LARGE (G-2)(MU) IN SU(5)XU(1) SUPERGRAVITY MODELS

We compute the supersymmetric contribution to the anomalous magnetic moment of the muon within the context of SU(5) X U(1) supergravity models. The largest possible contributions to a(mu)SUSY occur for the largest allowed values of tanbeta and can easily exceed the present experimentally allowed range, even after the CERN LEP lower bounds on the sparticle masses are imposed. Such tanbeta enhancement implies that a(mu)SUSY can greatly exceed both the electroweak contribution (almost-equal-to 1.95 X 10(-9)) and the present hadronic uncertainty (almost-equal-to 1.75 X 10(-9)). Therefore, the new E821 Brookhaven experiment (with an expected accuracy of 0. 4 X 10(-9)) should explore a large fraction (if not all) of the parameter space of these models, corresponding to slepton, chargino, and squarks masses as high as 200, 300, and 1000 GeV, respectively. Moreover, contrary to popular belief, the a(mu)SUSY contribution can have either sign, depending on the sign of the Higgs mixing parameter mu: a(mu)SUSY > 0 (< 0) for mu > 0 (mu < 0). The present a(mu) constraint excludes chargino masses in the range 45-120 GeV depending on the value of tanbeta, although there are no constraints for tanbeta less than or similar 8. We also compute a(tau)SUSY and find \a(tau)SUSY\ almost-equal-to (m(tau)/m(mu)2\a(mu)SUSY\ less than or similar 10(-5) and briefly comment on its possible observability.