Study of rare b decays with the DELPHI detector at LEP

Rare decays of beauty particles were studied in several charmless modes using the data collected with the DELPHI detector at LEP from 1991 to 1994. These decays are mediated by both tree levelb→u and one-loop penguinb→s, d transitions. Evidence for charmlessB decays was obtained in two body hadronic modes. The branching ratios ofB d,s 0 toπ + π − orK + π − andB u − toρ 0 π − orK*0 π − were found to be (2.8 −1.0 +1.5 ±0.2)×10−5 and (1.7 −0.8 +1.2 ±0.2)×10−4 respectively. The fraction of these decays with a charged kaon in the final state that is not from the spectators quark, was measured to be 0.58±0.18. Upper limits were set at 90% confidence level on the branching ratios for three and four body charmless hadronic decays in the range of (1–3)×10−4, for inclusive radiativeb→sγ decays at 5.4×10−4, for the exclusive radiative decaysB d 0 →K*(892)0 γ andB s 0 →φ(1020)γ at 2.1×10−4 and 7.0×10−4 respectively, and for dineutrino decays\(b \to s\nu \bar \nu \), in the exclusive channels\(B_d^0 \to K^* (892)^0 \nu \bar \nu \) and\(B_s^0 \to \phi (1020)\nu \bar \nu \) at 1.0×10−3 and 5.4×10−3 respectively. The limits on dineutrino decays constrain theories with a newU(1) gauge boson coupling predominantly to the third family of fermions.