  Angle-resolved photoemission spectroscopy (ARPES) is used to study the doping
dependence of the lifetime and the mass renormalization of the low energy
excitations in the high-Tc cuprate (Bi,Pb)_2Sr_2CaCu_2O_8 along the zone
diagonal. We find a linear energy de-pendence of the scattering rate for the
underdoped samples and a quadratic energy depend-ence in the overdoped case.
The mass enhancement of the quasiparticles due to the many body effects at the
Fermi energy is found to be in the order of 2 and the renormalization extends
over a large energy range for both the normal and the superconducting state.
The much discussed kink in the dispersion around 70 meV is interpreted as a
small additional effect at low temperatures.
