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

The differential production cross section d²σdΩdP 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 d²σdΩdP on the longitudinal and transverse components of the c.m. momenta of the produced particles, Pₗ and P_⊥, while holding all other variables fixed in the center-of-mass system. The ranges of the components measured were Pₗ=0.0-1.0 GeV/c and P_⊥²=0.1-1.5 (GeV/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 {C} {C}{}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 π^± and K^± were all found to have an unambiguous Gaussian dependence on P_⊥ over the entire range. In the formula d²σdΩdP=Bexp(-AP_⊥²), we found A≈3.5 (GeV/c)^-2 for π^± and K^-. However, for K⁺ we found A≈2.7 (GeV/c)^-2. In studying the dependence of d²σdΩdP on Pₗ, we found that the cross section was very strongly peaked about Pₗ≈0.5 GeV/c, with very few particles produced near Pₗ=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.