arXiv subjects

A. Ali

Publications and source records attributed to A. Ali.

154 records · Page 9Linked to original sources

arXiv1996

Heavy Flavour Decays - Introduction and Overview

We review some selected topics in the decays of heavy flavours, beauty and charm, which are of principal interest at HERA-B and HERA. The topics in $B$ physics include: an update on the quark mixing matrix and CP violating phases, issues bearing on an improved resolution on the CP-violating phase $Δ(sin 2 β)$, prospects of measuring radiative and semileptonic rare $B$ decays, the $B_s^{0}$ - $\overline{B_s^{0}}$ mixing ratio $x_s$, improved measurements of the $B_d^{0}$ - $\overline{B_d^{0}}$ mixing ratio $x_d$ and the $B$-hadron lifetimes, in particular $τ(Λ_b)$. In the charm sector, we have focussed on rare decays and $D^0$- $\overline{D^0}$ mixing, whose measurements will signal physics beyond the standard model.

hep-ph↗

arXiv1996

CP Violation and Flavour Mixing in the Standard Model -- 1996 Update

We review and update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model and estimate the resulting CP asymmetries in $B$ decays, taking into account recent experimental and theoretical developments. With the updated CKM matrix we present the currently-allowed range of the ratios $|V_{td}/V_{ts}|$ and $|V_{td}/V_{ub}|$, as well as the standard model predictions for the \bsbsbar\ mixing parameter $\xs$ (or, equivalently, $\delms$) and the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize the CP-asymmetries in $B$-decays.

hep-ph↗

arXiv1996

$B$ Decays, Flavour Mixings and CP Violation in the Standard Model

These lectures review the progress made in our present understanding of $B$ decays. The emphasis here is on applications of QCD to $B$ decays and the attendant perturbative and non-perturbative uncertainties, which limit present theoretical precision in some cases but the overall picture that emerges is consistent with the standard model (SM). This is illustrated by quantitatively analyzing some of the key measurements in $B$ physics.

hep-ph↗

arXiv1995

CP Violation and Flavour Mixing in the Standard Model

We review and update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model and estimate the resulting CP asymmetries in $B$ decays, taking into account recent experimental and theoretical developments. In performing our fits, we use inputs from the measurements of the following quantities: (i) $\abseps$, the CP-violating parameter in $K$ decays, (ii) $\delmd$, the mass difference due to the \bdbdbar\ mixing, (iii) the matrix elements $\absvcb$ and $\absvub$, (iv) $B$-hadron lifetimes, and (v) the top quark mass. The experimental input in points (ii) - (v) has improved compared to our previous fits. With the updated CKM matrix we present the currently-allowed range of the ratios $|V_{td}/V_{ts}|$ and $|V_{td}/V_{ub}|$, as well as the standard model predictions for the \bsbsbar\ mixing parameter $\xs$ (or, equivalently, $\delms$) and the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize the CP-asymmetries in $B$-decays. Various theoretical issues related to the so-called ``penguin-pollution," are of importance for the determination of the phases $α$ and $γ$ from the CP-asymmetries in $B$ decays, are also discussed. (to be published in the proc. of the Pisa heavy flavour physics conference.)

hep-ph↗

arXiv1995

Photon Energy Spectrum in $B \to X_s γ$ and Comparison with Data

A comparison of the inclusive photon energy spectrum in the radiative decay $\BGAMAXS$, measured recently by the CLEO collaboration, with the standard model is presented, using a $B$-meson wave function model and improving earlier perturbative QCD-based computations of the same. The dependence of the photon energy spectrum on the non-perturbative model parameters, $p_F$, the $b$-quark Fermi momentum in the $B$ hadron, and $m_q$, the spectator quark mass, is explicitly shown, allowing a comparison of these parameters with the ones obtained from the analysis of the lepton energy spectrum in semileptonic $B$ decays. Taking into account present uncertainties, we estimate $\BBGAMAXS = (2.55 \pm 1.28) \times 10^{-4}$ in the standard model, assuming $\absvts/\absvcb= 1.0$. Comparing this with the CLEO measurement $\BBGAMAXS = (2.32 \pm 0.67) \times 10^{-4}$ implies $\absvts/\absvcb= 1.1 \pm 0.43$, in agreement with the CKM unitarity.

hep-ph↗

arXiv1995

Estimates of the Weak Annihilation Contributions to the Decays $B \to ρ+ γ$ and $B \to ω+ γ$

The dominant long-distance contributions to the exclusive radiative decays $B \to ρ(ω) + γ$ involve photon emission from the light quarks at large distances, which cannot be treated perturbatively. We point out that this emission can be described in a theoretically consistent way as the magnetic excitation of quarks in the QCD vacuum and estimate the corresponding parity-conserving and parity-violating amplitudes using the light-cone QCD sum rule approach. These are then combined with the corresponding short-distance contribution from the magnetic moment operator in the same approach, derived earlier, to estimate the decay rates $Γ(B \to ρ(ω) + γ)$. The implications of this result for the determination of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements in radiative $B$ decays are worked out.

hep-ph↗

arXiv1994

Implications of the Top Quark Mass Measurement for the CKM Parameters, $x_s$ and CP Asymmetries

Motivated by the recent determination of the top quark mass by the CDF collaboration, $\mt =174 \pm 10 ^{+13}_{-12}$ GeV, we review and update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model. In performing our fits, we use inputs from the measurements of the following quantities: (i) $\abseps$, the CP-violating parameter in $K$ decays, (ii) $\delmd$, the mass difference due to the \bdbdbar\ mixing, (iii) the matrix elements $\absvcb$ and $\absvub$, and (iv) $B$-hadron lifetimes. We find that the allowed region of the unitarity triangle is very large, mostly due to theoretical uncertainties. (This emphasizes the importance of measurements of CP-violating rate asymmetries in the $B$ system.) Nevertheless, the present data do somewhat restrict the allowed values of the coupling constant product $f_{B_d}\sqrt{\hat{B}_{B_d}}$ and the renormalization-scale invariant bag constant $\hat{B}_K$. With the updated CKM matrix we present the currently-allowed range of the ratio $\vert V_{td}/V_{ts} \vert$, as well as the standard model predictions for the \bsbsbar\ mixing parameter $\xs$ and the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize the CP-asymmetries in $B$-decays. The ALEPH collaboration has recently reported a significant improvement on the lower limit on the $\bs$-$\bsb$ mass difference, $ΔM_s/ΔM_d > 11.3$ (95\% C.L.). This has interesting consequences for the CKM parameters which are also worked out. NOTE: this is a revised and updated version of our previous paper.

hep-ph↗

arXiv1994

Implications of the Top Quark Mass Measurements for the CKM Parameters, $\xs$ and CP Asymmetries

Motivated by the recent determination of the top quark mass by the CDF collaboration, $\mt =174 \pm 10 ^{+13}_{-12}$ GeV, we review and update constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model. In performing these fits, we use inputs from the measurements of $\abseps$, the CP-violating parameter in $K$ decays, $\xd = (\delm)/Γ$, the mixing parameter in \bdbdbar\ mixing, the present measurements of the matrix elements $\absvcb$ and $\absvub$, and the $B$-hadron lifetimes. The CDF value for $\mt$ considerably reduces the CKM-parameter space previously allowed. An interesting result of our analysis is that the present data can be used to restrict the coupling constant product ratio $f_{B_d}\sqrt{B_{B_d}}$ to the range 110-270 MeV -- in comfortable agreement with existing theoretical estimates of this quantity. We use the updated CKM matrix to predict the \bsbsbar\ mixing ratio $\xs$, as well as the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize CP-violating asymmetries in $B$-decays.

hep-ph↗

arXiv1994

Exclusive Radiative B-Decays in the Light-Cone QCD Sum Rule Approach

We carry out a detailed study of exclusive radiative rare $B$-decays in the framework of the QCD sum rules on the light cone, which combines the traditional QCD sum rule technique with the description of final state vector mesons in terms of the light-cone wave functions of increasing twist. The decays considered are: $B_{u,d} \to K^* +γ, B_{u,d}\to ρ+γ, B_d\to ω+γ$ and the corresponding decays of the $B_s$ mesons, $B_s\to ϕ+γ$ and $B_s\to K^*+γ$. Based on our estimate of the transition form factor $F_1^{B \to K^*\pg}(0) =0.32\pm0.05$, we find for the branching ratio $BR(B \to K^* + γ) = (4.8\pm 1.5)\times 10^{-5}$, which is in agreement with the observed value measured by the CLEO collaboration. We present detailed estimates for the ratios of the radiative decay form factors, which are then used to predict the rates for the exclusive radiative B-decays listed above. This in principle allows the extraction of the CKM matrix element $|V_{td}|$ from the penguin-dominated CKM-suppressed radiative decays when they are measured. We give a detailed discussion of the dependence of the form factors on the $b$-quark mass and on the momentum transfer, as well as their interrelation with the CKM-suppressed semileptonic decay form factors in $B\to ρ+\ell+ν$, which we also calculate in our approach.

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↗