arXiv subjects
Publications and source records attributed to Paolo Gambino.
We compute the O(alpha_s) corrections to the Wilson coefficients of the dimension five operators emerging from the Operator Product Expansion of inclusive radiative B decays. We discuss the impact of the resulting O(alpha_s Lambda_QCD^2/m_b^2) corrections on the extraction of m_b and mu_pi^2 from the moments of the photon spectrum.
The inclusive semileptonic branching ratio is often employed to normalize other inclusive B decays. Using recent determinations of the non-perturbative parameters of the Operator Product Expansion we compute the normalization factor for the branching ratio of B->X_s gamma and find a few percent enhancement with respect to previous determinations.
We compute the fully-differential B -> X_u l nu decay width to all orders in perturbation theory in the large-beta0 limit. Each of the five structure functions that build the hadronic tensor is expressed as a Borel integral, summing up O(CF beta0^{n-1} alpha_s^n) corrections for any n. We derive analytic expressions for the Borel transforms of both real and virtual diagrams with a single dressed gluon, and perform an all-order infrared subtraction, where the Borel parameter serves also as an infrared regulator. Expanding the result we recover the known triple-differential NLO coefficient, and obtain an explicit expression for the O(CF beta0 alpha_s^2) triple-differential NNLO correction. This result can be used to improve the determination of |V_{ub}| from inclusive B -> X_u l nu measurements at the B factories with a variety of kinematic cuts.
We report OPE predictions for hadronic mass and q^2 moments in inclusive semileptonic B decays without charm, taking into account experimental cuts on the charged lepton energy and on the hadronic invariant mass, and address the related theoretical uncertainty.
I briefly review the theoretical status of semileptonic and radiative B decays in 2005.
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 report the OPE-based predictions for a number of lepton energy and hadronic mass moments in the inclusive semileptonic B -> X_c \ellνdecays with a lower cut on the charged lepton energy. We rely on the direct OPE approach where no expansion in the charm mass is employed and the theoretical input is a limited set of underlying OPE parameters including m_b and m_c. A Wilsonian treatment with a `hard' cutoff is applied using running low-scale masses m_Q(μ) and kinetic expectation value μ_π^2(μ). This leaves for perturbative corrections only genuinely short-distance effects and makes them numerically small. Predictions are also given for the modified hadronic moments of the kinematic variable \cal N_X^2 which is a combination of M_X^2 and E_X. Measurement of such moments would allow a more reliable extraction of higher-order nonperturbative heavy quark parameters from experiment.
Overall, the Standard Model describes electroweak precision data rather well. There are however a few areas of tension (charged current universality, NuTeV, (g-2)_μ, b quark asymmetries), which I review emphasizing recent theoretical and experimental progress. I also discuss what precision data tell us about the Higgs boson and new physics scenarios. In this context, the role of a precise measurement of the top mass is crucial.
We compute the complete O(alphas^2) anomalous dimension matrix relevant for the b to s gamma, b to s gluon and b to s l+l- transitions in the standard model and some of its extensions. For radiative decays we confirm the results of Misiak and Munz, and of Chetyrkin, Misiak and Munz. The O(alphas^2) mixing of four-quark into semileptonic operators is instead a new result and represents one of the last missing ingredients of the next-to-next-to-leading-order analysis of rare semileptonic B meson decays.
Present global fits to electroweak data are characterized by two results that differ from Standard Model expectations by about 3 sigma, the NuTeV measurement of sin^2theta_W and the FB b quark asymmetries measured at LEP. I review possible SM and new physics explanations of these anomalies and the implications for the indirect determination of the Higgs mass.
The ratio between the photon spectrum in B -> X_s gamma and the differential semileptonic rate wrt the hadronic variable M_X/E_X is a short-distance quantity calculable in perturbation theory and independent of the Fermi motion of the b quark in the B meson. We present a NLO analysis of this ratio and show how it can be used to determine |V_ub/V_cb| independently of any model for the shape function. We also discuss how this relation can be used to test the validity of the shape-function theory on the data.
We compute the complete two-loop O(alpha) Wilson coefficients relevant for radiative decays of the B meson in the SM. This is a necessary step in the calculation of the O(alpha alpha_s^n ln^n m_b/M_W) corrections and improves on our previous analysis of electroweak effects in B -> X_s gamma. We describe in detail several interesting technical aspects of the calculation and include all dominant QED matrix elements. In our final result, we neglect only terms originated from the unknown O(alpha alpha_s) evolution of the Wilson coefficients and some suppressed two-loop matrix elements. Due to the compensation among different effects, we find that non-trivial electroweak corrections decrease the branching ratio by about 3.6% for a light Higgs boson, very close to our previous result. The corresponding up-to-date SM prediction for the branching ratio with E_gamma > 1.6 GeV is (3.61 +- 0.30) x 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.
We review theoretical and experimental results on CP violation summarizing the discussions in the working group on CP violation at the UK phenomenology workshop 2000 in Durham.
We compute the two-loop electroweak corrections to the radiative decays of the B meson in the SM. Electroweak effects reduce the Wilson coefficient C_7^eff(mw) by 2.6% for a light Higgs boson of about 100 GeV and are less important for a heavier Higgs. The leading term of a heavy top expansion of our result differs from the one obtained in the gaugeless approximation where only top quark Yukawa couplings are considered: we discuss the origin of the discrepancy and provide a criterion for the validity of the gaugeless approximation. As a byproduct of the calculation we also obtain the O(alpha) corrections to the Wilson coefficient of the four-fermion operator Q_2. A careful analysis of the interplay between electroweak and QCD effects leads to an overall 2% reduction of the total branching ratio for B -> X_s gamma due to purely electroweak corrections. For a light Higgs boson, the up-to-date SM prediction is BR_gamma=3.29 10^-4.
In a generic gauge theory the gauge parameter dependence of individual Green functions is controlled by the Nielsen identities, which originate from an enlarged BRST symmetry. We give a practical introduction to the Nielsen identities of the Standard Model (SM) and to their renormalization and illustrate the power of this elegant formalism in the case of the problem of the definition of mass.We prove to all orders in perturbation theory the gauge-independence of the complex pole of the propagator for all physical fields of the SM, in the most general case with mixing and CP violation. At the amplitude level, the formalism provides an intuitive and general understanding of the gauge recombinations which makes it particularly useful at higher orders. We also include in an appendix the explicit expressions for the fermionic two-point functions in a generic R_ξgauge.
Recent calculations of the two-loop electroweak effects enhanced by powers of the top mass have been implemented in the main electroweak libraries and have an important effect on the indirect determination of the Higgs mass MH. I briefly review the main results of these calculations and discuss in detail the residual uncertainties and their impact on the global fit to MH. The perspectives for the near future are also considered.
The electroweak corrections induced by a heavy top on the main precision observables are now available up to O(g^4 mt^2/mw^2). The new results significantly reduce the theoretical uncertainty and have a sizable impact on the determination of sin^2theta_eff. We give precise predictions for mw and the effective sine in different renormalization schemes, estimate their accuracy, and discuss the implications for the indirect determination of M_H. From the present data for sin^2theta_eff we obtain M_H = 127 +143 -71 GeV (or M_H < 430 GeV at 95% C.L.).