The $πNN$ and $πNN(1535)$ couplings in QCD

Abstract

The light cone QCD sum rules are derived for the $πNN$ coupling $g_{πNN}$ and $πNN(1535)$ coupling $g_{πNN^\ast}$. The contribution from the excited states and continuum is subtracted cleanly through double Borel transformation. We first improve the original sum rule for $g_{πNN}$ and determine the value of the pion wave function at the middle point $ϕ_π(u_0 ={1\over 2})=(1.5\pm 0.2)$ at $μ=1$GeV, which is a fundamental nonperturbative quantity like the quark condensate. Our calculation shows that the $πN N(1535)$ coupling is strongly suppressed.