  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.
