Strongest experimental constraints on SU(5)xU(1) supergravity models

We consider a class of well-motivated string-inspired flipped SU(5) supergravity models which include four supersymmetry-breaking scenarios: no-scale, strict no-scale, dilaton, and special dilaton, such that only three parameters are needed to describe all new phenomena (mt,tanβ,mg). We show that the CERN LEP precise measurements of the electroweak parameters in the form of the ε1 variable and the CLEO II allowed range for B(b→sγ) are at present the most important experimental constraints on this class of models. For mt approx-gt 155 (165) GeV, the ε1 constraint [at 90 (95)% C.L.] requires the presence of light charginos (mχ1± approx-lt 50--100 GeV depending on mt). Since all sparticle masses are proportional to mg, mχ1± approx-lt 100 GeV implies mχ10 approx-lt 55 GeV, mχ20 approx-lt 100 GeV, mg approx-lt 360 GeV, mq approx-lt 350 (365) GeV, meR approx-lt 80 (125) GeV, meL approx-lt 120 (155) GeV, and mnu approx-lt 100 (140) GeV in the no-scale (dilaton) flipped SU(5) supergravity model. The B(b→sγ) constraint excludes a significant fraction of the otherwise allowed region in the (mχ1±,tanβ) plane (irrespective of the magnitude of the chargino mass), while future experimental improvements will result in decisive tests of these models