Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Search for K̄N N bound state at J-PARC
F. Sakuma,∗1 T. Hashimoto,∗1 K. Itahashi,∗1 M. Iwasaki,∗1 Y. Ma,∗1 H. Ohnishi,∗1 S. Okada,∗2 H. Outa,∗1∗3forthetheJ-PARC
J-PARC E15
E15 Collaboration
Collaboration
M. Sato,Sato∗3for

∗1
∗2
∗3

RIKEN Nishina Center
AMO Physics Laboratory, RIKEN
Condensed Molecular Materials Laboratory, RIKEN- 119 -M(Λ(1405)+p)
M(K+p+p)M(Σ+p+π)Count per 20 MeV/c 220

Λp invariant mass(b)data16

Multi-NA sum
3NA (Λpn)
3NA (Σ0pn)

122NA (Λpns)
(x 5)

84

Count per 0.05

5CM0cosθnThe possible existence of strongly bound K̄ nuclear
states has been widely discussed as a consequence ofthe strongly attractive K̄N interaction in the I = 0channels.1) The investigation of those exotic states will
provide unique information of the K̄N interaction below the threshold, which is still not fully understood.
Among the K̄ nuclear-states, the simplest K̄-nuclear
cluster K̄N N , is of special interest because it is thelightest S = −1 K̄ nucleus. Many theoretical calculations based on the K̄N N − πΣN − πΛN coupled channels have predicted the existence of the bound state.2)
However, the calculated properties of the bound state,such as the binding energy (B.E.) and decay width (Γ),
strongly depend on K̄N interaction models.
Experimentally, there are several reports on the observation of peak structure with a B.E. of ∼ 100
MeV.3–5) In contrast, null results were also reported
by several experiments,6,7) and therefore the existenceof the K̄N N bound state has remained controversial.
The J-PARC E15 experiment was performed at theK1.8BR beam line using the K − + 3 He reaction at pK −= 1.0 GeV/c.8) The ﬁrst physics run of the E15 experiment was conducted in May 2013 with 5.3 × 109
kaons incident on 3 He. The results of a semi-inclusive
3He(K − , n)X measurement can be found in Ref9) . To
measure an expected decay mode of K̄N N → Λp, anexclusive 3 He(K − , Λp)n measurement was also performed by identifying two protons and one negative
pion in a cylindrical detector system (CDS) surrounding a liquid 3 He target system.10) The pπ − pair associated with a speciﬁc Λ decay in the ppπ − event
was reconstructed with a log-likelihood method, anda missing neutron was identiﬁed kinematically.
The distribution of the Λp invariant mass and theneutron emission angle of the obtained Λpn ﬁnal state
are shown in Fig. 1 (b) and (c), respectively, and a scatter plot of the two is given in Fig. 1 (a). A bump structure is observed at the mass threshold of K − + p + pin the Λp invariant mass spectrum, which is not reproduced by a global ﬁt based on multi-nucleon absorption
processes of K − . To explain the excess, a simple Swave Breit-Wigner structure over the three-body phasespace of the K − 3 He → Λpn reaction is assumed. A χ2
test was performed between the experimental data andthe simulated pole together with the multi-nucleon absorption processes, and the minimum χ2 point was ob2
tained with MX = 2355+6−8 (stat.) ± 12(syst.) MeV/c ,+192
ΓX = 110−17 (stat.) ± 27(syst.) MeV/c .

-1.0-0.8 (a)-0.6 CDS acceptance-0.4-0.2010152025

neutron emisson angle(c)2NA (Λpns)(x 5)

3NA (Σ0pn)

3NA (Λpn)

0.2
0.4
0.6
0.8
1.0

data
Multi-NA sum

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
Minv. Λp [GeV/c 2]

Fig. 1. (a) Distribution of the Λ invariant mass and theemission angle of the missing neutron, (b) Λ invariant
mass with simulated spectra obtained by the global ﬁt,and (c) angular distribution of the missing neutron.10)

A naive interpretation of the structure would be theK̄N N bound state, since the pole position is locatedbelow the mass threshold of K − +p+p (2370 MeV/c2 ).
It could also be a shallow bound or unbound resonanceof the Λ(1405)p system followed by Λp conversion, thethreshold of which is located at 2343 MeV/c2 if themass of Λ(1405) is assumed to be 1405 MeV/c2 . To
explore whether or not the observed structure is theK̄N N bound state in more detail, the second run was
carried out in 2015 with the accumulation of 30 times
more data on the Λpn ﬁnal state and the analysis is inprogress.
References
1) Y. Akaishi and T. Yamazaki, Phys. Rev. C 65, 044005(2002).
2) A. Dote, T. Inoue, and T. Myo, Prog. Theor. Exp. Phys.
2015 043D02 (2015).
3) M. Agnello et al., Phys. Rev. Lett. 94, 212303 (2005).
4) T. Yamazaki et al., Phys. Rev. Lett. 104, 132502 (2010).
5) Y. Ichikawa et al., Prog. Theor. Exp. Phys. 2015
021D01 (2015).
6) G. Agakishiev et al., Phys. Lett. B 742, 242 (2015).
7) A. O. Tokiyasu et al., Phys. Lett. B 728, 616 (2014).
8) K. Agari et al., Prog. Theor. Exp. Phys. 2012 02B011(2012).
9) T. Hashimoto et al., Prog. Theor. Exp. Phys. 2015
061D01 (2015).
10) Y. Sada et al., Prog. Theor. Exp. Phys. 2016 051D01(2016).