Axial resonances a1(1260), b1(1235) and their decays from the lattice

The light axial-vector resonances a(1) (1260) and b(1) (1235) are explored in N-f = 2 lattice QCD by simulating the corresponding scattering channels p pi and w pi. Interpolating fields (q) over barq and p pi or w pi are used to extract the s-wave phase shifts for the first time. The. and. are treated as stable and we argue that this is justified in the considered energy range and for our parameters m pi similar or equal to 266MeV and L similar or equal to 2 fm. We neglect other channels that would be open when using physical masses in continuum. Assuming a resonance interpretation a Breit-Wigner fit to the phase shift gives the a(1) (1260) resonance mass m(a1)(res) = 1.435(53)( +/-(0)(109)) GeV compared to m(a1)(exp) = 1.230(40) GeV. The a(1) width Gamma(a1)(s) = g(2) p/s is parametrized in terms of the coupling and we obtain g(a1p pi) = 1.71( 39) GeV compared to g(a1p pi) (exp) = 1.35(30) GeV derived from Gamma(exp)(a1) = 425(175) MeV. In the b(1) channel, we find energy levels related to pi(0)w(0) and b(1) (1235), and the lowest level is found at E-1 greater than or similar to m(w) + m(pi) but is within uncertainty also compatible with an attractive interaction. Assuming the coupling g(b1w pi) extracted from the experimental width we estimate m(b1)(res) = 1.414(36) (+/-(0)(83)).