Are there any "reversed" interactions that are forbidden ?

[Phil Gardner]

For any inelastic interaction between two subatomic particles in which
the final state is a set of two different particles it would seem that
we
can in a gedanken experiment run the interaction in reverse (eg photon +
photon ---> electron + positron in place of pair annihilation) and in
this experiment conserve all quantities that are conserved in the
original experiment. The total cross-section for any such interaction
will of course be changed, often by many orders of magnitude. Are there
any theoretical rules that forbid such "reversed" interactions ? If so,
do they have any experimental support ?

Phil Gardner <pej...@oznetcom.com.au>

[Stephen B. Selipsky]

Au contraire; "crossing symmetry" relates the quantum mechanical
amplitudes for process and reverse-process. (This is more general
than the two-particle example above; you can cross particles one at
a time between initial and final states.) Then you just need to
calculate the phase space factors to get the cross sections; they need
not differ greatly, although they can, or even have one direction
forbidden by kinematics if the phase space vanishes (for instance, n
decays to p e- nu_ebar, but proton doesn't have enough mass to decay
to n e+ nu_e , although the matrix elements or amplitudes are
basically the same).
---
Stephen B. Selipsky seli...@wuphys.wustl.edu
Washington Univ. Dept. of Physics Phone: (314) 935-4064
1 Brookings Dr., Campus Box 1105 Fax: (314) 935-6219
Saint Louis, MO 63130-4899 USA Home: (314) 863-7351