${\rho }^0$ Production and Possible Modification in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $p+p$ Collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$

We report results on $\rho (770{)}^0\to {\pi }^{+}{\pi }^{-}$ production at midrapidity in $p+p$ and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. This is the first direct measurement of $\rho (770{)}^0\to {\pi }^{+}{\pi }^{-}$ in heavy-ion collisions. The measured ${\rho }^0$ peak in the invariant mass distribution is shifted by $\sim 40\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ in minimum bias $p+p$ interactions and $\sim 70\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ in peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ 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.

Figure 1

The raw ${\pi }^{+}{\pi }^{-}$ invariant mass distributions after subtraction of the like-sign reference distribution for minimum bias $p+p$ (top) and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ (bottom) interactions. The inset plot corresponds to the raw ${\pi }^{+}{\pi }^{-}$ invariant mass (solid line) and the like-sign reference distributions (open circles) for peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions.

Figure 2

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 $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ (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 $\mathrm{B}\mathrm{W}\times \mathrm{P}\mathrm{S}$ functional form described in the text. The dashed lines represent the average of the ${\rho }^0$ mass measured in $e^{+}{e}^{-}$ [29]. The shaded areas indicate the ${\rho }^0$ mass measured in $p+p$ collisions [23]. The open triangles have been shifted downward on the abscissa by $50\mathrm{M}\mathrm{e}\mathrm{V}/c$ for clarity.

Figure 3

The $p_T$ distributions at $|y|<0.5$ for minimum bias $p+p$ and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. See the text for an explanation of the functions used to fit the data. The errors shown are statistical only and smaller than the symbols.

Figure 4

${\rho }^0/\pi$ ratios as a function of c.m. system energy. The ratios are from measurements in $e^{+}{e}^{-}$ collisions at 10.45 GeV [18], 29 GeV [19], and 91 GeV [20] c.m. system energy, $p+p$ at 6.8 GeV [21], 19.7 GeV [22], 27.5 GeV [23], and 52.5 GeV [24], $K^{+}p$ at 7.82 GeV [25] and ${\pi }^{-}p$ at 19.6 GeV [26]. The errors on the ratios at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ are the quadratic sum of the statistical and systematic errors. The ratios at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ are offset from one another for clarity.