X-ray Spectrocopy of Kaonic Atoms

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Strong interaction at low energy was studied by measuring X-rays of kaonic hydrogen atoms. The experiment was set up at the 12 GeV proton synchrotron of KEK.The importance of this experiment has long been recognized, and there were 3 previous trialsin late 70's and early 80's. However, due to experimental difficulties and due also to the small flux of kaon beam, nobody has succeeded in the observation of the kaonic hydrogen X-rays.In order to overcome difficulties, due to the large background caused by beam particles, and also to the small X-ray yields, we developed novel methods to suppress the X-ray background, and to enhance the signal to noise ratio.Namely, background was suppressedby i) measuring the reaction vertex, and by selecting the events originating from the hydrogen target, and ii) by applying event tagging to select events with 2 charged pions in the final state. We also developed a novel X-ray detector system consisting of 60 Si (Li) crystals, operating in a low-temperature hydrogen gas. The use of gas target, instead of liquid, should help improve signal to background ratio.In the first year, we concentrated on the detector development and construction. The Japanese group worked mostly on the hydrogen target, beam counters, Si (Li) detectors, and on the infrastructure in general. The Canadian team worked on the development and construction of the cylindrical vertex chambers and water Cherenkov detectors, both of which were essential in reducing the background. The completed detectors were transported to KEK,and were assembled there.In 1994, we started to debug the system using the kaon beam, and by the end of 1994 we started to take data. Preliminary analysis exhibited that we have succeeded in observing the kaonic hydrogen X-rays, although we need to collect higher statistics in order to draw final conclusions.