J-PARC

HADRON
EXPERIMENTAL
FACILITY
Advanced Research on Particle & Nuclear Physics
with High Intensity Hadron Beams

Welcome toHadron Experimental Facility
At the J-PARC Hadron Experimental Facility (HEF),
fundamental research on particle and nuclear physics
is conducted experimentally using hadron beams.
What are hadrons, nuclei, and elementary particles?
This brochure provides an overview of those and theresearch activities at HEF as clearly as possible.

History of the Universe
and the origin of matterOnion structure of material world

MatterOrdinary materials around us are composed of atoms when viewed indetail. For example, a lump of aluminum, the raw material of a 1-yencoin, is made by connecting aluminum atoms by a metallic bond. Whenthe atom is viewed in detail, we notice its structure is a positively
charged nucleus at the center and negatively charged electrons
around the nucleus. An electron is an “elementary” particle, since itcannot be divided into smaller constituents. However, the nucleus is notsince it is composed of protons and neutrons. Furthermore, a proton

AtomNucleus

Electronor neutron is composed of quarks. In modern physics, the quark isthought to be a true “elementary” particle. As shown above, the material
world has a hierarchical structure like an onion. At the Hadron
Experimental Facility (HEF), we investigate elementary particles which
lay deep inside the hierarchical structure, protons and neutrons, and thenuclei composed of protons and neutrons.

Tracing the history of the Universe throughthe study of nuclei and elementary particles
ProtonNeutronThe history of the Universe began at the Big Bang 13.8 billion years ago.
Just after the Big Bang, the Universe was very hot. Elementary particles
such as quarks and gluons were moving in a state similar to a soup. After
that, as the Universe cooled down, protons and neutrons were formed,
followed by nuclei and atoms. The great variety of materials around us
was finally created.

uQuarkuBirth of the Universe= Big BangdSoup of quarks and gluons= quark-gluon plasma

Hadron Experimental Facility

The study of nuclei and elementary particles is a path to investigate thehierarchical structure of matter, and in other words, a path to investigate
the history of the Universe.

Formation of protons andneutrons

Formation of nuclei

2Formation of atoms

Formation of stars

Birth of the EarthWhat are nuclei and elementary particles?
Quarks and leptons
the u quark is an anti-u quark. The four kinds of force are gravity, theelectromagnetic force, and the weak and strong forces. The weak force
acts during nuclear disintegration where one nucleus changes to adifferent nucleus. The strong force binds quarks to a proton or neutron
and makes a nucleus.

Modern physics has created the “Standard Model” of elementary
particles and forces. Elementary particles in the Standard Model,
which are not divisible, and are the smallest units of the matter, arethought to be composed of 6 types of quarks, 6 types of leptons, gauge
particles which mediate four kinds of force, and the Higgs. There exist
anti-particles in quarks and leptons; for example, the anti-particle ofquarks
up

u

charm

c

topt

leptonsdownstrangebottomdthe 1stgenerations

the 2ndgenerationb

the 3rdgenerationelectron

muon

tau

electron-neutrinoeemuon-neutrino

tau-neutrinoparticle which generates
elementary particles’ masses.

particles which mediate force.
gluon

gstrong force

photonelectromagnetic forceweakboson

w+w−
weak force z

graviton

Higgs

Hgravityuu

Nucleons and nuclei

uProtons and neutrons are collectively called nucleons. The nucleon is composedof three quarks. A proton is composed of two up quarks and one down quark,
while a neutron is composed of one up quark and two down quarks. Protons andneutrons are bound by the strong force forming an ordinary nucleus.

uprotond

nucleondunucleonprotondd

neutrondd
neutronOrdinary and non-ordinary materials(ordinary) nucleus

Materials around us are made of ordinary nuclei which are composed of only
protons and neutrons. This ordinary material is composed of the 1st generation
elementary particles, up and down quarks and electrons. There are non-ordinary
particles (matter) which are composed of the elementary particles other than the
1st generation in the particles, and can be produced by cosmic-rays or accelerators.
A Lambda (L) particle is composed of one up quark, one down quark, and one
strange quark as shown in a right figure.
Pion (π meson) was theoretically predicted by Dr. Hideki Yukawa andlater experimentally discovered. For example, a positively charged pion (π+) iscomposed of one up quark and one anti-down quark. There are a lot of mesons
which are composed of other than up and down (anti-up and anti-down) quarks.
A positively charged kaon (K+ meson), for example, is composed of one up quarkand one anti-strange quark.

(ordinary) nucleus

3uudussdLambda()uudLambda ()su ()
piondu ()kaonspion ()kaon ()Hadron Experimental Facility

What are hadrons?
Hadrons, Baryons, and Mesonsu

Nucleons, a Λ particle, and mesons, as already mentioned, are particles formed byquarks via the strong force. They are collectively called “hadrons”.
A particle composed of three quarks, such as a proton or a neutron, is called a“baryon”. Whereas a particle composed of only one quark and one anti-quark iscalled a “meson”.

sss

Xi0 ()Xi- ()

dudsds

Sigma0 ()Sigma- (–)duusu

Lambda ()sSigma+ ()uu

How to produce hadronssdu sd sd su su dd uuddd



proton (p)A lot of protons and neutrons exist around us, as components of nuclei. Other
hadrons are found in the cosmic-rays raining down from space. But the numberof the hadrons in cosmic-rays is very small and they are not easy-controlled foruse in experiments. The only way to obtain a large amount of specific hadrons
to study, is to produce them artificially using accelerators. When a proton beamaccelerated by the J-PARC accelerators hits a metal target, various types ofparticles (hadrons and leptons) are produced. Among them, particles suitable forthe purpose of the experiment are selected and used.

–neutron (n)baryons–mesonshadronsmuon ()pion ()neutrino ()proton (p)A nucleus in themetal targetkaon(K)

proton (p)anti-proton (p)neutron (n)neutron (n)Research using hadrons
mass is generated in matter. Experimental confirmation of the massgeneration mechanism in the strong force is also another one of mainsubjects. The experiment to confirm the model by Dr. Kobayashi andDr. Maskawa, who also won the Nobel Prize in 2008, is also beingconducted. At the HEF, research on nuclei and elementary particles
is being conducted from various aspects. Let's take a look at some ofthem starting on the next page.

What kind of research is conducted at the HEF using hadrons?
The HEF is not a research facility for industrial application but
a playground for basic science, where we consider the origin andevolution of our universe through the study of nuclei, elementary
particles, and the force between them. Properties of the strong force,
which works between hadrons and also forms them, is one of main
research subjects at the HEF. Dr. Yoichiro Nambu, who is one of
the Nobel Prize winners in 2008, proposed a mechanism by which

Hadron Experimental Facility

4

Hypernuclei­— into the unknown strange matter world — hypernucleus

What is a hypernucleus?

Strangeness

There are a lot of nuclear species (nuclides) in ordinary nuclei that are composed of protons
and neutrons. Non-ordinary nuclei which contain not only up and down quarks but also
strange quark(s), called hypernuclei, can be produced by the experiments on the Earth. At theHEF, hypernuclei are intensively studied.

ype hrnuclei

 particle
udsleipernuc hy

S= -2

Ordinariy nucle

S= -1

Number of neutrons

Courtesy of the Graduate School of Science,
Tohoku University

Numberof protons

S=0

What can we know?
Dr. Hideki Yukawa won the Nobel Prize for showing that attractive nuclear force which binds nucleons together and forms a nucleus can beexplained by pion exchange. However, it is known that when nucleons are brought closer, there is a strong repulsive force. The origin of nuclear
force and the reason for the strong repulsion at short distances, is expected to be clarified through the study of hypernuclei.

What are the contents of neutron stars?
Neutron stars are extremely dense matter in the Universe;the density is more than 1014 times that of the Sun. Baryons
which contain strange quark(s), called hyperons, such as Λand Ξ particles are thought to appear in the core of neutronstars as shown in the figure below. The contents and propertiesof the matter inside neutron stars can be clarified through thestudy of hyperon force and hypernuclei.
Neutron-rich nuclei and electrons

S-2S spectrometer
Hypernuclues
Outgoing(scattered) meson

Iron in ordinary state

Neutron-rich nuclei,electrons, andneutronsu s

How to produce hypernuclei

Hyperon(Σ, Λ, Ξ)containedmatter?

Atmosphere ofhydrogen and helium

Superfluid neutronsand protons
Radius of ~ 10 km

Hypernuclei are produced by K– meson beamhitting the target nucleus at the HEF. The
emitted K+ meson or pion from the reactionis measured. Since the probability to produce
hypernuclei is very small, a high intensity K–
meson beam at J-PARC makes the experimental
study of hypernuclei feasible.

Targetnucleus

u s  meson beam

5

Hadron Experimental Facility

The origin of hadron mass

— Experimental confirmation of the mass-generation model by Dr. Nambu —How was mass of the matter generated?
We take it for granted that we have mass in ourselves and in the materials around us. However, it is believed that when the Universe was born,the mass of all particles was zero. So how was the mass generated from massless particles? The model proposed by Dr. Yoichiro Nambu explains
its mechanism.

10-11seconds

10-4seconds

300,000 years

10 billion years

13.8 billion years♥
Generation ofthe mass

Big BangFormation ofprotons and neutrons

Formation of atoms

Birth of life

PresentThe model by Dr. Nambu

How to confirm?

The smallest component of matter is a quark. The quark cannot befound in isolation. Instead, if it is pulled out from a composite particle,
then it immediately pairs with the anti-quark (q̄q). Consequentlyq̄-q pairs are thought to be condensed in vacuum. The mechanismthat causes the q̄q condensation is the spontaneous chiral symmetry
breaking proposed by Dr. Nambu. In this mechanism, hadrons
are considered to obtain their large mass by interacting with thecondensed q̄q.

So how do we confirm this model? The hadron mass is predicted tobecome lighter when it is put in a high density environment since thecondensed q̄q pairs, which generate hadron mass, becomes fewer ina high density environment. At the HEF, we will confirm the modelby producing a φ meson inside a nucleus and measuring how its masschanges within the nucleus, a high density environment.

qqIn vacuum

At high density
Hadrons will become lighter !
In the experiment, φ mesons are produced by the primary proton beamfrom the accelerator hitting on the target nucleus. The mass of the φmeson is measured by detecting an electron and positron pair fromthe φ meson decay inside the nucleus using the spectrometer shownin the figure. If the model is correct, a lighter φ meson mass would bemeasured. This makes it possible toexperimentally clarify whether chiralsymmetry breaking really generates ahadron mass.

electron mesonpositron

proton beam

Hadron Experimental Facility

6

The origin of hadron mass

Hadron Experimental Facility— Experimental confirmation of the mass-generation model by Dr. Nambu —Beamline & main subject

How was mass of the matter generated?
We take it for granted that we have mass in ourselves and in the materials around us. However, it is believed that when the Universe was born,the mass of all particles was zero. So how was the mass generated from massless particles? The model proposed by Dr. Yoichiro Nambu explains
its mechanism.

10-11seconds

10-4seconds

300,000 years

10 billion years

13.8 billion years♥
Generation ofthe mass

Big BangFormation ofprotons and neutrons

Formation of atoms

Birth of life

Hadron Experimental Facility (HEF)— The origin of matter investigated by various particles beams —PresentA
BC
DEF

Beam particles

Beam momentum

Beam intensity

K1.8

hypernuclei (S=–2)▶(charged) pion, (charged) kaon, anti-proton, etc.

▶≤ 2 GeV/c▶~ 106 K–/s

K1.8BR

strangeness & hadron physics▶(charged) pion, (charged) kaon, anti-proton, etc.

▶≤ 1.2 GeV/c▶~ 106 K–/s

KLCP violation▶neutral kaon▶~ 2 GeV/c▶~ 107 K0/s

K1.1

hypernuclei (S=–1)▶(charged) pion, (charged) kaon, anti-proton, etc.

▶≤ 1.1 GeV/c▶~ 106 K–/s

high-phadron mass, hadron physics▶primary proton, (charged) pion▶≤ 31 GeV/c▶< 1012/s

COMETmuon to electron transition▶muon▶≤ 77 MeV/c▶~ 1011/s

Switch yard
Hadron Experimental Hall &South Experimental Building
Main Ring Accelerator

Slow extraction point
The model by Dr. Nambu

How to confirm?

The smallest component of matter is a quark. The quark cannot befound in isolation. Instead, if it is pulled out from a composite particle,
then it immediately pairs with the anti-quark (q̄q). Consequentlyq̄-q pairs are thought to be condensed in vacuum. The mechanismthat causes the q̄q condensation is the spontaneous chiral symmetry
breaking proposed by Dr. Nambu. In this mechanism, hadrons
are considered to obtain their large mass by interacting with thecondensed q̄q.

So how do we confirm this model? The hadron mass is predicted tobecome lighter when it is put in a high density environment since thecondensed q̄q pairs, which generate hadron mass, becomes fewer ina high density environment. At the HEF, we will confirm the modelby producing a φ meson inside a nucleus and measuring how its masschanges within the nucleus, a high density environment.

AB

qqC
Production target T1DE
LinacIn vacuum

At high density

Primary proton beam

Hadrons will become lighter !
In the experiment, φ mesons are produced by the primary proton beamfrom the accelerator hitting on the target nucleus. The mass of the φmeson is measured by detecting an electron and positron pair fromthe φ meson decay inside the nucleus using the spectrometer shownin the figure. If the model is correct, a lighter φ meson mass would bemeasured. This makes it possible toexperimentally clarify whether chiralsymmetry breaking really generates ahadron mass.

electron mesonpositron

3 GeV Rapid Cycling
Synchrotron (RCS)

FNeutrino beamMaterial and Life
Science Experimental
Facility (MLF)
diameter of 500m

Main Ring
Synchrotron (MR)

proton beam

Hadron Experimental
Facility (HEF)

Japan Proton Accelerator Research Complex (J-PARC)
Hadron Experimental Facility

678

Hadron Experimental Facility

Asymmetry of matter and anti-matter in the Universe— from confirmation of Kobayashi-Maskawa model to search for new phenomenon —Why doesn’t anti-matter exist?

CP conversion

Anti-matter, composed of anti-particles, is almost non-existent in nature, unlike
matter. The asymmetry of matter and anti-matter is called “CP symmetryviolation”. Why did this asymmetry occur in the Universe?

particle

Kobayashi-Maskawa model
Dr. Makoto Kobayashi and Dr. Toshihide Maskawa predicted three generationsof quark pairs; each pair has two quarks. They also showed that CP symmetry
violation is naturally explained in this case.

anti-particle

Wts

We can investigate how much CP symmetry is broken by measuring particle
decay phenomena or decay probabilities. A rare kaon decay measurement at theHEF is one of the experiments to clarify whether the Kobayashi-Maskawa model
is correct or if there is a new phenomenon which cannot be explained by theStandard Model (SM).

Z0
Diagram in the Standard Model

Experiment to search for a rare decay of neutral kaons

sIn this experiment, a very rare decay in the SM, where a long-lived neutral kaonˉ ), will be(KL0 ) decays to a neutral pion (π0 ), neutrino (ν), and anti-neutrino (νobserved. If a supersymmetric particle which is not included in the SM exists,
the probability of this rare decay would be enhanced.

ũDetection of rare decay events

Beam halo particle
detectorCsI Crystal calorimeterTwo gamma-rays from neutral pion
decay are measured.

Neutron detector

Hadron Experimental Facility9d

New diagram for CP violation beyond the SM

Charged particle
detector

Neutral kaon beamThe probability of this rare decay is predicted to be one in several billion by
the SM. A huge number of neutral kaons can be produced at J-PARC using
the world highest intensity proton beam. The past experiment at the 12 GeV
proton synchrotron in KEK Tsukuba Campus achieved the detection sensitivityof one in tens of millions. In the KOTO (K0 at Tokai) experiment at J-PARC,
the achieved sensitivity is expected more than a thousand times that of the pastexperiment. Therefore, the rare decay event is expected to be detected for the firsttime.

Gamma-ray detectordSearch for muon to electron transition phenomenon— Direct evidence of the fraying of the Standard Model —Feature of muons
The Standard Model (SM) is the basis of modern particle physics. However, it isthought to be only an approximate model at low energy, of a more fundamental
and unknown theory of the elementary particles. A muon is one of the leptons
in the SM, and about 200 times more massive than an electron of the same
lepton family. Charged leptons (electron, muon, and tau) can not change into
other leptons without accompanying neutrinos*1. However, the muon to electrontransition is suggested to occur in theories beyond the SM. The experimental
observation of the muon to electron transition would be evidence for theexistence of an unknown theory beyond the SM.
*1 The transition may occur with a quite low probability through the neutrino oscillation phenomenon
discovered and established recently.

Looking at the muon to electrontransition in detail, unknown particles
and/or interactions are related.

COMET Experiment
In the COMET (Coherent Muon to Electron Transition) experiment, muon to electron transition (reaction that a muon changes into anelectron) event is searched for using a pulsed muon beam obtained at the HEF. Since this transition is a very rare event, huge numbersof muons are required. The events should be measured during the timing between the beam pulses to avoid huge background events(noise) which originate from the beam. The key to success for the experiment is to form a muon beam of very high intensity in a clean
pulse shape. In order to identify the “muon to electron transition” event, electrons with a characteristic momentum are selected. The
momentum of the electron is measured with a high momentum-resolution detector in order to discriminate the signal electrons frombackground electrons from known reactions. In the COMET experiment, we aim to achieve the sensitivity of 10 -16, one in ten thousandof that in the past experiments, by using state-of-art technology both in the beam and detectors.

Pion production target

Proton beam

Muon transportsolenoid magnet

Pion capturesolenoid magnet

Muon stopping target

Electron transportsolenoid magnet

Electron

Pion to muon decay
Electron detector-Detector solenoid magnet-Straw-tube tracking detector-LYSO calorimeter

J-PARC Hadron Experimental Facility http://j-parc.jp/March 2023