Study of the reactionsπ−p→π+π−nandπ−p→K+K−nat 1.98 and 2.41 GeV/c

Abstract

We have studied the reactions ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}n$ and ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{K}^{+}{K}^{\ensuremath{-}}n$ at 1.98 and 2.41 GeV/c over a dimeson mass range from 0.75 to 1.23 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$. Final-state neutrons were detected near ${0}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$ corresponding to $|t\ensuremath{-}{t}_{min}|<0.003$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ near a dimeson mass of 1.0 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$. Charged particles were detected in the Argonne Effective Mass Spectrometer and/or scintillation detectors surrounding the hydrogen target. The mass of the dimeson was calculated from the beam and neutron four-vectors, the latter being determined by the measured flight time of the neutrons, with a mass resolution of $\ensuremath{\sigma}=4$ ${\mathrm{M}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$. One of the prominent features of the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ mass spectrum is a sharp break near 0.95 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$. This feature and the rapid rise of the ${K}^{+}{K}^{\ensuremath{-}}$ spectrum near threshold are analyzed in terms of the parameters of the ${S}^{*}$ resonance. In the kinematic region covered by our data the contribution of the $\ensuremath{\rho}$ meson is small. The 62000 ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ events (11000 three-constraint and 51000 one-constraint) and th470 ${K}^{+}{K}^{\ensuremath{-}}$ events (all four-constraint) were analyzed with both a Breit-Wigner and a $K$-matrix formulation with various parameterizations of the background. Consistent values of the ${S}^{*}$ pole parameters were obtained in all of the various types of analyses for both beam momenta. An average of the parameters from these fits gives the position of the ${S}^{*}$ pole to be (969 \ifmmode\pm\else\textpm\fi{} 5) - $i$ (15 \ifmmode\pm\else\textpm\fi{} 4) ${\mathrm{M}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$, compared to the present world average of (993.2 \ifmmode\pm\else\textpm\fi{} 4.4) - $i$ (20.0 \ifmmode\pm\else\textpm\fi{} 3.7) ${\mathrm{M}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$.