arXiv subjects
Publications and source records attributed to Mohammad R. Ahmady.
The light-front wavefunction for mesons predicted by holographic light-front QCD is used to obtain the light-cone distribution amplitudes for the light vector mesons and the transition form factors for B and $B_s$ decays to $ρ$, $K^*$ and $ϕ$ mesons. Here, I present some of our predictions as compared to lattice QCD, OCD sum rules and the available experimental data.
The tree level FCNC due to the presence of an additional generation of vector quarks result in the leading order nonspectator contributions to rare B-> K^*+photon decay mode. These tree level contributions are sensitive only to b-> s nonunitary parameter U^{sb} and therefore, provide a direct constraint on this model parameter. We obtain the isospin asymmetry between \bar B^0->\bar K^{*0}+photon and B^--> K^{*-}+photon to be Δ_{0-}=-0.03*Re (U^{sb}/(a^c_7V_{tb}V^*_{ts})) and the direct CP asymmetry between B^+->K^{*+}+photon and B^--> K^{*-}+photon to be A_{CP}^{VQM}=0.27|U^{sb}/(V_{tb}V_{ts}^*)|*sinθ*sin{ϕ_s}, where θis the weak phase of U^{sb} and ϕ_s is the strong phase of decay amplitude. We predict a direct CP asymmetry of around a few percent if the current experimental difference between Δ_{0-} and Δ_{0+} is to be explained by the presence of the additional 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^* γdecay is obtained. It is shown that this additional contribution is sensitive to the nonunitarity parameter U^{sb}, 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 resultson the isospin asymmetry becomes available, one can constrain the U^{sb} independent of the mass of the extra quarks and so a much more stringent acceptablemodel parameter space could be obtained.
We investigate the leading effects of extending the Standard Model of electroweak interactions by an extra iso-singlet up- and down- type quark pair on various distributions and total branching ratio of the inclusive B-> X_s l^+ l^- (l =e,μ) rare B decays. The presence of the extra vector-like down quark $D$ results in the non-unitarity of the extended quark mixing matrix V, which in turn leads to b-> s FCNC at the tree level proportional to (V^\dagger V)_{sb}. On the other hand, the effective penguin and box vertex functions are sensitive to the mass of the extra iso-singlet up quark m_U. The experimental upper bound on BR(B-> X_s μ^+ μ^-) is used to constrain the parameters of the model. It is shown that the shapes of the differential branching ratio and forward-backward asymmetry distribution are very sensitive to the value of the model parameters. We also calculate the CP aymmetry distribution of the dileptonic decay in the vector-like quark model. It is shown that, for a typical choice of the model parameters, asymmetries up to around 10% can be achieved for certain values of the dilepton invariant mass.
The nonperturbative content of the QCD vacuum permits the occurrence of QCD-vaccum condensate contributions to penguin amplitudes. We calculate the dimension-4 $ $ and $<α_s G^2>$ contributions to the effective Wilson coefficients for penguin four-Fermi operators, and discuss how such contributions may contribute to nonleptonic B decays.
We examine our previous conjecture that the eta' intermediate resonance has the dominant role in the long distance contributions to B decay into two photons and a strange final state hadron. We calculate the branching ratio of the exclusive B-> K eta'-> K photon photon decay using the nonspectator mechanism for eta' production in charmless hadronic B decays. It is shown that the obtained branching ratio B^eta'(B-> Kγγ)~ 8.7 X 10^{-7} is more than twice as large as the eta_c contribution to this decay mode.
We calculate the leading nonperturbative contributions to the effective Wilson coefficient of the four-fermion operators arising from the QCD penguin, and we demonstrate how the usual perturbative one loop contribution is augmented by nonperturbative condensates. These corrections, which are obtained by quark and gluon condensate insertions into the quark loops, enter at the next-to-leading logarithm precision. Our results indicate for the charmed quark loop that the gluon condensate contribution is quite sensitive to the momentum transfer to the quark-antiquark pair.
We calculate the branching ratio and direct CP asymmetry in nonleptonic two body B decays B^+- ->π^+- η'. It is shown that the tree diagram and gluon fusion mechanism via penguin diagram have comparable contributions to these decays which, as a result, could provide an interesting venue for investigating CP violation. Our estimate shows that the direct CP asymmetry in the above decays could be as large as 75% which along with a branching ratio B(B^- ->π^- η')=3.4 X 10^{-6} should be accessible to experiment in the near future.
We investigate the momentum dependence of the g*-g-eta' vertex form factor in the presence of the nonperturbative QCD vacuum condensate. The behaviour of this form factor is crucial for, among other things, some proposed mechanisms devised to explain the large branching ratio for fast eta' production in charmless B decays observed by CLEO. We show that the leading momentum dependence of this vertex form factor is not altered when the contribution of nonperturbative quark condensate is considered.
We calculate the inclusive decays $B\to X_sψ$ and $B\to X_sη_c$ using factorization assumption. To investigate the bound state effect of the decaying B meson in these inclusive decays we take into account the motion of the $b$ quark using a Gaussian momentum distribution model. The resulting correction to free quark decay approximation is around 6% at most. Utilizing a potential model evaluation of the ratio of the decay constants $f^2_{η_c}/f^2_ψ$, it is shown that the ratio $R=Γ(B\to X_sη_c )/Γ(B\to X_sψ)$ can be used as a possible test of factorization assumption.
We examine the leptonic forward-backward and polarization (left-right) asymmetries in the dileptonic B -> X_s l^+ l^- decay when the momentum dependence of $ψ$ and $ψ'-γ$ conversion strength is taken into account. The results indicate only a small shift in the asymmetry distributions.
We analyse the long-distance contribution to $B\to X_s\ell^+\ell^-$ differential decay rate when the momentum dependence of $ψ$ and $ψ'$-$γ$ conversion strength is taken into account. The results indicate that the resonance to nonresonance interference in the dilepton invariant mass distribution is substantially reduced.
We use the Dirac equation with a ``(asymptotically free) Coulomb + (Lorentz scalar) linear '' potential to estimate the light quark wavefunction for $ q\bar Q$ mesons in the limit $m_Q\to \infty$. We use these wavefunctions to calculate the Isgur-Wise function $ξ(v.v^\prime )$ for orbital and radial ground states in the phenomenologically interesting range $1\leq v.v^ \prime \leq 4$. We find a simple expression for the zero-recoil slope, $ξ^ \prime (1) =-1/2- ε^2 <{r_q}^2>/3$, where $ε$ is the energy eigenvalue of the light quark, which can be identified with the $\barΛ$ parameter of the Heavy Quark Effective Theory. This result implies an upper bound of $-1/2$ for the slope $ξ^\prime (1)$. Also, because for a very light quark $q (q=u, d)$ the size $\sqrt {<{r_q}^2>}$ of the meson is determined mainly by the ``confining'' term in the potential $(γ_\circ σr)$, the shape of $ξ_{u,d}(v.v^\prime )$ is seen to be mostly sensitive to the dimensionless ratio $\bar Λ_{u,d}^2/σ$. We present results for the ranges of parameters $150 MeV <\bar Λ_{u,d} <600 MeV$ $(\barΛ_s \approx \barΛ_{u,d}+100 MeV)$, $0.14 {GeV}^2 \leq σ\leq 0.25 {GeV}^2$ and light quark masses $m_u, m_d \approx 0, m_s=175 MeV$ and compare to existing experimental data and other theoretical estimates. Fits to the data give: ${\barΛ_{u,d}}^2/σ=4.8\pm 1.7 $, $-ξ^\prime_{u,d}(1)=2.4\pm 0.7$ and $\vert V_{cb} \vert \sqrt {\frac {τ_B}{1.48 ps}}=0.050\pm 0.008$ [ARGUS '93]; ${\barΛ_{u,d}}^2/σ= 3.4\pm 1.8$, $-ξ^\prime_{u,d}(1)=1.8\pm 0.7$ and $\vert V_{cb} \vert \sqrt { \frac {τ_B}{1.48 ps}}=0.043\pm 0.008$ [CLEO '93]; ${\barΛ_{u,d}}^2/
The decay rates for the exclusive B decays $B \rightarrow K η_c $ and $B \rightarrow K^* η_c$ are calculated in the context of the heavy quark effective theory. We obtain $Γ(B \rightarrow K η_c )/Γ(B\rightarrow Kψ)=1.6 \pm 0.2$ and $Γ(B\rightarrow K^*η_c)/Γ(B \rightarrow K^*ψ)=0.39 \pm 0.04$. These results lead to estimates $BR(B \rightarrow K η_c)=(0.11 \pm 0.02) \% $ and $BR(B \rightarrow K^* η_c)= (0.05 \pm 0.01) \% $ if we use the central current experimental values for $B\rightarrow (K,K^*)ψ$ branching ratios.
We estimate the non-leptonic B decays $B \rightarrow (ψ,ψ^\prime , χ_{1c})+K^i$, where $K^i$ are various K-meson resonances. We use the model of Isgur, Wise, Scora and Grinstein in the context of heavy quark effective theory, to calculate the hadronic matrix elements. Our estimates show that a substantial fraction of $B \rightarrow X_s ψ$ results in higher resonances of K-meson and besides $B \rightarrow K(K^*) ψ$, a considerable fraction of $B \rightarrow X_s (c\bar c)$ goes to $B \rightarrow (K,K^*)+( ψ^{\prime},χ_{1c})$.
We use the new symmetries in the heavy quark limit to calculate the exclusive B-decays $B \rightarrow K^i ψ$ where $K^i$ are higher resonances of K-meson. We then estimate the long distance contributions to $B \rightarrow K^i γ$ decays via $ψ- γ$ conversion. The results point to around $30 \%$ uncertainty in the top quark mass determined by measurements of rare B-decays $B \rightarrow K^i γ$.
We use the new symmetries in the heavy quark limit to calculate the exclusive B-decays $B \rightarrow K ψ(ψ^{\prime})$ and $B \rightarrow K^* ψ(ψ^{\prime})$. The estimated decay rates are compared with experimental values. We also estimate rare $B \rightarrow K^* γ$ decay rate, based on these experimental data and heavy quark symmetry.