We report the result of a search for the pair production of the light supersymmetric partner of the top quark (t˜1) in 5.4+/-0.3 fb-1 of data from the DO detector at a pp¯ center-of-mass energy of 1.96 TeV at the Fermilab Tevatron collider. The scalar top quarks are assumed to decay into a b quark, a charged lepton and a scalar neutrino ( n&d5; ), and the search is performed in the electron plus muon final state. No significant excess of events above the standard model prediction is detected and new exclusion limits at the 95% C.L. are set for a portion of the ( mt&d5;1,m n&d5; ) mass plane.