Exclusive Production of Heavy Flavours in Proton-Antiproton Collisions
Theoretically it is well founded that the large momentum-transfer region of exclusive processes like pp̄ → 2 hadrons shotdd be dominated by perturbative QCD1. Or, to be more precise, the corresponding scattering amplitude is known to be a convolution of a process specific, perturbatively calculable hard scattering amplitude — which is the sum of all tree diagrams — with process independent quark distribution amplitudes — which account for the nonperturbative formation of quarks into hadrons. For the reactions we are interested in, namely pp̄ → B f B̄; f , where B f is a baryon containing at least one heavy quark f(= s,c,b) a full calculation along this so called “hard scattering scheme” would require an enormous effort due to the tremendous number of diagrams (O(105)) contributing. This huge number of diagrams can be reduced considerably by assuming baryons to be bound states of a pointlike quark and a spatially extended diquark. As an additional advantage one thereby includes non-perturbative (higher twist) effects which, as e.g. spin measurements have revealed2, are still present in the available large momentum-transfer exclusive scattering data. The existence of diquarks, although not deduced from QCD, is also strongly suggested by many effects they can explain such as baryon production in hard collisions, scaling violations in the structure functions of deep inelastic lepton-hadron scattering, or static properties of baryons. Recently it has been shown3 that a consistent description of the electromagnetic proton form factor and γγ → pp̄ cross sections can be achieved within the hard scattering scheme including diquarks.
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