Observation of a charged charmoniumlike structure in e+e- to pi+pi-J/psi at \sqrt{s}=4.26 GeV

TL;DR

This paper reports the observation of a charged charmoniumlike structure, Z_c(3900), in e+e- collisions at 4.26 GeV, providing evidence for an exotic hadron state that challenges conventional quark models.

Contribution

First observation of a charged charmoniumlike state Z_c(3900) in e+e- collisions, with detailed measurements of its mass, width, and production ratio.

Findings

Z_c(3900) observed at ~3.9 GeV/c^2

Measured mass of (3899.0±3.6±4.9) MeV/c^2

Production ratio of (21.5±3.3±7.5)%

Abstract

We study the process e+e- to pi+pi-J/psi at a center-of-mass energy of 4.260 GeV using a 525 pb^{-1} data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be (62.9\pm 1.9\pm 3.7) pb, consistent with the production of the Y(4260). We observe a structure at around 3.9 GeV/c^2 in the \pi^\pm J/psi mass spectrum, which we refer to as the Z_c(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the \pi^\pm J/psi invariant mass spectrum, neglecting interference, results in a mass of (3899.0\pm 3.6\pm 4.9) MeV/c^2 and a width of (46\pm 10\pm 20) MeV. Its production ratio is measured to be R=\frac{\sigma(e+e- to \pi^\pm Z_c(3900)^\mp to pi+pi-J/psi))} {\sigma(e+e- to pi+pi-J/psi)}=(21.5\pm 3.3\pm 7.5)%. In all measurements the first errors are statistical and the second are systematic.