## Dissertations, Theses, and Masters Projects

1998

Dissertation

#### Degree Name

Doctor of Philosophy (Ph.D.)

Physics

#### Abstract

Brookhaven National Laboratory AGS Experiment 871 (E871) has carried out a search for the very rare, GIM and helicity suppressed decay of the long-lived neutral kaon into an electron-positron pair. This decay is predicted within the Standard Model to occur with a branching ratio of {dollar}(9.0 \pm 0.4)\times 10\sp{lcub}-12{rcub}.{dollar} Data were taken during the 1995 and 1996 AGS HEP running periods on the B5 beamline. A signal of (4 {dollar}\pm{dollar} 2) {dollar}K\sbsp{lcub}L{rcub}{lcub}0{rcub} \to e\sp+e\sp-{dollar} events is seen with a physics background expectation of 0.2 events. The ratio of the partial decay widths {dollar}\Gamma(K\sbsp{lcub}L{rcub}{lcub}0{rcub} \to e\sp+e\sp-)/\Gamma(K\sbsp{lcub}L{rcub}{lcub}0{rcub}\to\mu\sp+\mu\sp-){dollar} is determined to be {dollar}(1.3\sbsp{lcub}-0.6{rcub}{lcub}+0.8{rcub} \pm 0.2)\times 10\sp{lcub}-3{rcub}.{dollar} Using the Particle Data Group value for the branching fraction of {dollar}K\sbsp{lcub}L{rcub}{lcub}0{rcub}\to\mu\sp+\mu\sp-{dollar} this corresponds to a branching fraction of {dollar}B(K\sbsp{lcub}L{rcub}{lcub}0{rcub}\to e\sp+e\sp-) = (9.4\sbsp{lcub}-4.6{rcub}{lcub}+5.9{rcub}) \times 10\sp{lcub}-12{rcub}.{dollar} This result represents the lowest branching fraction ever measured in particle physics.

#### DOI

https://dx.doi.org/doi:10.21220/s2-h69n-4s15