is suppressed by the more favoured reactions with only two particles in the
final state.Panofsky et al. stopped low energy p - in a high-pressure liquid hydrogen target and detected the resulting photons in a Geiger counter array. Knowing the pions to be bosons and considering conservation laws, the p -p system can result in either n g (radiative capture) or n p 0 - followed by p 0-> g g - (single charge exchange reaction). Since the stopped pions are temporarily bound in a hydrogen electron shell, they form pionic hydrogen. The original excited state loses energy through the emission of Auger electrons until the pion reaches the K-shell and finally react with the proton. The relative strength of both reactions thus is proportional to the lifetime of the 1s state of pionic hydrogen [Ras76]. The Panofsky ratio then is defined as
.
)
data. Anderson and Fermi [And52] gave a theoretical calculation of the cross
sections but could not match Panofsky's result (P=0.94±0.20)
[Pan50] with Steinberger's [Bis50] value for the pion photoproduction cross
section s g . They suggested a different value for
P, therefore, and acknowledged the difficulties in the extrapolation of
the s g value to the threshold of vanishing kinetic
energy.
With better statistics and improved detector resolution later experiments
established the value 1.546(±0.009) [Spu77] for P, which is in
remarkable agreement with theoretical predictions [Ras76]. Besides the p N
scattering length, P also gives information about the
-quark
contribution of the proton, the so-called [Sigma]-term [Gas91]. The field of
pion photoproduction is still highly active, since basic symmetries such as
time reversal invariance, parity conservation and isospin invariance of the
strong interaction can be tested [Mat97, Sig96].