Light bottom squark and gluino confront electroweak precision measurements

Possibility of light sbottom with mass $2\sim 5.5 \gev$ and light gluino of mass $12\sim 16 \gev$ is studied in light of the electroweak precision measurements. Such light particles have been motivated to explain the excess of $b$ quark production cross section at Tevatron. The electroweak observables may be affected by the sbottom and gluino through the SUSY-QCD corrections to the $Zbb$ vertex. We examine, in addition to the SUSY-QCD corrections, the electroweak corrections to the gauge boson propagators from the stop which may be allowed to be light from the SU(2)$_L$ symmetry. We find that the possibility is strongly disfavored from the electroweak precision measurements unless the heavy mass eigenstate of sbottoms are lighter than $180\gev$ and the left-right mixing of the stops is sufficiently large. This implies that one of the stops should be lighter than about $98\gev$.