Observation of the two-photon decay of a light penetrating particle

Abstract

Coincident two-photon events, emerging from a 2 m long decay region, and pointing back to the SIN 590 MeV proton beam dump were detected in a thin-foil optical spark chamber. There was a significant excess of photons at small angles (<7°): 19 were observed, whereas (2.0 ± 0.5) were measured with cosmic rays, and additional (2.5 ± 1.0) were accelerator background. The visible γ-ray energies were estimated by multiple scattering to be 〉 E γ1 〈 = (83 ± 20) MeV, 〉 E γ2 〈 = (84 ± 15) MeV. The invariant 2γ mass is < 1 MeV. The 2γ event rate did not change significantly, when a 20 cm thick iron wall was placed in front of the decay region, but vanished with the wall put at its end. Presumably a light boson χ 0 comes from the beam dump, penetrates the shielding, and decays: χ 0 → 2 γ The measured rate of (14.5 ± 5.0) events in 129 Coulomb fixes a combination of production cross section and life-time. If the new boson χ 0 was the axion, one can solve for the Higgs parameter X = 3.0 ± 0.3, and infer τ a ≈ 7 ms, and m a = (250 ± 25) keV.