c̄c purity of ψ(3770) and ψ′ challenged

It is suggested that the resonance ψ(3770) may contain a sizeable (O(10%) in terms of the probability weight factor) four-quark component with the up- and down-quarks and antiquarks in addition to the cc̄ pair, which component in itself has a substantial part with isospin I=1. Furthermore such a four-quark part of the wave function should also affect the properties of the ψ′ charmonium resonance through the ψ(3770)-ψ′ mixing previously considered in the literature. It is argued that an admixture of extra light quark pairs can explain a possible discrepancy between the theoretical expectations and the recent data on the non-DD̄ decay width of the ψ(3770) and the ratio of the yield of charged and neutral D meson pairs in its decays, as well as on the extra rate of the ψ′ direct decay into light hadrons and the rate of the decay ψ′→π0J/ψ. It is further argued that the suggested four-quark component of the wave function of the ψ(3770) should give rise to a measurable rate for the decays ψ(3770)→ηJ/ψ and ψ(3770)→π0J/ψ.