Abstract
In June-July 1952 an expedition was organized in Cagliari (Sardinia) by the Universities of Bristol, Padova and Roma, in collaboration with other European universities. The expedition was organized for the purpose of launching balloons, capable of reaching altitudes of 25 to 30 km, carrying photographic emulsions sensitive to ionizing radiation. In one of these emulsions (G5 Ilford nuclear emulsion, 1200
μ
thick) we have observed an event which we interpret as the decay at rest of a ז-meson (Brown, Camerini, Fowler, Muirhead, Powell & Ritson 1949; and other references given below). The event is shown in figure 40, plate 12. Particle 1 enters the surface of the emulsion and stops in
P
. The increase of ionization and scattering establishes the direction of motion of the particle which produced the track. Three other particles leave
P
at different angles (particles 2, 4 and 5). The tangents at
P
of the trajectories of these particles are co-planar within 1 degree. Particle 2 has a residual range of ~ 690
μ
, and at its end disintegrates into an ionizing particle, track 3, (which leaves the emulsion after a path of 510
μ
) and an unknown number of neutral particles. By scattering and ionization measurements it was shown that the mass of particle 2 was less than 700
m
c
; it cannot therefore have been a proton. The most probable interpretation, as discussed later in detail, is that particle 3 is a
μ
-meson, and particle 2 a
π
+
meson. As far as we know this is the first observation of a ז-disintegration with an associated π-
μ
decay.