Search forμ→e+e+e

Abstract

We have made a search for the neutrinoless decay mode $\mu \to e+e+e$. Positive poins from the Berkeley 184-in. synchrocyclotron were stopped in a lithium target. The detection of three decay particles not in coincidence with an incoming pion served to trigger two thin-plate spark chambers viewing the decays. The energy of the decay particles was measured with a combination of NaI counters and range telescopes. The spark chamber pictures were first scanned for events with three tracks meeting at a common point in the target. A requirement that no angle between any of the three tracks could be greater than 160° eliminated essentially all of the main source of background, namely, knock-on collisions of electrons passing through the target. Further requirements for the three tracks to be coplanar and for each particle to have at least 16 MeV eliminated all but one ambiguous event. For this event insufficient energy information was available. If we assume that one event to be $\mu \to e+e+e$, and with a calculated detection efficiency of 0.0083, an upper limit for the branching ratio for this mode can be set at 1.5×${10}^{-7\mathrm{}}$ with a 90% confidence level.