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We calculate the E1 transition widths of higher vector charmonium states into the spin-triplet 2P states in three typical potential models, and discuss the possibility to detect these 2P states via these E1 transitions. We attempt to clarify the nature of some recently observed X,Y,Z states by comparing them with these 2P charmonium states in these E1 transitions. In particular, the calculated branching ratios of ψ(4040), ψ(4160) → χ ′ c J γ (J=0,1,2) are found to be in the range of 10 −4 -10 −3 , and sensitive to the 3S-2D mixing of ψ(4040) and ψ(4160). The mixing angle may be constrained by measuring ψ(4040), ψ(4160) → Z(3930)γ, if Z( 3930) is identified with the χ ′ c 2 state, and then be used in measuring χ ′ c 0,1 states. These processes can be studied experimentally at e + e − colliders such as BEPCII/BESIII and CESR/CLEO.

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