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Precision observables in inclusive decays of the B meson and their prediction

Final Activity Report Summary - PRECISION B-PHYSICS (Precision observables in inclusive decays of the B meson and their prediction)

Precision experiments in the last 20 years have verified the structure of the Standard Model (SM) of fundamental interactions to very high accuracy. Though very successful, the SM is an incomplete theory and should be seen as a low-energy effective theory. On the other hand, the very success of the SM as a quantum field theory restricts considerably the spectrum of its possible extensions. In particular, precision experiments are sensitive to new mass scales, often beyond the range of present accelerators, because of the exchange of virtual heavy particles. Precision measurements are therefore complementary to direct searches of new particles.

After the end of LEP and waiting for the LHC - the next big experiment at CERN - the main testing grounds for the SM are now the B factories and the neutrino experiments. The B factories produce a vast amount of b quarks and are designed to study the b properties with high accuracy and to detect the most unlikely (rare) of its decay modes. The latter are very interesting because in new physics scenarios they might avoid the suppression they undergo in the SM.

The main questions we would like to answer with the data from the B factories are:
1) Is the mechanism that gives rise to CP violation in the SM correct?
2) Is there any hint of new physics in the rare B decays?

Unfortunately, due to the poorly understood strong interactions that bind the quarks inside the hadrons, it is generally difficult to extract from the data information on CP violation and on the fundamental interactions that give rise to rare decays. A great deal of effort is necessary to take into account perturbative and non-perturbative QCD effects and a number of techniques are used to this effect.

The advantage of inclusive decays is that perturbative and non-perturbative contributions to them can be systematically computed in the context of an Operator Product Expansion, that exploits the large hierarchy between the b quark mass and the scale typical of hadronic interactions. The extraction of the Vub and Vcb elements of the CKM matrix from inclusive semileptonic B decays within this framework is central in the analysis of the so-called CKM unitarity triangle, that parameterises the violation of CP in the SM. However, the uncertainty in this extraction is dominated by theoretical errors. The present project aimed:
i) at reducing the theoretical error in the analysis of inclusive semileptonic and radiative B decays; and
ii) at constraining new physics models from the study of rare B decays.

This has been accomplished through the calculation of certain small effects due to virtual particles, a typical quantum phenomenon. In particular, we have computed QCD corrections to the most general semileptonic distributions, we have studied the moments of charmless semileptonic distributions, and we have analysed the constraints on new physics from combining B to Xs gamma and B to Xs l+l-. The results have been published in first-class research journals, such as Nuclear Physics B, Physical Review Letters, and Journal of High Energy Physics.