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Register a new userEstimate of BR(B -> X_s gamma) at O(alpha_s^2)
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Combining our results for various O(alpha_s^2) corrections to the weak radiative B-meson decay, we are able to present the first estimate of the branching ratio at the next-to-next-to-leading order in QCD. We find BR(B -> X_s gamma) = (3.15 +_ 0.23) x 10^-4 for E_gamma > 1.6 GeV in the B-meson rest frame. The four types of uncertainties: non-perturbative (5%), parametric (3%), higher-order (3%) and m_c-interpolation ambiguity (3%) have been added in quadrature to obtain the total error.
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We examine the effects of R-parity violating (RPV) supersymmetry on the two-photon B decays B -> X_s gamma gamma and B_s -> gamma gamma. We find that, although there are many one-loop RPV diagrams that can contribute to these two-photon B decays, the RPV effect is dominated by a single diagram. This diagram, named here lambda-irreducible, has a distinct topology which is irrelevant for the b -> s gamma amplitude at one-loop and has thus a negligible effect on the one-photon decay B -> X_s gamma. We show that the lambda-irreducible RPV diagram can give BR(B_s -> gamma gamma) ~ 5*10^(-6) and BR(B -> X_s gamma gamma) ~ 6*10^(-7), which is about 16 and 5 times larger than the SM values, respectively. Although the enhancement to the decay width of B -> X_s gamma gamma is not that dramatic, we find that the energy distribution of the two photons is appreciably different from the SM, due to new threshold effects caused by the distinct topology of the RPV lambda-irreducible diagram. Moreover, this diagram significantly changes the forward-backward asymmetry with respect to the softer photon in B -> X_s gamma gamma. Thus, the RPV effect in B -> X_s gamma gamma can be discerned using these observables.
The logarithmic contributions to the massive twist-2 operator matrix elements for deep-inelastic scattering are calculated to $O(alpha_s^3)$for general values of the Mellin variable $N$.
The B -> X_s l+ l- decay rate is known at the next-to-next-to-leading order in QCD. It is proportional to alpha_em (mu)^2 and has a +- 4% scale uncertainty before including the O(alpha_em log(M_W^2/m_b^2)) electromagnetic corrections. We evaluate these corrections and confirm the earlier findings of Bobeth et al. >. Furthermore, we complete the calculation of logarithmically enhanced electromagnetic effects by including also QED corrections to the matrix elements of four-fermion operators. Such corrections contain a collinear logarithm log(m_b^2/m_l^2) that survives integration over the low dilepton invariant mass region 1 GeV^2 < q^2 < 6 GeV^2 and enhances the integrated decay rate in this domain. For the low-q^2 integrated branching ratio in the muonic case, we find B (B -> X_s mu^+mu^-) = (1.59 +- 0.11) 10^(-6), where the error includes the parametric and perturbative uncertainties only. For B (B -> X_s e^+e^-), in the current BaBar and Belle setups, the logarithm of the lepton mass gets replaced by angular cut parameters and the integrated branching ratio for the electrons is expected to be close to that for the muons.
Precise predictions are provided for the production of a $mathrm{Z}$-boson and a $mathrm{b}$-jet in hadron-hadron collisions within the framework of perturbative QCD, at $mathcal{O}(alpha_s^3)$. To obtain these predictions we perform the first calculation of a hadronic scattering process involving the direct production of a flavoured-jet at next-to-next-to-leading order accuracy in massless QCD, and extend techniques to also account for the impact of finite heavy-quark mass effects. The predictions are compared to CMS data obtained in $mathrm{pp}$ collisions at a centre-of-mass energy of $8~mathrm{TeV}$, which are the most precise data from Run I of the LHC for this process, where a good description of the data is achieved. To allow this comparison we have performed an unfolding of the data, which overcomes the long-standing issue that the experimental and theoretical definitions of jet flavour are incompatible.
Using the theoretical and experimental results on $B to X_s gamma$, a four-generation SM is analyzed to constrain the combination of the $4times 4$ Cabibbo-Kobayashi-Maskawa factor $V_{t^prime s}^* V_{t^prime b}$ as a function of the $t^prime$--quark mass. It is observed that the results for the above--mentioned physical quantities are essentially different from the previous predictions for certain solutions of the CKM factor. Influences of the new model is used to predict CP violation in $B to X_s gamma$ decay at the order of $A_{CP}=5%$, stemming from the appearance of complex phases of $V_{t^prime s}^* V_{t^prime b}$ and of Wilson coefficients $C_7$, $C_8$, in the related process. The above mentioned physical quantities can serve as efficient tools in search of the fourth generation.
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