Physical mechanism of the linear beam-size effect at colliders.

We present qualitative but precise description of the linear beam–size effect predicted for the processes in which unstable but long–living particles collide with each other. We derive physically pronounced equation for the events rate which proves that the linear beam–size effect corresponds to the scattering of one beam of particles on the decay products of the other. We compare this linear beam–size effect with the known logarithmic beam–size effect measured in the experiments on a single bremsstrahlung at VEPP-4 and HERA.