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M. R. Ahmady

Publications and source records attributed to M. R. Ahmady.

At least 19 recordsLinked to original sources
arXiv2014

The isospin asymmetry in $B \to K^* μ^+ μ^-$ using AdS/QCD

We compute the isospin asymmetry distribution in the rare dileptonic decay $B \to K^* μ^+ μ^-$, in the dimuon mass squared ($q^2$) region below the $J/Ψ$ resonance, using non-perturbative inputs as predicted by the anti-de Sitter/Quantum Chromodynamics (AdS/QCD) correspondence and by Sum Rules. We predict a positive asymmetry at $q^2=0$ which flips sign in the region $q^2 \in [1,2]~\mbox{GeV}^2$ to remain small ($\le 2\%$) and negative for larger $q^2$. While our predictions are distinct as $q^2 \to 0$, they become hardly model-dependent $q^2 \ge 4~\mbox{GeV}^2$. We compare our predictions to the most recent LHCb data.

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arXiv2009

Estimating the annihilation decay B_s -> rho gamma with factorization

The branching ratio for the rare two-body B_s -> rho gamma decay is calculated using the factorization assumption. This transition is dominated by the annihilation diagrams and, in principle, prone to receiving substantial contributions from new physics. We estimate Br(B_s -> rho gamma) = 1.6 x 10^-9 within the Standard Model and investigate the sensitivity of this decay mode to the effects of two new physics scenarios: vector quark model and supersymmetry. Our results indicate that the shift in branching ratio is at most around 10% with the addition of vector quarks and is negligibly small in the constrained minimal supersymmetric extension of the Standard Model.

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arXiv2007

Constraints on the mSUGRA parameter space from NLO calculation of isospin asymmetry in B -> K* gamma

The contributions of supersymmetric particles in the isospin symmetry violation in B -> K* gamma decay mode are investigated. The model parameters are adopted from minimal Supergravity with minimal flavor violation. A complete scan of the mSUGRA parameter space has been performed, using the next to leading supersymmetric contributions to the relevant Wilson coefficients. The results are compared to recent experimental data in order to obtain constraints on the parameter space. We point out that isospin asymmetry can prove to be an interesting observable and imposes severe restrictions on the allowed parameter space, in particular for large values of tan(beta). The constraints obtained with isospin asymmetry also appear as more restricting than the ones from the branching ratio of B -> X_s gamma.

hep-ph↗
arXiv2006

Constraints on mSUGRA from isospin asymmetry in B -> K* gamma

We study the isospin asymmetry as an observable in exclusive B -> K* gamma decay that can be used to study the effects of physics beyond the Standard Model. We present predictions for isospin asymmetry in Supersymmetric extension of the Standard Model with minimal flavor violation, using parameter values allowed by current experimental constraints. This observable can provide very tight constraints on the mSUGRA parameter space.

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arXiv2006

Improvement of Renormalization-Scale Uncertainties Within Empirical Determinations of the b-Quark Mass

Accurate determinations of the MS-bar b-quark mass $m_b(m_b)$ from $σ(e^+e^-\to{\rm hadrons})$ experimental data currently contain three comparable sources of uncertainty; the experimental uncertainty from moments of this cross-section, the uncertainty associated with $α_s(M_z)$, and the theoretical uncertainty associated with the renormalization scale. Through resummation of all logarithmic terms explicitly determined in the perturbative series by the renormalization-group (RG) equation, it is shown that the renormalization-scale dependence is virtually eliminated as a source of theoretical uncertainty in $m_b(m_b)$. This resummation also reduces the estimated effect of higher-loop perturbative contributions, further reducing the theoretical uncertainties in $m_b(m_b)$. Furthermore, such resummation techniques improve the agreement between the values of the MS-bar b-quark mass extracted from the various moments of $R(s)=σ(e^+e^-\to{\rm hadrons})/σ_{pt}$ [$σ_{pt}=4πα^2/(3s)$], obviating the need to choose an optimummoment for determining $m_b(m_b)$. Resummation techniques are also shown to reduce renormalization-scale dependence in the relation between b-quark MS-bar and pole mass and in the relation between the pole and $1S$ mass.

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arXiv2003

Heavy Baryonic Decays of Λ_b \to Λη^{(\prime)} and Nonspectator Contribution

We calculate the branching ratios of the hadronic Λ_b decays to ηand η^\prime in the factorization approximation where the form factors are estimated via QCD sum rules and the pole model. Our results indicate that, contrary to B\to Kη^{(\prime)} decays, the branching ratios for Λ_b\toΛηand Λ_b\toΛη^\prime are more or less the same in the hadronic Λ_b transitions. We estimate the branching ratio of Λ_b\toΛη^{(\prime)} to be 10.80 (10.32)\times 10^{-6} in QCD sum rules, and 2.78 (2.96)\times 10^{-6} in the pole model. We also estimate the nonfactorizable gluon fusion contribution to Λ_b\toΛη^\prime decay by dividing this process into strong and weak vertices. Our results point to an enhancement of more than an order of magnitude due to this mechanism.

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arXiv2003

Optimal Renormalization-Group Improvement of the Perturbative Series for the e^+ e^- -Annihilation Cross-Section

Using renormalization-group methods, we derive differential equations for the all-orders summation of logarithmic corrections to the QCD series for R(s) = sigma(e^+ e^- --> hadrons)/sigma(e^+ e^- --> mu^+ mu^-), as obtained from the imaginary part of the purely-perturbative vector-current correlation function. We present explicit solutions for the summation of leading and up to three subsequent subleading orders of logarithms. The summations accessible from the four-loop vector-correlator not only lead to a substantial reduction in sensitivity to the renormalization scale, but necessarily impose a common infrared bound on perturbative approximations to R(s), regardless of the infrared behaviour of the true QCD couplant.

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arXiv2002

Renormalization-Group Improvement of Effective Actions Beyond Summation of Leading Logarithms

Invariance of the effective action under changes of the renormalization scale $μ$ leads to relations between those (presumably calculated) terms independent of $μ$ at a given order of perturbation theory and those higher order terms dependent on logarithms of $μ$. This relationship leads to differential equations for a sequence of functions, the solutions of which give closed form expressions for the sum of all leading logs, next to leading logs and subsequent subleading logarithmic contributions to the effective action. The renormalization group is thus shown to provide information about a model beyond the scale dependence of the model's couplings and masses. This procedure is illustrated using the $ϕ_6^3$ model and Yang-Mills theory. In the latter instance, it is also shown by using a modified summation procedure that the $μ$ dependence of the effective action resides solely in a multiplicative factor of $g^2 (μ)$ (the running coupling). This approach is also shown to lead to a novel expansion for the running coupling in terms of the one-loop coupling that does not require an order-by-order redefinition of the scale factor $Λ_{QCD}$. Finally, logarithmic contributions of the instanton size to the effective action of an SU(2) gauge theory are summed, allowing a determination of the asymptotic dependence on the instanton size $ρ$ as $ρ$ goes to infinity to all orders in the SU(2) coupling constant.

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arXiv2002

Pade/renormalization-group improvement of inclusive semileptonic B decay rates

Renormalization Group (RG) and optimized Pade-approximant methods are used to estimate the three-loop perturbative contributions to the inclusive semileptonic b \to u and b \to c decay rates. It is noted that the \bar{MS} scheme works favorably in the b \to u case whereas the pole mass scheme shows better convergence in the b \to c case. Upon the inclusion of the estimated three-loop contribution, we find the full perturbative decay rate to be 192π^3Γ(b\to u\barν_\ell\ell^-)/(G_F^2| V_{ub}|^2) = 2065 \pm 290{\rm GeV^5} and 192π^3Γ(b\to c\ell^-\barν_\ell)/(G_F^2|V_{cb}|^2)= 992 \pm 198 {\rm GeV^5}, respectively. The errors are inclusive of theoretical uncertainties and non-perturbative effects. Ultimately, these perturbative contributions reduce the theoretical uncertainty in the extraction of the CKM matrix elements |V_{ub}| and |V_{cb}| from their respective measured inclusive semileptonic branching ratio(s).

hep-ph↗
arXiv2002

Renormalization-Mass Scale Dependence in QCD Contributions to Semileptonic $b \to u$ Decay

QCD contributions to the $b \to u \ell^- \barν_\ell$ decay rate, which are known to two-loop order in the $\bar{MS}$ scheme, exhibit sufficient dependence on the renormalization mass $μ$ to compromise phenomenological predictions for inclusive semileptonic $B \to X_u$ processes. Such scale dependence is ameliorated by the renormalization-group (RG) extraction and summation of all leading and RG-accessible subleading logarithms occurring subsequent to two-loop order in the perturbative series. This optimal RG-improvement of the known portion of the perturbative series virtually eliminates $μ$-dependence as a source of theoretical uncertainty in the predicted semileptonic $B \to X_u$ inclusive rate.

hep-ph↗
arXiv2002

Three Loop Estimate of the Inclusive Semileptonic $b\to c$ Decay Rate

The renormalization-scale ($μ$) dependence of the two-loop inclusive semileptonic $b\to c\ell^-\barν_\ell$ decay rate is shown to be significant in the pole mass scheme, and the decay rate is shown to be poorly convergent in the MS-bar scheme. Three-loop contributions to the decay rate are estimated by developing Pade approximant techniques particularly suited to perturbative calculations in the pole mass scheme. An optimized Pade estimate of the three-loop contributions is obtained by comparison of the Pade estimates with the three-loop terms determined by renormalization-group invariance. The resulting three-loop estimate in the pole-mass scheme exhibits minimal sensitivity to the renormalization scale near $μ=1.0 GeV$, leading to an estimated decay rate of $192π^3Γ(b\to c\ell^-\barν_\ell)/(G_F^2|V_{cb}|^2)=992\pm 217 GeV^5$ inclusive of theoretical uncertainties and non-perturbative effects.

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arXiv2002

Closed-Form Summation of RG-Accessible Logarithmic Contributions to Semileptonic B-Decays and Other Perturbative Processes

For any perturbative series that is known to $k$-subleading orders of perturbation theory, we utilise the process-appropriate renormalization-group (RG) equation in order to obtain all-orders summation of series terms proportional to $α^n \log^{n-k}(μ^2)$ with $k = {0,1,2,3}$, corresponding to the summation to all orders of the leading and subsequent-three-subleading logarithmic contributions to the full perturbative series. These methods are applied to the perturbative series for semileptonic $b$-decays in both MS-bar and pole-mass schemes, and they result in RG-summed series for the decay rates which exhibit greatly reduced sensitivity to the renormalization scale $μ$. Such summation via RG-methods of all logarithms accessible from known series terms is also applied to perturbative QCD series for vector- and scalar-current correlation functions, the perturbative static potential function, the (single-doublet standard-model) Higgs decay amplitude into two gluons, as well as the Higgs-mediated high-energy cross-section for $W^+W^-\to ZZ$ scattering. The resulting RG-summed expressions are also found to be much less sensitive to the renormalization scale than the original series for these processes.

hep-ph↗
arXiv2002

Constraints on Higher-Order Perturbative Corrections in $b\to u$ Semileptonic Decays from Residual Renormalization-Scale Dependence

The constraint of a progressive decrease in residual renormalization scale dependence with increasing loop order is developed as a method for obtaining bounds on unknown higher-order perturbative corrections to renormalization-group invariant quantities. This technique is applied to the inclusive semileptonic process $b\to u \barν_\ell\ell^-$ (explicitly known to two-loop order) to obtain bounds on the three- and four-loop perturbative coefficients that are not accessible via the renormalization group. Using the principle of minimal sensitivity, an estimate is obtained for the perturbative contributions to $Γ(b\to u \barν_\ell\ell^-)$ that incorporates theoretical uncertainty from as-yet-undetermined higher order QCD corrections.

hep-ph↗
arXiv2001

B -> X_s l+ l- in the vectorlike quark model

We extend the standard model by adding an extra generation of isosinglet up- and down-type quark pair which engage in weak interactions only via mixing with the three ordinary quark families. It is shown that the generalized $4\times 4$ quark mixing matrix, which is necessarily nonunitary, leads to nonvanishing flavor changing neutral currents. We then proceed to investigate various distributions and total branching ratio of the inclusive B -> X_s l+ l- (l =e,μ) rare B decays in the context of this 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 which are constrained by the experimental upper bound on BR(B -> X_s μ^+ μ^-). We also indicate that, for certain values of the dileptonic invariant mass, CP asymmetries up to 10% can be obtained.

hep-ph↗
arXiv2000

Reducing the Theoretical Uncertainty in Extracting |V_{ub}| from the Inclusive Semileptonic b to u Decay Rate

Utilizing asymptotic Pade-approximant methods, we estimate the three-loop order MS-bar coefficients of alpha_s^3[\log(μ^2/m^2_b(μ)]^k terms [k = {0,1,2,3}] within the $b \to u\ell^-\barν_\ell$ decay rate. Except for the coefficient of the k = 0 term, all other coefficients may also be obtained via renormalization-group (RG) methods. The relative errors in asymptotic Pade-approximant estimates of the k = {1,2,3} terms are found to be 5.1% or less, thereby providing an estimate of the theoretical uncertainty in the asymptotic Pade-approximant estimate of the RG-inaccessible k = 0 term. By a judicious choice in the renormalization scale parameter, we are able to extract |V_{ub}| from the inclusive decay rate to within 7% theoretical uncertainty.

hep-ph↗
arXiv2000

Estimate of the Three-Loop Perturbative Contribution to Inclusive Semileptonic b to u Decays

We utilize asymptotic Pade-approximant methods to estimate the three-loop order MS bar-scheme coefficients within the inclusive semileptonic b to u 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 $μ\ge m_b$ domain. Given the input values $α_s(M_Z) = 0.118 \pm 0.004$ and $m_b(m_b) =4.17 \pm 0.05 GeV$, the three-loop expression for the purely-perturbative contribution to the 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 $192π^3Γ(b\to u\barν_\ell\ell^-)/(G_F^2| V_{ub}|^2) = 2070 {\rm GeV^5} \pm 16%$, inclusive of theoretical uncertainties from series truncation, the input parameters, and the estimation procedure.

hep-ph↗
arXiv2000

A more careful estimate of the charm content of eta'

We estimate the quantity |f_eta'^(c)| which is associated with the charm content of eta' meson from the experimentally known ratio R=B(psi ->eta' gamma )/B(psi ->eta_c gamma). It is shown that due to the off-shellness of the c\bar c component of eta', which has been overlooked so far, f_eta'^(c) is further suppressed. Assuming that psi ->eta' gamma decay is dominated by psi ->eta_c transition, we obtain |f_eta'^(c)| = 2.4 MeV which could imply that the (b -> c\bar cs) mechanism does not play a major role in the B -> K eta' decay mode.

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