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Brookhaven National Laboratory experiment E949 was designed to search for the rare K meson decay K⁺ → π⁺ , neutrino, and anti-neutrino, a decay sensitive to physics beyond the Standard Model. While previous data analyses dealt with the high π⁺ momentum region accessible for this reaction, this thesis concentrates on the lower range between 140 and 199 MeV/c. Analysis of this low π⁺ momentum region was performed to search for additional evidence of the process K⁺ → π⁺, neutrino, and anti-neutrino. A blind analysis technique was used to avoid bias when developing the selection criteria used to suppress the competing background processes. The blind analysis technique was based on identifying background sources a priori and only examining the signal region once all selection criteria and background estimates had been finalized. The background estimates were performed using a technique known as a "bifurcation method", which relied on using two uncorrelated selection criteria to suppress each background source. The analysis of an exposure of 1.71 x 10¹² K⁺ decays resulted in an observation of three events with an estimated background of 0.927 ± 0.168(stat.)⁺³²⁰-₀.₂₃₇(sys.) events and a single event sensitivity of (4.28 ± 0.43)x 10-¹⁰. Using a likelihood method, the three candidate events observed here were combine with the previous E787 and E949 results, yielding a branching ratio of ϐ(K⁺ → π⁺, neutrino, and anti-neutrino decay of (1.73+1.15-1.05) x 10⁻¹⁰ at the 68% confidence level. This branching ratio is consistent with the prediction of the Standard Model, (0.85 ± 0.07) x 10⁻¹⁰.

Brookhaven National Laboratory experiment E949 was designed to search for the rare K meson decay K⁺ → π⁺ , neutrino, and anti-neutrino, a decay sensitive to physics beyond the Standard Model. While previous data analyses dealt with the high π⁺ momentum region accessible for this reaction, this thesis concentrates on the lower range between 140 and 199 MeV/c. Analysis of this low π⁺ momentum region was performed to search for additional evidence of the process K⁺ → π⁺, neutrino, and anti-neutrino. A blind analysis technique was used to avoid bias when developing the selection criteria used to suppress the competing background processes. The blind analysis technique was based on identifying background sources a priori and only examining the signal region once all selection criteria and background estimates had been finalized. The background estimates were performed using a technique known as a "bifurcation method", which relied on using two uncorrelated selection criteria to suppress each background source. The analysis of an exposure of 1.71 x 10¹² K⁺ decays resulted in an observation of three events with an estimated background of 0.927 ± 0.168(stat.)⁺³²⁰-₀.₂₃₇(sys.) events and a single event sensitivity of (4.28 ± 0.43)x 10-¹⁰. Using a likelihood method, the three candidate events observed here were combine with the previous E787 and E949 results, yielding a branching ratio of ϐ(K⁺ → π⁺, neutrino, and anti-neutrino decay of (1.73+1.15-1.05) x 10⁻¹⁰ at the 68% confidence level. This branching ratio is consistent with the prediction of the Standard Model, (0.85 ± 0.07) x 10⁻¹⁰.

Brookhaven National Laboratory experiment E949 was designed to search for the rare K meson decay K⁺ → π⁺ , neutrino, and anti-neutrino, a decay sensitive to physics beyond the Standard Model. While previous data analyses dealt with the high π⁺ momentum region accessible for this reaction, this thesis concentrates on the lower range between 140 and 199 MeV/c. Analysis of this low π⁺ momentum region was performed to search for additional evidence of the process K⁺ → π⁺, neutrino, and anti-neutrino. A blind analysis technique was used to avoid bias when developing the selection criteria used to suppress the competing background processes. The blind analysis technique was based on identifying background sources a priori and only examining the signal region once all selection criteria and background estimates had been finalized. The background estimates were performed using a technique known as a "bifurcation method", which relied on using two uncorrelated selection criteria to suppress each background source. The analysis of an exposure of 1.71 x 10¹² K⁺ decays resulted in an observation of three events with an estimated background of 0.927 ± 0.168(stat.)⁺³²⁰-₀.₂₃₇(sys.) events and a single event sensitivity of (4.28 ± 0.43)x 10-¹⁰. Using a likelihood method, the three candidate events observed here were combine with the previous E787 and E949 results, yielding a branching ratio of ϐ(K⁺ → π⁺, neutrino, and anti-neutrino decay of (1.73+1.15-1.05) x 10⁻¹⁰ at the 68% confidence level. This branching ratio is consistent with the prediction of the Standard Model, (0.85 ± 0.07) x 10⁻¹⁰. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Recent results at the LHCb and B-factory experiments have suggested that rare processes in B →Vγ and B → V ℓ¯ℓ decays, where V is a vector meson, show some deviation from Standard Model predictions. Although these anomalies are not yet at the level to constitute a formal discovery, they are certainly suggestive of potential New Physics effects in flavour-changing neutral currents. However, explanations within the Standard Model cannot yet be ruled out. This thesis contributes to the understanding of such anomalies in two ways. Firstly, the angular distribution of the B → KJ (→ Kπ)ℓ1¯ℓ2 decay is derived, for the full dimension-six effective weak Hamiltonian, using a generalisation of the helicity formalism to effective theories mediating b → sℓ1¯ℓ2 transitions. This approach sheds light on the origin of the underlying structure, and in the process extends the general angular distribution to decays in which the two leptons in the final state, ℓ1¯ℓ2, are not necessarily identical. An additional benefit of the derivation of the angular distribution presented in this manner is that it lends itself to a moments analysis of the decay. It is shown how the angular distribution changes in the presence of new operators, predicted to be vanishingly small in the Standard Model. Such operators could be sizeable in the presence of New Physics, but using a moments analysis enables the contribution of such operators to be assessed. Secondly, an analysis is presented of the three-particle vector and axial meson distribution amplitudes. It is shown that the distribution amplitudes of both particles are, up to QCD corrections, nearly identical. These results are applied to a new calculation of the long-distance charm loop contribution to radiative B → Vγ decays, and it is shown that the approximate symmetry can be exploited to provide an improved theoretical control in the search for New Physics contributions to right-handed currents in radiative decays.

The flavor changing neutral current process b → dγ is a sensitive probe to the Standard Model of elementary particle physics. Using a sample of 386 × 10⁶ B meson pairs accumulated by the Belle detector at the KEKB e⁺e⁻ collider, we measure the branching fractions for the exclusive modes B⁻ → ρ⁻γ, B̅⁰ → ρ⁰γ and B̅⁰ → ωγ as follows: B(B⁻ → ρ⁻γ) = 0.55 _−0.36^+0.42_−0.08^+0.09 B(B̅⁰ → ρ⁰γ) = 1.25 _−0.33^+0.37_−0.06^+0.07 B(B̅⁰ → ωγ) = 0.56 _−0.27^+0.34_−0.10^+0.05 where the first error on each value is statistical and the second is systematic. Assuming that these three modes are related by isospin conservation rules, we find the combined branching fraction B(B̅ → (ρ,ω)γ) = 1.32 _−0.31^+0.34_−0.09^0.10. This result is used to determine the ratio of CKM matrix elements, |V_td/V_ts| = 0.199 _−0.025^+0.026_−0.015^0.018. / Ph. D.