The peak structure observed in the 𝑲− + 𝟑𝐇𝐞 → 𝚲 + 𝒑 + 𝒏 reaction at J-PARC
implies production of a 𝑲− 𝒑𝒑 bound state
T. Sekihara1)
1): Hadron Nuclear Physics Gr., JAEA

Atomic nuclei are many-body bound systems of nucleons, i.e.,
protons and neutrons, due to the attractive interaction between
them, which we call the nuclear force. The nuclear force takes
place by the exchange of mesons such as pion between nucleons, as Yukawa predicted in 1935. After the discovery of pion in
1947, more than 150 mesons have been discovered and listed inthe Mesons Summary Tables published in the Review ofParticle Physics.
Inside ordinary atomic nuclei, mesons are not explicit degreesof freedom but exist only as “virtual” states which bind
nucleons (Fig.1). However, one may theoretically consider asituation that a meson really exists in an atomic nucleus as anexplicit degrees of freedom and composes an exotic bound state
together with nucleons, which we call “a mesic nucleus”. Studyof mesic nuclei has a potential to extend physics of atomicnuclei, because it strongly reflects the meson-nucleon
interaction in addition to the nuclear force.
We expect that mesic nuclei exist if the interaction between a
constituent meson and a nucleon is attractive enough. Among
mesons, 𝐾 − has been attracting more attention for the last
decade because it was predicted that the interaction between 𝐾 −meson and proton is strongly attractive and hence there exists abound state of a 𝐾 − meson and two protons, often called as “a𝐾 −𝑝𝑝 bound state”. Based on this prediction, there have been
many theoretical studies on the properties of the 𝐾 −𝑝𝑝 boundstate as well as experimental searches for it.
In order to discover this 𝐾 −𝑝𝑝 bound state in experiments, afacility where a large number of strange quarks can be generated
is necessary. For this reason, we performed the E15 experimentat J-PARC (Japan Proton Accelerator Research Complex), in
which we observed the 𝐾 − 3He → Λ𝑝𝑛 reaction by applying
3.4 × 109 𝐾 − particles with momentum 1 GeV/c on the 3He
target [1]. In the J-PARC E15 experiment, we found a peakstructure in the Λ𝑝 invariant-mass spectrum near the 𝐾 −𝑝𝑝
threshold (2.37 GeV) as shown in Fig.2. Since the peak exists
near the 𝐾 −𝑝𝑝 threshold, this could be a signal of a 𝐾 −𝑝𝑝
bound state.
Furthermore, we performed a theoretical calculation of the𝐾 − 3He → Λ𝑝𝑛 reaction to investigate the origin of the peakstructure observed in the J-PARC E15 experiment [2]. We took
into account a scenario that a 𝐾 −𝑝𝑝 bound state is generated anddecays into Λ𝑝, and calculate the Λ𝑝 invariant-mass spectrum ofthe reaction. The numerical result is shown with the red thickline in Fig.2. From the comparison, the signal in the J-PARC
E15 experiment is qualitatively reproduced by the assumptionthat a 𝐾 − 𝑝𝑝 bound state is generated in the reaction. Thisimplies an existence of a 𝐾 −𝑝𝑝 bound state in the J-PARC E15experiment.
We note that the E15 data shown in Fig.2 were accumulated
in May 2013 as the 1st run. The E15 2nd run was completed inDec. 2015, in which we accumulated about 30 times more data
than that in the 1 st run. Right now we are analyzing these E15
2nd run data to conclude more clearly the peak structure of thespectrum and the existence of a 𝐾 −𝑝𝑝 bound state, which is a
new form of atomic nuclei composed of a 𝐾 − meson together
with nucleons.

Fig.1 (Left) Bound state of two protons (p) and two neutrons (n) as
a 4He nucleus. (Right) Bound state of a 𝐾 − and two protons as akaonic nucleus. In both figures, the wavy lines represent interaction
between particles by the exchange of “virtual” mesons.

Fig.2 Spectrum for the Λ𝑝 invariant mass in the 𝐾 − 3He → 𝛬𝑝𝑛reaction. Red thick line represents the theoretical result in theassumption that the 𝐾 − 𝑝𝑝 bound state is generated. Black dots andblue band are experimental data and fit to them, respectively. Notethat the experimental results are scaled so as to compare with thetheoretical one. The vertical dotted line denotes the 𝐾 − 𝑝𝑝 threshold.

References[1] Y. Sada et al., Prog. Theor. Exp. Phys. 2016, 051D01 (2016).
[2] T. Sekihara, E. Oset and A. Ramos, Prog. Theor. Exp. Phys.
2016, 123D03 (2016).