Please send me E-mail, dow@ubpheno.physics.buffalo.edu, no later than Monday, February 13, 2006, 2 pm, with your thoughts on:

When measuring the polarisation of the muons that are produced when a charged pion at rest decays (pi+ -> mu+ nu_mu) one always finds the muon spin to be oriented opposite to the direction of motion of the outgoing muon. One never observes the case where the spin is oriented in the direction of motion of the muon. Angular momentum conservation allows for both cases to be realized in nature.

I suggest to make a drawing of the pion decay at rest with the muon and neutrino momenta being opposite to each other (momentum conservation). Indicate the direction of the muon and neutrino spins with respect to their momenta. Angular momentum conservation allows for two possible scenarios. Pick one possibility and perform a mirror transformation (vex(x) -> -vec(x)). What do you find ? What behaviour with respect to space reflection (vec(x) -> -vec(x)) do you conclude for the interaction responsible for this decay ?