In this paper we describe BR-4, a computational model of scientific discovery in particle physics. The system incorporates operators for determining quantum values of known particles, formulating new quantum properties, positing new particles, andpred icting reactions among particles. BR-4 carries out heuristic search guided by constraints that its theory be consistent andcomplete with respect to observedreactions. We show that this control scheme is sufficient to model, with some manual intervention, an extendedp eriodin the history of particle physics, including the discovery of the neutrino and the postulation of baryon, lepton, andelectron numbers. In closing, we compare BR-4 to other discovery systems and suggest directions for future research.