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

We compute the supersymmetric contribution to the anomalous magnetic m oment of the muon within the context of SU(5) X U(1) supergravity mode ls. The largest possible contributions to a(mu)SUSY occur for the larg est allowed values of tanbeta and can easily exceed the present experi mentally allowed range, even after the CERN LEP lower bounds on the sp article masses are imposed. Such tanbeta enhancement implies that a(mu )SUSY can greatly exceed both the electroweak contribution (almost-equ al-to 1.95 X 10(-9)) and the present hadronic uncertainty (almost-equa l-to 1.75 X 10(-9)). Therefore, the new E821 Brookhaven experiment (wi th an expected accuracy of 0. 4 X 10(-9)) should explore a large fract ion (if not all) of the parameter space of these models, corresponding to slepton, chargino, and squarks masses as high as 200, 300, and 100 0 GeV, respectively. Moreover, contrary to popular belief, the a(mu)SU SY contribution can have either sign, depending on the sign of the Hig gs mixing parameter mu: a(mu)SUSY > 0 (< 0) for mu > 0 (mu < 0). The p resent a(mu) constraint excludes chargino masses in the range 45-120 G eV depending on the value of tanbeta, although there are no constraint s for tanbeta less than or similar 8. We also compute a(tau)SUSY and f ind \a(tau)SUSY\ almost-equal-to (m(tau)/m(mu)2\a(mu)SUSY\ less than o r similar 10(-5) and briefly comment on its possible observability.