K sub L sup 0 yields. mu. sup + e sup + at the Fermilab main injector

Some of the attempts to replace the Standard Model with more satisfactory theories result in predictions that lepton flavor (separate lepton number) is not absolutely conserved. A new generation of experiments is underway to study decays in which lepton flavor conservation might be violated, such as K{sub L}{sup 0} {yields} {mu}{sup {minus plus}}e{sup {plus minus}}, K{sup +} {yields} {pi}{sup +}{mu}{sup +}e{sup {minus}}, {mu} {yields} e{gamma}, {mu} {yields} eee, and {mu}{sup {minus}}A {yields} e{sup {minus}}A. Since the form of a lepton number violating interaction is unknown, it is useful to probe all of these decays, and perhaps decays of B and D mesons, to whatever level is technically feasible. However, here we concentrate on the decay K{sub L}{sup 0} {yields} {mu}{sup {minus plus}}e{sup {plus minus}}. The three most recent experiments on this process have been E780 at BNL, E791 at BNL, and KEK E137, which currently report (90% C.L.) upper limits on the branching fraction of 1.9 {times} 10{sup {minus}9}, 2.2 {times} 10{sup {minus}10}, and 4.3 {times} 10{sup {minus}10} respectively. While E780 is complete, the two other experiments are in the process of collecting larger samples and a combined sensitivity of about 2 {times} 10{sup {minus}11} is likely (by sensitivity wemore » mean the branching fraction which would produce one observed event). Any new experiment should aim at a sensitivity of 10{sup {minus}13}. There are several places which might offer a suitable beam: the BNL AGS with the Booster, the Fermilab Main Injector (MI), and the TRIUMF KAON facility. 5 refs.« less