  Searches for a scalar top quark and a scalar bottom quark have been performed
using a total data sample of 56.8 pb-1 at a centre-of-mass energy of 183 GeV
collected with the OPAL detector at LEP. No candidate events were observed.
Combining this result with those obtained at lower centre-of-mass energies, the
95% C.L. lower limit on the scalar top quark mass is 85.0 GeV if the mixing
angle between the supersymmetric partners of the left- and right-handed states
of the top quark is zero. The lower limit is 81.3 GeV, even if the scalar top
quark decouples from the Z boson. These limits were obtained assuming that the
scalar top quark decays into a charm quark and the lightest neutralino, and
that the mass difference between the scalar top quark and the lightest
neutralino is larger than 10 GeV. Assuming a relatively light scalar neutrino,
The complementary decay mode of the scalar top quark in which it decays into a
bottom quark, a charged lepton and a scalar neutrino was also studied. From a
similar analysis, a mass limit on the light scalar bottom quark was set at 82.7
GeV for a mass difference between the scalar bottom quark and the lightest
neutralino larger than 7 GeV, and at 84.0 GeV for the mass difference larger 10
GeV and the lightest neutralino heavier than 30 GeV. These limits were obtained
assuming that the scalar bottom quark decays into a bottom quark and the
lightest neutralino, and that a mixing angle between the supersymmetric
partners of the left- and right-handed states of the bottom quark is zero.
