# Synopsis: Catching $Z$’s in Particle Colliders

Electron-positron collisions provide new information on a possible four-quark particle called ${Z}_{c}\left(3900\right)$.

In April 2013, particle physicists made an unexpected discovery: a particle, called ${Z}_{c}\left(3900\right)$, that appears to be composed of four quarks rather than the usual two or three. The BESIII Collaboration—one of the two groups to first spot ${Z}_{c}\left(3900\right)$—has now explored a separate set of reactions that may lead to the production of these four-quark states. As reported in Physical Review Letters, they do find strong signatures of a particle, but its mass isn’t exactly that of ${Z}_{c}\left(3900\right)$. Regardless of its true identity, the detected entity may give a better understanding of how four quarks might bind together in these unusual particles.

The original evidence for the ${Z}_{c}\left(3900\right)$ particle comes from electron-positron collisions. At an energy of $4.26$ giga-electron-volts (GeV), these collisions can produce a particle called $Y\left(4260\right)$, which decays some of the time into a $J/\mathrm{\Psi }$ meson and two pions. In these decay chains, physicists uncovered evidence of another particle, the ${Z}_{c}\left(3900\right)$, with a mass of $3.9\phantom{\rule{0.333em}{0ex}}{\text{GeV/c}}^{2}$. It is still unclear whether ${Z}_{c}\left(3900\right)$ is a true four-quark state or a “molecule” composed of two two-quark states.

Looking for new insight into this problem, the BESIII experiment at the Beijing Electron Positron Collider has analyzed a different decay route for $Y\left(4260\right)$, which results in a pair of $D$ mesons and one pion. The data showed a peak at a specific energy, implying a particle with a mass of $3.885\phantom{\rule{0.333em}{0ex}}{\text{GeV/c}}^{2}$ was created. The mass discrepancy with the ${Z}_{c}\left(3900\right)$ is small but significant (2 sigma), so the researchers refrain from identifying their particle as ${Z}_{c}\left(3900\right)$. However, they did measure the total angular momentum and parity of their particle, which could help discriminate this particle from other potential four-quark particles in the same mass range. – Michael Schirber

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