Search for R-parity breaking sneutrino exchange at LEP

We report on a search for R-parity breaking effects due to supersymmetric tau-sneutrino exchange in the reactions e+e− → e+e− and e+e− → μ+μ− at centre-of-mass energies from 91 GeV to 172 GeV, using the L3 detector at LEP. No evidence for deviations from the Standard Model expectations of the measured cross sections and forward-backward asymmetries for these reactions is found. Upper limits for the couplings λ131 and λ232 for sneutrino masses up to mντ ≤ 190 GeV are determined from an analysis of the expected effects due to tau sneutrino exchange.