Abstract
We report results on ρ(770)0 → π+ π- production at midrapidity in p + p and peripheral Au + Au collisions at psNN = 200 GeV. This is the first direct measurement of ρ(770)0 → π+ π- in heavy-ion collisions. The measured ρ0 peak in the invariant mass distribution is shifted by ∼40 MeV=c2 in minimum bias p + p interactions and ∼70 MeV=c2 in peripheral Au + Au collisions. The ρ0 mass shift is dependent on transverse momentum and multiplicity. The modification of the ρ0 meson mass, width, and shape due to phase space and dynamical effects are discussed.