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Register a new userConstraint on compactification scale via recently observed baryonic $Lambda_brightarrow Lambda ell^+ ell^-$ channel and analysis of the $Sigma_b rightarrow Sigma ell^+ ell^-$ transition in SM and UED scenario
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We obtain a lower limit on the compactification scale of extra dimension via comparison of the branching ratio in the baryonic $Lambda_brightarrow Lambda mu^+ mu^-$ decay channel recently measured by CDF collaboration and our previous theoretical study. We also use the newly available form factors calculated via light cone QCD sum rules in full theory to analyze the flavour changing neutral current process of the $Sigma_b rightarrow Sigma ell^+ ell^-$ in universal extra dimension scenario in the presence of a single extra compact dimension. We calculate various physical quantities like branching ratio, forward-backward asymmetry, baryon polarizations and double lepton polarization asymmetries defining the decay channel under consideration. We also compare the obtained predictions with those of the standard model.
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We comparatively analyze the flavor changing neutral current process of the $Lambda_b rightarrow Lambda ell^+ ell^-$ in the standard model as well as topcolor-assisted technicolor model using the form factors calculated via light cone QCD sum rules in full theory. In particular, we calculate the decay width, branching ratio and lepton forward-backward asymmetry related to this decay channel. We compare the results of the topcolor-assisted technicolor model with those of the standard model and debate how the results of the topcolor-assisted technicolor model depart from the standard model predictions. We also compare our results on the differential branching ratio with recent experimental data provided by CDF and LHCb Collaborations.
We comparatively analyze the rare $Lambda _brightarrow Lambda ell^+ ell^-$ channel in standard model, supersymmetry and Randall-Sundrum model with custodial protection (RS$_c$). Using the parametrization of the matrix elements entering the low energy effective Hamiltonian in terms of form factors, we calculate the corresponding differential decay width and lepton forward-backward asymmetry in these models. We compare the results obtained with the most recent data from LHCb as well as lattice QCD results on the considered quantities. It is obtained that the standard model, with the form factors calculated in light-cone QCD sum rules, can not reproduce some experimental data on the physical quantities under consideration but the supersymmetry can do it. The RS$_c$ model predictions are roughly the same as the standard model and there are no considerable differences between the predictions of these two models. In the case of differential decay rate, the data in the range $4 $ GeV$^2/$c$^4leq$ $q^2 leq 6$ GeV$^2/$c$^4$ can not be described by any of the considered models.
We work out the semileptonic $Lambda_brightarrow Lambda ell^+ ell^-$ transition in standard as well as different supersymmetric models. In particular, considering the parametrization of the matrix elements entered the low energy effective Hamiltonian in terms of form factors in full QCD, we calculate the amplitude and differential decay rate responsible for this decay channel in supersymmetric models. We then use the form factors calculated via light cone QCD sum rules in full theory to analyze the differential branching ratio and lepton forward-backward asymmetry of this decay channel in different supersymmetric models and compare the obtained results with those of the standard model. We also discuss how the results of different supersymmetric models deviate from the standard model predictions and which SUSY scenarios are favored.
We discuss the general properties of the amplitude of the $Bto l^+l^-l u$ decays and calculate the related kinematical distributions $d^2Gamma/dq^2dq^2$, $q$ the momentum of the $l^+l^-$ pair emitted from the electromagnetic vertex and $q$ the momentum of the $l u$ pair emitted from the weak vertex. We emphasize that electromagnetic gauge invariance imposes essential constraints on the $Bto gamma^*l u$ amplitude at small $q^2$ which in the end yield the behaviour of the differential branching fraction as $dGamma(Bto l^+l^-l u)/dq^2propto 1/q^2$ and a mild logarithmic dependence of $Gamma(Bto l^{+}l^{-}l u)$ on the lepton mass $m_l$. Consequently, (i) the main contribution to the decay rate $Gamma(Bto mu^+mu^-e u_e )$ comes from the region of light vector resonances $rho^0$ and $omega$, $q^2simeq M_rho^2, M_omega^2$ and (ii) the decay rate $Gamma(Bto e^{+}e^{-}mu u_mu)$ receives comparable contributions from the region of small $q^2$ and from the resonance region. As the result, the decay rate $Gamma(Bto e^+e^-mu u_mu)$ is only a factor $sim 2$ larger than $Gamma(Bto mu^+mu^-e u_e)$. We perform a detailed analysis of the uncertainties in the theoretical predictions for the decays $Bto l^+l^-l u$ in the Standard Model. We found that the theoretical expectations for such decays in the Standard Model are only marginally compatible with the recent upper limits of the LHCb collaboration.
Using the responsible form factors calculated via full QCD, we analyze the $Lambda_{b}rightarrow Lambda ell^{+}ell^{-}$ transition in the standard model containing fourth generation quarks (SM4). We discuss effects of the presence of $t$ fourth family quark on related observables like branching ratio, forward-backward asymmetry, baryon polarization as well as double lepton polarization asymmetries. We also compare our results with those obtained in the SM as well as with predictions of the SM4 but using form factors calculated within heavy quark effective theory. The obtained results on branching ratio indicate that the $Lambda_{b}rightarrow Lambda ell^{+}ell^{-}$ transition is more probable in full QCD comparing to the heavy quark effective theory. It is also shown that the results on all considered observables in SM4 deviate considerably from the SM predictions when $m_{t}geq 400 GeV$.
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