Question

There is a narrow beam of negative pions with kinetic energy T equal to the rest energy of these particles. Find the ratio of fluxes at the sections of the beam separated by a distance l = 20 m. The proper mean lifetime of these pions is τ₀ = 25.5 ns.

Answer 1

Here  so the life time of the pion in the laboratory frame is

The law of radioactive decay implies that the flux decrease by the factor.