Search

A search was made for electron pairs originating from a 2 m decay region. They were detected in a counter-triggered thin-foil spark chamber, and had to point back to the 590 MeV proton beam dump at SIN. During 246 h of running time, in which 72 C of protons were discharged onto the target, 5 candidates were registered - only slightly more than what one expects from cosmic rays. The excess of (2.4 ± 3.4) events places a 90% confidence limit on the axion partial decay rate, a0→ēe, ofΓēe⩽20 m−1aMeVs. This is about 10−7times the theoretical estimate, and hence: ma< 2 me.

Muon electron pairs were detected in an Al multiplate spark chamber, exposed to a neutrino beam from the CERN PS. The leptons were not accompanied by other particles, except occasionally by protons. The background came mainly from muon associated p0 production, with one decay gamma lost. It was determined empirically, together with the small contribution from ?e reactions. For electron energies above 2 GeV the background is 5.7±1.5 events, whereas 18 (µe)-candidates have been observed. Hence the effect is established, with a rate of about 10-4 as compared to the muonic reactions above 3 GeV. Charm creation as the origin of this (µe)-production process is excluded; heavy neutral lepton production does not fit the kinematics observed. Instead the events are compatible with the two-body decay of an object with variable invariant mass of order 1 GeV, possibly resulting from axion interactions.