Behavior of Pionic Hydrogen Atoms in Gas Phase

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Molecular and atomic capture process of negative pions has not yet been completely understood, especially in hydrogen-containing molecules. The transfer of pions captured by hydrogen to other atoms plays an important role in the overall process, although it complicates the process and makes the mechanism hard to understand. The pion transfer will be considerably affected by the relation between the chemical state of the Z-H bond and the isolation of a pionic hydrogen atom from the bond, because the transfer is considered to occur only from an excited pionic hydrogen atom.Recently we selectively measured the pion capture probability on a particular hydrogen atom in the molecule using deuterated compounds for alcohols and carboxylic acids [Phys.Rev.Lett., 76,2460 ('96)], and proposed a combined model which involves a large mesomolecular (LMM) model and a external transfer process [Phys.Rev.A,53,130 ('96)].Here we carried out the gas phase experiments to investigate the behavior (the capture, formation, isolation, and decay or external transfer processes) of pionic hydrogen atoms without the effects of the inter-molecular interactions. The pionic X rays and pi^0 decay gamma rays from gas samples were measured with a newly designed high pressure gas chamber made of beryllium. The samples were mixtures of CH_4/C_2H_4/C_2H_6/H_2 and rare gas/SF_6. The measurements are now in progress at the pimu-channel of the 12-GeV proton synchrotron in the National Laboratory for High Energy Physics.