Experimental study of Deeply-bound Pionic Atoms

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The aim of this project is to search for deeply-bound pionic atoms, such as 1s and 2p states of pionic Pb, using a novel nuclear-reaction method. The study of such states using the conventional pionic X-ray spectroscopy is impossible, since pions will be absorbed by the nucleus before they can reach the deeply bound states.We carried out Pb(d,2p) experiments using the SPES IV spectrometer at Saclay (France), Pb(n,d) experiments using the MRS spectrometer at TRIUMF (Canada), and Pb(d,^3He) experiments using the SPES I spectrometer at Saclay. Corresponding theoretical works have been done also.1. (d,2p) reactionThe experiment reached the sensitivity with which we should be able to find the pionic-atom peak, if the formation cross section was as large as was theoretically predicted. However, no peak was observed.2. (n,d) and (d,^3He) theoryThe reason for the non-observation of peaks in the (d,2p) experiment was later understood through detailed theoretical works, and (n,d) or(d,^3He) reactions were found to be more suited for the production of deeply-bound pionic atoms.3. (d,^3He) reactionData were taken at 3ﾟ in the laboratory frame. Theory guides us to go to smaller angles to increase the cross section, but due to experimental difficulties, we were unable to go to much smaller angles. No peak was identified in the spectrum, but as a by product, the pion production cross section for p(d,^3He)pi^0 was measured for the fist time.4. (n,d) reactionNear the pion production threshold, a structure, whose energy and cross section were consistent with theoretical prediction, was seen. However, the statistical significance was too small to draw strong conclusions, and the use of neutron beam makes it difficult to much improve the statistical accuracy.