A search for stable particles heavier than the proton and for q = 2/3 quarks produced in e+ e- annihilation

Abstract We have searched 1.4 × 10 6 e + e − annihilation events for particles with 1–3 GeV/ c 2 mass and charge Q = − 2 3 from the processes e + e − →q q X and e + e − → q q . Upper limits of RQ ∼ 10 −4 for each process are presented which improve the previous limits on free quark production in electromagnetic interactions by 2 orders of magnitude.