Search for Lepton Flavor Violation in ϒ (3 S )→e±μ∓

We report on the first search for electron-muon lepton flavor violation (LFV) in the decay of a b quark and b antiquark bound state. We look for the LFV decay ϒ (3 S )→e^±μ^∓ in a sample of 118 million ϒ (3 S ) mesons from 27 fb^-1 of data collected with the BABAR detector at the SLAC PEP-II e⁺e^- collider operating with a 10.36 GeV center-of-mass energy. No evidence for a signal is found, and we set a limit on the branching fraction B [ϒ (3 S )→e^±μ^∓]<3.6 ×10^-7 at 90% C. L. This result can be interpreted as a limit Λ_NP/g_NP²>80 TeV on the energy scale Λ_NP divided by the coupling-squared g_NP² of relevant new physics (NP).