  Nodal quasiparticles of Bi2Sr2CaCu2O8 have been studied by angle-resolved
photoemission spectroscopy with high momentum and energy resolution. Low-energy
tunable photons have enabled us to resolve a small nodal bilayer splitting,
unmasking the intrinsic single-particle scattering rate. The nodal scattering
rate is abruptly suppressed upon the superconducting transition, and shows a
linear energy dependence at low energies, indicating the nontrivial effect of
elastic scatterings on the quasiparticles. With increasing energy, the
antibonding-band scattering rate becomes higher than the bonding one. The
observations imply the character of the scatterers dominant at low energies.
