With data samples collected with the BESIII detector operating at the BEPCII storage ring at center-ofmass energies from 4.009 to 4.420 GeV, the process e+e-→ γX(3872) is observed for the first time with a statistical significance of 6.3σ. The measured mass of the X(3872) is (3871.9 ± 0.7stat± 0.2syst) MeV/c2, in agreement with previous measurements. Measurements of the product of the cross section σ[e+e-→ γX(3872)] and the branching fraction B[X(3872) → π+π-J/ψ] at center-of-mass energies 4.009, 4.229, 4.260, and 4.360 GeV are reported. Our measurements are consistent with expectations for the radiative transition process Y(4260) → γX(3872).
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© 2014 American Physical Society.