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 level b-->u and one-loop penguin b-->s, d transitions. Evidence for charmless B decays was obtained in two body hadronic modes. The branching ratios of B-d,s(0) to pi(+)pi(-) or K(+)pi(-) and B-u(-) to rho(0) pi(-) or K(*0)pi(-) were found to be (2.8(-1.0)(+1.5) +/- 0.2) x 10(-5) and (1.7(-0.8)(+1.2) +/- 0.2) x 10(-4) respectively. The fraction of these decays with a charged kaon in the final state that is not from the spectator s 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) x 10(-4), for inclusive radiative b-->s gamma decays at 5.4 x 10(-4), for inclusive radiative b-->s gamma decays at 5.4 x 10(-4), for the exclusive radiative decays B-d(0)-->K*(892)(0) gamma and B-s(0)-->phi(1020)gamma at 2.1 x 10(-4) and 7.0 x 10(-4) respectively, and for dineutrino decays, b--><s nu(nu)over bar>, in the exclusive channels B-d(0)-->K*(892)(0) nu<nu(nu)over bar> and B-2(0)-->phi(1020)<(nu nu)over bar> at 1.0x10(-3) respectively. The limits on dineutrino decays constrain theories with a new U(1) gauge boson coupling predominantly to the third family of fermions.