Jun 2 – 7, 2019
Carnegie Mellon University
America/New_York timezone

## $K^-pp$ Bound System at J-PARC

Jun 6, 2019, 9:45 AM
30m
McConomy Auditorium (CMU Cohon University Center)

### McConomy Auditorium

#### CMU Cohon University Center

Invited Few Body Systems

### Speaker

Dr Fuminori Sakuma (RIKEN)

### Description

The possible existence of deeply-bound $\bar K$-nuclear bound states
has been widely discussed as a consequence of the strongly attractive
$\bar K N$ interaction in I = 0 channels.
The investigation of those exotic states will provide unique information
of the $\bar K N$ interaction below the threshold, which is still not
fully understood up to date.
Furthermore, the grate interest of those exotic states is that they
might form high-density nuclear matter where the chiral symmetry is
expected to be restored.

Among the $\bar K$-nuclear bound states, the $\bar K NN$ system with $I = 1/2$ and $J^{P} = 0^-$ (symbolically denoted as $K^-pp$'') is of special interest because it is the lightest $S = -1$ $\bar K$ nucleus. Many theoretical works agree on the existence of the bound state, however, the binding energies and the decay widths are scattered. Experimentally, there are several reports on observation of a$K^-pp$'' candidate with the binding energy of around 100 MeV.
On the other hand, several groups concluded that the reactions
can be understood without the inclusion of a bound state.

To clarify whether or not the $K^-pp$'' bound state exists, we performed an exclusive measurement of the $^3$He$(K^-, \Lambda p)n$ reaction at 1.0 GeV/$c$ at J-PARC (J-PARC E15 xperiment). In the experiment, we observed a distinct peak in the $\Lambda p$ invariant-mass spectrum well below the mass threshold of $K^- + p + p$, of which the simplest and natural interpretation is a kaonic-nuclear bound-state$K^-pp$''.
The obtained binding energy and the width are $47 \pm 3 ({\rm stat.}) ^{+3}_{-6}({\rm syst.})$ MeV/$c^2$ and $115 \pm 7 ({\rm stat.}) ^{+10}_{-20}({\rm syst.})$MeV/$c^2$, respectively.
This result is experimentally solid as against the previously reported
results.
In addition, we observed $\Lambda(1405)pn$ final state in $K^-$ $+$
$^3$He reactions by reconstructing $\pi^{\mp}\Sigma^{\pm}pn$ events,
which is of special importance to understand the production mechanism of
the $K^-pp$'' state such as theoretically predicted $\Lambda(1405) N \to \bar KNN$ doorway process.

We will discuss the possible existence of the $K^-pp$'' state from both aspects of production and decay:$K^-pp$'' and $\Lambda(1405)p$
production, and $\Lambda p$ non-mesonic and $(\pi \Sigma)^{0} p$ mesonic
decay, respectively.