Effective theory approach to new physics in $b\to u$ and $b\to c$ leptonic and semileptonic decays

Abstract

Recent measurements of exclusive $B^{-}\to {\tau }^{-}\nu$ and $B^0\to {\pi }^{+}{l}^{-}{\bar{\nu }}_l$ decays via the $b\to ul\nu$ transition process differ from the standard model expectation and, if they persist in future $B$ experiments, will be a definite hint of the physics beyond the standard model. Similar hints of new physics have been observed in $b\to c$ semileptonic transition processes as well. BABAR measures the ratio of branching fractions of $B\to (D,D^{\ast })\tau \nu$ to the corresponding $B\to (D,D^{\ast })l\nu$, where $l$ represents either an electron or a muon, and finds $3.4\sigma$ discrepancy with the standard model expectation. In this context, we consider a most general effective Lagrangian for the $b\to ul\nu$ and $b\to cl\nu$ transition processes in the presence of new physics and perform a combined analysis of all the $b\to u$ and $b\to c$ semi-(leptonic) data to explore various new physics operators and their couplings. We consider various new physics scenarios and give predictions for the $B_c\to \tau \nu$ and $B\to \pi \tau \nu$ decay branching fractions. We also study the effect of these new physics parameters on the ratio of the branching ratios of $B\to \pi \tau \nu$ to the corresponding $B\to \pi l\nu$ decays.

Comments: 32 pages, 8 figures, published version