The decay η ′ c → η c π π. Abstract It is pointed out that the decay of the recently observed charmonium η ′ c resonance, η ′ c → η c π π is simply related to the well studied decay ψ ′ → J/ψ π π and can thus be used for absolute normalization of other decay modes of the η ′ c. The total rate of the discussed decay should be approximately three to four times the corresponding rate for the ψ ′ r...