Mass-renormalized electronic excitations at ($\pi$, 0) in the   superconducting state of $Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}$
  Using high-resolution angle-resolved photoemission spectroscopy on $Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}$, we have made the first observation of a mass renormalization or "kink" in the E vs. $\vec k$ dispersion relation localized near $(\pi, 0)$. Compared to the kink observed along the nodal direction, this new effect is clearly stronger, appears at a lower energy near 40 meV, and is only present in the superconducting state. The kink energy scale defines a cutoff below which well-defined quasiparticle excitations occur. This effect is likely due to coupling to a bosonic excitation, with the most plausible candidate being the magnetic resonance mode observed in inelastic neutron scattering. 
A. D. Gromko, A. V. Fedorov, Y. -D. Chuang, J. D. Koralek, Y. Aiura,   Y. Yamaguchi, K. Oka, Yoichi Ando, D. S. Dessau