A solvable model for irreversible quantum phenomena

Recently a modified equation of motion for density matrices has been proposed and checked against upper bonds for the violation of quantum mechanics in various phenomenological situations. Difficulties have been also pointed out. A general feature of this approach is that the correspondence between symmetries and quantum mechanical conservation laws is lost. On the other hand, in the case of a specific model , in which rotational invariance is preserved, but conservation of quantum-mechanical angular momentum is released, it is shown, in this letter, that the modified evolution equation exhibits irreversible evolution of pure states into mixed states of maximal entropy. Consequences of our approach concerning some views about Bell's theorem are also discussed