Extension of
the J-PARC Hadron Experimental Facility- summary report -April 30, 2017

Committee for the study of the extension of
the Hadron Experimental Facility

1The committee for the study of the extension of the Hadron Experimental
Facility was formed under the Hadron Hall Users’ Association in August, 2015.
This document is a summary of the discussions among the committee members,
and documented by a part of the members listed below.
Hiroyuki Fujioka
Kenneth Hicks
Yoichi Igarashi
Kenta Itahashi
Takeshi K. Komatsubara
Kouji Miwa
Tomofumi Nagae
Satoshi N. Nakamura
Hajime Nanjo∗
Hiroyuki Noumi
Hiroaki Ohnishi∗∗Shinji Okada
Atsushi Sakaguchi
Shinya Sawada
Suguru Shimizu
Kotaro Shirotori
Hitoshi Takahashi
Toshiyuki Takahashi
Hirokazu Tamura
Kiyoshi Tanida
Mifuyu Ukai

(Kyoto U.)(Ohio U.)(IPNS, KEK)
(Nishina Center, RIKEN)(IPNS, KEK)
(Tohoku U.)(Kyoto U.)(Tohoku U.)(Kyoto U.)(RCNP, Osaka U. / IPNS, KEK)
(Nishina Center, RIKEN / RCNP, Osaka U.)(RIKEN)(Osaka U.)(IPNS, KEK)
(Osaka U.)(RCNP, Osaka U.)(IPNS, KEK)(IPNS, KEK)
(Tohoku U.)(ASRC, JAEA)(IPNS, KEK)
*present address: Osaka U.
**present address: ELPH, Tohoku U.

2

This document is also supported by the following physicists.
Patrick Achenbach
Shuhei Ajimura
Yuya Akazawa
Claude Amsler
Kazuya Aoki
Hidemitsu Asano
Sakiko Ashikaga
Bernd Bassalleck
Elena Botta
Wen-Chen Chang
Hiroyuki Ekawa
Alessandro FelicielloYu Fujii
Manami Fujita
David Gill
Toshi Gogami
Yuji Goto
Philipp Gubler
Taku Gunji
Hiroyuki Harada
Toru Harada
Shoichi Hasegawa
Tadashi Hashimoto
Michael Hasinoﬀ
Shuhei Hayakawa
Satoru Hirenzaki
Ryotaro Honda
Tomoaki Hotta
Sanghoon Hwang
Masaya Ichikawa
Yudai Ichikawa
Masaharu Ieiri
Masami Iio
Kentaro Inoue
Takatsugu Ishikawa
Masahiko Iwasaki(U. of Mainz)(RCNP, Osaka U.)(Tohoku U.)(Stefan Meyer Institute for Subatomic Physics)(IPNS, KEK)
(RIKEN)(Kyoto U.)(U. New Mexico)(Torino U. / INFN-Torino)(Institute of Physics, Academia Sinica)(Kyoto U.)(INFN-Torino)(Tohoku Medical and Pharmaceutical U.)(Tohoku U.)(TRIUMF)(Tohoku U.)(Nishina Center, RIKEN)(Yonsei U.)(CNS, U. Tokyo)(JAEA)(Osaka EC U.)(JAEA)(RIKEN)(U. British Columbia)(Osaka U.)(Nara Woman U.)(Tohoku U.)(RCNP, Osaka U.)(Korea Researh Institute of Standards and Science)(Kyoto U.)(JAEA)(IPNS, KEK)
(ARL, KEK)
(RCNP, Osaka U.)(ELPH, Tohoku U.)(RIKEN)3

Shunsuke Kanatsuki
Hiroki Kanda
Masashi Kaneta
Yohji Kato
Shinji Kinbara
Takeshi Koike
Yusuke Komatsu
Yoshihoro Konishi
Ami Koshikawa
Geiyoub Lim
Toru Matsumura
Michifumi Minakawa
Yoshiyuki Miyachi
Yuhei Morino
Norihoto Muramatsu
Ryotaro Muto∗
Sho Nagao
Fujio Naito
Yoshiyuki Nakada
Manami Nakagawa
Ken’ichi Nakano
Kazuma Nakazawa
Megumi Naruki
Hidekatsu Nemura
Masayuki Niiyama
Tadashi Nomura
Susumu Oda
Yuji Ohashi
Makoto Oka
Haruhiko Outa
Kyoichiro Ozawa
Jen-Chieh Peng

(Kyoto U.)(Tohoku U.)(Tohoku U.)(IPNS, KEK)
(Gifu U.)
(Tohoku U.)(RCNP, Osaka U.)(Tohoku U.)(Kyoto U.)(IPNS, KEK)
(National Defense Academy of Japan)(IPNS, KEK)
(Yamagata U.)(IPNS, KEK)
(ELPH, Tohoku U.)(IPNS, KEK)
(Tohoku U)(ACCL, KEK)
(Osaka U.)(Osaka U.)(Tokyo I. Technology)(Gifu U.)
(Kyoto U.)(U. Tsukuba)(Kyoto U.)(IPNS, KEK)
(Kyushu U.)(RCNP, Osaka U.)(Tokyo I. Technology / JAEA)(RIKEN)(IPNS, KEK)
(U. Illinois)

4

Takao Sakaguchi
Yasuhiro Sakemi
Hiroyuki Sako
Fumiaki Sakuma
Masaharu Sato
Susumu Sato
Yoshinori Sato
Michiko Sekimoto
Toshiaki Shibata
Hajime Shimizu
Takao Shinkawa
Koji Shiomi
Myint Kyaw Soe
Yorihito Sugaya
Hitoshi Sugimura
Yasuyuki Sugiyama
Makoto Tabata
Yasuhisa Tajima
Tomonori Takahashi
Minoru Takasaki
Makoto Takizawa
Kazuhiro Tanaka
Manabu Togawa
Junji Tojo
Atsuhi Tokiyasu
Shoko Tomita
Yuichi Toyama
Akihisa Toyoda
Hiroo Umetsu
Hiroaki Watanabe(BNL)(CNS, U. Tokyo)(JAEA)(RIKEN)(RIKEN)(JAEA)(IPNS, KEK)
(RIKEN)(Tokyo I . Technology)(ELPH, Tohoku U.)(National Defense Academy of Japan)(IPNS, KEK)
(Gifu U.)(RCNP, Osaka U.)(ACCL, KEK)(ACCL, KEK)
(Chiba U.)(Yamagata U.)(RCNP, Osaka U.)(IPNS, KEK)(Showa Pharmaceutial U.)(IPNS, KEK)
(Osaka U.)(Kyushu U.)(ELPH , Tohoku U.)(Tohoku U.)(Tohoku U.)(IPNS, KEK)
(Tohoku U.)(IPNS, KEK)5

Takumi Yamaga
Hitoshi Yamamoto
Takeshi Yamamoto
Taku Yamanaka
Shigehiro Yasui
Satoshi Yokkaichi
Junya Yoshida
Tamaki Yoshioka(RCNP, Osaka U.)(Tohoku U.)(IPNS, KEK)
(Osaka U.)(Tokyo I. Technology)(RIKEN)(Gifu U.)
(Kyushu U.)*present address: ACCL, KEK

6

Contents
1 Overview

8

2 Facility

11

3 Particle Physics

15

4 Hadron and Nuclear Physics
18
4.1 how hadrons are produced and excited . . . . . . . . . . . . . . . 18
4.2 understanding of the baryon-baryon interaction which makes atomic
nuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.3 understanding of baryonic matter with super-high density - why
heavy neutrons stars exist . . . . . . . . . . . . . . . . . . . . . . 21

7

1

Overview

What are the most fundamental elements from which matter is made ? What
laws of nature do the elements obey ? How is matter developed from them ?
These are the questions that are at the heart of physics research. In particle
and nuclear physics, we try to reveal the origin of matter, how the matter isdeveloped, and the mechanism of the relevant phenomena. We summarize ourmain subjects as follows.
• Fundamental particles and interactions appeared in the early universe at atime of high temperature and are behind the physical laws of the universe.
Through elucidation of these particles and interactions, we shed light onunanswered questions in the universe, such as the mechanism to realize thedominance of matter over anti-matter in the universe and the character ofdark matter.
• Because the couplings among strongly-interacting quarks and gluons arecomplicated, it is still unclear how hadrons, as a many-body system ofquarks, and nuclear matter, as a hadronic complex, are mathematicallydescribed. We investigate the nature of hadrons and nuclear matter invarious environments, such as the high temperature in the early universe
and the high density in the core of neutron stars, to clarify the mechanism
which forms a variety of materials such as the stars and human beings in
the universe.
We are challenging these subjects with the high-intensity hadron beams produced at the Hadron Experimental Facility of Japan Proton Accelerator Research Complex (J-PARC).
J-PARC provides the world highest-power beams for particle and nuclear
experiments. J-PARC was constructed as the project with the highest priorityof the nuclear physics community in Japan with strong support from the highenergy physics community. J-PARC attracts scientists from abroad and is aninternational core of excellence in Asia.
At present, two low-momentum charged-kaon beam lines (K1.8 and K1.8BR)
and one neutral-kaon beam line are being operated in the north and south areasof the experimental hall, respectively, All of these beam lines are connected tothe primary target that produces secondary particles. In the south area, a highmomentum beam line and a muon beam line will be ready for operation in thenear future. From experiments with charged kaon beams, fruitful results havebeen reported; for example, candidates for deeply bound kaon-nuclear states,
the discovery of charge symmetry breaking in the level structure of hypernuclei,
8and the observation of hypernuclei with two strange quarks. With a neutralkaon beam, a search for the rare decay KL0 → π 0 ν ν̄, which is sensitive to new
physics beyond the Standard Model of particle physics, is on the right track; thisexperiment has already achieved the world highest sensitivity to the branching
fraction.
A lot of results on nuclear and particle physics have been achieved from theexperiments at the Hadron Experimental Facility of J-PARC. However, we also
recognized that an extension of the facility is necessary for further researches in
the future.
Only one production target is placed and is shared by the secondary beamlines in the present hall. About 50 % of the beam power of the primary proton
from the J-PARC Main Ring accelerator is used to produce secondary particles,
and the remaining protons are transported to the beam dump. The secondary
beam lines were built in the limited space of the present hall. If the facility isextended to install the second and third production targets, and to construct
new secondary beam lines for utilization of the high-power beam of J-PARC,
we can expand our researches substantially. The merits of the extension are as
follows.
Firstly, the new beam lines will enable us to promote new measurements
which are impossible in the present facility. For the strangeness nuclear physics
and hadron physics described in the following sections, construction of thehigh-intensity high-resolution beam line (HIHR) and the high-momentum massseparated beam line (K10), with unprecedented capability, are highly anticipated. Since these new beam lines do not fit in the existing hall, the extension
is necessary.
Secondly, we can improve the sensitivities of rare kaon-decay searches. The
extraction angle of the existing neutral beam line is chosen to be 16 degrees
due to the limitation of the space of the present hall. Extension of the hallenables us to optimize the flux of kaons and neutrons in the neutral beam; withan extraction angle of 5 degrees, we can utilize a more intense kaon beam andthereby dramatically increase the sensitivities.
Thirdly, more flexible operation of charged kaon beam lines will be realized.
The existing K1.8 beam line is optimized for the studies of the nuclei with two
strange quarks, and the studies for the nuclei with a single strange quark are
performed at the branch line (K1.8BR), which shares the upstream part of theK1.8 beam line. Since those two beam lines cannot be operated at the sametime, we are forced to perform the K1.8 and K1.8BR experiments one-by-one.
With a new low-momentum beam line (K1.1), which provides kaon beams with9

higher quality (e.g., better K/π ratio) and intensity, the experiments with two
beam lines can be performed simultaneously.
Fourthly, flexible operation can be realized also for high momentum beamlines. In the extended hall, both the existing high-momentum beam line and a
new high-momentum beam line (K10), which provides separated charged particles up to 10 GeV/c, are placed in the south side, with the low momentum
beam lines (K1.8, K1.1, and HIHR) in the north side. These five beam lines canbe operated simultaneously and thus can accommodate the increasing demands
from users.
This report describes the project to extend the Hadron Experimental Facility
and construct new beam lines. These plans had already been considered in theinitial plan of J-PARC conceptual designs, and are updated and revised in thisreport to reflect recent progress.
This is a field-crossing project of particle and nuclear physics. The nuclear
physics community in Japan supports it as the top-priority project, and the particle physics community also recognized it as an important project. The project
was discussed in ”Master Plan 2014” by the Science Council of Japan, and wasreported in the revised version of ”Roadmap 2014” issued in September 2015by Ministry of Education, Culture, Sports, Science and Technology (MEXT) of
Japan. In this opportunity, we aim to motivate the project and then elucidate
the mysteries in particle and nuclear physics described above.

10

2

FacilityAt the Hadron Experimental Facility of J-PARC, the primary proton beam of
30 GeV is slowly extracted from the Main Ring accelerator and transported tothe production target (T1) in the experimental hall. Various secondary particles
such as K and π mesons produced in the target are transported through thesecondary beam lines to the experimental area, and are used for particle andnuclear physics experiments. The construction of the facility was started in
2004, and the first beam was extracted to the hall on January 27th, 2009. The
formal beam operation for users started from January 2010.
The layout of the present experimental hall is shown in Fig 1. The sizeof the hall is 58 m in width and 56 m in length, and has a semi-undergroundstructure with the height of 16 m above and 6 m under the ground. At present
three secondary beam lines: K1.8 and K1.8BR beam lines in the north area andKL beam line in the south area are being operated. Hadron-nuclear physics
experiments are being carried out by using charged K − mesons in the K1.8
and K1.8BR beam lines, although these two beam lines cannot be operated
simultaneously. In the KL beam line, a particle-physics experiment using neutral
KL mesons is being performed. In the south area of the hall, in the next few
years, a high-momentum (High-p) beam line as a branch of the primary proton
beam and a COMET beam line for a particle-physics experiment with muons in
the South Experimental Building will start the operation.

Figure 1: Layout of the present experimental hall.
In the plan of facility extension, the following new secondary beam lines will11

be constructed for new experiments in order to expand the potential for particle
and nuclear physics.
1. New beam lines optimized for physics goals
new KL beam line : high-intensity neutral KL meson
HIHR beam line : high-intensity and high-resolution charged π meson
2. Beam lines to provide new secondary particles
K10 beam line : high-momentum (2-10 GeV/c) charged K meson, πmeson, and anti-proton
3. A beam line to extend S = −1 strange nuclear physics
K1.1 beam line : low-momentum (<1.2 GeV/c) charged K meson
The hall is extended to the downstream by 105 m (Fig. 2). The existing
primary beam line (A line) is extended, and two new production targets (T2and T3) are installed. The technologies necessary to extend the facility, suchas the beam optics to irradiate primary protons to two or more targets and the
movement of the existing beam dump to the downstream, have already been
established. The K1.1 and K10 beam lines are extracted from the T2 target
station, and the HIHR and new KL beam lines are extracted from the T3 target
station. The beam dump to absorb primary protons is located downstream of theT3 target. The detector for the new KL experiment will be installed downstreamof the beam dump, so that the experimental area will be extended by 48 m.
The K1.8, High-p, and COMET beam lines that already exist in the hall continue to be operated. Thus, in the extended experimental hall, seven secondary
beam lines in total can be operated. We will push forward with the strangenessnuclear-physics experiments at the K1.8, K1.1, and HIHR beam lines in thenorth area, the hadron-nuclear physics experiments at the high-p and K10 beamlines in the south area, and particle-physics experiments at the COMET and
new KL beam lines. Table 1 summarizes the parameters of these beam lines.
A test beam line is planned to be constructed in the experimental area that ispreviously used for the KL experiment.
The physics goals of the experiments in each beam line is described in thesubsequent sections. In brief,• strangeness nuclear physics to understand many-body systems of hadrons.
K1.1 beam line : to investigate the baryon-baryon interactions and thecharacteristics of baryons in the nuclear medium by using hyperons.
12

Table 1: Beam lines in the Hadron Experimental Facility.
particlemomentum
beam lines in the present hall
K1.8
K ±, π±< 2.0 GeV/c
K1.8BR
K ±, π±< 1.1 GeV/c
KL
KL
2.1 GeV/c in ave.
High-pp
π±<31 GeV/c−
COMET µnew beam lines in the extended area
K1.1
K ±, π±< 1.2 GeV/c
0.7∼0.8 GeV/cnumber of particles

characteristics

106 K − /spill
105 K − /spill
107 KL /spill1010 p /spill
107 π /spillseparated
separated
to 16 degrees
primary protons

106 K − /spillseparated
lower momentum[ K1.1BR ]separated
×10 better ∆p/p
separatedto 5 degrees
n/KL optimized

HIHR

π±

< 2.0 GeV/c

2.8 × 108 π/spill

K10
new KLK ±, π±, pKL< 10 GeV/c
5.2 GeV/c in ave.

107 K − /spill
108 KL /spill13µ− → e− conversion

Figure 2: Layout plan of the extended experimental hall.
HIHR beam line : to investigate the baryon-baryon interactions in highdensity by measuring the hyper-nuclear energy levels.
• hadron-nuclear physics to understand quark many-body systems.
K10 beam line : to investigate the eﬀective degrees of freedom to describe hadrons and the characteristics in the nuclear medium.
• particle physics to search for physics beyond the Standard Model.
new KL beam line : to understand the dominance of matter over antimatter in the universe by detecting 100 events of CP violating rare
decay.

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3

Particle Physics

The following two Nobel Prizes in Physics illustrate the current issues on experimental particle physics related to the extension. One is to Makoto Kobayashi
and Toshihide Maskawa in 2008 for their discovery, in 1973, of the origin of thebroken CP symmetry which predicts the existence of at least three families ofquarks in nature. The other is to François Englert and Peter W. Higgs in 2013 forthe theoretical discovery of a mechanism that contributes to our understandingof the origin of mass of subatomic particles.
The Advanced Information in 2008 1 explains this award as:
In 1967, Andrei Sakharov (the Nobel Prize in Peace in 1975)
pointed out in a famous work that CP violation must be the causeof the asymmetry in the universe. It contains more matter thanantimatter. The CP violation that the KM (Kobayashi-Maskawa)
Model gives rise to is most probably not enough to explain this phenomenon. To find the origin of this CP violation we probably haveto go beyond the Standard Model. Such an extension should exist
for other reasons as well. It is believed that at higher energies othersectors of particles, so heavy that the present day accelerators havebeen unable to create them, will augment the model. It is naturalthat these particles will also cause CP violations and in the tumultuous universe just after the Big Bang these particles could havebeen created. These particles would have been part of the hot early
universe and could have influenced it, by an as yet unknown mechanism, to be dominated by matter. Only future research will tell us
if this picture is correct.
The Advanced Information in 2013 2 explains this award as:
All measurements to date confirm that the properties of the newly
discovered particle are consistent with those expected for the fundamental scalar boson predicted by the BEH (Brout-Englert-Higgs)
mechanism. The discovery is a milestone for particle physics anda tremendous success for the Standard Model. However, far fromclosing the book it opens a number of new exciting possibilities:
1“The Nobel Prize in Physics 2008 - Advanced Information”.
Nobelprize.org. Nobel Media AB 2014. Web. 19 Apr 2016.
<http://www.nobelprize.org/nobel prizes/physics/laureates/2008/advanced.html>
2“The Nobel Prize in Physics 2013 - Advanced Information”.
Nobelprize.org. Nobel Media AB 2014. Web. 17 Apr 2016.
<http://www.nobelprize.org/nobel prizes/physics/laureates/2013/advanced.html>

15

Theorists believe that the SM most probably is but a low-energy
approximation of a more complete theory. If this were not so, quantum mechanical corrections to the Higgs mass would drive mH (theHiggs mass) towards the Planck scale - unless unnatural cancellations occur. Therefore, extensions of the SM are proposed, keeping
the successful features of the SM but at the same time introducing
new physics in a way, which stabilizes mH at its low value, whichis in accordance with SM expectations.
Both achievements are implemented in the framework of the Standard Model(SM) of particle physics and well describe the experiments of elementary particles. On the other hand, there remains questions that cannot be explained by
the SM, and the searches for new physics beyond the SM have been pursued intensively. The questions include the mechanism to produce the matter-dominant
universe and the origin of the low mass of the Higgs particle. The CP-violation
is a must for the matter-dominant universe, but the size of the CP violation
implemented in the SM is too small to explain the matter dominance in our
universe. A new mechanism should be introduced, and new particles at a muchhigher energy-scale may contribute to the new breaking of CP symmetry. Such
particles would also provide the mechanism to stabilize the Higgs mass to thevalue observed. In summary, the key to answer the questions is at the higher
energy-scale.
To search for new physics beyond the SM, there are two approaches in theaccelerator-based particle physics experiments. One is the Energy Frontier, suchas the ATLAS and CMS experiment in the LHC, which aims to produce newparticles directly by colliding the beams to maximize the center-of-mass energy.
The other is the Intensity Frontier, in which a high energy scale is realized ina short time-period due to the Uncertainty Principle in Quantum Mechanics
between time and energy. Heavy new particles can contribute to rare processes
and make phenomena diﬀerent from the SM predictions. High intensity beams
are necessary in the experiments to study rare processes to see a golden needle
in a haystack and to explore new physics beyond the SM.
In the search for new physics, the usual processes caused by the SM areconsidered to be the background. Thereby, if such SM processes were suppressed
due to symmetry principles, new physics eﬀects can be enhanced because they
do not need to follow the suppression mechanisms of the SM. Furthermore, theprocesses should be predicted by the SM as precisely as possible; when we observea deviation from the SM in a process, it is not so meaningful if the deviation iswithin the uncertainty of the SM prediction.
The particle-physics experiments performed in the Hadron Experimental Fa16

cility of J-PARC are exactly what we pursue in the Intensity Frontier. In thepresent hall, two experiments using K mesons are being conducted; they are
directly connected to the extension of the facility. Also, a new COMET experiment to search for µ-e conversion is being prepared. They are complementary to
the experiments using B mesons in other facilities; by comparing their physics
eﬀects the new physics scenario can be identified.
One of the kaon experiments at the present hall is the E14 KOTO experiment
to search for the decay KL → π 0 ν ν̄, whose branching-fraction prediction in the
SM is very small, 3 × 10−11 , and thus the decay has never been observed. Since
this decay is strongly suppressed, due to the hierarchy at the flavor transitionsin the Kobayashi-Maskawa matrix, it is sensitive to the new physics processes
that does not obey such suppression. This decay is also a CP-violating process
and a new source of CP violation can be explored. The uncertainty in the SM
prediction, 2%, is small, and some new physics theories predict a branching
fraction of an order-of-magnitude larger. The KOTO experiment will searchat the sensitivity of O(10−11 ) in order to detect eﬀects from new physics. In
the extended facility, a new beam line and a new detector will be constructedaiming to measure the branching fraction of the decay with higher statistics as
the KOTO step-2 experiment. The step-2 experiment plans to detect O(100)events of this decay even with a branching ratio similar to that in the SM.
The other kaon experiment at the present hall is is the E36 experiment whichuses charged-kaon decay to explore the breaking of of the universality of lepton
flavor. In the experiment, the ratio of the branching fractions of the K + → µνµand K + → eνe decays is measured and is compared to the SM expectation. The
decay K + → eνe is suppressed due to the helicity suppression in the structure of
the weak interaction in the SM. The sensitivity to new physics is enhanced withthe ratio of the two branching fractions, since the theoretical as well as systematic
uncertainties are reduced. In the extended facility, the collaboration plans toperform the E06 TREK experiment to study the breaking of time-reversal (T)symmetry with the same spectrometer magnet. The transverse polarizationof the muon from the K + → π 0 µ+ ν decay in the decay plane is measured.
Since the polarization is a T violating observable, a polarization larger than the
SM prediction would indicate T violation. T violation is connected to the CP
violation through the CPT theorem, and is expected to be a key to solve themystery of a matter-dominant universe.

17

4

Hadron and Nuclear Physics

What is the origin of all the matter in the universe? - this is a simple question
which humankind has asked since the ancient times. The research with highenergy accelerators in the 20th century found that the six leptons (an electron
and its sisters, and neutrinos) and the six quarks are the elementary particles
which cannot be divided anymore. The interactions between these particles arecaused by exchanging gauge bosons. The strong interaction between quarks
is carried by gluons and described by the quantum chromo-dynamics (QCD).
Hadrons, such as protons and neutrons, are made from quarks, and atomic
nuclei are made of protons and neutrons. Atomic nuclei make up atoms andmolecules by capturing electrons, and then various kinds of materials, which areconstituent of stars and humankind, are produced. This hierarchy traces thehistory of the creation of materials of the universe from its beginning, startingwith the Big Bang. One of the most important issues of modern hadron andnuclear physics is to investigate various aspects of particles with quark multibody systems and to understand mechanisms of their production.
Because quarks are confined in hadrons, there is not enough knowledge
on how hadrons are made up from quarks and on the characteristics of thequark/hadronic systems, depending on the temperature and density. Thus, theproject of extending the Hadron Experimental Facility addresses three issues;
the first is how hadrons are produced and excited , the second is understanding of the baryon-baryon interaction which makes atomic nuclei , andthe third is understanding of baryonic matter with super-high density - why
heavy neutrons stars exist . In particular, on the second and third issues, understanding of the short-range interaction is a key to the characteristics of thehigh-density hadronic matter, and is the major subject.
In the following subsections, the overview and plans of addressing these issues
are described.

4.1

how hadrons are produced and excited

The dynamics of quarks and gluons are described by the theory of quantum
chromodynamics (QCD). The coupling constant of QCD strongly depends on
its energy, which introduces a scale parameter, i.e. ΛQCD . The strength of theparameter is known to be about 0.2 GeV. The physical picture of the strongly interacting matter is very diﬀerent below and above ΛQCD . Above ΛQCD , phenomena can be described well by perturbative QCD. However, below ΛQCD , due tothe large coupling constant, non-perturbative eﬀects cannot be neglected. Twocharacteristic phenomena appear in this low energy QCD regime. One is con-18

finement and the other is generation of the dynamical mass of the hadron .
One of the ways to solve low energy QCD is indeed the lattice QCD calculations.
Nowadays, the lattice QCD calculations are well developed and mass spectra ofground state hadrons are reproduced. However, mass spectra for excited state
hadrons still cannot be described. On the other hand, a model based on constituent quarks (a.k.a. the quark model) also succeeds in describing groundstate hadrons; however, excited-state baryons, such as Λ(1405) and Roper resonance(N*(1440)), cannot be described very well. Recently, new exotic hadrons,
i.e. X, Y , Z and penta-quark hadrons (Θ+ and Pc+ ), are discovered in highly
excited hadrons. Those states also cannot be described with the lattice QCD
calculations, nor with the simple quark model. Thus none of the models or theories can reach a perfect description of the hadron spectra including excited-state
hadrons. To understand hadron spectra including the excited-state hadrons, weneed to shed light on the eﬀective degrees of freedom inside the hadrons and themechanism of how the hadrons are excited.
One of the ways to understand the dynamics inside a baryon is to replace
one of two light-quarks in the baryon to heavy quarks, such as a strange quark
and/or a charm quark. The color magnetic interaction is proportional to the
1/mq , and the interaction between a heavy-quark and a light-quark is going to
be reduced. Thus the correlation between light quarks (di-quark correlation)
in a baryon will be enhanced. The strength of the correlation would depend
not only on the baryon-excited energy but also on the decay branch and the
production cross section. At J-PARC, spectroscopic studies of charmed baryon
and multi-strangeness baryon (Ξ∗ and Ω∗ ) will be performed.
Spontaneous breaking of chiral symmetry is one of the fundamental conceptsof hadron physics and characterizes the QCD vacuum structure. The restorationof chiral symmetry is predicted in high-density or high-temperature QCD matter.
It is easily expected that the properties of hadrons, such as the mass and decay
width, will be modified under such extreme conditions. However, the pattern ofthe modification is predicted to be diﬀerent in diﬀerent hadrons (in particular
for mesons). At J-PARC, precise measurements of meson spectra in the nuclear
matter will be performed with light vector mesons (ϕ, ρ, ω), pseud-scalar mesons(η, η ′ ) and light-heavy mesons (D mesons). Comparisons of the in-medium
modification of these mesons will reveal the mechanism of partial restoration ofchiral symmetry in nuclear matter. Precise measurements of the light vectormeson spectral functions in nuclei will be performed in the High-p beam line.
Extending the facility will open a new horizon, i.e. D mesons in nuclei by
the measurements of D − D meson production near the production thresholdusing an intense and high-energy anti-proton beam. Understanding of the meson
property in nuclei and/or nuclear matter is needed to describe hadrons in the19extremely high-density matter, i.e. the core of neutron stars.

4.2

understanding of the baryon-baryon interaction which makes
atomic nuclei

Nucleon scattering experiments have indicated that the nuclear force is strongly
repulsive at ranges shorter than 1 fm. Due to small nucleon momenta, thestructure of ordinary nuclei is determined mainly by the strength of the nuclear
force averaged over short and long ranges and is not aﬀected by details of theshort-range parts. Theoretical models for the nuclear force are based on themeson exchange picture proposed by Yukawa about eighty years ago. They
include a phenomenological short-range repulsive core at ranges shorter than
1 fm where the two nucleons overlap with each other. So far, these models havesucceeded when they are used to describe nuclei.
In contrast, the short-range parts of the nuclear force play essential rolesin the high-density nuclear matter realized in the core of neutron stars. It
would be natural to describe the nuclear force using the degrees of freedom ofquarks and gluons rather than the meson exchange picture at the ranges shorter
than 1 fm, where two nucleons are overlapping and their structure aﬀects thenuclear force. In addition, since the Fermi energy of nucleons in high-density
nuclear matter increases with density, hyperons are expected to appear at a
certain density. Thus, the nuclear force should be generalized to other baryonbaryon interactions (all the forces between the baryon octet members including
hyperons) in studies of high-density nuclear matter.
Theoretical studies on the baryon-baryon interactions based on quark models suggest that the strength of repulsive cores depends on the spin and flavorof the two baryons. It is expected that there is a very large repulsive corein a specific state, such as Σ+ p in the spin singlet channel, due to the Paulieﬀect between quarks, while there would be an attractive core due to the colormagnetic interaction by one-gluon exchange in the H dibaryon channel. The
spin-orbit interaction, which is important at short ranges, has been, so far, explained with heavy meson exchange. However, if the spin-orbit force is due togluon exchange, a quite-diﬀerent behavior from the nucleon-nucleon case is expected in the spin-orbit force of the hyperon-nucleon (Y N ) interaction. These
quark-model predictions are in qualitative agreements with the baryon-baryon
interactions obtained from lattice QCD simulations.
Existing hyperon-nucleon scattering data have been limited in the measured
momentum ranges as well as in the amount of statistics. Nevertheless, thebaryon-baryon interaction models based on a one-meson exchange picture havebeen developed by using spectroscopic information from hyper-nuclear struc20

ture and by supplementing the scarce hyperon-nucleon data with more plentiful
nucleon-nucleon scattering data and with the help of flavor SU (3) symmetry.
These models, unfortunately, have no predictive power for the short-range partsof the hyperon-nucleon interaction, although they can reproduce the Y N scattering data and hyper-nuclear information rather well. Since the existing Y N
scattering data are not so sensitive to the short-range parts of interaction, newdata are essential to reveal these interesting features.
In this situation, we plan to introduce the most advanced techniques to theexperiments and drastically improve the Y N scattering data both in quality andin quantity. The new scattering data, also compared with lattice QCD calculations, enables us to establish the baryon-baryon interaction models covering theshort-range parts. As described in the next subsection, the Y N N three-body
repulsive force that cannot be reduced into the two-body baryon-baryon interactions is considered to play an important role in describing the core of neutronstars. Experimental verification of the three-body force eﬀects also requires wellestablished two-body baryon-baryon interaction models covering the short-range
parts.

4.3

understanding of baryonic matter with super-high density why heavy neutrons stars exist

Nucleus can be characterized with features such as constant density (saturationof density) and constant binding-energy per nucleon (saturation of binding energy). It is an important scientific issue to deduce these characteristics of nuclei
from the nuclear force models, and many eﬀorts have been paid to solve it. It has
been clarified that the two-body nuclear force is not enough and the introduction of three-body forces is essential to reproduce high-precision experimental
results of nuclear binding energies. Recent theoretical calculations indicate thatthe three-body nuclear force is repulsive in heavy nuclei and it should give moresignificant repulsion in the higher-density nuclear matter such as deep inside ofneutron stars. Modern theoretical calculations predict that the three-body repulsive baryonic-force plays an important role in deciding the maximum allowed
mass of neutron stars.
The Fermi energy of neutrons becomes quite high deep inside of neutronstars, so that a neutron converts to a hyperon such as a Λ particle to reduce
the total energy when the density of a neutron star becomes higher than athreshold value. The interaction between a Λ and a nucleon is attractive as
we learned from various studies of hypernuclei. Thus, neutrons convert to Λ
particles when the density becomes larger than a few times of the normal nuclear
density. We naturally expect that hyperons exist deep inside of neutron stars.
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Rich NN scattering data enable us to construct realistic nuclear force models.
Spectroscopy of hypernuclei gives information on the Y N /Y Y interactions. Theinternal structure of a neutron star is theoretically studied with the equation ofstate for high-density nuclear (hadronic) matter using realistic nuclear models
with the knowledge of the Y N /Y Y interactions. Established models predictthat hyperons such as Λ particles appear in the central part of neutron stars
and a predicted heaviest neutron star has about 1.5 solar mass. It had been
consistent with the results of neutron mass measurements for long time, however
two neutron stars with two solar mass were observed after 2010. Based onconventional theoretical models, such a heavy neutron star should collapse to a
black hole because the equation of state of the star becomes too soft if it contains
hyperons. The discrepancy between the observation and theoretical prediction
cannot be solved even if the three-body nuclear force is taken into account.
This is called as a hyperon puzzle and is a big challenge to nuclear physics
researchers. It reveals that something important is missing in our understandingof hadronic interactions. The equation of state for high-density hadronic mattershould be revised and updated.
A promising approach to solve the hyperon puzzle is to introduce a threebody repulsive force in the baryonic interaction with hyperons. This is a naturalextension of the three-body repulsive nuclear force which is known to be necessary in the experiments. We intend to prove the existence of this three-body
repulsive baryonic force by high precision experiments at J-PARC. Even if theexperiments clarify that such three-body force is not the source of the hyperon
puzzle, it would open a door to more exotic scenario such as the co-existence of
nuclear matter and quark matter.
A three-body repulsive baryonic force aﬀects the single particle energy of a
hyperon which is bound deep inside of a hypernucleus. An eﬀect on the Λ binding
energy is expected to be about a half MeV and it is smaller than the achievableaccuracy of the established hypernuclear spectroscopic techniques with meson
beams. The HIHR beam line, which will be newly constructed in the extended
hall, enables us to measure hypernuclei with ten times better accuracy (<0.1
MeV) by using the dispersion matching technique. High precision spectroscopy
can be expanded from light hypernuclei (A∼10) to heavy ones (A > 200). Sucha series of systematic measurements will reveal the existence of a three-body
repulsive baryonic force and solve the hyperon puzzle of heavy neutron stars.

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