Search for Single Leptoquark and Squark Production in Electron-Photon Scattering at sqrt(s_ee) = 189 GeV at LEP

Abstract

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks in R-parity violating SUSY models has been performed using e+e- collisions collected with the OPAL detector at LEP at an e+e- centre-of-mass energy sqrt(s_ee) of 189 GeV. The data correspond to an integrated luminosity of about 160 pb-1. The dominant process for this search is e q -> LQ/squark -> e q, nu q, where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95% confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings lambda to fermions larger than sqrt{4 pi alpha_em}, the mass limits range from 121 GeV/c^2 to 175 GeV/c^2 (149 GeV/c^2 to 188 GeV/c^2) depending on the branching ratio beta of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings lambda for leptoquarks and lambda'_1jk for squarks, and on beta as a function of the scalar leptoquark/squark mass.