Title: The $Z_c(3900)$ as a Tetra-Quark State


Abstract: The BESIII Collaboration (Phys. Rev. Lett. 110, 252001

(2013)) and the Belle Collaboration (Phys. Rev. Lett. 110,

252002 (2013)) found a structure at about 3.9 GeV, called the

Z(3900), which  canot be a conventional $c\bar{c}$ state. There

have been theoretical studies that it is a D+-Do molecular

state. We have used the method of QCD Sum Rules to test the

possibility that the  $Z(3900)$, now called the $Z_c(3900)$,

is a tetra-quark state. I shall review the method of QCD sum

rules and show that our calculation demonstrates that the

$Z_c(3900)$ is a tetra-quark state.