Rare B Decays at Belle

Belle is an experiment designed to make measurements of time-dependent CP asymmetries in B physics. Important measurements of these phenomena have recently been reported.1 In addition, owing to the unparalleled performance of KEKB,2 and the excellent performance of the Belle detector, numerous interesting rare B decay modes can be studied. The Belle detector is described in detail elsewhere.3 It consists of a silicon vertex detector, a central drift chamber (CDC), aerogel Cerenkov counters (ACC), time-of-flight scintillation counters (TOF) and an electromagnetic calorimeter made of CsI(Tl) crystals enclosed in alternating layers of resistive plate chambers and iron for KL/μ detection and to return the flux of the 1.5 T magnetic field. The responses of the CDC, ACC and TOF are combined to provide clean identification of charged kaons, pions and protons, crucial for rare hadronic B decays. Another important feature of rare decay analyses, is the suppression of background from e+e− → qq̄ (continuum) events. One commonly used tool is a likelihood ratio combining information from the reconstructed B direction and a Fisher discriminant of modified Fox-Wolfram moments.4 Signal candidates are identified using two kinematic variables: mbc = √