  The neutron spin resonance is a collective magnetic excitation that appears
in copper oxide, iron pnictide, and heavy fermion unconventional
superconductors. Although the resonance is commonly associated with a
spin-exciton due to the $d$($s^{\pm}$)-wave symmetry of the superconducting
order parameter, it has also been proposed to be a magnon-like excitation
appearing in the superconducting state. Here we use inelastic neutron
scattering to demonstrate that the resonance in the heavy fermion
superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$ with $x=0,0.05,0.3$ has a ring-like
upward dispersion that is robust against Yb-doping. By comparing our
experimental data with random phase approximation calculation using the
electronic structure and the momentum dependence of the $d_{x^2-y^2}$-wave
superconducting gap determined from scanning tunneling microscopy for
CeCoIn$_5$, we conclude the robust upward dispersing resonance mode in
Ce$_{1-x}$Yb$_{x}$CoIn$_5$ is inconsistent with the downward dispersion
predicted within the spin-exciton scenario.
