RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-4. Hadron Physics

Results of the K − pp bound-state search in J-PARC E151stT. Hashimoto,∗1 K. Itahashi,∗1 M. Iwasaki,∗1 Y. Ma,∗1 H. Ohnishi,∗1 S. Okada,∗1 H. Outa,∗1 F. Sakuma,∗1M. Sato,∗1 M. Tokuda,∗1 and Q. Zhang,∗1 for the J-PARC E15 collaboration∗1

structed from a π − p pair detected using the CDS to
be almost free from the background. Missing-neutron
events were identified by the 3 He(K − , Λp)X missingmass, with ∼ 20% Σ0 p-event contamination (Σ0 →Λγ). The preliminary Λp invariant-mass spectrum, obtained with ∼ 10 MeV/c2 resolution, consists of a flatcomponent widely distributed in the three-body phasespace and an enhancement around the threshold. Thethreshold structure mainly originates from events witha low-momentum transfer, and it might be evidence
for the K − pp bound state.
In 2015, we plan to conduct the 2nd-stage physics
run with 10 times the number of beam-kaons in the 1ststage, as well as calibration runs with hydrogen anddeuteron targets. With the new data set, we will further investigate the structure around the K − pp binding threshold.
Binding Energy [GeV]
0×102302520100
2.25 2.3 2.35 2.4 2.45 2.5

15

M(Λ(1405)+p)

M(Σ +N+π)M(Λ+N+π)40

M(Σ +N)

60

M(Λ+p)

80

105

2002

Counts / 10 MeV/c2

120

0.1

M(K+p+p)

140

0.2

0
K s -tagged

M(K+p+p)160

Arbitrary unit

0.3

d2σ/dΩ/dM × ACDS (μb/sr/(MeV/c2))

The interaction between a nucleon and an anti-kaon(K̄), the lightest hadron with a strange quark, is one of
the keys to understanding meson-baryon interactions
in low energy quantum chromodynamics (QCD) incorporating three flavors in the nuclear system. Precise
measurements of elementary K̄N interactions and investigations of K̄-nuclear bound systems (K̄ nuclei)
are currently hot topics. The K − pp state, a boundsystem of a negative kaon and two protons, is theoretically considered the simplest K̄ nucleus,1) and thus
it has attracted special interest. We are searching for
the K − pp bound state by using an in-flight reaction as
the J-PARC E15 experiment.2) The first physics dataacquisition (E151st ) was performed in May 2013, with
5 × 109 kaons at 1 GeV/c on a liquid 3 He target.
Figure 1 shows the obtained semi-inclusive neutronspectrum at θnlab = 0◦ .3) Here, we detected the neutron
by using a plastic scintillator array ∼ 15 m away fromthe target. To reconstruct the reaction vertex for thetime-of-flight measurement, at least one charged track
is required in a cylindrical detector system (CDS) surrounding the target. The K 0 -tagged spectrum, shownin the inset of Fig. 1, is attributed to the chargeexchange K − p → Ks0 n reaction, and demonstrates
that the detector resolution and the missing-mass scale
are well understood.
The observed yield in the deeply bound region, corresponding to K − pp binding energies larger than 80
MeV, was in good agreement with the evaluated backgrounds originating from 1) accidental hits and neutral
particles other than neutrons, 2) reactions on the target cell, and 3) neutrons produced via charged Σ decay.
In this mass region, upper limits on the production
cross section of a bound state were evaluated, assuming
a K − pp → Λp isotropic decay. They were determined
to be 30-270 µb/sr, depending on the mass and the decay width. The upper limits obtained are much smaller
than the theoretical calculation.4) The ratios of the upper limits to cross sections of the quasi-elastic channels
are (0.5–5)% (quasi-elastic) and (0.3–3)% (charge exchange). These ratios are rather small compared tothe sticking probability of the usual hypernucleus formation. On the other hand, a significant excess wasobserved around the K − pp binding threshold. However, in spite of the observed large yield corresponding
to ∼ 1 mb/sr, structures suggested in theoretical spectral functions4,5) cannot be identified from only thesemi-inclusive measurement.
We also analyzed the Λp events detected using theCDS to investigate the K − pp decay. A Λ was recon-

2.1
3

2.2-

2.3

2.4

2.5

2.6

0
2.7

2He(K ,n)X missing mass (GeV/c )Fig. 1. 3 He(K − , n)X semi-inclusive missing-mass distribution at θnlab = 0◦ 3) . ACDS denotes the charged-taggingacceptance of the CDS. The inset shows the Ks0 -tagged
spectrum.

References
1) T. Yamazaki and Y. Akaishi: Phys. Lett. B 535 (2002)
70.
2) M. Iwasaki et al.: J-PARC proposal P15, “A searchfor deeply-bound kaonic nuclear states by in-flight3
He(K − , n) reaction” (2005).
3) T. Hashimoto et al.: arXiv:1408.5637 (2014).
4) T. Koike and T. Harada: Phys. Rev. C 80 (2009)055208.
5) J. Yamagata-Sekihara, D. Jido, H. Nagahiro, and S.
Hirenzaki: Phys. Rev. C 80 (2009) 045204.

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