  Quasiparticle dispersion in $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ is investigated with
improved angular resolution as a function of temperature and doping. Unlike the
linear dispersion predicted by the band calculation, the data show a sharp
break in dispersion at $50\pm10$ $meV$ binding energy where the velocity
changes by a factor of two or more. This change provides an energy scale in the
quasiparticle self-energy. This break in dispersion is evident at and away from
the d-wave node line, but the magnitude of the dispersion change decreases with
temperature and with increasing doping.
