Capture of NegativeKParticles by Bound and Free Protons in Emulsion

Abstract

Data are presented from the captures of negative $K$ particles by bound and free protons in nuclear emulsion. Only those captures in which a charged $\pi$ meson and one additional charged particle are emitted are included in the study. A model is presented for $K^{-}$-particle capture on bound protons in which the external energies of the emitted $\Sigma$ hyperons and $\pi$ mesons are modified from the values for captures on free protons by the internal proton momenta and the Coulomb and nuclear potentials. The model is used to explain the observed ${\Sigma }^{+}$- and ${\Sigma }^{-}$-hyperon and ${\pi }^{-}$- and ${\pi }^{+}$-meson energy distributions. A Coulomb potential of 10±3 Mev is estimated from the relative positions of the high-energy ends of the ${\Sigma }^{+}$- and ${\Sigma }^{-}$-hyperon energy distributions. This value suggests that most of these captures were on the heavy elements of the emulsion. This potential reduces the ratio of ${\Sigma }^{-}$ to ${\Sigma }^{+}$ hyperons, which escape the nucleus, from the value of 2 measured on free protons to the value 0.83±0.25 for protons bound in emulsion nuclei. The sum of the binding energies of the last proton in the capture nucleus and the excitation energy of the residual nucleus has a distribution which is peaked at about 20 Mev. Conservation of energy and charge are applied to the identification of the $\Sigma$ hyperons that end without making a visible star. The prong distribution for stars made by ${\Sigma }^{-}$-hyperon captures in emulsion nuclei can be interpreted as a composite of two distributions: one, a line spectrum of zero-prong events, when the ${\Lambda }^0$ or ${\Sigma }^0$ hyperon and neutron escape; the other, a spectrum of many-prong events, when the ${\Lambda }^0$ or ${\Sigma }^0$ hyperon or neutron (or both) are absorbed. The number of ${\Sigma }^{-}$ hyperons that were captured to give stars of zero or one prong is 0.65±0.10. Seven ${\Sigma }^{+}$ hyperons that decayed into protons at rest are used to find a ${\Sigma }^{+}$-hyperon mass of 2329.5±1.0 $m_e$. Two examples of $K^{-}$ captures on free protons in the emulsion give ${\Sigma }^{-}$ hyperon masses of 2347.4±3.5 $m_e$ and 2341.8±1.5 $m_e$.