Search for kaonic nuclear states: cold compressed matter?

par Ken Suzuki (Vienne)

vendredi 26 juin 2009, 14:30 → 15:30 Europe/Paris

Salle Master, batiment 20 (IPHC/DRS)

Recently a possible existence of exotic nuclear systems involving antikaon, $\bar{K}$ ($K^-$ or $\bar{K0}$), as a constituent has been widely discussed. Such kaonic system is first mentioned by Wycech in 1986, and the present stream of activity arises from the calculation by Akaishi and Yamazaki in late 90s. They were stimulated by the new KpX experiment at KEK which solved so-called "kaonic hydrogen puzzle", constructed phenomenologically a $\bar{K}$N interaction whose I=0 channel turned out to be strongly attractive. Using this interaction they predicted that antikaon plays glue-like role to bind nucleons ($K^-pp$, $K^-ppp$, ..) and $K^-ppn$ system might have a binding energy deep enough (~100 MeV) so that the $\Sigma\pi$ decay channel is prohibited and its width is suppressed to the order of 20 MeV. An existence of such kaonic states has been examined by many experiments, some are dedicated experiments, some are reanalysis of existing data, and indeed some indications have been obtained. However they lack for either enough statistical significance or/and unique interpretation, and moreover the indicated masses do not agree each other. The situation slightly resembles the one of the Pentaquark. Theoretical understanding is also similar. Obtained binding energies and widths vary. As precision calculation is required in the SU(3) sector of low energy QCD, it is realized again that the present database to constrain the ${\bar{K}$N interaction is not good enough. However all major calculations predict that such bound state is at least shallowly formed. It is also predicted that such antikaonic nuclei do not have density saturation as in the case of normal nucleus. The density increases towards the center of the system where the antikaon locates beyond the normal nuclear density. This would agree on the observation of "shrinkage" effect in Hypernuclei and would be understood that the antikaon has a different quark constituent (s ubar / sdbar) as nucleon's one (u d) and therefore is not constraint by the the Pauli principle of u d quark. In this point of view the kaonic nuclear state gives us an experimental access to a cold compressed baryonic matter of well defined quantum state. In the talk, our experimental program to aim at the first conclusive measurement of the such state which will take at GSI (Germany) employing the FOPI apparatus, together with overview of the updated experimental and theoretical works, will be discussed.