Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Advanced Meson Science Laboratory – Hadron part –M. Iwasaki, ∗1,∗2,∗3 H. Asano, ∗2 T. Hashimoto, ∗4,∗2 M. Iio, ∗5,∗2 K. Itahashi, ∗2 Y. Ma, ∗2 Y. Matsuda, ∗6,∗2T. Nishi, ∗2 H. Ohnishi, ∗7,∗2 H. Outa, ∗2 S. Okada, ∗2 F. Sakuma, ∗2 Y. Sada, ∗8,∗2 M. Sato, ∗5,∗2 andK. Tsukada ∗7,∗2The Advanced Meson Science Laboratory waslaunched in 2002 based on a proposal to pursue hadron
physics research using the meson as a probe into nuclei to study the strong interaction. At the same time,
we are requested to commit to the operation of theRIKEN-RAL branch and promotion of muon science
research. The branch is in the Rutherford Appleton
Laboratory (RAL) and operated under the UK-Japan
scientiﬁc agreement on muon science. Thus, our Laboratory is composed of two subgroups to conduct hadron
physics research using mesons and to conduct muonscience studies covering fundamental particle and nuclear physics to condensed matters physics. Because
the research activity in muon science is separately summarized by P. King et al. in this volume, let us describe
hadron physics research in this article.
By embedding mesons into nuclei and studying theproperty change of those particles in nuclear media,
one can study the physics at densities beyond standard
nuclear density, e.g., in neutron star matter, as well as
the origin of matter (hadron) mass, in which the standard scenario is that the hadron masses are generated
by the spontaneous chiral symmetry breaking of thevacuum. For these studies, we have been conducting
experiments at the world’s leading accelerator facilities
such as Japan Proton Accelerator Research Complex(J-PARC) for K-mesons having s-quarks, and RI Beam
Factory (RIBF) for π-mesons consisting of the lightest
u- and d-quarks together with their anti-quarks.
Deeply bound pionic atoms are quite interesting systems, in which the π is captured by the nucleus owing
to Coulomb interaction and its wave function overlaps
with that of the nucleus at the surface. Before ourbreakthrough experiment at GSI,1,2) it was believedthat the pionic atom cannot be realized if there exist
substantial wave-function overlaps between the pionand nucleus. This misunderstanding was strengthenedby the fact that the pionic-atom formation by the pion
atomic capture at rest is limited up to medium-heavy
nuclei, since no observation was realized in heavy nuclei previously.
In our GSI experiment, direct pion formation inside
nuclei by feeding it into the atomic state was realized
for the ﬁrst time by introducing the (d, 3 He) reaction
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8

to lead target at the recoilless condition. In this neutron pickup reaction, d + “n” → π − + 3 He, the pion
can be produced almost at rest for a speciﬁc deuteron
momentum (momentum transfer to the pion qπ ∼ 0);
therefore, it is likely to be captured, and the eﬃcient
atomic-state formation is realized.
At RIBF, we extended our research to conduct highprecision isotope-dependence studies on several Sn isotopes to acquire more quantitative information on theatomic shifts systematically. To achieve high precision,
we introduced dispersion-matching optics to the BigRIPS spectrometer for the ﬁrst time by taking advantage of the high-intensity beam available at RIBF. We
succeed in demonstrating the outstanding performanceof the dispersion-matching optics at the BigRIPS spectrometer. In fact, in the on-line monitor level, the formation of ground state of the pionic atom, together
with its several exited states, has already been identiﬁed clearly.
To our surprise, a theoretically unexpected excitedstate formation was also identiﬁed clearly, with the formation yield exhibiting an angular dependence. Thisangular dependence helps us to identify the quantum
number of those excited states without ambiguity. Our
analysis shows that the deduced isospin dependence ofthe energy shifts of the pionic-atom ground states is
consistent with the standard scenario of the hadronmass-generation mechanism. We are working on publishing this result.
In the case of the π-meson, the available density
range to be studied is limited to the nuclear surface
density, which is well below the normal nuclear-matter
density. This is because of the repulsive nature of theπN strong interaction. Pion wave functions are pushed
away from nuclei, and form a hybrid state in between
the atomic and nuclear bound state. Thus, it is quite
interesting to know if it is possible to form a pure nuclear meson bound state deep inside nuclei.
In this context, a very interesting meson is theη  (958)-meson. It is known that its mass is very difﬁcult to explain in a simple quark picture, and themass deviation of this speciﬁc meson is explained by
the UA (1)-anomaly. We have just started a new seriesof experiments to study a bound system of an η  in anucleus. The ﬁrst experiment was conducted in GSIby measuring excitation spectra of carbon nuclei in12C(p, d) reactions near the η  -emission threshold. We
achieved a very good resolution of ∼2.5 MeV (σ) anda good statistical sensitivity at a level better than 1%.
The measured excitation spectra agree well with theoretical predictions but do not show distinct structures

Chief Scientist of Advanced Meson Science Laboratory(2002–present)
RIKEN Nishina CenterTokyo Institute of TechnologyJAEA
KEK
University of Tokyo
Tohoku University
Osaka University

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Research Activities 1991–2016

due to formation of the η  -nucleus bound systems. The
result is interpreted in terms of an interaction between
an η  and a nucleus, setting stringent constraints inthe optical potential for the ﬁrst time.3) Based on theresults, a second experiment is being prepared in FAIRat GSI.
Another very interesting meson is the anti-kaon(K) in the second-lightest K-meson group having thestrange (s)-quark as a constituent quark, namely K −0and K . The KN interaction was rather poorly
known, in contrast to the πN interaction. The interaction seems to be strongly attractive based on thescattering data; therefore, it can form a nuclear boundstate, which requires the ground state of the kaonichydrogen atom to have a strong upward shift due tothe pole below the K N threshold. On the other hand,previous kaonic hydrogen atom data seem to be attractive only very weakly, which is entirely inconsistent
with scattering data. We played a very important role
again to solve this puzzling situation.
At KEK, we conducted an x-ray measurement of
the kaonic hydrogen atom to solve the KN interaction
puzzle. We utilized a gaseous hydrogen target for theﬁrst time to avoid the Stark eﬀect occurring in highdensity targets, which reduces the x-ray yield substantially. To compensate for the small number of available
kaons at rest in the gas, we placed multi-channel x-ray
detectors directly exposed to the hydrogen gas locatedinside the target cell. In the experiment, we also identiﬁed two charged pions as decay products of the kaonichydrogen atom. Using these two charged pions, we selected the ﬁducial volume for the target and identiﬁedbackground-free events by choosing the ﬁnal states, inwhich no γ-ray can be generated because of the severe
x-ray background source. Our work on kaonic hydrogen atom spectroscopy resulted in very clear spectra
for the ﬁrst time, and the strongly attractive interaction between a kaon and nucleon in the I = 0 channel
is conﬁrmed,4,5) together with the scattering data.
This conﬁrmation of strong attraction of the KN interaction in the I = 0 channel opens up another very
curious question. The attraction is so strong that it
is more natural to form a bound state between a kaonand proton. In fact, there is a well-known resonancecalled Λ(1405), the mass of which is located just below
the mass threshold of the kaon and proton, M (K − p);the resonance is assumed to be an exited state of amember of the Λ hyperon, i.e., it is an excited udsquark baryon system. Thus, it is very natural to ask
whether Λ(1405) can be interpreted as a bound stateof a kaon and proton due to the strong interaction, i.e.,Λ(1405) = “K − p”. If this is true, then the kaon canform a variety of nuclear bound states together withvarious nuclear systems. The strong KN attractive interaction might help form a high-density nuclear object
beyond the standard nuclear density spontaneously. It
might also help the study of the in-medium propertychange of mesons in nuclei.
Therefore, a variety of experimental studies havebeen conducted by a number of experimental groups
to identify the simplest kaonic nuclear bound state,“K − pp.” The detection of the kaonic nuclear state formation is diﬃcult from the kaon absorption at rest because the kaon mainly reacts with one of the nucleonsin the mesonic channel and produces a hyperon (Λ orY ) as K − N → π Λ, without forming a kaonic nuclearstate. It can also be absorbed by two nucleons simultaneously as K − N N → Λ N , and this process produces
huge backgrounds. The direct kaon production channel is also attempted via the p p → K + + “K − pp”
reaction. However, this channel has large ambiguity
due to the presence of N ∗ (1410) resonance, which candecay strongly to K + Λ. Obviously, no K − (nor K)
is generated in this reaction channel, and the channel
is energetically easier to be produced compared to theK + K − -pair production. One can easily be misled by
the reaction chain of p p → N ∗ (1410) + p → (K + Λ) + p
to be a “K − pp” formation signal, if one believes theΛ p in the ﬁnal state (wrong pair) is the decay productof “K − pp.” There are also other experimental studies to search for the kaonic bound state, but those arelimited by either null results or insuﬃcient statistics.
Therefore, there is no convincing and conclusive experimental evidence of the existence of the kaonic nuclear
bound state.
We employed an entirely diﬀerent approach at JPARC K1.8BR beam-line in our experiment E15. We
bombarded a K − beam on a 3 He target to knockout a√neutron from the target nucleus at 1 GeV/c ( sKN ∼2−3−
1.8 GeV/c ), i.e., K + He → K ps ps + n (ps denotes spectator proton). The cross section of this reaction is rather high, because of the presence of the Y ∗resonance near 1.8 GeV/c2 , which decays strongly toKN . There are several key advantages in this reaction
channel to search for the kaonic bound state. First, therecoil kaon momentum (or momentum transfer), qK , isas small as ∼200 MeV/c (∼ pF ) in this reaction; therefore, one can expect very eﬃcient nuclear formation as
K − ps ps → “K − pp. Another advantage is that thepresence and the commitment of K − in this channel
is secured from the beginning. Still another advantage is that the two- (or multi-) nucleon absorption
reaction can be expected to have a small cross section.
Finally, we can cover the target region with a cylindrical detector system (CDS) to identify the ﬁnal stateof “K − pp” with sensitivity to the decay process. We
also placed large-volume neutron counter arrays in theforward direction 15 m away from the target system
to identify neutrons in the production channel with ahigh missing-mass resolution of about 10 MeV/c2 .
The pilot run of J-PARC E15 (E151st ) showed quite
remarkable results. The semi-inclusive forward neutron spectrum shows a large yield below the mass
threshold of M (K − pp) as a long tail from the quasielastic kaon scattering, implying the existence of strong

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RIKEN Accel. Prog. Rep. 50 (2017)

peak centroid is located around ∼40 MeV, which ismuch deeper than that of the normal nuclear system
about 10 MeV.
The existence of the structure above the thresholdprovides further conﬁrmation that the structure below
the threshold is actually the nuclear bound state of“K − pp,” in which the constituent particles do not lose
their identity in the system. Generally, the peaks in a
mass spectrum are isolated in the case of baryonic resonance. In contrast, nuclear-state formations are always
associated with the so-called quasi-free (QF) processes
in the unbound energy region, which indicate that theconstituent particles can be dissolved. In this case, thestructure above the threshold can be interpreted as theinitial kaon backscattered at an energy above the kaon
mass in the QE channel, followed by internal conversion (IC) with two spectator protons resulting in Λp in
the ﬁnal state. Thus, this successive reaction can betreated as a QF process of the 3 He(K − , Λp)n reaction
channel.
To ﬁnalize the present study, we are analyzing theangular distribution of the particles in the ﬁnal state,
to study the form factor, spin, and parity of the observed state, and to prepare an independent analysisof the data so as to reach a conﬁrmative result on thekaonic nuclear bound state “K − pp.” The peak in thebound region would suggest that the ∼ on-shell K − (orK) can form a nuclear bound state (Boson & Fermion
hybrid system), where u- and u-quarks coexist, at-least
within a time scale allowed by the width of the boundstate. It is quite important and interesting to know
how the hadron identity is conserved even in nuclear
media.
In the future, we wish to study π, η  , and K-mesonbound states more precisely and to extend our study
to nuclear bound states of other mesons, such as η andφ, for a global understanding of the hadron mass generation mechanism and the quark-conﬁnement mechanism. We believe that the mesonic nuclear state study
is a door to access the mystery of the origin of mattermass in the universe and a laboratory-based showcase
to understand the physics of very-high-density matter,such as the core of a neutron star.

KN attractive interaction.6) An even more impressive
spectrum was obtained in the Λp invariant mass spectrum of the Λ p n ﬁnal state, in which we observed an
event concentration near the M (K − pp) threshold, andthe centroid of the event concentration is well within
the bound region.7) Thus, we conducted a new beamtime for further study, especially focusing on the Λ p n
ﬁnal state (E152nd ).

(CM)

counts per 20 MeV/c2

250200150

100500

2.0

2.2

2.4

2.6

2.8

3.0

Minv. p [GeV/c 2]

Fig. 1. A preliminary Λp invariant mass spectrum ofthe Λ p n ﬁnal state of the forward neutron emission(cos θnCM > 0.75) events, in which a strong event concentration is found. The spectrum is ﬁtted by a BreitWigner formula for the bound region (red) and a Gaussian formula for the unbound region (blue), together
with a smooth background. The dashed line indicatesthe mass threshold of M (K − pp).

We are still in an analysis phase, but the preliminary result is truly astonishing. As shown in Fig. 1,the event concentration near the M (K − pp) threshold
is not a single peak structure, as we simply assumed
in our previous publication,7) but it has clear internal structures separated by the threshold energy indicated by the dashed line. First, the only reasonable
explanation of the peak-structure formation below theM (K − pp) threshold is the kaonic nuclear bound state
formation of “K − pp.” Events below the threshold canbe generated when virtual kaons below the rest mass
are produced in a quasi-elastic (QE) reaction. The
peak structure can only be formed when there exists
a resonance pole below the threshold, while a smooth
tail is formed below the threshold if a pole does notexist. The Λp pair in the ﬁnal state, together withthe forward neutron, ensures that the backscatteredK − interacts with the other two spectator protons.
0Thus, K − , K , or Λ(1405) escaping channels are naturally suppressed substantially, in contrast to the semiinclusive missing-mass spectra of 3 He(K − , n)X.6) TheReferences
1) T. Yamazaki et al., Z. Phys. A 355, 219 (1996).
2) H. Gilg et al., Phys. Rev. C 62, 025201 (2000); K. Itahashi et al., ibid. 62, 025202 (2000).
3) Y. K. Tanaka, K. Itahashi, H. Fujioka et al., Phys.
Rev. Lett. 117, 202501 (2016).
4) M. Iwasaki, R. S. Hayano, T. M. Ito et al., Phys. Rev.
Lett. 78, 3067 (1997).
5) T. M. Ito, R. S. Hayano, S. N. Nakamura et al., Phys.
Rev. C 58, 2366 (1998).
6) T. Hashimoto, S. Ajimura, G. Beer et al., Prog. Theor.
Exp. Phys. 6, 061D01 (2015).
7) Y. Sada, S. Ajimura, M. Bazzi et al., Prog. Theor. Exp.
Phys. 5, 051D01 (2016).

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