We present evidence for the flavor-changing neutral current decay B[yields] K*[ell][sup+][ell][sup -] and a measurement of the branching fraction for the related process B[yields] K[ell][sup+][ell][sup -], where[ell][sup+][ell][sup -] is either an e[sup+]e[sup -] or[mu][sup+][mu][sup -] pair. These decays are highly suppressed in the Standard Model, and they are sensitive to contributions from new particles in the intermediate state. The data sample comprises 123 x 10[sup 6][Upsilon](4S)[yields] B[bar B] decays collected with the BABAR detector at the PEP-II e[sup+]e[sup -] storage ring. Averaging over K* isospin and lepton flavor, we obtain the branching fractions[Beta](B[yields] K[ell][sup+][ell][sup -])= (0.65[sub -0.13][sup+0.14][+-] 0.04) x 10[sup -6][Beta](B[yields] K*[ell][sup+][ell][sup -])= (0.88[sub -0.29][sup+0.33][+-] 0.10) x 10[sup -6], where the uncertainties are statistical and systematic, respectively. The significance of the B[yields] K[ell][sup+][ell][sup -] signal is over 8[sigma], while for B[yields] K*[ell][sup+][ell][sup -] it is 3.3[sigma].

We report the first evidence for the decay? → p?+?− from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is?(?+ → p?+?−) = [8.6{sub -5.4}{sup +6.6}(stat) ± 5.5(syst)] x 10−8. The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state,?+ → pP°, P° →?+?− with a P° mass of 214.3 ± 0.5 MeV/c2 and branching ratio?(?+ → pP°, P° →?+?−) = [3.1{sub -1.9}{sup +2.4}(stat) ± 1.5(syst)] x 10−8.

This thesis describes the search for KL 2![pi]0[pi]0[mu]+[mu]-, which was conducted using data collected by the KTeV Experiment at Fermi National Accelerator Laboratory in Batavia, Illinois. Although this decay mode is possible within the Standard Model, it is limited to a very narrow band of phase space. The HyperCP Experiment has recently observed three [Sigma]+ 2!p[mu]+[mu]- events within a narrow dimuon mass range of 213.8 MeV/c2 to 214.8 MeV/c2. This suggests that the process could occur via a neutral intermediary particle, [Sigma]+ 2!pX0 2!p[mu]+[mu]-, with an X0 mass of 214.3 MeV/c2 ± 0.5 MeV/c2 . Since the X0 has a light mass and a low interaction probability, then it is most likely a new neutral boson that exists beyond the Standard Model. Recent theoretical predictions suggest that the decay mode KL 2![pi]0[pi]0[mu]+[mu]- can also occur via the aforementioned neutral boson: KL 2![pi]0[pi]0X02![pi]0[pi]0[mu]+[mu]-. Therefore, in addition to a Standard Model measurement, the search for KL 2![pi]0[pi]0[mu]+[mu]- is also carried out in an effort to address the viability of X0 in explaining the HyperCP phenomena. Measurement of the upper limits presented in this thesis were performed using blind analysis techniques. The upper limit of the decay KL 2![pi]0[pi]0[mu]+[mu]- has been measured in this thesis to be BR(KL 2![pi]0[pi]0[mu]+[mu]- )

We report evidence for the B meson decays, B[sup[+-]][yields] J/[psi][eta]K[sup[+-]] and B[sup 0][yields] J/[psi][eta]K[sub S][sup 0], using 90 million B[bar B] events collected at the[Upsilon](4S) resonance with the BABAR detector at the PEP-II e[sup+]e[sup -] asymmetric-energy storage ring. We obtain preliminary branching fractions in the charged and neutral channels of (10.8[+-] 2.3(stat.)[+-] 2.4(syst.)) x 10[sup -5] and (8.4[+-] 2.6(stat.)[+-] 2.7(syst.)) x 10[sup -5], respectively.