Experimental study of kaonic nuclei and kaonic atoms at J-PARC

J-PARCにおけるK中間子原子核とK中間子原子の実験的研究

橋本 直   

Hashimoto, Tadashi

An antikaon, the lightest meson with a strange quark, is a unique candidate for forming a nuclear-bound state because the KN interaction is known to be strongly attractive. Such an exotic state with a meson as a constituent particle has been searched for many years, but no conclusive result has been obtained so far. We have investigated the KNN system, the simplest kaonic nucleus consisting of one antikaon and two nucleons, using an in-flight K beam on helium-3 in J-PARC E15. We exclusively analyzed Lambda-p pairs, decay products of KNN, with a requirement of a missing neutron and found a clear peak structure below the KNN binding threshold. Since this peak position is independent of the momentum transfer to the KNN system, we interpreted it as evidence of the kaonic nuclear-bound state. Very recently, we also took a small amount of data with a helium-4 target as J-PARC T77 and found a sub-threshold peak structure in the Lambda-d invariant-mass distribution, which could be signals of KNNN. Another approach is to evaluate the basic KN interaction better. Especially, X-ray spectroscopy of kaonic atoms is essential to have established the strongly attractive interaction. In this direction, we have recently succeeded in dramatically improving the precision of X-ray measurements from kaonic helium atoms in J-PARC E62 using a novel cryogenic detector, TES. In this contribution, we will discuss an overview of above experimental results and future prospects for the systematic study of kaonic systems.