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

The differential production cross section $\frac{d^2\sigma }{d\Omega \mathrm{dP}}$ 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 $\frac{d^2\sigma }{d\Omega \mathrm{dP}}$ on the longitudinal and transverse components of the c.m. momenta of the produced particles, $P_l$ and $P_{\perp }$, while holding all other variables fixed in the center-of-mass system. The ranges of the components measured were $P_l=0.0-1.0\mathrm{}$ GeV/c and ${P_{\perp }}^2=0.1-1.5\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. 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 Čerenkov 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 ${\pi }^{\pm }$ and $K^{\pm }$ were all found to have an unambiguous Gaussian dependence on $P_{\perp }$ over the entire range. In the formula $\frac{d^2\sigma }{d\Omega \mathrm{dP}}=B\exp (-A{P_{\perp }}^2)$, we found $A\approx 3.5$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{-2\mathrm{}}$ for ${\pi }^{\pm }$ and $K^{-}$. However, for $K^{+}$ we found $A\approx 2.7$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{-2\mathrm{}}$. In studying the dependence of $\frac{d^2\sigma }{d\Omega \mathrm{dP}}$ on $P_l$, we found that the cross section was very strongly peaked about $P_l\approx 0.5$ GeV/c, with very few particles produced near $P_l=0\mathrm{}$. 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.