  We study the threshold conditions of spatial self organization combined with
collective coherent optical backscattering of a thermal gaseous beam moving in
a high Q ring cavity with counter propagating pump. We restrict ourselves to
the limit of large detuning between the particles optical resonances and the
light field, where spontaneous emission is negligible and the particles can be
treated as polarizable point masses. Using a linear stability analysis in the
accelerated rest frame of the particles we derive an analytic bounds for the
selforganization as a function of particle number,average velocity, temperature
and resonator parameters. We check our results by a numerical iteration
procedure as well as by direct simulations of the N-particle dynamics. Due to
momentum conservation the backscattered intensity determines the average force
on the cloud, which gives the conditions for stopping and cooling a fast
molecular beam.
