ARPES matrix element and the waterfall effect in the cuprates.

The high-energy kink (HEK) or the 'waterfall' effect as seen in angle-resolved photoemission spectra (ARPES) in the cuprates has the potential of revealing important information about the dressing of quasiparticles by electronic excitations [1,2,3]. However, recently it has been suggested that matrix element effects radically modify the experimental spectra in $\rm{Bi_2Sr_2CaCu_2O_8}$ (Bi2212), and it has been questioned whether the HEK exists [4]. Here we discuss how the interplay between the matrix element and self-energy effects shapes the ARPES spectra. Both the ARPES matrix element and the self-energy are found to be necessary for understanding the experimental spectra. Work supported in part by the USDOE. [1] R. S. Markiewicz {\it et al.}, Phys. Rev. B {\bf{76}}, 174514 (2007). [2] A. Macridin {\it et al.},Phys. Rev. Lett. {\bf 99}, 237001 (2007). [3] Tanmoy Das {\it et al.}, cond-mat:0807.4257. [4] D.S. Inosov {\it et al.}, Phys. Rev. Lett. {\bf 99}, 237002 (2007).