Direct observation and partial-width measurement of γγ decay of charmonium states

Abstract As part of the charmonium formation experiment at the CERN Intersecting Storage Rings, we studied the reaction p p →c c →γγ in an antiproton momentum scan through the η c , χ 1 , and χ 2 regions. We report events observed in the η c and χ 2 regions, whilst no event was observed in the χ 1 region, as expected for a spin-1 state.