Search for the η_c^′ (2¹S₀) charmonium resonance

We report on a search by Fermilab experiment E835 for the η_c^′ (2¹S₀) charmonium resonance in the process p̅ p⃗η_c^′→γγ. No signal was observed and, based on 34 pb^-1 integrated luminosity, we determine the following upper limits (90% confidence level) to the product of the branching ratios for a resonance mass in the region 3575–3660 MeV/c²: Br(η_c^′→p̅ p)×Br(η_c^′→γγ)<12.0×10^-8 for Γ=5 MeV; <5.9×10^-8 for Γ=10 MeV; <4.8×10^-8 for Γ=15 MeV. Combining the present data with those of the predecessor experiment, E760, the upper limits become 8.0×10^-8, 5.0×10^-8, and 4.5×10^-8, respectively. In the restricted region 3589–3599 MeV/c², where a candidate was reported by the Crystal Ball experiment, we obtain the following limits from the combined E760–E835 experiments: Br(η_c^′→p̅ p)×Br(η_c^′→γγ)<5.6×10^-8 for Γ=5 MeV; <3.7×10^-8 for Γ=8 MeV. A comparison of these with other experimental results is presented.