One of the problems performing a PWA of a system with neutral mesons
decaying into 
's is the combinatorial background.
This background strongly depends on the resolution of the electromagnetic
calorimeter. In COMPASS we expect a higher resolution with
increasing energy and thus will benefit in the signal/background
ratio, in particular in the 
system.
Also important are statistical limitations.
For this specific channel it prevented to measure
the phase of the 
wave with respect to the less intensive reference wave.