arXiv subjects
Publications and source records attributed to Mikolaj Misiak.
The latest Belle and BaBar measurements of the inclusive B -> X_s l^+ l^- branching ratio have smaller errors and lower central values than the previous ones. We point out that these results indicate that the sign of the b -> s gamma amplitude is the same as in the SM. This underscores the importance of B -> X_s l^+ l^- in searches for new physics, and may be relevant for neutralino dark matter analyses within the MSSM.
We evaluate the three-loop matching conditions for the dimension-five operators that are relevant for the b -> s gamma decay. Our calculation completes the first out of three steps (matching, mixing and matrix elements) that are necessary for finding the next-to-next-to-leading QCD corrections to this process. All such corrections must be calculated in view of the ongoing accurate measurements of the B -> Xs gamma branching ratio.
Several years ago, Stefan Pokorski, Manfred Muenz and us outlined a program for calculation of the NLO QCD corrections to the weak radiative B meson decay B -> X_s gamma. Very recently, just before the 60th birthday of Stefan Pokorski, this program has been formally completed. In the present paper, we summarize the existing results and discuss perspectives for further improvement of the accuracy of the Standard Model prediction for BR[ B -> X_s gamma].
We evaluate two-loop b -> s gamma matrix elements of all the four-quark operators containing no derivatives. Contrary to previous calculations, no expansion in the mass ratio mc/mb is performed, and all the possible Dirac and flavor structures are included. Consequently, we are able to provide the last item in the NLO analysis of B -> Xs gamma that has been missing so far, namely the two-loop matrix elements of the QCD-penguin operators. Due to smallness of the Wilson coefficients of those operators in the Standard Model, their effect on the branching ratio is small: a reduction by roughly 1%. We find BR[ B -> Xs gamma]_{E_gamma > 1.6 GeV} = (3.57 +_ 0.30)*10^{-4}.
The charm-loop contribution to B -> Xs gamma is found to be numerically dominant and very stable under logarithmic QCD corrections. The strong enhancement of the branching ratio by QCD logarithms is mainly due to the b-quark mass evolution in the top-quark sector. These observations allow us to achieve better control over residual scale dependence at the next-to-leading order. Furthermore, we observe that the sensitivity of the matrix element < Xs gamma | (sc)_(V-A)(cb)_(V-A) | b > to mc/mb is the source of a sizeable uncertainty that has not been properly taken into account in previous analyses. Replacing mc^pole/mb^pole in this matrix element by the more appropriate mc(mu)/mb^pole with mc < mu < mb causes an 11% enhancement of the SM prediction for BR[B -> Xs gamma]. For E_gamma > 1.6 GeV in the B-meson rest frame, we find BR[ B -> Xs gamma]_{E_gamma > 1.6 GeV} = (3.60 +_ 0.30) * 10^-4. The difference between our result and the current experimental world average is consistent with zero at the level of 1 sigma. We also discuss the implementation of new physics effects in our calculation. The lower bound on the charged Higgs boson mass in 2HDM(II) is found the be higher than 350 GeV.
The experimental and theoretical status of the inclusive decay B -> X_s gamma is briefly summarized. Results from a very recent theoretical analysis are reported. An ~11% increase in the SM prediction for BR[ B -> X_s gamma] is found after replacing m_c^pole/m_b^pole by m_c(mu)/m_b^1S in the NLO QCD correction. The well-known enhancement of the branching ratio by QCD logarithms is identified as an effect of m_b-evolution in the top-quark contribution to the amplitude. This observation helps controlling the residual scale-dependence. The present prediction for the "total" branching ratio differs by 1.4 sigma from the experimental world average.
We evaluate the important two-loop matrix element of the operator (cbar gamma^mu P_L b) (sbar gamma_mu P_L b) contributing to the inclusive radiative decay B -> X_s gamma. The calculation is performed in the NDR scheme, by means of asymptotic expansions method. The result is given as a series in z = m_c^2/m_b^2 up to O(z^6). We confirm the result of Greub, Hurth and Wyler obtained by a different method up to 0(z^3). Higher-order terms are found to be numerically insignificant.
Status of the theoretical B -> X_s gamma and B -> X_s l^+ l^- analyses is reviewed. Recently completed perturbative calculations are mentioned. The level at which non-perturbative effects are controlled is discussed.
Theory of charmless radiative B decays is reviewed. Existence of uncontrolled non-perturbative effects in the inclusive rate at O(alpha_s) is reminded.
We calculate two-loop matching conditions for all the operators that are relevant to B -> X_s l^+ l^- decay in the Standard Model. In effect, we are able to remove the +_16% uncertainty in the decay spectrum, which was mainly due to the renormalization-scale dependence of the top-quark mass. We find 1.46 * 10^-6 for the branching ratio integrated in the domain 0.05 < m_{l^+l^-}^2/m_b^2 < 0.25, for l= e or mu. There remains around 13% perturbative uncertainty in this quantity, while the non-perturbative effects are expected to be smaller.
We evaluate the O(alpha_em/alpha_s) correction to the rate of B -> X_s gamma decay, i.e. we resum all the 0[ alpha_em ln M_W^2/m_b^2 x (alpha_s ln M_W^2/m_b^2)^n ] corrections for n=0,1,2,... Our calculation differs from the previously available one by that it takes into account the complete relevant set of operators. The correction is found to be negligible, i.e. it is below 1%, in accordance with the former results.
The $c \to u γ$ transition does not occur at the tree level in the Standard Model and is strongly GIM-suppressed at one loop. The leading QCD logarithms are known to enhance the amplitude by two orders of magnitude. We point out that the amplitude increases further by two orders of magnitude after including the complete 2-loop QCD contributions. Nevertheless, $ΔS =0$ radiative decays of charmed hadrons remain dominated by the $c \to d \bar{d} u γ$ and $c \to s \bar{s} u γ$ quark subprocesses.
We evaluate matching conditions for the Wilson coefficients of operators mediating the $b \to s γ$ and $b \to s gluon$ transitions in a large class of extensions of the Standard Model. The calculation is performed at the leading order in flavor-changing couplings and includes two-loop QCD corrections. These corrections can be numerically important when approximate cancellations occur among the new physics contributions and/or the SM one.
QCD corrections are evaluated to FCNC processes like $B \to X_s ν\barν$, $K \to πν\barν$, $B \to l^+ l^-$ or $K_L \to μ^+ μ^-$, i.e. to processes mediated by effective operators containing neutrino currents or axial leptonic currents. Such operators originate from $W$-box and $Z$-penguin diagrams in the Standard Model. QCD corrections to them are given by two-loop diagrams. We confirm results for those diagrams which are already present in the literature. However, our analytical expressions for the Wilson coefficients disagree, due to a subtlety in regulating spurious IR divergences. The numerical effect of the disagreement is rather small. The size of the perturbative QCD corrections compared to the leading terms depends on the renormalization scheme used at the leading order. It varies from 0 to around 15% for a reasonable class of schemes. The uncertainty originating from uncalculated higher-order (three-loop) QCD corrections is expected to be around 1% .
We present our results for three-loop anomalous dimensions necessary in analyzing $B{\to}X_sγ$ decay at the next-to-leading order in QCD. We combine them with other recently calculated contributions, obtaining a practically complete next-to-leading order prediction for the branching ratio ${\cal B}(B{\to}X_sγ) = (3.28 \pm 0.33) \times 10^{-4}$. The uncertainty is more than twice smaller than in the previously available leading order theoretical result. The Standard Model prediction remains in agreement with the CLEO measurement at the $2 σ$ level.
We consider Flavour Changing Neutral Current processes in the framework of the supersymmetric extension of the Standard Model. FCNC constraints on the structure of sfermion mass matrices are reviewed. Furthermore, we analyze supersymmetric contributions to FCNC transitions which remain in the limit of flavour-conserving sfermion mass matrices. Implications of the FCNC constraints on the structure of sfermion mass matrices for SUSY breaking and sfermion mass generation are discussed. We conclude that the supersymmetric flavour problem is intriguing but perhaps not as severe as it is commonly believed.
We consider |ΔF| = 1 (F = S,C or B) nonleptonic effective hamiltonian in a renormalization scheme which allows to consistently use fully anticommuting gamma_5 at any number of loops, but at the leading order in the Fermi coupling G_F. We calculate two-loop anomalous dimensions and one-loop matching conditions for the effective operators in this scheme. Finally, we transform our results to one of the previously used renormalization schemes, and find agreement with the original calculations.
We derive an algorithm for automatic calculation of perturbative $β$-functions and anomalous dimensions in any local quantum field theory with canonical kinetic terms. The infrared rearrangement is performed by introducing a common mass parameter in all the propagator denominators. We provide a set of explicit formulae for all the necessary scalar integrals up to three loops.