Rare B-meson decays at the crossroads

Abstract

Experimental era of rare [Formula: see text]-decays started with the measurement of [Formula: see text] by CLEO in 1993, followed by the measurement of the inclusive decay [Formula: see text] in 1995, 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 [Formula: see text] at about 1 part in 10[Formula: see text] 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 [Formula: see text]-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 [Formula: see text]-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.