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We introduce the minimal momentum subtraction (MiniMOM) scheme for QCD. Its definition allows the strong coupling to be fixed solely through a determination of the gluon and ghost propagators. In Landau gauge this scheme has been implicit in the early studies of these propagators, especially in relation to their non-perturbative behaviour in the infrared and the associated infrared fixed-point. Here we concentrate on its perturbative use. We give the explicit perturbative definition of the scheme and the relation of its β-function and running coupling to the MS scheme up to 4-loop order in general covariant gauges. We also demonstrate, by considering a selection of N f = 3 examples, that the apparent convergence of the relevant perturbative series can in some (though not all) cases be significantly improved by re-expanding the MS coupling version of this series in terms of the MiniMOM coupling, making the MiniMOM coupling also of potential interest in certain phenomenological applications.

Employing our previous framework to treat non-perturbative effects selfconsistently, including duality violations, we update the determination of the strong coupling, α s , using a modified version of the 1998 OPAL data, updated to reflect current values of exclusive mode hadronic τ decay branching fractions. To account for non-perturbative effects, non-linear, multi-parameter fits are necessary. We have, therefore, investigated the posterior probability distribution of the model parameters underlying our fits in more detail. We find that OPAL data alone provide only weak constraints on some of the parameters needed to model duality violations, especially in the case of fits involving axial vector channel data, making additional prior assumptions on the expected size of these parameters necessary at present. We provide evidence that this situation could be greatly improved if hadronic spectral functions based on the high-statistics BaBar and Belle data were to be made available.2

In the past decade, one of the major challenges of particle physics has been to gain an in-depth understanding of the role of quark flavor. In this time frame, measurements and the theoretical interpretation of their results have advanced tremendously. A much broader understanding of flavor particles has been achieved; apart from their masses and quantum numbers, there now exist detailed measurements of the characteristics of their interactions allowing stringent tests of Standard Model predictions. Among the most interesting phenomena of flavor physics is the violation of the CP symmetry that has been subtle and difficult to explore. In the past, observations of CP violation were confined to neutral K mesons, but since the early 1990s, a large number of CP-violating processes have been studied in detail in neutral B mesons. In parallel, measurements of the couplings of the heavy quarks and the dynamics for their decays in large samples of K, D, and B mesons have been greatly improved in accuracy and the results are being used as probes in the search for deviations from the Standard Model. In the near future, there will be a transition from the current to a new generation of experiments; thus a review of the status of quark flavor physics is timely. This report is the result of the work of physicists attending the 5th CKM workshop, hosted by the University of Rome "La Sapienza", September 9-13, 2008. It summarizes the results of the current generation of experiments that are about to be completed and it confronts these results with the theoretical understanding of the field which has greatly improved in the past decade. (C) 2010 Elsevier B.V. All rights reserved

We apply an analysis method previously developed for the extraction of the strong coupling from the OPAL data to the recently revised ALEPH data for nonstrange hadronic τ decays. Our analysis yields the values α s (m 2 τ ) = 0.296±0.010 using fixed-order perturbation theory, and α s (m 2 τ ) = 0.310±0.014 using contourimproved perturbation theory. Averaging these values with our previously obtained values from the OPAL data, we find α s (m 2 τ ) = 0.303 ± 0.009, respectively, α s (m 2 τ ) = 0.319 ± 0.012. We present a critique of the analysis method employed previously, for example in analyses by the ALEPH and OPAL collaborations, and compare it with our own approach. Our conclusion is that non-perturbative effects limit the accuracy with which the strong coupling, an inherently perturbative quantity, can be extracted at energies as low as the τ mass. Our results further indicate that systematic errors on the determination of the strong coupling from analyses of hadronic τ -decay data have been underestimated in much of the existing literature.

We investigate the possibility of qq bb tetraquark bound states using n f = 2 + 1 lattice QCD ensembles with pion masses 164, 299, and 415 MeV. Motivated by observations from heavy baryon phenomenology, we consider two lattice interpolating operators both of which are expected to couple efficiently to tetraquark states: one with diquark-antidiquark and one with a mesonmeson structure. Using nonrelativistic QCD to simulate the bottom quarks, we study the udbb, sbb channels with = u, d, and find unambiguous signals for strong-interaction-stable J P = 1 + tetraquarks. These states are found to lie 189(10) and 98(7) MeV below the corresponding free two-meson thresholds.

We present a new framework for the extraction of the strong coupling from hadronic τ decays through finite-energy sum rules. Our focus is on the small, but still significant non-perturbative effects that, in principle, affect both the central value and the systematic error. We employ a quantitative model in order to accommodate violations of quark-hadron duality, and enforce a consistent treatment of the higher-dimensional contributions of the Operator Product Expansion to our sum rules. Using 1998 OPAL data for the non-strange isovector vector and axial-vector spectral functions, we find the n f = 3 values α s (m 2 τ) = 0.307 ±0.019 in fixed-order perturbation theory, and 0.322±0.026 in contour-improved perturbation theory. For comparison, the original OPAL analysis of the same data led to the values 0.324 ± 0.014 (fixed-order) and 0.348 ± 0.021 (contour-improved).