Supersymmetric contributions to muon g-2 and the electroweak precision measurements

In view of the recent measurement of the muon g-2 and finalization of the LEP electroweak data, we re-examine all the new particle contributions to those observables in the MSSM. The SM fits the latest electroweak data excellently with the observed top-quark mass and the Higgs-boson mass of around 100 GeV, and so does the MSSM in the decoupling limit. The MSSM gives a slightly better fit to the data than the SM (\Delta \chi^2_{min} \sim -2) when relatively light left-handed sleptons of mass \sim 200 GeV and a light chargino of mixed higgsino-wino character (\mu/M_2 \sim 1) with mass \sim 100 GeV co-exist. The improvement in the fit diminishes quickly for wino- or higgsino- dominant charginos, for heavier charginos, and for lighter sleptons. We find that the MSSM contributions to the muon g-2 is most efficient when the light chargino has a mixed character. If \tan\beta < 10, the set of a light mixed chargino and light sleptons that is favored by the electroweak data is also favored by the g-2 data for \mu >0. Models with light gauginos (\mu/M_2 > 10) or light higgsinos (\mu/M_2 < 0.1) give significant contributions to muon g-2 only for large \tan\beta (\simgt 15).