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Ahmed Ali

Publications and source records attributed to Ahmed Ali.

102 records · Page 6Linked to original sources
arXiv2006

$ B \to (ρ, ω)γ$ Decays and CKM Phenomenology

We review and update the branching ratios for the $B \to (ρ,ω) γ$ decays, calculated in the QCD factorization approach in the next-to-leading order (NLO) in the strong coupling $α_s$ and to leading power in $Λ_{\rm QCD}/m_b$. The corrections take into account the vertex, hard-spectator and annihilation contributions and are found to be large. Theoretical expectations for the branching ratios, CP-asymmetry, isospin- and $SU (3)_{\rm F}$-violating ratios in the $B \to ργ$ and $B \to ωγ$ decays are presented and compared with the available data.

hep-ph↗
arXiv2006

A Brief Account of $B \to K^* \ell^+\ell^-$ Decay in Soft-Collinear Effective Theory

A brief account of the study of rare B decay $B \to K^\ast \ell^+ \ell^-$ using soft-collinear effective theory (SCET) is presented. Theoretical underpinning of this work is a factorization formula, derived to leading power in $1/m_b$ and valid to all orders in $α_s$. Partially integrated branching ratio in the dilepton squared mass range $1 {GeV}^2 \le q^2 \le 7 {GeV}^2$ and the forward-backward (FB) asymmetry of the leptons are calculated. For the zero-point of the FB asymmetry, we get $q_0^2=(4.07^{+0.16}_{-0.13}) {GeV}^2$. The scale-related uncertainty of $q_0^2$ is improved compared to the earlier estimate of the same.

hep-ph↗
arXiv2005

Helicity analysis of the decays B --> K^* ell^+ ell^- and B --> rho ell nu_ell in the large Energy Effective Theory

We calculate the independent helicity amplitudes in the decays B --> K^* ell^+ ell^- and B --> rho ell nu_ell in the so-called Large-Energy-Effective-Theory (LEET). Taking into account the dominant O(alpha_s) and SU(3) symmetry-breaking effects, we calculate various Dalitz distributions in these decays making use of the presently available data and decay form factors calculated in the QCD sum rule approach. Differential decay rates in the dilepton invariant mass and the Forward-Backward asymmetry in B --> K^* ell^+ ell^- are worked out. We also present the decay amplitudes in the transversity basis which has been used in the analysis of data on the resonant decay B --> K^* J/psi (--> ell^+ ell^-). Measurements of the ratios R_i(s) equivalent to d Gamma_{H_i}(s)(B --> K^* ell^+ ell^-)/ d Gamma_{H_i}(s)(B --> rho ell nu_ell), involving the helicity amplitudes H_i(s), i = 0,+1, -1, as precision tests of the standard model in semileptonic rare B-decays are emphasized. We argue that R_0(s) and R_{-}(s) can be used to determine the CKM ratio |V_{ub}|/|V_{ts}| and search for new physics, where the later is illustrated by supersymmetry.

hep-ph↗
arXiv2003

CKM Phenomenology and B-Meson Physics - Present Status and Current Issues

We review the status of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements and the CP-violating phases in the CKM-unitarity triangle. The emphasis in these lecture notes is on $B$-meson physics, though we also review the current status and issues in the light quark sector of this matrix. Selected applications of theoretical methods in QCD used in the interpretation of data are given and some of the issues restricting theoretical precision on the CKM matrix elements discussed. The overall consistency of the CKM theory with the available data in flavour physics is impressive and we quantify this consistency. Current data also show some anomalies which, however, are not yet statistically significant. They are discussed briefly. Some benchmark measurements that remain to be done in experiments at the $B$-factories and hadron colliders are listed. Together with the already achieved results, they will provide unprecedented tests of the CKM theory and by the same token may lead to the discovery of new physics.

hep-ph↗
arXiv2002

Implications of B -> rho gamma measurements in the Standard Model and Supersymmetric Theories

We study the implications of the recently improved upper limits on the branching ratios for the decays B -> rho gamma, expressed as R(rho gamma/K^* gamma) = BR(B -> rho gamma)/BR(B -> K^* gamma) <0.047. We work out the constraints that the current bound on R(rho gamma/K^* gamma) implies on the parameters of the quark mixing matrix in the standard model (SM). Using the present profile of the unitarity triangle, we predict this ratio to be R(rho gamma/K^* gamma) = 0.023 +/- 0.012. We also work out the correlations involving R(rho gamma/K^* gamma), the isospin-violating ratio Delta (rho gamma), and the direct CP-violating asymmetry A_CP (rho gamma) in B -> rho gamma decays in the SM, in the minimal supersymmetric extension of the SM (MSSM), and in an extension of the MSSM involving an additional flavor-changing structure in b -> d transitions.

hep-ph↗
arXiv2000

CP Violation and Quark Mixing

Measurements of CP asymmetries in B decays will soon be made at $B$ factories and hadron machines. In light of this, we review and update the profile of the CKM unitarity triangle and the resulting CP asymmetries in B decays. This is done both in the standard model and in several variants of the minimal supersymmetric standard model (MSSM), which are characterized by a single phase in the quark flavour mixing matrix. After imposing present constraints on the parameters of these models, the predicted ranges of $\sin 2 β$ in the standard model and in the MSSM are found to be similar. However, these theories may be distinguished by future precise measurements of the other two CP-violating phases $α$ and $γ$.

hep-ph↗
arXiv1999

Precision Flavour Physics and Supersymmetry

We review the salient features of a comparative study of the profile of the CKM unitarity triangle, and the resulting CP-violating phases $α$, $β$ and $γ$ in B decays, in the standard model and in several variants of the minimal supersymmetric standard model (MSSM), reported recently by us. These theories are characterized by a single phase in the quark flavour mixing matrix and give rise to well-defined contributions in the flavour-changing-neutral-current transitions in K and B decays. We analyse the supersymmetric contributions to the mass differences in the Bd-Bd(bar) and Bs-Bs(bar) systems, $ΔM_d$ and $ΔM_s$, respectively, and to the CP-violating quantity $|epsilon|$ in K decays. Our analysis shows that the predicted ranges of $β$ in the standard model and in MSSM models are very similar. However, precise measurements at B-factories and hadron machines may be able to distinguish these theories in terms of the other two CP-violating phases $α$ and $γ$. (Contribution to the Festschrift for L.B. Okun, to appear in a special issue of Physics Reports, eds. V.L. Telegdi and K. Winter)

hep-ph↗
arXiv1995

Rare Radiative $B$ Decays in the Standard Model

A status report on the theory and phenomenology of rare radiative $B$ decays in the standard model is presented with emphasis on the measured decays $B \to X_s γ$ and $B \to K^* γ$. Standard model is in agreement with experiments though this comparison is not completely quantitative due to imprecise data and lack of the complete next-to-leading order contributions to the decay rates. Despite this, it is possible to extract non-perturbative parameters from the shape of the photon energy spectrum in $B \to X_s γ$, such as the $b$-quark mass and the kinetic energy of the $b$ quark in $B$ hadron. The measured decay rate $\BBGAMAXS =(2.32\pm 0.67) \times 10^{-4}$ can also be used to extract the CKM ratio $\absvts/\absvcb$, yielding $\absvts/\absvcb =1.10 \pm 0.43$. Issues bearing on the determination of the parameters of the CKM matrix from the CKM-suppressed decays $B \to X_d + γ$ and $B \to (ρ,ω) + γ$ are also discussed. It is argued that valuable and independent constraints on the CKM matrix can be obtained from the measurements of these decays, in particular those involving neutral $B$-mesons (to appear in the Proceedings of the XXXth Rencontres de Moriond ``Electroweak Interactions and Unified Theories'', Les Arcs, France, March 1995).

hep-ph↗
arXiv1994

An Update of the CKM Matrix

We update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model using the latest experimental and theoretical results as input. We present the 95$\%$ C.L. allowed region of the unitarity triangle and the corresponding ranges for the ratio $\vert V_{td}/V_{ts} \vert$ and for the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize CP-violating rate asymmetries in $B$-decays. The SM prediction for the $\bs$-$\bsb$ mixing ratio $\xs$ is also presented.

hep-ph↗
arXiv1993

Towards a Model-Independent Analysis of Rare $B$ Decays

Motivated by the experimental accessibility of rare $B$ decays in the ongoing and planned experiments, we propose to undertake a model-independent analysis of the inclusive decay rates and distributions in the processes \bgamaxs~ and \Bsell ~($B=B^\pm$ or $B^0_d$). We show how measurements of the decay rates and distributions in these processes would allow us to extract the magnitude and sign of the dominant Wilson coefficients of the magnetic moment operator $\mb \bar{s}_L σ_{μν} b_R F^{μν}$ and the four-fermion operators $(\bar{s}_L γ_μb_L)(\bar{\ell} γ^μ \ell)$ and $(\bar{s}_L γ_μb_L)(\bar{\ell} γ^μγ^5 \ell)$. Non-standard-model effects could thus manifest themselves at low energy in rare $B$ decays through the Wilson coefficient having values distinctly different from their standard-model counterparts. We illustrate this possibility using the examples of the two-doublet Higgs models and the minimal supersymmetric models. The dilepton invariant mass spectrum and the forward-backward asymmetry of $\ell^+$ in the centre-of-mass system of the dilepton pair in the decay \Bsell ~are also worked out for the standard model and some representative solutions for the other two models.

hep-ph↗
arXiv1992

Radiative Rare B Decays into Higher K-Resonances

We estimate the contributions of higher K-resonances to the radiative rare decays b -> s gamma. We use the spin symmetry for heavy quarks to reduce the number of independent form factors to four Isgur-Wise functions, which are estimated from the wave function model of Isgur, Scora, Grinstein, and Wise. Our results suggest a substantially larger branching fraction for the decay B -> K*2(1430) gamma than previous investigations.

hep-ph↗