  A $\rho(770)^0$ mass shift of about -40 MeV/$c^2$ was measured in $p+p$
collisions at $\sqrt{s_{_{NN}}} = $ 200 GeV at RHIC. Previous mass shifts have
been observed at CERN-LEBC-EHS and CERN-LEP. We will show that phase space does
not account for the $\rho(770)^0$ mass shift measured at RHIC, CERN-LEBC-EHS
and CERN-LEP and conclude that there are significant scattering interactions in
$p+p$ collisions.
