Pion, Kaon, and Antiproton Production in the Center-of-Mass in High-Energy Proton-Proton Collisions

Abstract

The differential production cross section $\frac{{d}^{2}\ensuremath{\sigma}}{d\ensuremath{\Omega}\mathrm{dP}}$ has been measured for pions, kaons, and antiprotons produced in 12.5-GeV/c proton-proton collisions. In this experiment we studied the dependence of $\frac{{d}^{2}\ensuremath{\sigma}}{d\ensuremath{\Omega}\mathrm{dP}}$ on the longitudinal and transverse components of the c.m. momenta of the produced particles, ${P}_{l}$ and ${P}_{\ensuremath{\perp}}$, while holding all other variables fixed in the center-of-mass system. The ranges of the components measured were ${P}_{l}=0.0\ensuremath{-}1.0$ GeV/c and ${{P}_{\ensuremath{\perp}}}^{2}=0.1\ensuremath{-}1.5$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. The 12.5-GeV/c extracted proton beam of the Argonne ZGS impinged upon a liquid-hydrogen target. The produced particles were detected by a spectrometer containing two bending magnets and \ifmmode \check{C}\else \v{C}\fi{}erenkov counters and scintillation counters in coincidence. The incident proton flux was determined by monitor scintillators calibrated during gold-foil irradiations. The cross sections for the production of ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${K}^{\ifmmode\pm\else\textpm\fi{}}$ were all found to have an unambiguous Gaussian dependence on ${P}_{\ensuremath{\perp}}$ over the entire range. In the formula $\frac{{d}^{2}\ensuremath{\sigma}}{d\ensuremath{\Omega}\mathrm{dP}}=B\mathrm{exp}(\ensuremath{-}A{{P}_{\ensuremath{\perp}}}^{2})$, we found $A\ensuremath{\approx}3.5$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{\ensuremath{-}2}$ for ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${K}^{\ensuremath{-}}$. However, for ${K}^{+}$ we found $A\ensuremath{\approx}2.7$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{\ensuremath{-}2}$. In studying the dependence of $\frac{{d}^{2}\ensuremath{\sigma}}{d\ensuremath{\Omega}\mathrm{dP}}$ on ${P}_{l}$, we found that the cross section was very strongly peaked about ${P}_{l}\ensuremath{\approx}0.5$ GeV/c, with very few particles produced near ${P}_{l}=0$. This shows that there is no tendency for particles to be produced at rest in the center-of-mass system. (Such production is predicted by the statistical model.) Instead, particles come out in two clouds or "fireballs" following the two departing baryons. These fireballs have a mass of about 2100 MeV.