Subatomic Physics

266 P , C, CP, andT
long-lived component that decays into three pions? At the time of Gell-Mann and
Pais’ paper, neutral kaons were known to decay with a lifetime of about 10 −10 sec. A
longer-lived component was found by a Columbia–Brookhaven group using a cloud
chamber. (27) The experimental
arrangement is sketched in
Fig. 9.11. A 90 cm cloud
chamber was exposed to the
neutral beam emitted from a
copper target hit by 3 GeV
protons. Charged particles
were eliminated by a sweeping
magnet. The 6 m flight path
from target to chamber corresponded
to about 100 mean
lives for the known decay component;
the K 0 1 component
hence was absent in the chamber.
The observation of many
V events that could not be fitted
kinematically by two-pion
decays established the existence
of a long-lived three-
and con-
pion decay of K 0 2
stituted a clear verification of
the brilliant proposal by Gell-
Mann and Pais. Later experiments
substantiated this conclusion,
and the mean lives
of the two components were
found to be τ(K 0 2 )=0.517 ×
10 −7 sec and τ(K 0 1 )=0.894×
10 −10 sec.
Figure 9.11: Observation of the long-lived neutral kaon
component, K0 2 , by a Columbia–Brookhaven group in a
cloud chamber. [K. Lande et al., Phys. Rev. 103, 1901
(1956); 105, 1925 (1957).] The charged particles are swept
out of the beam by a magnet; the neutral particles in the
beam are observed after a flight of about 3×10−8 sec. The
observed V events cannot be explained by two-particle decays.
Figure 9.12: Observation of the K 0 component of an initially
pure K 0 beam.
2. Hypercharge Oscillations. (28) Equation (9.72) predicts that a particle that
was dropped into one well at time t = 0 will continuously oscillate between the two
wells, with a circular frequency given by Eq. (9.74). If neutral kaons were stable,
they would do the same. However, they decay, and the oscillations are damped.
Consider a situation where at time t = 0 a K0 was produced, as described by
Eq. (9.85). After a time that is long compared to τ(K 0 1 ), all K0 1s will have decayed,
27K. Lande, E. T. Booth, J. Impeduglia, L. M. Lederman, and W. Chinowsky, Phys. Rev. 103,
1901 (1956); 105, 1925 (1957).
28A. Pais and O. Piccioni, Phys. Rev. 100, 1487 (1955).