New physics effects in tree-level decays

TL;DR

This paper examines the potential impact of new physics on tree-level B-meson decays and finds that sizable contributions could introduce uncertainties in the CKM angle gamma measurement, highlighting the need for more precise constraints.

Contribution

It demonstrates that current experimental data do not exclude significant new physics effects in tree-level Wilson coefficients, affecting the precision of CKM angle gamma determination.

Findings

Sizable new physics contributions in C1 and C2 are not excluded by data.

Such contributions can cause an uncertainty of about 4° in gamma.

Improved measurements of semi-leptonic CP asymmetries could better constrain new physics effects.

Abstract

We critically review the assumption that no new physics is acting in tree-level $B$-meson decays and study the consequences for the ultimate precision in the direct determination of the CKM angle $\gamma$. In our exploratory study we find that sizable universal new physics contributions, $\Delta C_{1,2}$, to the tree-level Wilson coefficients $C_{1,2}$ of the effective Hamiltonian describing weak decays of the $b$ quark are currently not excluded by experimental data. In particular we find that Im $\Delta C_1 $ and Im $\Delta C_2 $ can easily be of order $\pm 10%$ without violating any constraints from data. Such a size of new physics effects in $C_1$ and $C_2$ corresponds to an intrinsic uncertainty in the CKM angle $\gamma$ of the order of $|\delta \gamma| \approx 4^\circ$, which is slightly below the current experimental precision. The accuracy in the determination of $\gamma$ can be improved by putting stronger constraints on the tree-level Wilson coefficients, in particular $C_1$. To this end we suggest a more refined theoretical study as well as a more precise measurements of the observables that currently provide the strongest bounds on hypothetical new weak phases in $C_1$ and $C_2$. We note that the semi-leptonic CP asymmetries seem to have the best prospect for improving the bound on the weak phase in $C_1$.