Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity 1 . The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high transition temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle resolved photoemission spectroscopy to probe electron dynamicsvelocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity. We investigated the electronic quasiparticle dispersions in three different families of hole-doped cuprates, Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) and Pb doped Pb-Bi2212, Pb-doped Bi 2 Sr 2 CuO 6 (Pb-Bi2201) and La 2-x Sr x CuO 4 (LSCO). Except for the Bi2201 (overdoped, T c =7K) data and that in Fig. 3b, recorded at the beam-line 5.4 of the Stanford Synchrotron Radiation Laboratory (SSRL), all the data were recorded at the Advanced Light Source (ALS), as detailed elsewhere 2 . The top panels of figure 1 report the momentum distribution curve (MDC) derived dispersions along the (0, 0)-(π, π) direction for LSCO (panel a) and Bi2212 (panel b) superconducting state and for Pb-Bi2201 normal state (panel c) vs the rescaled momentum, k ' , defined by normalizing to one the momentum k relative to the Fermi momentum k F , (k-k F ), at the binding energy E=170meV. A "kink" in the dispersion around 50-80meV, highlighted by thick arrows in the figure, is the many-body effect of

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Quasiparticle dispersion in Bi2Sr2CaCu2O8 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+/-15 meV binding energy where the velocity changes by a factor of 2 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.