Mohammad R. Ahmady

Examination of the resonance contributions to dileptonic rare B decays.

We analyze the long-distance contribution to the {ital B}{r_arrow}{ital X}{sub {ital s}}l{sup +}l{sup {minus}} differential decay rate when the momentum dependence of the {psi} and {psi}{sup {prime}}-{gamma} conversion strength is taken into account. The results indicate that the resonance to nonresonance interference in the dilepton invariant mass distribution is substantially reduced. {copyright} {ital 1996 The American Physical Society.}

Estimate of the three-loop perturbative contribution to inclusive semileptonic b→u decays

Abstract We utilize asymptotic Pade-approximant methods to estimate the three-loop order MS -scheme coefficients within the inclusive b→u ν l l − decay rate for four and five active quark flavours. The estimates we obtain for the three renormalization-group-accessible coefficients within the three-loop contribution are all found to be within 5.1% of their true values, using a least-squares procedure in conjunction with an asymptotic Pade-approximant estimate of the three-loop term over the entire μ ≥ m b domain. Given the input values α s ( M Z )=0.118±0.004 and m b (m b )=4.17±0.05 GeV , the three-loop expression for the purely-perturbative contribution to the b→u ν l l − decay rate is minimally sensitive to renormalization scale at μ=1.775 GeV , at which scale the three-loop contribution is estimated to be only 1.4% of the leading tree-order contribution. We estimate the full perturbative decay rate to be 192π 3 Γ(b→u ν l l − )/ G F 2 |V ub | 2 =2070 GeV 5 ±16% , inclusive of theoretical uncertainties from series truncation, the input parameters, and the estimation procedure.

Isgur-Wise function in a relativistic model for a qQ-bar system.

Fits to the data give {bar {Lambda}} {sub {ital u},}{ital d}{sup 2}/{sigma}=4.8{plus_minus}1.7, {minus}{xi}{sub {ital u},}{ital d}{sup {prime}}(1)=2.4{plus_minus}0.7, and {vert_bar}{ital V}{sub {ital c}{ital b}}{vert_bar} {radical}{tau}{sub {ital B}}/1.48 ps =0.050{plus_minus}0.008 [ARGUS 1993]; {bar {Lambda}} {sub {ital u},}{ital d}{sup 2}/{sigma}=3.3{plus_minus}1.2, {minus}{xi}{sub {ital u},}{ital d}{sup {prime}}(1)=1.8{plus_minus}0.5, and {vert_bar}{ital V}{sub {ital c}{ital b}}{vert_bar} {radical}{tau}{sub {ital B}}/1.48 ps =0.043{plus_minus}0.005 [CLEO 1993]; {bar {Lambda}} {sub {ital u},}{ital d}{sup 2}/{sigma}=2.0{plus_minus}0.7, {minus}{xi}{sub {ital u},}{ital d}{sup {prime}}(1)=1.3{plus_minus}0.3, and {vert_bar}{ital V}{sub {ital c}{ital b}}{vert_bar}{ital F}(1)=0.037{plus_minus}0.002 [CLEO 1994] [existing theoretical estimates for {ital F}(1) fall in the range 0.86{lt}{ital F}(1){lt}1.01.] Our model seems to favor the CLEO 1994 data set in two respects: the fits are better and the resulting ranges for the model parameters ({bar {Lambda}}{sub {ital u},}{ital d}{sub ,}{sigma}) are more in line with independent theoretical estimates.

ISOSPIN SYMMETRY BREAKING IN B → K*γ DECAY DUE TO AN EXTRA GENERATION OF VECTOR QUARKS

The extra contributions due to an extra generation of vector-like quarks to the isospin symmetry breaking observable associated with the radiative B → K*γ decay is obtained. It is shown that this additional contribution is sensitive to the nonunitarity parameter Usb, which is a measure of the the strength of the non-zero tree-level flavor changing neutral current in this model. The significance of this result is that, once accurate experimental results on the isospin asymmetry becomes available, one can constrain the Usb independent of the mass of the extra quarks and so a much more stringent acceptable model parameter space could be obtained.

Decays eta c--> gamma gamma and eta b--> gamma gamma : A test for potential models.

We use a simple perturbation theory argument and measurements of charmonium leptonic widths

Non-Leptonic B Decays into K-Resonances

\Gamma (\psi_{NS} \rightarrow e^+e^-)

The Decay

to estimate the ratio \mbox{

\eta_c \rightarrow \gamma \gamma

R_\circ \equiv {\vert \Psi _{\eta_{c1S}}(0) \vert}^2 /{\vert\Psi_{\psi_{1 S}}(0)\vert}^2

: A Test for Potential Models

} in the general context of non- relativistic potential models. We obtain

A theoretical prediction for exclusive decaysB→K(K*)η c

R_\circ = 1.4 \pm 0.1