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Theoretical Physics II

Special Seminar by Dr. Robert Feger (Vanderbilt University) on March 20 at 10:15 am


"Complete Michel Parameter Analysis of the Inclusive Semileptonic b->c Transition"


The inclusive semileptonic decay B -> X_c l nu yields the best extracted value of V_cb with an uncertainty of less than 2%, which is due to its precise theoretical description by the heavy-quark expansion (HQE) and the large amount of data collected by the B factories Babar and Belle. Besides the precision determination of V_cb and the HQE parameters it also enables us to test for new physics effects. We have performed an analysis, which is known from the leptonic sector as "Michel parameter analysis''. Contributions from possible new physics effects are derived with effective field theory methods, yielding an enhanced b->c current, with not only a right-handed vector contribution, but also scalar and tensor couplings. We repeated the computations of the decay with the enhanced current up to O(1/m_b^2) in the HQE and up to O(alpha_s) in the perturbative corrections.

A moment analysis of the moments of the lepton energy spectrum and the hadronic invariant mass has become a reliable tool in both the theoretical evaluation and the experimental determination. This combines in an HQE fit, which has been improved frequently, yielding a precise determination of the HQE parameters and especially the best extracted value for V_cb. The HQE fit may as well serve as a test for new physics effects in these moments. The theoretical analysis of the moments revealed a low sensitivity of the moments on the non-vector currents. Thus we performed the fit with only a possible right-handed vector current contribution.

Time: 10:15 am, Tue 03/20/2012
Room: Seminar room 22.00.017

T. Flacke

By T. Flacke