FLAVOR-CHANGING-NEUTRAL-CURRENT PROCESSES IN B-PHYSICS

Recent progress in flavour-changing-neutral-current (FCNC) processes is reviewed with particular emphasis on the role of QCD. Standard model (SM) is consistent with data. In particular, the rate and final-state distributions in radiative rare B decays, B --> X(s) + gamma) are well accounted for in the SM in terms of the short-distance contribution. The CLEO measurement of the inclusive rate B(B --> X(s) gamma) = (2.32+/-0.67) x 10(-4) yields \V-ts\/\V-cb\ = 0.85 +/- 0.23, in agreement with unity expected from the unitarity of the CKM matrix. Likewise, most QCD-based theoretical estimates, using short-distance dominance, yield for the exclusive-to-inclusive decay rate ratio B(B --> K* + gamma)/B(B --> X(s) + gamma) = 0.1 - 0.2, in agreement with the data giving 0.19 +/- 0.1 for the same quantity. Along the same lines, the present measurements of the branching ratio B(B --> K* + gamma) and. upper limits on the branching ratios B(B --> rho + gamma) and B(B --> omega + gamma) can be interpreted in terms of an upper limit on the CKM matrix element ratio, yielding \V-td\/\V-ts\ less than or equal to 0.75. This bound is less restrictive than the unitarity bounds and the ones that follow from the lower limit on the B-0 - (B) over bar(0) mixing ratio Delta M(s)/Delta M(d) > 11.3, reported recently by the ALEPH collaboration. The potential of rare B decays in searching for physics beyond the standard model is also reviewed with emphasis on the decays B --> X(s) + gamma and B --> X(s) l(+)l(-).