Search for η c ( 2 S ) → K + K − η ′ decay

Using ( 2.712 ± 0.014 ) × 10 9 ψ ( 2 S ) events collected with the BESIII detector operating at the BEPCII, we search for the η c ( 2 S ) → K + K − η ′ decay. Its decay branching fraction is measured to be ( 11.11 ± 4.6 7 ( stat ) ± 1.8 2 ( syst ) ± 4.2 4 ( extr ) ) × 10 − 4 , where the first uncertainty is statistical, the second is systematic, and the third uncertainty is from the branching fraction of the ψ ( 2 S ) → γ η c ( 2 S ) decay. The statistical significance is 2.8 σ . The upper limit on the product branching fraction B [ ψ ( 2 S ) → γ η c ( 2 S ) ] × B [ η c ( 2 S ) → K + K − η ′ ] is set to be 0.94 × 10 − 6 at 90% confidence level. In addition, the branching fractions of χ c 1 → K + K − η ′ and χ c 2 → K + K − η ′ are updated to be ( 8.48 ± 0.10 ( stat ) ± 0.47 ( syst ) ) × 10 − 4 and ( 1.53 ± 0.04 ( stat ) ± 0.08 ( syst ) ) × 10 − 4 , respectively. The precision is improved by twofold.