Testing the Standard Model with radiative B decays

par Simon Akar (CPPM)

lundi 17 févr. 2014, 14:00 → 15:00 Europe/Paris

Amphiteatre (CPPM)

In the last decades Standard Model (SM) predictions have been extensively tested, and even though several questions remain without answer, no contradictory experimental result arises to date. Before the start of the B Factories, BaBar in SLAC National Laboratory California, and Belle at KEK, in Japan, the quark flavor sector described by electroweak interactions, from which emerges the essential phenomenon of CP violation, was one of the least tested. The first observation of CP violation in weak interactions was performed in 1964 in decays of neutral K mesons. During the same decade, Andrei Sakharov showed that CP violation was one of the necessary conditions in order to explain why the Universe evolved from its initial matter-antimatter symmetric state to the one that exists today. However, the mechanism allowing for CP violation in the SM is not sufficient to explain the observed asymmetry between matter and antimatter, which could indicate that the SM is an effective model for a more fundamental theory. The absence of results coming from direct searches of New Physics (NP) effects performed in high-energy colliders, indicates that if such NP exist, corresponding effects would arise beyond the electroweak scale. Some specific decay processes, referred to as penguin diagrams, in which CP violation occurs and that are accessible at low-energy colliders such as it is the case in the B factories, allow to probe NP effects via indirect searches. In this context, the "radiative penguin" transition b —> s gamma which proceeds via an electroweak-interaction loop diagram is a promising process; it underlies the decays B —> Xs gamma, where Xs is a hadronic final state. In B0 (B0bar) transitions with radiated photons, the SM predicts a right (left) helicity of these photons. A hadronic correction of order (ms/mb) needs to be taken into account, introducing a small left (right) handed component. Measurement of the photon polarization would be a strong test of the SM, since non-SM processes such as supersymmetry can introduce diagrams with different polarization. As the polarization of the photon cannot be directly measured, different experimental methods exist that give access to this information through other phenomena. In this seminar, results exploiting the mixing-induced CP asymmetry, S, in B0 —> Ks rho0 gamma decays at BaBar will be detailed.

Slides Seminaire_CPPM.pdf