Transfer mechanisms for pionic hydrogen in organic liquids

Abstract

The transfer of negative pions in mixtures of bromodecane and carbon tetrachloride has been investigated by combined measurements of /r 0, s produced from nuclear capture on hydrogen and pionic x rays following Coulomb capture on bromine and chlorine. A significant fraction of pion transfer occurs via an external-transfer process. The results represent the first direct observation of transfer in condensed matter. PACS numbers: 36. lO.GvIn recent years a considerable quantity of data has been acquired on the capture of negative pions in molecular systems. Despite this, a number of important questions regarding the molecular interactions of pions remain unanswered. Chief among these are the distribution of pions within the molecule immediately following molecular capture, and the nature and significance of subsequent processes involving transfer of pions from hydrogen to atoms of higher nuclear charge. The study of transfer processes involving pionic hydrogen is useful in understanding similar processes for muonic atoms. This is especially significant because pion transfer occurs only from excited states of the exotic atom, the analogous process of muon transfer from excited states being critical in muon-catalyzed fusion.Experiments which measure charge-exchange (K~,TT°) probabilities in simple systems such as mixtures of hydrogen with noble or other gases suggest that a significant proportion of pions which are initially captured on hydrogen are subsequently transferred to higher-Z atoms before nuclear capture occurs. 1,2 Such a transfer is believed to take place by a process in which the small neutral pn ~ atom first breaks away from the hydrogen molecule (in which "atomic" capture initially occurs) and then transfers the pion to a higher-Z atom in a subsequent collision, 3,4 i.e., pn~+ Z-• Zn~+p. The large increase in binding energy once the n~ is transferred to a higher-Z atom ensures that the transfer is essentially one-way. This process is called an "external-transfer" process if the pion is transferred outside of the molecule onto which it was originally captured.In more complex molecular systems (e.g., organic compounds) more involved mechanisms come into play. Whether the pion undergoes atomic capture on hydrogen instead of on other atoms in the molecule depends on the local electronic wave functions, but only the broad principles are understood. The details of the subsequent pion transfer process are even less well understood and there is dispute about the nature of the transfer mechanism itself. It has been suggested 5,6 that an alternative pion transfer mechanism is important in these systems. In