Novel hard exclusive QCD phenomena with COMPASS - measuring nondiagonal GPDs, high energy color transparency, etc.

We argue that COMPASS experiment with hadron beams can collect (already collected ?)  the data which would allow to study a novel  type of pQCD processes in which  a high energy projectile splits into two hadrons with large p_t  and comparable longitudinal momenta and a recoil nucleon (Delta -isobar) - for example \pi^- +n --> \pi^-\pi^-- +p, \pi^-p--> \pi^-\pi^0+p,.... ( 2--> 3 processes).  Use of the nuclear targets  allows to determine starting at what p_t  point-like configurations dominate  large angle 2 --> 2 processes, leading to the  onset of the color transparency regime and hence to applicability of the GPD based description of the processes.  In contrast with previously proposed approaches, the discussed reaction allows to decouple the effects of the transverse size of configurations  from effects of the space-time evolution of these configurations. As a result in these reaction three important    QCD topics could be explored - 1) nucleon nondiagonal generalized parton distributions, 2) high - energy color transparency dynamics  3) Checking for the first time  pQCD predictions for the simplest large angle elastic scattering process \pi +(q\bar q) --> \pi + \pi .