A beam of particles moving with uniform velocity collides with a collection of target part...

A beam of particles moving with uniform velocity collides with a collection of target particles that are at rest in a particular system. Let σ0 be the collision cross section observed in this system. In another system, the incident particles have a normalized velocity β1 and the target particles a normalized velocity β2. If σ is the observed cross section in this system, show that

Remember that collision rate must be invariant under a Lorentz transformation.