We report results on rho(770)^0 -> pi+pi- production at midrapidity in p+p and peripheral Au+Au collisions at sqrt(s_NN) = 200 GeV. This is the first direct measurement of rho(770)^0 -> pi+pi- in heavy-ion collisions. The measured rho^0 peak in the invariant mass distribution is shifted by ~40 MeV/c^2 in minimum bias p+p interactions and ~70 MeV/c^2 in peripheral Au+Au collisions. The rho^0 mass shift is dependent on transverse momentum and multiplicity. The modification of the rho^0 meson mass, width, and shape due to phase space and dynamical effects are discussed.
The raw $\pi^{+} \pi^{-}$ invariant mass distributions after subtraction of the like-sign reference distribution for minimum bias p+p (top) and peripheral Au+Au (bottom) interactions.
The raw $\pi^{+} \pi^{-}$ invariant mass (solid line) and the like-sign reference distributions (open circles) for peripheral Au+Au collisions.
The $\rho^{0}$ mass as a function of $p_{T}$ for minimum bias $p$+$p$ (filled circles), high multiplicity $p$+$p$ (open triangles), and peripheral Au+Au (filled squares) collisions. The error bars indicate the systematic uncertainty. Statistical errors are negligible. The $\rho^{0}$ mass was obtained by fitting the data to the BW×PS functional form described in the text. The dashed lines represent the average of the $\rho^{0}$ mass measured in $e^{+} e^{−}$. The shaded areas indicate the ρ0 mass measured in $p$+$p$ collisions. The open triangles have been shifted downward on the abscissa by $50$ MeV/$c$ for clarity.
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