Determinations of the angular and energy dependence of hard constituent scattering from $\pi^{0}$ pair events at the CERN Intersecting Storage Rings

The authors present data on proton-proton collisions, obtained at the CERN Intersecting Storage Rings, in which two roughly back-to-back\pi^{0}s of high transverse momentum (p/sub T/) were produced. The angular distribution of the dipion axis relative to the collision axis is found to be independent of both the effective mass m of the dipion system and the centre-of-mass energy \sqrt{s} of the proton- proton collision. The cross-sections d sigma /dm at the two values of \sqrt{s} satisfy a scaling law of the form d sigma /dm=G(x)/m/sup n/, where x=m(\pi^{0},\pi^{0})/ \sqrt{s} and n =6.5+or-0.5. They show from the data that the leading\pi^{0} carries most of the momentum of the scattered parton. Given this fact, the axis of the dipion system follows closely the direction of the scattered constituents, and they exploit this to determine the angular dependence of the hard-scattering subprocess. They also compare our data with the lowest order QCD predictions using structure functions as determined in deep-inelastic scattering and fragmentation and functions from electron-positron annihilation.