Study of S=-2 Nuclei with Emulsion=Scintillating-Fiber Hybrid Method (KEK-PS E373)

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KEK-PS E373 is an experiment to explore the world of double strangeness.

In 1977, R.Jaffe theoretically predicted that the H-dibaryon which is a six quark(uuddss) state in a bug may exist stably against the strong interaction.
As a six-quark(uuddss) state, two-baryon systems such as Lambda+Lambda, Xi-+p, Xi0+n are known to exist. R.Jaffe pointed out that the mass of the H-dibaryon may be smaller than those of these two-baryon states.
Although so many experiments have been carried out to search for the H-dibaryon until now, the exisitence is not yet confirmed.
On the other hand, the experiment KEK-PS E176 was carried out in 1980's, and refound a Lambda-Lambda hypernucleus which is stable against the strong interaction after an absence of a quarter of a century. As for the species of the nucleus, there remained two kinds of interpritation. The right figure shows the two weak-decay of the Lambda-Lambda hypernucleus in the emulsion of E176. The lower limit for the mass of the H-dibaryon was determined from the measured mass of the Lambda-Lambda hypernucleus. But whether Lambda-Lambda interacion is attractive or repulsive is not yet known due to the two kinds of interpritaion.
KEK-PS E373 searches S=-2 nuclei in nuclear emulsion to study them with ten times higher statics than the past experiments. It is expected to get the definitive information about the existence of the H-dibaryon, the Lambda-Lambda interaction and the Xi-N interaction.

The 1.67GeV/c K- beam was focused on a diamond target. The Xi- hyperon produced via the 'p'(K-,K+)Xi- quasi-free reaction in the target might be slowed and brought to rest in a nuclear emulsion stack placed downstream of the target. We are searching S=-2 nuclei produced from Xi- capture points. Identification of the (K-,K+) reaction is performed by a spectrometer system. Among the (K-,K+) reaction events, "good" events are selected by the image of scintillating fiber block detectors (SciFi-block) installed both upstream and downstream of the emulsion. In order to measure the position and the angle of the Xi- track, a high resolution tracking detector using coherent bundles of scintillating micro fibers(Fiber-Bundle) had been developed. It was installed between the target and the emulsion stack. Fast protons and pi- from the decay of hypernuclei can be detected with SciFi-block. This method of using nuclear emulsion with different detectors is called Hybrid-Emulsion method.

From '98 to '00, the experiment was carried out with 69l emulsion. It is expected that tens of double hypernuclei are found and that we can get the final answer about Lambda-Lambda interaction.

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(ichikawa@nh.scphys.kyoto-u.ac.jp)