Search for light neutral objects photoproduced in a crystal strong field and decaying into e+e− pairs

We describe a search for a light neutral object X0 possibly produced by ≈ 100 GeV photons interacting with the QED strong field encountered along the <110 > axis of a cooled Ge crystal. We look at the e+e− decay mode, in short (10−18−10−12 s) as well as long (10−13−10−8 s) lifetime detection conditions. For a mass of 1.8 ±0.2MeV/c2 (“Darmstadton” mass) we find no evidence for X0 production. The upper limit for the ratio between e+e− decay rate and pair creation is given as a function of the X0 lifetime. It reaches ≈ 5 × 10−4 and ≈ 2 × 10−5 in the short and long lifetime mode, respectively, including the crystal absorption contribution. However, for the long-lifetime mode and 108 incident photons, 9 events are compatible with neutral objects having masses ranging from 2.1 to 3.5 MeVc2. A Monte Carlo simulation of the background gives only 1 event in the same energy range.