K− pp bound system at J-PARC

The (K) over bar NN bound system, symbolically denoted as "K- pp", is the simplest (K) over bar -nuclear bound system which has been widely discussed as a consequence of the strongly attractive (K) over barN interaction in I = 0 channels. Many theoretical works have pointed out the existence of the "K- pp" bound system, but the calculated properties such as the binding energy and the width spread out due to the uncertainty of the (K) over barN interaction below the (K) over bar + N mass threshold. Experimentally, there are several reports on observation of a "K- pp" candidate with the binding energy of around 100 MeV, however, no definitive evidence was available so far. At J-PARC, we conducted a experimental search for the "K- pp" bound system using K- + He-3 reaction at 1.0 GeV/c where the "K- pp" is expected to be directly produced via the (K-, n) reaction. We finally observed a bound state below the K- p + p mass threshold in the Lambda pn final state, which can be interpreted as the "K- pp" bound state. The possible existence of the "K- pp" state is discussed from both aspects of production and decay.