| Abstract: | We study e+e- --> pi+pi-h_c at center-of-mass energies from 3.90 GeV to 4.42
GeV using data samples collected with the BESIII detector operating at the
Beijing Electron Positron Collider. The Born cross sections are measured at 13
energies, and are found to be of the same order of magnitude as those of e+e-
--> pi+pi-J/psi but with a different line shape. In the pi^pm h_c mass
spectrum, a distinct structure, referred to as Z_c(4020), is observed at 4.02
GeV/c^2. The Z_c(4020) carries an electric charge and couples to charmonium. A
fit to the pi^pm h_c invariant mass spectrum, neglecting possible
interferences, results in a mass of (4022.9pm 0.8pm 2.7) MeV/c^2 and a width
of (7.9pm 2.7pm 2.6) MeV for the Z_c(4020), where the first errors are
statistical and the second systematic. No significant Z_c(3900) signal is
observed, and upper limits on the Z_c(3900) production cross sections in
pi^pm h_c at center-of-mass energies of 4.23 and 4.26 GeV are set. |
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