Indication for $\pi^+ \pi^-$ scattering in $p+p$ collisions at   $\sqrt{s_{_{NN}}} = $200 GeV
  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. 
P. Fachini, R. S. Longacre, Z. Xu and H. Zhang