Search for the ηc′ (2 1S0) charmonium resonance

We report on a search by Fermilab experiment E835 for the ηc′ (21S0) charmonium resonance in the process p̅ p⃗ηc′→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(ηc′→p̅ p)×Br(ηc′→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(ηc′→p̅ p)×Br(ηc′→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.