Collisions of partons of momenta 
and 
in the
center-of-mass system yield an intermediate state of mass M where,
and,
A binary decay into massless final state particles yields,
where 
is the decay angle in the rest frame of the intermediate state. Thus
the geometry of final-state particle detection depends only on 
and 
. Considering
for simplicity the case
, one finds that the opening angle between the final state particles is,
