Search for anomalous production of high mass photon pairs in e +e − collisions at LEP

Abstract

A search for events with photon pairs of large invariant mass is described based on a data sample of 43 pb −1 collected with the OPAL detector. This search is motivated by the L3 observation of four events of the type e +e − → l + l − γγ with an invariant mass of the two photons, m γγ , clustering around 60 GeV. In the OPAL data, four ℓ +ℓ −γγ events are found with m γγ above 40 GeV. The number of observed events is consistent with the QED expectation and no obvious resonance structure is seen. Three candidate qq̄γγ events with m γγ above 40 GeV are found and no νν̄nγγ candidate with m γγ > 5 GeV is found. From these searches an upper limit is set on any anomalous production of ℓ +ℓ − γγ (ℓ = e, μ, τ) events, where the ℓ +ℓ − pair comes from a virtual Z 0 and m γγ is near 60 GeV , of B( Z 0 → Z ∗0γγ → ℓ +ℓ −γγ) < 6.0 × 10 −7 at the 95% CL. In a similar search in the reaction e +e − → γγγ, seven events are observed with a photon pairing of mass near 60 GeV compared to an expected background of 2.7 ± 0.4 from QED. In a search for the production of a 60 GeV resonance X in two-photon collisions, an upper limit is set on Γ X B 2(X → γγ) of 2.6 MeV at the 95% CL. A search is also performed for ℓ +ℓ −X (ℓ = e, μ) events in non-γγ decay modes of X. No indication of a 60 GeV resonance is seen. Combining these two searches, Γ X values greater than 110 MeV are excluded at the 95% CL for ℓ +ℓ − γγ (ℓ = e, μ) cross-sections greater than 0.05 pb.