Search for Scalar Top and Scalar Bottom                                        
Quarks at ECM=189GeV at LEP                                                    
  
Searches for a scalar top quark and a scalar bottom quark                      
have been performed using a data sample of 182 inverse pico-barns              
at a centre-of-mass energy of 189 GeV                                          
collected with the OPAL detector at LEP.                                       
No evidence for a signal was found.                                            
The 95% C.L. lower limit                                                       
on the scalar top quark mass is                                                
90.3 GeV if the mixing angle between the supersymmetric partners               
of the left- and right-handed states of the top quark is zero.                 
In the worst case, when the scalar top quark decouples                         
from the Z boson, the lower limit is 87.2 GeV.                                 
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.                                              
The complementary decay mode of the scalar top quark                           
decaying into a bottom quark,                                                  
a charged lepton and a scalar neutrino has also been studied.                  
From a search for the scalar bottom quark,                                     
a mass limit of 88.6 GeV was obtained if the mass difference                   
between the scalar bottom quark and the lightest                               
neutralino is larger than 7 GeV.                                               
  
 Parallel session :11                  
 Plenary  session :7a