The $\pi NN$ and $\pi NN(1535)$ couplings in QCD

Abstract

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