Anatomy of the $\rho$ resonance from lattice QCD at the physical point

Abstract

We propose a strategy to access the $q\bar{q}$ component of the $\rho$ resonance in lattice QCD. Through a mixed action formalism (overlap valence on domain wall sea), the energy of the $q\bar{q}$ component is derived at different valence quark masses, and shows a linear dependence on $m_\pi^2$. The slope is determined to be $c_1=0.505(3)\,{\rm GeV}^{-1}$, from which the valence $\pi \rho$ sigma term is extracted to be $\sigma_{\pi \rho}^{(\rm val)}=9.82(6)$ MeV using the Feynman-Hellman theorem. At the physical pion mass, the mass of the $q\bar{q}$ component is interpolated to be $m_\rho=775.9\pm 6.0\pm 1.8$ MeV, which is close to the $\rho$ resonance mass. We also obtain the leptonic decay constant of the $q\bar{q}$ component to be $f_{\rho^-}=208.5\pm 5.5\pm 0.9$ MeV, which can be compared with the experimental value $f_{\rho}^{\rm exp}\approx 221$ MeV through the relation $f_{\rho}^{\rm exp}=\sqrt{Z_\rho}f_{\rho^\pm} $ with $Z_\rho\approx 1.13$ being the on-shell wavefunction renormalization of $\rho$ owing to the $\rho-\pi$ interaction. We emphasize that $m_\rho$ and $f_\rho$ of the $q\bar{q}$ component, which are obtained for the first time from QCD, can be taken as the input parameters of $\rho$ in effective field theory studies where $\rho$ acts as a fundamental degree of freedom.