Rare B-Meson Decays at the Crossroads

TL;DR

This paper reviews key experimental measurements of rare B-meson decays, compares them with Standard Model predictions, discusses recent anomalies, and highlights the need for improved theoretical and experimental efforts to explore potential new physics.

Contribution

It provides a comprehensive overview of recent rare B-decay measurements, contrasts them with SM estimates, and discusses the implications of observed mismatches for future research.

Findings

Experimental measurements have reached unprecedented precision.

Several recent anomalies are observed but not yet statistically significant.

Future experiments like Belle II are crucial for clarifying current issues.

Abstract

Experimental era of rare $B$-decays started with the measurement of $B \to K^* \gamma$ by CLEO in 1993, followed two years later by the measurement of the inclusive decay $B \to X_s \gamma$, which serves as the standard candle in this field. The frontier has moved in the meanwhile to the experiments at the LHC, in particular, LHCb, with the decay $B^0 \to \mu^+ \mu^-$ at about 1 part in $10^{10}$ being the smallest branching fraction measured so far. Experimental precision achieved in this area has put the standard model to unprecedented stringent tests and more are in the offing in the near future. I review some key measurements in radiative, semileptonic and leptonic rare $B$-decays, contrast them with their estimates in the SM, and focus on several mismatches reported recently. They are too numerous to be ignored, yet , standing alone, none of them is significant enough to warrant the breakdown of the SM. Rare $B$-decays find themselves at the crossroads, possibly pointing to new horizons, but quite likely requiring an improved theoretical description in the context of the SM. An independent precision experiment such as Belle II may help greatly in clearing some of the current experimental issues.