arXiv subjects

A. Ali

Publications and source records attributed to A. Ali.

At least 145 records · Page 8Linked to original sources

arXiv1998

Perturbative QCD and Power Corrected Hadron Spectra and Spectral Moments in the Decays $B \to X_s \ell^+ \ell^-$

Leading order (in $α_s$) perturbative QCD and power ($1/m_b^2)$ corrections to the hadronic invariant mass and hadron energy spectra in the decay $B \to X_s \ell^+ \ell^-$ are reviewed in the standard model using the heavy quark expansion technique (HQET). In particular, the first two hadronic moments $ $ and $ $, $n=1,2$, are presented working out their sensitivity on the HQET parameters $λ_1$ and $\barΛ$. Data from the forthcoming B facilities can be used to measure the short-distance contribution in $B \to X_s \ell^+ \ell^-$ and determine the HQET parameters from the moments $ $. This could be combined with complementary constraints from the decay $B \to X \ell ν_\ell$ to determine these parameters precisely.

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arXiv1998

Towards a Theory of Charmless Non-Leptonic Two-Body B Decays

We address a number of theoretical and phenomenological issues in two-body charmless non-leptonic B decays in the context of a factorization model. A classification of the exclusive decays involving tree and penguin amplitudes is reviewed. The role of QCD anomaly in exclusive decays involving an $η$ or $η'$ is elucidated and comparison is made with the existing data. We argue that the factorization approach accounts for most of the observed two-body B decays.

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arXiv1998

A Theoretical Reappraisal of Branching Ratios and CP Asymmetries in the Decays $B \to (X_d,X_s) \ell^+ \ell^-$ and Determination of the CKM Parameters

We present a theoretical reappraisal of the branching ratios and CP asymmetries for the decays $ B \to X_q \ell^+ \ell^-$, with $q=d,s$, taking into account current theoretical uncertainties in the description of the inclusive decay amplitudes from the long-distance contributions, an improved treatment of the renormalization scale dependence, and other parametric dependencies. Concentrating on the partial branching ratios $Δ{\cal B}(B \to X_q \ell^+ \ell^-)$, integrated over the invariant dilepton mass region $1 {GeV}^2 \leq s \leq 6 {GeV}^2$, we calculate theoretical precision on the charge-conjugate averaged partial branching ratios $<Δ{\cal B}_q >= (Δ{\cal B}(B \to X_q \ell^+ \ell^-) + Δ{\cal B}(\bar{B} \to \bar{X}_q \ell^+ \ell^-))/2$, CP asymmetries in partial decay rates $(a_{CP})_q=(Δ{\cal B}(B \to X_q \ell^+ \ell^-) - Δ{\cal B}(\bar{B} \to \bar{X}_q \ell^+ \ell^-))/(2 < Δ{\cal B}_q >)$, and the ratio of the branching ratios $Δ{\cal R} = < Δ{\cal B}_d >/< Δ{\cal B}_s>$. For the central values of the CKM parameters, we find $<Δ{\cal B}_s > =(2.22^{+0.29}_{-0.30}) \times 10^{-6}$, $<Δ{\cal B}_d > =(9.61^{+1.32}_{-1.47}) \times 10^{-8}$, $(a_{CP})_s =-(0.19^{+0.17}_{-0.19})%$, $(a_{CP})_d =(4.40^{+3.87}_{-4.46})%$, and $Δ{\cal R} =(4.32 \pm 0.03)%$. The dependence of $<Δ{\cal B}_d>$ and $Δ{\cal R}$ on the CKM parameters is worked out and the resulting constraints on the unitarity triangle from an eventual measurement of $Δ{\cal R}$ are illustrated.

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arXiv1998

Phenomenological Profiles of the Inclusive Hadron Spectra in the Decay $B \to X_s \ell^+ \ell^-$

Hadron spectra and hadronic moments in the decay $B \to X_s \ell^+ \ell^-$ are calculated taking into account both the short-distance and long-distance contributions in the decay amplitude using a Fermi motion (FM) model to incorporate the $B$-meson wave-function effects. The measured branching ratios for the inclusive decays $B \to X_s+ (J/ψ,ψ^\prime,...)\to X_s \ell^+ \ell^-$ are used to fix the normalization of the long-distance contribution. The momentum distribution of the $J/ψ$ measured by the CLEO collaboration is fitted in the FM model which is then used to calculate the hadronic spectra from the resonant contribution also away from the $J/ψ$-resonance. We also study the effect of various descriptions of the resonant and non-resonant $c\bar{c}$ contributions in $B \to X_s \ell^+ \ell^-$ existing in the literature on the hadron energy and invariant mass spectra, and in the Forward-Backward asymmetry. Selective cuts on the hadron and dilepton invariant masses can be used to reduce the $B\bar{B}$ background and resonant contribution and, as an example, we work out the hadron spectra with the experimental cuts used by the CLEO collaboration in searching for the decay $B \to X_s \ell^+ \ell^-$. We show that data from the forthcoming B facilities could be used effectively to measure the short-distance contribution in $B \to X_s \ell^+ \ell^-$, enabling precise determination of the FM model and heavy quark effective theory parameters $λ_1$ and $\barΛ$.

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arXiv1998

Quark Mixing and CP Violation

We review the historical developments leading to the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. Present status of the CKM matrix from direct measurements is summarized, giving also the present profile of the unitarity triangle. CP Violation in the $K^0$ - $\bar{K^0}$ complex and in $B$-meson decays are discussed in the context of the CKM matrix.

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arXiv1998

CP-Violating Asymmetries in Charmless Non-Leptonic Decays $B \to PP, PV, VV$ in the Factorization Approach

We present estimates of the direct (in decay amplitudes) and indirect (mixing- induced) CP-violating asymmetries in the non-leptonic charmless two-body decay rates for $B \to PP$, $B \to PV$ and $B \to VV$ decays and their charged conjugates, where P(V) is a light pseudoscalar (vector) meson. These estimates are based on a generalized factorization approach making use of next-to-leading order perturbative QCD contributions which generate the required strong phases. No soft final state interactions are included. We study the dependence of the asymmetries on a number of input parameters and show that there are at least two (possibly three) classes of decays in which the asymmetries are parametrically stable in this approach. The decay modes of particular interest are: $\optbar{B^0} \to π^+ π^-$, $\optbar{B^0} \to K_S^0 π^0$, $\optbar{B^0} \to K_S^0 η^\prime$, $\optbar{B^0} \to K_S^0 η$ and $\optbar{B^0} \to ρ^+ ρ^-$. Likewise, the CP-violating asymmetry in the decays $\optbar{B^0} \to K_S^0 h^0$ with $h^0=π^0,K_S^0, η,η^\prime$ is found to be parametrically stable and large. Measurements of these asymmetries will lead to a determination of the phases $\sin 2α$ and $\sin 2 β$ and we work out the relationships in these modes in the present theoretical framework. We also show the extent of the so-called "penguin pollution" in the rate asymmetry $A_{CP}(π^+ π^-)$ and of the "tree shadow" in the asymmetry $A_{CP}(K_S^0η^\prime)$ which will effect the determination of $\sin 2 α$ and $\sin 2 β$ from the respective measurements. CP-violating asymmetries in $B^\pm \to π^\pm η^\prime$, $B^\pm \to K^{*\pm} η$, $B^\pm \to K^{*\pm} η^\prime$ and $B^\pm \to K^{*\pm}ρ^0$ are potentially interesting and are studied here.

hep-ph↗

arXiv1998

Experimental Tests of Factorization in Charmless Non-Leptonic Two-Body B Decays

Using a theoretical framework based on the next-to-leading order QCD-improved effective Hamiltonian and a factorization Ansatz for the hadronic matrix elements of the four-quark operators, we reassess branching fractions in two-body non-leptonic decays $B \to PP, PV, VV$, involving the lowest lying light pseudoscalar $(P)$ and vector $(V)$ mesons in the standard model. Using the sensitivity of the decay rates on the effective number of colors, $N_c$, as a criterion of theoretical predictivity, we classify all the current-current (tree) and penguin transitions in five different classes. The recently measured charmless two-body $B \to PP$ decays $(B^+ \to K^+ η^\prime, B^0 \to K^0 η^\prime, B^0 \to K^+π^-, B^+ \to π^+ K^0$ and charge conjugates) are dominated by the $N_c$-stable QCD penguins (class-IV transitions) and their estimates are consistent with data. The measured charmless $B \to PV$ $(B^+ \to ωK^+, ~B^+ \to ωh^+)$ and $B\to VV$ transition $(B \to ϕK^*)$, on the other hand, belong to the penguin (class-V) and tree (class-III) transitions. The class-V penguin transitions are in general more difficult to predict. We propose a number of tests of the factorization framework in terms of the ratios of branching ratios for some selected $B \to h_1 h_2$ decays involving light hadrons $h_1$ and $h_2$, which depend only moderately on the form factors. We also propose a set of measurements to determine the effective coefficients of the current-current and QCD penguin operators. The potential impact of $B \to h_1 h_2$ decays on the CKM phenomenology is emphasized by analyzing a number of decay rates in the factorization framework.

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arXiv1998

Inclusive Decay Rate for $B \to X_d + γ$ in Next-to-Leading Logarithmic Order and CP Asymmetry in the Standard Model

We compute the decay rate for the CKM-suppressed electromagnetic penguin decay $B \to X_d + γ$ (and its charge conjugate) in NLO QCD, including leading power corrections in $1/m_b^2$ and $1/m_c^2$ in the standard model. The average branching ratio of the decay $B \to X_dγ$ and its charge conjugate is estimated to be in the range $6.0 \times 10^{-6} \leq \leq 2.6 \times 10^{-5}$, obtained by varying the CKM-Wolfenstein parameters $ρ$ and $η$ in the range $-0.1 \leq ρ\leq 0.4$ and $0.2 \leq η\leq 0.46$ and taking into account other parametric dependence. In the stated range of the CKM parameters, we find the ratio $R(dγ/sγ) = / $ to lie in the range between 0.017 and 0.074. Theoretical uncertainties in this ratio are found to be small. Hence, this ratio is well suited to provide independent constraints on the CKM parameters. The CP-asymmetry in the $B \to X_d γ$ decay rates is found to be in the range $(7 - 35)%$. Both the decay rates and CP asymmetry are measurable in forthcoming experiments at $B$ factories and possibly at HERA-B.

hep-ph↗

arXiv1998

Perturbative QCD- and Power-Corrected Hadron Spectra and Spectral Moments in the Decay $B \to X_s \ell^+ \ell^-$

We compute the leading order (in $α_s$) perturbative QCD and power ($1/m_b^2)$ corrections to the hadronic invariant mass and hadron energy spectra in the decay $B \to X_s \ell^+ \ell^-$ in standard model. This is done both by using the heavy quark expansion technique (HQET) and a perturbative-QCD improved Fermi motion (FM) model which takes into account $B$-meson wave-function effects. The corrections in the hadron energy ($E_H$) spectrum are found to be small over a good part of this spectrum in both the methods. However, the expansion in $1/m_b$ in HQET fails near the lower kinematic end-point and at the $c\bar{c}$ threshold. The hadronic invariant mass ($S_H$) spectrum is calculable only over a limited range $S_H > \barΛm_B$ in the heavy quark expansion, where $\barΛ \simeq m_B-m_b$. We also present results for the first two hadronic moments $< S_H^n>$ and $< E_H^n>$, $n=1,2$, working out their sensitivity on the HQET and FM model parameters. For equivalent values of these parameters, the moments in these methods are remarkably close to each other. Using the FM model, we study the effect of the experimental cuts, used recently by the CLEO collaboration in searching for the decay $B \to X_s \ell^+ \ell^-$, on the hadron spectra and hadronic invariant mass moments. The constraints following from assumed values of $< S_H^n>$ on the HQET parameters $λ_1$ and $\barΛ$ are worked out. Data from the forthcoming B facilities could be used to measure the short-distance contribution in $B \to X_s \ell^+ \ell^-$ and determine the HQET parameters $λ_1$ and $\barΛ$. This could be combined with complementary constrains in $B \to X \ell ν_\ell$ to determine them precisely.

hep-ph↗

arXiv1998

Hadron Spectra and Spectral Moments in the Decay B \to X_s \ell^+ \ell^- using HQET

We compute the leading order (in alpha_s) perturbative QCD and power (1/m_b^2) corrections to the hadronic invariant mass and hadron energy spectra in the decay B \to X_s \ell^+ \ell^- in standard model using the heavy quark expansion technique (HQET). Results for the first two hadronic moments < S_H^n> and , n=1,2, are presented here working out their sensitivity on the HQET parameters lambda_1 and \barΛ. Data from the forthcoming B facilities can be used to measure the short-distance contribution in B \to X_s \ell^+ \ell^- and determine the HQET parameters from the moments . This can be combined with the analysis of semileptonic decays B \to X \ell ν_\ell to determine them precisely.

hep-ph↗

arXiv1998

Contribution of $b \to sgg$ through the QCD anomaly in exclusive decays $B^{\pm}\to (η^{\prime},η)(K^{\pm}, K^{\pm})$ and $B^{0}\to (η^{\prime},η)(K^{0},K^{0})$

We compute the decay rates for the exclusive decays $B^{\pm} \to (η^{\prime},η) (K^{\pm}, K^{*\pm})$ and $B^{0}\to (η^{\prime},η) (K^{0}, K^{*0})$ in a QCD-improved factorization framework by including the contribution from the process $b\to sgg \to s (η^{\prime}, η)$ through the QCD anomaly. This method provides an alternative estimate of the contribution $b \to s c\bar{c} \to s(η,η^\prime)$ to these decays as compared to the one using the intrinsic charm content of the $η^{\prime}$ and $η$ mesons determined through the decays $J/ψ\to (η,η^\prime ,η_c) γ$. The resulting branching ratios are compared with the CLEO data on $B^{\pm} \to η^{\prime} K^{\pm}$ and $B^{0} \to η^{\prime} K^{0}$ and predictions are made for the rest.

hep-ph↗

arXiv1998

Flavour Changing Neutral Current Processes and CKM Phenomenology

Some selected topics in flavour changing neutral current processes in $B$ decays are discussed in the context of the Standard Model. The emphasis here is on the determination of the CKM matrix elements from the ongoing and forthcoming experiments. The topics discussed include: (i) CKM-allowed radiative decays $B \to X_s + γ$, (ii) CKM-suppressed radiative decays $B \to X_d + γ$, $B^\pm \to ρ^\pm + γ$ and $B^0 \to (ρ^0,ω) + γ$, (iii) $B^0 - \bar{B^0}$-mixings, and (iv) the Fleischer-Mannel bound in $B \to Kπ$ decays.

hep-ph↗

arXiv1997

An analysis of two-body non-leptonic B decays involving light mesons in the standard model

We report a theoretical analysis of the exclusive non-leptonic decays of B mesons into two light mesons,some of which have been measured recently by the CLEO collaboration. Our analysis is carried out in the context of an effective Hamiltonian based on the Standard Model using next-to-leading order perturbative QCD calculations. Using a factorization ansatz for the hadronic matrix elements, we show that existing data are accounted for in this approach. Thus, theoretical scenarios with a substantially enhanced Wilson coefficient of the chromomagnetic dipole operator (as compared to the SM) and/or those with a substantial color-singlet $c\bar{c}$ component in the wave function of $η^\prime$ are not required by these data. Implications of some of these measurements for the parameters of the CKM matrix are presented.

hep-ph↗

arXiv1997

Theory of Rare B Decays

We discuss some selected topics in rare B decays in the context of the standard model and compare theoretical estimates with available data. Salient features of the perturbative-QCD and power corrections in the decay rate for $B \to X_s + γ$ are reviewed and this framework is used to determine the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $| V_{ts} |$. We review estimates of the ratio $R_{K^*} \equiv Γ(B \to K^* + γ)/Γ(B \to X_s + γ)$ in a number of theoretical models, which give a consistent account of this quantity. Issues bearing on the photon energy spectrum in $B \to X_s + γ$ are also discussed. The CKM-suppressed decays $B \to X_d + γ$, $B^\pm \to ρ^\pm + γ$, and $B^{0} \to (ρ^{0}, ω) + γ$ are reviewed with particular emphasis on the long-distance contributions in the exclusive decays. The impending interest in these decays in determining the parameters of the CKM matrix is emphasized. Finally, the semileptonic decays $B \to X_s \ell^+ \ell^-$ are also discussed in the context of the SM.

hep-ph↗

arXiv1997

Flavour Changing Neutral Current Processes in B Decays

We review the theory and experimental measurements in the flavour changing neutral current (FCNC) processes involving B decays in the context of the standard model (SM). The role of these processes in determining the weak mixing matrix elements is stressed.

hep-ph↗

arXiv1997

Power Corrections in the Decay Rate and Distributions in $B \to X_s \ell^+ \ell^- $ in the Standard Model

We investigate the leading power corrections to the decay rates and distributions in the decay $B \rightarrow X_s \ell^+ \ell^- $ in the standard model (SM) using heavy quark expansion (HQE) in $(1/m_b)$ and a phenomenological model implementing the Fermi motion effects of the b quark bound in the B hadron. In the HQE method, we find that including the leading power corrections the decay width decreases by about 4% and the branching ratio by about 1.5% from their (respective) parton model values. The dilepton invariant mass spectrum is found to be stable against power corrections over a good part of this spectrum. However, near the high-mass end-point this distribution becomes negative with the current value of the non-perturbative parameter $λ_2$ (the $λ_1$-dependent corrections are found to be inoccouos), implying the breakdown of the HQE method in this region. Our results are at variance with the existing ones in the literature in both the decay rate and the invariant dilepton mass distribution calculated in the HQE approach. As an alternative, we implement the non-perturbative effects in the decay $B \rightarrow X_s \ell^+ \ell^- $ using a phenomenologically motivated Gaussian Fermi motion model. We find small corrections to the branching ratio, but the non-perturbative effects are perceptible in both the dilepton mass distribution and the Forward-Backward asymmetry in the high dilepton mass region. Using this model for estimating the non-perturbative effects, we present the decay rates and distributions for the inclusive process $B \rightarrow X_s \ell^+ \ell^- $ in the SM.

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arXiv1996

B Decays in the Standard Model - Status and Perspectives

These lectures review some of the progress made in the quantitative understanding of B decays. The emphasis here is on applications of QCD using perturbative and non-perturbative techniques. In some cases, however, phenomenological models must at present be invoked to make meaningful comparison with data. The resulting picture is consistent with the standard model (SM) and this agreement is quantified in terms of the branching ratios, mixing probabilities, and lifetimes which measure the charge current and effective flavour changing neutral current transitions involving B hadrons. This, in turn, enables a determination of five of the nine elements of the quark mixing matrix. We discuss several proposals on improving the precision on the parameters of this matrix in forthcoming experiments. Issues intimately related to the quark mixing matrix such as the profile of the unitarity triangle and CP-violating asymmetries in B decays are discussed. In particular, we emphasize the role of rare B decays and B^0 - $\overline{B^0}$ mixings in testing the SM quantitatively and in searching for physics beyond the SM.

hep-ph↗

arXiv1996

Rare B Decays in the Standard Model

We discuss the electromagnetic-penguin-dominated radiative B decays $B \to X_s + γ, ~B^{\pm (0)} \to K^{*\pm (0)} + γ$, and $B_s \to ϕ+ γ$ in the context of the standard model (SM) and their Cabibbo-Kobayashi-Maskawa (CKM)-suppressed counterparts, $B \to X_d + γ$, $B^\pm \to ρ^\pm + γ, ~B^{0} \to (ρ^{0}, ω) + γ$, and $B_s \to K^{* 0} + γ$, using QCD sum rules for the exclusive decays. The importance of these decays in determining the parameters of the CKM matrix is emphasized. The semileptonic decays $B \to X_s \ell^+ \ell^-$ are also discussed in the context of the SM and their role in determining the Wilson coefficients of the effective theory is stressed. Comparison with the existing measurements are made and SM-based predictions for a large number of rare B decays are presented.

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