Search for the decays Bz + yy and Bf --+ yy

Abstract A search for the decays B& + yy in 2.95 million hadronic Z decays has been performed using the L3 detector at LEl? No candidates are found in the signal region and upper limits have been set on the branching ratios: Br( B: + yy) < 3.9 x 10m5 and Br(Bf + yy) < 14.8 x lo-’ at 90% CL. These are the first limits set on these exclusive rare decays. 1. Introduction Measurements of rare B hadron decays are impor- tant discriminators to test the Standard Model (SM) as well as to probe the physics beyond it. Of par- ticular interest are the decays Bz, -+ yy for which there has been no experimental ‘measurement until now. The decays correspond to a second order weak transition, including gluonic penguins, followed by an- nihilation, often referred to as an “effective” flavor- changing weak neutral current process, as shown by some diagrams in Fig. 1. The expected branching ra- tio for this decay mode is N 10V7 in the SM, al- though there are large theoretical uncertainties [ l-41. As pointed out by Falk [5] exclusive decay rates are extremely difficult to compute reliably. Observation of these decays could lead to a direct measurement of the Cabibbo-Kobayashi-Maskawa (CKM) matrix el- ements 1 V,, 1 and 1 I& 1. In contrast to the theoretical sit- uation, experimentally B” -+ yy is a relatively clean channel to study. It is believed that for some non-standard scenarios, such as two-Higgs-doublets and minimal supersym- I