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We employ Momentum-Resolved Electron Energy Loss Spectroscopy (M-EELS) on Bi2.1Sr1.9CaCu2O8+x to resolve the issue of the kink feature in the electron dispersion widely observed in the cuprates. To this end, we utilize the GW approximation to relate the density response function measured in in M-EELS to the self-energy, isolating contributions from phonons, electrons, and the momentum dependence of the effective interaction to the decay rates. The phononic contributions, present in the M-EELS spectra due to electron-phonon coupling, lead to kink features in the corresponding single-particle spectra at energies between 40 meV and 80 meV, independent of the doping level. We find that a repulsive interaction constant in momentum space is able to yield the kink attributed to phonons in ARPES. Hence, our analysis of the M-EELS spectra points to local repulsive interactions as a factor that enhances the spectroscopic signatures of electron-phonon coupling in cuprates. We conclude that the strength of the kink feature in cuprates is determined by the combined action of electron-phonon coupling and electron-electron interactions. 

Even the particle world is not immune to identity politics. Bosons have been in a bit of an identity crisis, or so it has seemed since 1989 ( 1 ). Quantum mechanics requires bosons made of two paired electrons to either condense into a superfluid with a well-defined phase with zero electrical resistance or localize in an insulating state with infinite resistance. The direct transition from superconducting to insulating states was widely observed in a range of thin films ( 2 – 4 ). The most popular model for explaining these observations ( 5 ) claims that the destruction of superconductivity occurs when the resistance of the thin film exceeds a critical value. For bosons on the brink of localization, electrically insulating behavior is observed if the resistance is greater than the quantum of resistance, R q = h /4 e 2 , otherwise superconductivity persists, where h is Planck's constant and e is the electric charge. On page 1505 of this issue, Yang et al. ( 6 ) offer a counterexample by establishing that a bosonic metallic phase disrupts the superconductor-insulator transition (SIT) in the high-temperature superconductor YBa 2 Cu 3 O 7– x (YBCO).