  We used resonant inelastic x-ray scattering (RIXS) with and without analysis
of the scattered photon polarization, to study dispersive spin excitations in
the high temperature superconductor YBa2Cu3O6+x over a wide range of doping
levels (0.1 < x < 1). The excitation profiles were carefully monitored as the
incident photon energy was detuned from the resonant condition, and the spin
excitation energy was found to be independent of detuning for all x. These
findings demonstrate that the largest fraction of the spin-flip RIXS profiles
in doped cuprates arises from magnetic collective modes, rather than from
incoherent particle-hole excitations as recently suggested theoretically
[Benjamin et al. Phys. Rev. Lett. 112, 247002(2014)]. Implications for the
theoretical description of the electron system in the cuprates are discussed.
