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Articles Particle Physics Quantum-chromodynamic Estimates for Heavy-particle Production (1978)
Quantum-chromodynamic Estimates for Heavy-particle Production (1978) |
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The associated production of new, heavy, flavors of quarks in photon and hadron beams affords the opportunity to study the hadronic interactions of massive quarks. A small, but growing, body of experimental information on these processes already exists. There are indications for the production of charm by virtual photons in the observation of events of the type 
. (1) The observation of a charmed antibaryon, 
(2.26), has been claimed in photoproduction (2) but it has not yet been possible to deduce the corresponding cross section. Indirect evidence for charm in photoproduction can be obtained from the energy dependence of 
near charm threshold (3) or, perhaps, from the behavior of 
. (4) There is evidence for the production of charm in hadron-hadron collisions from cosmic-ray experiments (5) but, at this time, no accelerator experiment using hadronic beams has reported a charm signal. Perhaps the most restrictive bound on this cross section comes from an emulsion exposure (6) which gives
However, the observation of 
and 
production in hadron beams has become quite commonplace and there has been a recent report of an enhancement, 
, (7) which presumably is the harbinger of still another flavor of quark.
There have been several model calculations of the cross sections for processes involving heavy particles but there does not appear to be a theoretical consensus on what the underlying production mechanisms should be. We shall discuss here a model for the associated production of heavy quarks which is applicable either in photon-hadron or hadron-hadron collisions. We assume that the production of heavy quarks occurs through the interaction of the fundamental fields in quantum chromodynamics (QCD)---quarks and gluons---and that the cross section is dominated by the lowest-order perturbation-theory contribution. For real or virtual photons this assumption means that the dominant internal production mechanism is 
where 
is a vector gluon and 
a heavy quark. In hadron-hadron collisions we assume that the important mechanisms are 
and 
where 
denotes a light quark 
. The model is largely motivated by a similar calculation of large-
hadron production in QCD. (8) , (9) A naive justification for the approach can be found if the threshold invariant mass of the produced hadrons which carry the new flavor is large enough that the renormalization-group-improved QCD coupling constant 
is small. In practice, the requirement that 
) be small may be satisfied in the production of charm or of a heavier flavor such as that associated with the . One advantage of treating simultaneously photon-hadron and hadron-hadron collisions is that the dependence of the cross section on the main input to the calculation, the distribution 
of gluons in a hadron, is different in the two cases.
The outline of the rest of this paper is as follows: In Sec. II, we discuss our calculation for photon-hadron collisions and introduce in more detail the assumptions we make. The different gluon distributions used in the calculation are presented and we make several comparisons between our calculations and those based on the generalized-vector-meson-dominance model. In Sec. III, we present our calculations for hadron-hadron collisions. The constraints on our model from the experimental bound (1.1) are discussed. Section IV gives a brief discussion of possible effects associated with higher-order corrections to our calculation. We include appendices which give the details of our perturbation-theory calculations of , , and .
