Search for the electron-positron decay of axions and axionlike particles at a nuclear power reactor at Bugey

A search for the decay of isoscalar penetrating particles into an e⁺e-pair has been performed at the nuclear power reactor Bugey 5, situated close to Lyon (France). The detector set-up, consisting of an arrangement of large-area multiwire proportional chambers, and the data analysis are described. No excess of decay events over background has been observed. For standard axions, therefrom, stringent upper limits for the branching ratio 〈Γ_a/Γ_M^γ, averaged over the entire reactor gamma spectrum of magnetic multipolarity, have been obtained. Analysis in terms of the decay χ→e⁺+e^- of a more general axionlike boson χ yields new constraints for the decay constant f_χ which enhance existing laboratory limits considerably.