${\rho }^0$ Meson Production in the $pp\to pp{\pi }^{+}{\pi }^{-}$ Reaction at $3.67\mathrm{G}\mathrm{e}\mathrm{V}/c$

Total and differential cross sections for the exclusive reaction $pp\to pp\rho$ observed via the ${\pi }^{+}{\pi }^{-}$ decay channel have been measured at $p_{\mathrm{b}\mathrm{e}\mathrm{a}\mathrm{m}}=3.67\mathrm{G}\mathrm{e}\mathrm{V}/c$. The observed total meson production cross section is determined to be $(23.4\pm 0.8\pm 8)\mu \mathrm{b}$ and is significantly lower than typical cross sections used in model calculations for heavy-ion collisions. The differential cross sections measured indicate a strong anisotropy ($\sim \cos {\theta }_{\rho }^{CM}$) in the ${\rho }^0$ meson production.

Figure 1

Proton-pion invariant mass distributions $M_{p\pi }^{\mathrm{i}\mathrm{n}\mathrm{v}}$ selected according to the absolute value of the four-momentum transfer $t$: $M_{p_1{\pi }^{+}}^{\mathrm{i}\mathrm{n}\mathrm{v}}$ with the lower $\mid t\mid$ value (left frame) and $M_{p_2{\pi }^{+}}^{\mathrm{i}\mathrm{n}\mathrm{v}}$ with the larger $\mid t\mid$ (right frame). The curves are fits to the data as explained in the text.

Figure 2

${\pi }^{+}{\pi }^{-}$ invariant mass distributions obtained in four different CM angular bins as indicated in the figure. The dashed line represents the smoothed invariant mass distribution for the $0<\cos ({\theta }^{\mathrm{C}\mathrm{M}})<0.2$ normalized to the maximum.

Figure 3

Upper frame: $M_{{\pi }^{+}{\pi }^{-}}$ distribution measured for $\cos ({\theta }^{\mathrm{C}\mathrm{M}})>0.5$. The solid line shows the fit using a relativistic Breit-Wigner function added onto a third order polynomial for the background, as described in the text. The dotted line shows the meson line shape, and the dashed line presents the $M_{{\pi }^{+}{\pi }^{-}}^{\mathrm{i}\mathrm{n}\mathrm{v}}$ distribution obtained for $\cos ({\theta }^{\mathrm{C}\mathrm{M}})<0.4$ normalized to the same maximum. Lower frame: The ${\rho }^0$ (circles) and $\omega$ (triangles) differential cross sections as a function of the meson CM emission polar angle. The solid and dashed lines show fits using the first three even Legendre polynomials.