Two-pion exchange for coupled-channel scattering of two heavy mesons

To improve the theoretical understanding of multiquark states like Zb(10610) and Zb(10650), we calculate the heavy-meson heavy-(anti)meson scattering potential up to next-to-leading order, O(Q2), within chiral effective field theory (chi EFT) employing a power counting scheme that explicitly keeps track with the large momentum scale Q similar to p(where delta = mV - mP is the vector-pseudoscalar mass difference ffiffiffiffiffiffiffi 2 mu delta and mu their reduced mass) introduced by the coupled channel dynamics. We provide expressions for the two-pion exchange (TPE) terms up to O(Q2) and their partial-wave decomposition. We show that these potentials are well approximated by contact terms at O(Q2), with minor residual nonanalytic TPE contributions, supporting chi EFT convergence in the theoretical predictions for Zb(10610) and Zb(10650), as well as their spin partners. These findings are also relevant for D(*)D(*) scattering, especially for the Tcc state, for both physical and lattice quantum chromodynamics (QCD) data with moderately larger pion masses. We further demonstrate that the differences between isovector and isoscalar potentials for heavy mesons are naturally explained by the TPE contributions.