Charmoniumlike Channels $1^{+}$ with Isospin $1$ from Lattice and Effective Field Theory
Many exotic charmoniumlike mesons have already been discovered experimentally, of which the $Z_c$ mesons with $I=1$ are prominent examples. We investigate $J^{PC}=1^{+\pm}$ states with flavor $\bar cc\bar qq$ ($q=u,d$) in $I=1$ using lattice QCD. This is the first study of these mesons employing more than one volume and involving frames with nonzero total momentum. We utilize two $N_f=2+1$ CLS ensembles with $m_{\pi}\simeq 280\,$MeV. The simulations are performed with unphysical light quark masses at a single lattice spacing of $a\simeq 0.086\,$fm and omit $\psi(2S)\pi$, $\psi(3770)\pi$ and three-particle decay channels, so our results provide only qualitative insights. Resulting eigenenergies are compatible or just slightly shifted down with respect to noninteracting energies, where the most significant shifts occur for certain $D\bar D^*$ states. Both channels $1^{+\pm}$ have a virtual pole slightly below the threshold if $D\bar D^*$ is assumed to be decoupled from other channels. In addition, we perform a coupled channel analysis of $J/\psi\pi$ and $D\bar D^*$ scattering with $J^{PC}=1^{+-}$ within an effective field theory framework. The $J/\psi\pi$ and $D\bar D^*$ line shapes from BESIII and finite-volume energies from several lattice QCD simulations, including this work, are fitted simultaneously. All fits yield two poles relatively close to the $D\bar D^*$ threshold and reasonably reproduce the experimental $Z_c$ peaks. They also reproduce lattice energies up to slightly above the $D\bar D^*$ threshold, while reproduction at even higher energies is better for fits that put more weight on the lattice data. Our findings suggest that the employed EFT can reasonably reconcile the peaks in the experimental line shapes and the lattice energies, although those lie close to noninteracting energies. We also study $J/\psi\pi$ scattering in s wave and place upper bounds on the phase shift.