Final report of the committee on Future Projects inHigh Energy Physics
Masaki Ishitsuka, Hirokazu Ishino, Masaya Ishino, Wataru Ootani,
Takashi Obina, Masaaki Kitaguchi, Ryuichiro Kitano, Hiroshi Sakai,
Tomoyuki Sanuki, Nanae Taniguchi, Makoto Tomoto,Takeshi Nakadaira, Hajime Nanjo, Takeo Higuchi, Shigeki Matsumoto,
Shigetaka Moriyama, Tamaki Yoshioka
06 September 20171

Preface
In February 2012, the subcommittee on Future Projects in High Energy Physics
made the recommendation that ”should a new particle such as a Higgs boson witha mass below approximately 1 TeV be confirmed at LHC, Japan should aim for an
early realization of an e+ e− linear collider” and ”should the neutrino mixing angle
θ13 be confirmed as large, Japan should aim to realize a large-scale neutrino detector
allowing studies on CP symmetry through neutrino oscillations”. In the same year,
there were two extremely important results in physics: the discovery of the Higgs
boson and the establishment of three-generation neutrino mixing.
The charge of the committee on Future Projects in High Energy Physics is to
update the 2012 report based on recent progress in the field and to examine thefuture plans of the Japanese physics community on a time scale longer than 10 years
in the future. The most important task is to report on future large-scale projects
while taking into account their competitive and cooperative relationships with otherprojects around the world. The committee investigated progress in physics since2012 and discussed new future plans that Japan should realize and lead.
It is also important to continuously advance particle physics research on a varietyof fronts, including for example, the Belle II experiment which will start physics
running in the next fiscal year, and medium- to small-scale experiments at J-PARC
and other facilities. Realization of future large-scale projects may depend on the
results obtained from these smaller-scale projects. In addition, a coherent strategy
for cosmological observations and underground particle physics experiments needs tobe adopted since these fields are becoming more and more important. The committee
has therefore surveyed and examined these issues.
The committee on Future Projects in High Energy Physics consists of members
the High Energy Committee and relevant persons appointed thereby. This reporthas been prepared mainly by the latter group based on the discussion among allcommittee members.

2

Contents
1 Status and Prospects for Particle Physics
1.1 Current Status of Particle Physics . . . . . . . . . . . . . . . . . . . .
1.2 Prospects for Particle Physics . . . . . . . . . . . . . . . . . . . . . .

88102 Energy Frontier Physics
14
2.1 International Linear Collider . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 LHC Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Future Accelerators and Novel Acceleration Techniques . . . . . . . . 23
3 Neutrino Oscillation and Proton Decay
25
3.1 Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Future Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4 Flavor Physics
31
4.1 SuperKEKB / Belle-II Experiment . . . . . . . . . . . . . . . . . . . 32
4.2 Kaon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.3 Muon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.4 Neutron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5 Non-accelerator Particle Physics
38
5.1 Particle Physics in Underground Laboratories . . . . . . . . . . . . . 38
5.2 Cosmological Observations . . . . . . . . . . . . . . . . . . . . . . . . 42
6 Human Resource and Technology Development

347

Recommendation
In 2012, not only was a Higgs boson with a mass of 125 GeV discovered at the LHC,
but three-generation neutrino mixing was also established. Taking full advantage ofthe opportunities provided by these discoveries the committee makes the following
recommendations concerning large-scale projects, which comprise the core of futurehigh energy physics research in Japan.
• With the discovery of the 125 GeV Higgs boson at the LHC, constructionof the International Linear Collider (ILC) with a collision energy of
250 GeV should start in Japan immediately without delay so as to guidethe pursuit of particle physics beyond the Standard Model through detailed
research of the Higgs particle. In parallel, continuing studies of new physics
should be pursued using the LHC and its upgrades.
• Three-generation neutrino mixing has been established and has provided a pathto study CP symmetry in the lepton sector. Therefore, Japan should promote the early realization of Hyper-Kamiokande as an international
project due to its superior proton decay sensitivity, and should continue tosearch for CP violation with the T2K experiment and upgrades of the J-PARC
neutrino beam.
The High Energy Committee should pursue all available options to achieve the early
realization of these key, large-scale projects.
It is important to complete construction of SuperKEKB and start physics studies
as scheduled. Some of the medium- and small-scale projects currently under consideration have implicit potential to develop into important research fields in the future,as happened with neutrino physics. They should be promoted in parallel in orderto pursue new physics from various directions. Flavor physics experiments, such as
muon experiments at J-PARC, searches for dark matter and neutrinoless double beta
decay, observations of CMB B-mode polarization and dark energy, are considered to
be projects that have such potential.
Furthermore, accelerator R&D should be continued to dramatically increase particle collision energies in preparation for future experimental eﬀorts that may indicate
the existence of new particles and new phenomena at higher energy scale.

410-32cm
19

Theory of Everything

1032cm

10 GeV Quantum Gravity

GroundInflation-28
Unification
10 cm
Proton Decay
1015 GeV

1028cmDark Energy

Large Scale
StructureSpace

Dark Matter

10-24cm0νββ1011 GeV
B, D, K,
Hierarchyν, μ, n
Problem Dark Matter

10-20cm

Underground1024cmGalaxyFlavorEnergy Frontier1020cmHiggs Particle

107 GeVElectroweak
Symmetry Breaking
Nucleus10-16cm
103 GeV

Solar System
Human Being

1016cm1cm
10-13 GeV

Figure 1: The figure shows the hierarchy of energy scales in the study of matter andphysical processes as represented by an ouroboros (Sheldon Glashow, who received
the Nobel Prize in Physics in 1979, introduced the idea to map the energy scales
onto the ouroboros, and the idea is reused here). Going backward in time, thescale of the universe shrinks smaller and smaller until finally reaching the moment
when the universe was born. The physics of both extremely large and extremely
small scales are connected and an understanding of microscopic physics is therefore
essential to understanding large scale physics. In other words, in order to understandthe physics of the beginning of universe, we must also understand the microscopic
world of elementary particles. It has long been a dream of physicists to construct”The Theory of Everything”, and we are approaching this goal from many fronts
including, energy-frontier experiments, flavor experiments, underground experiments
and cosmological experiments.
5

2012

2015

2018

2021

2024

2027

2030

2033

2036

2039

ghZZ ~0.4%, Yb,Yc,Yτ~1-2%, ghgg ~2%+ -Br (H → inv.) < 0.3%,∆ΦCP∼ 4°,Γh~3%

constructione e (ILC)

Λ(Higgs composite) < 3TeV , mA(SUSY) < 1TeV*(2040-2050)

energyfrontierp-p (LHC)SUSY < 2.5TeV

Higgs
discovery

SUSY < 3 TeV

T2K

θ13
measured

CPV 3σneutrino 
proton decay
Hyper KCPV 5σconstruction

SN

st
Bu r

SN relic
Neutrino

Mass Hierarchy
5 ab-1Super KEKB

flavorconstruction

KOTO

± 6°

± 1.5°

R(D*) : ± 9%

± 3.2%± 2.1%< 14.7x10-9< 4.7x10-9

Br(B→K(*)νν)± 10%polarization F_L(B→K(*)νν)

± 8%

10-9

10-11

MEG
10-13

μ → eγProton Decay~1035 yr

50 ab-1

Φ3 : ± 14°

Br(τ→μγ) : 45x10-9

10-8O(100 events)
MEG II
10-14

Muon rare decay

DeeMe DeeMe 10-14
10-13 COMET 10-15

μ → e conv.

COMET
10-17

μ g-2 : 0.1ppm
μ EDM : 5x10-22 ecm

Muonneutron
dipole momentn : 10-26 ecm
degenerated

0ν ββ

FCC
HE-LHC

Yt , Yb , Yτ , Yμ ~ 10%

Yt , Yb , Yτ , Yμ ~ 20%

n : 10-27 ecm

n : 10-28 ecm

inverted hierarchy~ 40meV~ 20meVundergroundexperiments
Dark Matter

10-45cm2

10-46cm2

10-47cm2

POLARBEAR-2
GroundBIRD

CMB

10-48cm2

Simons Observatory
Simons Array

LiteBIRDCMB Stage-4

σ(r) = O ( 0.01 ) ( > 20)

Dark Energy

Primordial
B.H.

DESI, LSSTSuMIRe2012

2015

Euclid2018

2021

σ(r) = O ( 0.001 )>= 2σ ( Σ mν) = 15 meVσ(Nν)~ O ( 0.01 )σ ( Σ mν) = 40 meV

Cosmology

10-49cm2

σ(w) = 0.01
WFIRST-AFTA

2024

2027

2030

2033

2036

2039

running period of experiments
approved
* ILC(2040-2050)∆mt ∼50MeV, Yt ∼3−6%, λhhh∼15−30%,m(WIMP, stop, Higgsino) < 250GeV, gttZ(gttγ) < 1%

proposing planning experiment

Figure 2: Summary of timelines of ongoing and future projects described in thereport. Sensitivities numbers are shown at the times when the sensitivity is expected
to be realized (2012 – 2040).

6
1

The goal ; Theory of Everything
Approaches from Particle Physics

E

, &V

GK

2
K GF
KM E FL G L
K GKGF

K &GMHD FV
L GF DK&

VL
GK EE LH G D EF G (C F

VF O H
K

L

F

L GF
V
G
F G &2
N GD L GF

L

D

EMGF

M

FGE D
F
F

K

F
L GK
K

F F
FGE D

L G F
K G L
F D
KL
ML GFK)DGMF

GF FMEN GD L GF

F D L GF

F G L
D
L F
MF N KL H G M L GF

L E

H G D EF G (C F

F ML FG &2
N GD L GF F 2
E L P

FG E P F
E L P

F O &2 N GD L GF

)

F
K

F
L GF
V
G
F G &2
N GD L GF

D

L K

L

C

LL

V
S (

(
V
UUU K DD L GF

BG

L

F

S (

GD L GF

F ML
K O

FG

F

F ML

FG

LGL D E KK
V
FME
G
F
L GFK

F
L GF
V
F G L
FL
L GFK

F ML FGH
K GF
E KM E FLVGF G L
F ML FG E KK K

F
G

BG

F D L GF

&GKEGDG

VL
G
K EE L

F ML

S (

, 2

WT

2
K GF
KM E FL G L
K GKGFGC E LLK K D
GMHD FV
K D&GMHD F&2V
F O &2 N GD L GF

L M

E

G
F G
K EE L
LO F
E LL
F
FL
E LL

PL

F ML FG &2
N GD L GF

F ML GF

MF

F
L GF
V
G
F G L
FL
L GFK

K &2

F ML FG
DD L GFVH GLGF-

F ML FGH
K GF
E KM E FLVGF G L
F ML FG E KK KF

HLGF FME
N GD L GF
D HLG
F K K

L

KL

G
F G
K EE L
LO F
E LL
F
FL
E LL

ML GF GC E LLH EG DN L L GF D O N

IM L GF GGC E LLF D L GF

KL L

F G L
D
L F
MF N K

Approaches from Cosmology

Figure 3: List of future experiments that will probe ”physics beyond the standard
model”. Projects described in this report are listed vertically and the horizontal
axis lists major physics topics to be explored in order to achieve this goal. Each cell
describes the target physics parameters for each of these studies.

7

1

Status and Prospects for Particle Physics

1.1

Current Status of Particle Physics

Particle physics has evolved to understand the elementary units of matter as wellas the basic principles of the interactions between them. The standard model of
elementary particles is based on the local gauge symmetry, SU(3)C ×SU(2)L ×U(1)Y ,and successfully describes all physics processes at energies up to the electroweak
scale (approximately 100 GeV). Recently the discovery of the Higgs boson by theLarge Hadron Collider (LHC) experiments revealed that spontaneous breaking of
the electroweak symmetry is caused by the condensation of the Higgs field.
In spite of great success of the standard model, it is known to inadequately
describe physics processes up to the Planck scale. Indeed the model’s mass hierarchy
problem, which requires unnaturally large radiative corrections to the Higgs mass,can be solved naturally if there are new particles just above the electroweak energy
scale. A large number of new physics models, such as Super-Symmetric models ormodels with Extra-Dimensions, which include such particles have been proposed, andtherefore direct searches for new particles using energy-frontier accelerators is highly
important. Accordingly, further increases to the instantaneous luminosity of the
LHC as well as the extremely clean environment of the International Linear Collider(ILC) will play a key role in extending the reach of searches for new particles.
It is also important to precisely measure the couplings between the Higgs bosonand all the gauge bosons and the fermions as well as to further clarify the natureof the Higgs field. The precision of coupling constant measurements is improving
as the integrated luminosity of LHC increases. It will improve sharply when theHigh-Luminosity LHC (HL-LHC) and ILC projects start operations. Diﬀerences in
the measured coupling constants from the predictions for the standard model Higgs
boson will tell us about physics beyond the standard model.
At the same time, research in the flavor violation and the CP asymmetry ofquarks and leptons has been evolving to clarify the origin of matter in the universe
and the origin of quark mixing. As Japan is operating two world class accelerators,
KEKB and J-PARC, it leads the field of the flavor physics and is expected to play
an even greater role in future. If new physics exists at TeV energy scales, it may

8

eﬀect the branching ratio of rare decay processes or induce new sources of CP violation. In particular, eﬀorts to achieve higher precision measurements of various decay
modes of B, D, K, τ, µ have been performed. Recently several experimental results
report deviations from the predictions of the standard model. It is therefore highly
important to establish the validity of these anomalies and confirm their existencein other physical processes, since they may ultimately guide us to the origin of themechanisms and shape of physics beyond the standard model. To achieve this goal,
many eﬀorts to improve the performance of both accelerators and detectors making
this measurements are underway.
The discovery of neutrino oscillations indicates that neutrinos have tiny masses.
This fact implies the existence of physics beyond the standard model. In addition,
the pattern of flavor mixing in the neutrino sector is largely diﬀerent from that in thequark sector. This may suggest that the origin of the neutrino masses is diﬀerent fromthat of the quark masses. Indeed, if neutrinos are Majorana fermions, their masses
could be related to physics at extremely high energy scales, as predicted by the seesaw
mechanism. While the neutrino mixing angle of θ13 has been measured by the T2K
experiment and reactor neutrino experiments, there are still important questions inneutrino physics, including whether or not oscillations violate CP, whether neutrinos
are Dirac or Majorana fermions, the absolute value of the neutrino masses and thenature neutrino mass hierarchy (that is, whether it is normal or inverted). The
search for proton decay, which is considered a signature of GUT models, has been
explored by Super-Kamiokande with the highest sensitivity in the world. So far aproton decay signal has not been observed and the lower limit on the proton lifetime
has been established as 1034 years for the decay mode p → e+ + π 0 .
Connections between cosmology and particle physics are ever increasing. Cosmology has reached the level of a precision science due to recent improvements incosmological parameter measurements and starting with the results from the Planck
project we have gained deep insight into the universe. In particular, the energy densities of dark energy and dark matter are known to be 69% and 26% of the totalenergy density of the universe, while baryonic matter accounts for only 5%. In addition, cosmic density fluctuations have been observed to be nearly scale-invariant,pointing strongly to inflation scenarios in the early universe. These results provide
the basis for constructing the standard model of cosmology but pose new problems
for particle physics at the same time. It is currently impossible to understand thegeneration of the observed matter-antimatter asymmetry in the universe, the originof its dark matter, the mechanism of inflation, the nature of dark energy, as wellas many other topics in cosmology within the framework of the standard model ofparticle physics. Clearly some new physics beyond the standard model must have9

played an important role in the evolution of the universe. In order to solve thesecosmological problems, new knowledge of physics beyond the standard model fromcurrent and future experiments, such as the LHC and the ILC, is indispensable.

1.2

Prospects for Particle Physics

The standard model of particle physics, as an excellent eﬀective theory up to theelectroweak energy scale, has been completed by the discovery of the Higgs boson in2012 and subsequent measurements of its mass, spin, and parity. At the same time,
pathways to discover new particles and phenomena beyond the standard model andto elucidate physics at higher energy scales have have become clear. Energy-frontier
experiments have adopted an extremely eﬀective approach of seeking to discover
new particles directly and to reveal the vacuum structure by precisely measuring the
production and decay of the Higgs boson.
The direct discovery of new particles is the most important approach to pursue
with energy-frontier accelerators. As the luminosity of the LHC increases, the frequency of collisions with partons carrying higher momenta increases and the ability
to discover new particles, such as super-symmetric particles, is extended. The LHC
experiments have a high potential to discover new colored particles, while the ILC,
which will observe collisions in a cleaner environment, has better sensitivity to find
new particles without color. It is necessary to exhaustively survey the widest possible
parameter space using both the LHC and the ILC.
When new particles are discovered, their interactions and quantum numbers need
to be determined to clarify the laws of physics behind them. In addition, the entire
landscape of new physics should be explored by searching for as yet undiscovered
particles predicted by the theoretical frameworks describing those laws. The ILC
and the High-Luminosity LHC will be used to extract as much physics as possible,
but if it turns out that the next generation of accelerators is necessary to reachbeyond the standard model, the energy scale of those machine should be clearly
determined, and the technical developments will be advanced in parallel.
The other important approach is to measure various Higgs particle production
and decay processes and to determine the coupling constant of the Higgs boson with
W/Z bosons, the top, bottom, and charm quarks, as well as the τ and µ leptons.
Such measurements will determine if there is a universal relationship between themass of the elementary particles and the coupling constant as expected from thestandard model, or if diﬀerent relationships are seen between bosons and fermions,
up and down-type fermions, or the second and third generation fermions. If the
Higgs vacuum shows such an unexpected property, it will open up a path to elucidate
10physics beyond the standard model.
The LHC is only the accelerator that can currently generate Higgs particles, andthe precision of its coupling constant measurements will be improved intermittently
as the luminosity is increased in preparation for the High-Luminosity LHC project.
The ILC project is going to measure the decay rates of Higgs boson produced byelectron and positron collisions in an exceedingly clean environment and will thereby
achieve the most precise coupling constant measurements. Now that the mass of
Higgs particles has been revealed, the ILC accelerator is essential to the constructionof tera-scale physics models beyond the standard model and therefore needs to berealized as soon as possible.
At energy-frontier experiments, the typical approach to search for new phenomena
at high energy scales is the direct search for new particles. However, in terms of acomplementary approach, there are various indirect ways to explore phenomena athigh energy scales without increasing the energy of particle accelerators. It is possible
to find quantum eﬀects from new physics by increasing beam intensities and searchingfor rare phenomena that cannot be explained by the standard model. New physics
can be identified over energies up to about 10 TeV with precise measurements ofB meson, D meson, τ -lepton and other decay processes, and CKM matrices using
the Belle II and LHCb experiments. Furthermore, similar studies are possible withmeasurements of the rare kaon decay processes in the KOTO and NA 62 experiments.
The full search for physics beyond the standard model will combine the results ofthese varied approaches.
Searching for symmetry breaking is an eﬀective experimental technique to probe
new physics at higher energy scales. Flavor symmetry is an imperfect symmetry, but
lepton flavor is a conserved quantum number in the standard model. The branching
fractions of charged-lepton-flavor-violating processes are very small even if the lightneutrino mass is taken into account. An upper limit on the branching ratio of theµ → eγ process, which is one of the charged-lepton-flavor-violating processes, hasbeen produced by the MEG experiment. The discovery of new physics processes
which break flavor symmetry as result of physics at higher energy scales is expectedwith the improved sensitivity of searches for µ → eγ process with the MEG-II experiment, µ → e conversion with the COMET and Mu2e experiments, and rare τ
lepton decays with the Belle II and LHCb experiments. CP symmetry is not a strict
symmetry, and the degree to which it is broken as introduced by the KobayashiMaskawa theory is too small to explain the matter-antimatter asymmetry of theuniverse. Though the standard model predicts a minuscule neutron electric dipole
moment, there is no reason to expect CP violation is small at higher energy scales.
Accordingly, there is the possibility to discover evidence of CP symmetry violation11

from new physics processes by precise measurements of B decays with Belle II, rarekaon decays with KOTO, and precision measurements of the neutron electric dipole
moment with experiments at TRIUMF, PSI, and J-PARC. Hints of baryon-numberviolating processes predicted by GUT at even higher energy scales (i.e. greater than
1015 GeV) may be obtained from observations of proton decay with extremely large
scale neutrino detectors. The decay process p → ν̄ + K + is predicted by various
SUSY-GUT scenarios and will be searched for at the Hyper-Kamiokande and DUNE
experiments with much higher sensitivity than the Super-Kamiokande experiment.
Furthermore, the Hyper-Kamiokande experiment will be able to measure the lifetimeof the proton via the p → e+ + π 0 process, which is predicted by many GUT models,with one order of magnitude better sensitivity than the current upper limit.
Observations of neutrino oscillations strongly support the presence of physics beyond the standard model. To understand the phenomenon in detail, further studiesof the neutrino are necessary. Three-generation neutrino mixing has been establishedby recent measurements by the T2K and reactor neutrino experiments, and in addition, hints of CP violation in the neutrino sector have been reported by the T2K
experiment. It is important to continually increase the intensity of the J-PARC neutrino beam and advance studies of neutrino oscillation, including the search for CP
violation at the T2K extension T2K-II, on the way to realizing Hyper-Kamiokande.
Precise measurements of CP violation and the determination of the neutrino mass
hierarchy will be carried out by future experiments, namely Hyper-Kamiokande with
the J-PARC neutrino beam and its competitor, the DUNE experiment in the U.S.
A full picture of neutrino flavor mixing will be revealed with these experiments.
Hyper-Kamiokande has better sensitivity to the CP violation search and has broader
physics capabilities, including supernova neutrino observations, and an early start ofthe project is essential. It is important to proceed with further upgrades of the JPARC neutrino beam while improving results from existing projects. It is considered
as one of the major goals of future projects in particle physics to obtain a unified
picture of the flavor structure and origin of CP violation through the combinationof knowledge of flavor mixing in the lepton and quark sectors. In addition, searches
for neutrinoless double β−decays (0νββ) by, for example, KamLAND-Zen are important to settle the issue of whether the neutrino mass is Majorana- or Dirac-type.
If the neutrino mass is Majorana-type, it supports the notion that the tiny neutrino
masses are a natural consequence of the seesaw mechanism, and the baryon-antibaryon asymmetry can be explained by the leptogenesis scenario.
Significant progress is also expected in understanding the properties of dark matter. There are various dark matter candidates, including weakly interacting massive
particles (WIMPs), coherent oscillation of scalar condensation, and others. Among12them, there is also a scenario in which thermally produced WIMPs were not entirely annihilated in the early universe and left its thermal bath. In order for suchthermally-produced WIMPs to be the main component of dark matter, their pairannihilation cross section should be at about 1 pb, which corresponds to the crosssection obtained from exchanging a particle with a mass of order 100 GeV to 1 TeV.
Accordingly, the dark matter particle may be embedded in a particle physics modelthat resolves the hierarchy problem. In fact, the cross section of supersymmetricdark matter particles scattering oﬀ nuclei may be large enough to be detected bycurrent and future direct dark matter detection experiments. Hence, the determination of the nature of dark matter particles by future direct detection experimentswith large-scale detectors, such as the XMASS, LZ and XENONnT is anticipated.
At the LHC, dark-matter particles may be found in measurements of missing
transverse momentum. With the discovery of such a candidate, detailed studies of
its properties will be necessary to confirm whether it is really consistent with thedark matter in the universe. For a weakly interacting dark matter candidate, those
properties can be investigated in further detail with the ILC if the production crosssection is large enough. Such studies provide important information to determine theproduction and interaction cross section of dark matter particle. This is essential fordark matter searches and to our understanding of the thermal history of the universe.
Progress is also expected in the understanding of the evolution of the universe
through inflation, mainly from observations of the cosmic microwave background(CMB). Especially, the B-mode polarization of the CMB can be produced fromprimordial gravitational waves due to the oscillation of spacetime during inflation.
Therefore, the observation of this polarization, if it is made, will strongly support
the inflation scenario, and in addition, it will provide information on the energy scaleof inflation. If the tensor-to-scalar ratio, which represents the intensity of primordial
gravitational waves, is larger than 0.03 the Simons Array ground-based experimentis expected to discover the B-mode polarization. Future projects searching for thispolarization are being considered both in and out of Japan using scientific satellites,
which will enable all-sky observations without interference from atmosphere. These
projects will search for B-mode polarization with sensitivity to tensor-to-scalar ratios
of 0.001 and can potentially validate major inflation models. In Japan, the LiteBIRD
project aims for an early realization of these measurements by focusing on specificscientific goals. In addition, progress in the study of dark energy and the origin of theaccelerating expansion of the universe, is expected from the SuMIRe project, whichis in operation at the Subaru Telescope, and from future wide sky survey projects
on both the ground and in the space.

13

2Energy Frontier Physics

Overview
Research at energy frontier aims to discover new particles and phenomena as well as
determine their properties and the laws of nature governing them using high energy
particle accelerators. Numerous accomplishments in this area include the discoveryof quarks, leptons, and gauge bosons as well as the measurement of their quantum
numbers, coupling strengths, symmetries of their interactions and the number ofparticle generations. Precision measurements performed at the LEP, the SLC, andthe Tevatron provided firm ground for the gauge sector that forms the basis of thestandard model. After the discovery of the Higgs boson with a mass of 125 GeV
in the year 2012, its spin-parity has been determined to be 0+ and the precision of
the measurements of the couplings to the gauge bosons and the fermions has been
improving. These eﬀorts have advanced our understanding on the origin of the massof elementary particles and the structure of the vacuum. The next targets for energyfrontier experiments are the direct search for new particles and phenomena and theprecise measurement of the coupling constants of the Higgs boson to the elementary
particles in order to determine whether there is any deviation from the prediction of
the standard model.
In order to study higher energy phenomena at hadron colliders such as the LHC,
either the beam energy must be increased with stronger bending magnets to keep
particles on their circular trajectory or the luminosity must be increased to obtainhigher collision rate of high-energy partons. In contrast, for lepton colliders, suchas a linear collider, which do not suﬀer from acceleration losses due to synchrotron
radiation, the collision energy must be increased, or alternatively a heavy lepton(muon) collider must be used.
Since the beams at hadron colliders consist of protons or antiprotons, which arecomposite particles, a wide variety of physics phenomena can be studied from a broad
perspective depending on the energies carried by the colliding partons, including thestandard model, the top quark, the production and decay of the Higgs boson, andsearches for new particles. On the other hand, there are issues that come from multiple interactions occuring in the same beam bunch (pile-up) and large backgrounds
coming from non-colliding partons. Since all the beam energy at lepton colliders isconverted into the reaction, the final states are clean and thus precision measurements can be performed. Once new particles have been discovered, their properties
can be studied in detail at such machines. In addition, the beam energy and thepolarization can be adjusted to separate the intermediate and final states of the in-14

teractions, enabling the determination of the symmetry in the reaction as well as thequantum numbers of the new particles.

2.1

International Linear Collider

The ideal machine to further study the Higgs boson discovered at the LHC is considered to be the International Linear Collider (ILC). At the ILC linear accelerators will
be installed in a tunnel to accelerate electrons and positrons in opposite directions
and produce head-on collisions. Unlike circular colliders, the ILC can start with anaccelerator length providing the energy required for the initial target of the physics
study, and subsequently extend its length to increase the energy according to theresults from the initial study. The center-of-mass energy of 250 GeV can be realizedwith a 20 km straight machine and 500 GeV can be reached if it is extended to 30 km.
A future option to upgrade to 1 TeV or even higher is also within the ILC’s scope
when possible improvements in accelerator technologies are considered.
The initial target of the ILC is to clarify the Higgs boson. The properties of the
Higgs, boson such as its couplings to other elementary particles will be studied indetail with a center-of-mass energy of 250 GeV, which will subsequently determine
the future direction of searches for physics beyond the standard model.
The cross-section of e+ e− → Zh is maximized at the center-of-mass energy of
250 GeV, and 5 × 105 Higgs bosons will be produced with an integrated luminosity of
2 ab−1 in a clean environment with a good signal-to-noise ratio. The Higgs couplings
can be precisely measured in a model-independent manner through the analysis of
the recoiling Z-boson from this process. The gauge couplings can be determined witha precision of 1% and the Yukawa couplings to the bottom quark, charm quark, and τ
lepton can be determined to 1–2%. The hγγ coupling can also be determined to 1–2%
by taking advantage of the synergy with the LHC. Even if the Higgs boson decays
to invisible particles such as dark matter, the branching ratio sensitivity will bebetter than 0.3%. The mass and total width of the Higgs boson can be determined
to within 15 MeV and 3%, respectively. The CP phase angle of the Higgs boson
can be measured with a precision of 4◦ . Any deviation from the expected coupling
constants of the standard model will be a clear evidence of new physics. Furthermore,
the pattern of such a deviation will help pin down the new physics model. If the
Higgs boson is a composite particle, its energy scale can be probed up to 3 TeV. If
the Higgs boson is an elementary particle as in SUSY models, the mass of the heavy
Higgs boson can be probed up to 1 TeV.
New particles that may escape detection at the LHC could be discovered at ILC
through their direct production. For new particles that do not create a mass peak,15

such as supersymmetric particles and light dark matter particles with no color charge,
it may be diﬃcult to discover them at LHC due to overwhelming backgrounds. At alepton collider, though, their discovery would be certain if the collision energy exceeds
their production threshold. New particles without color charge can be discovered
via their decay products or using initial state radiation and it may be possible todetermine the properties of dark matter particles through detailed measurements ofthis kind. In addition, phenomena or particles that are not currently predicted inany model may also be discovered. Furthermore, new physics at energy scales much′higher than the collision energy, such as the Z bosons up to and exceeding 10 TeV,
can be studied indirectly through comparison with precise theoretical calculations.
The possibilities to study fundamental physics which play an essential role in thevery early universe, such as those predicted in Grand Unified Theory (GUT), arealso being discussed.
The ability to extend the energy of the ILC is a great advantage it has over circularcolliders. After detailed studies of the Higgs boson at a center-of-mass energy of
250 GeV, various physics can be studied by increasing the energy to exceed the energy
thresholds for important processes, including top-pair production at around 350 GeV
and e+ e−→ Zhh and e+ e−→ tt̄h at around 500 GeV. The measurement of the triple
Higgs self-coupling λhhh performed mainly through the double Higgs radiative processe+ e−→ Zhh is considered to be one of the most important goals of the ILC since itwould allow us to directly probe the origin of the electroweak symmetry breaking.
It can be measured with a precision of 15–30% with an integrated luminosity of
4 ab−1 around 500 GeV. The top-Yukawa coupling can also be determined throughthe reaction e+ e− → tt̄h with a precision of 6(3)% at a center-of-mass energy of500(550) GeV.
Overall Plan, Framework, Budget and Schedule
ILC will be able to make significant progress, not only in the understanding of elementary particle physics and cosmology, but also in advanced technologies such as
super-conducting accelerators. Furthermore, it would lead to the construction of a
new international research center integrating academic, educational, and technological activities.
From 2000 to 2002, the world-wide high energy physics community, mainly represented by Asia, the U.S. and Europe, established a consensus with the aim to
jointly constructing an electron-positron linear collider as a priority for its next main
project. According to the agreement, the physics studies at ILC and the designof the accelerator and detectors have been performed as fully international activities. Notably, after the Global Design Eﬀort (GDE) was established in 2005 bymajor particle physics laboratories operating high energy accelerators (KEK, DESY,
16

Fermilab), global activities had been promoted intensively to further the detailed
technical design of ILC. Consequently, the Technical Design Report (TDR) was published in June 2013. More than 1000 researchers and engineers, and more than 300
laboratories, universities (over 40 in Japan), and many private companies participated in these R&D eﬀorts. As the Asian research center, the Japanese ILC teamincluding KEK plays a leading role. Synchrotron light facilities, based on the similar
superconducting accelerator technology to that is developed by the ILC project, arebeing constructed in Germany and the U.S. The facility in Germany started operation in July 2017. Detector R&D has been advanced by two large international
collaborations, corresponding to two experimental proposals.
The GDE was dissolved with the completion of the TDR and two new organizations were set up in 2013. One is the Linear Collider Collaboration (LCC), which
coordinates international R&D eﬀorts for next-generation linear accelerator projects,
and the other is the Linear Collider Board (LCB), which oversees the LCC. In 2013,
the ILC site evaluation committee of Japan announced the result of their site assessment that the Kitakami site is evaluated to be the best domestic candidate site for
the ILC. Accordingly, the LCC started site-specific design studies for Kitakami site.
The construction of the ILC in Japan is supported in Europe, Asia and the U.S. aspart of their respective particle physics research strategies.
The schedule of the ILC project aims to start the operations sometime in the late
2020s to the early 2030s following three development phases, the pre-preparation
phase, the preparation phase and the construction phase. In the pre-preparation
phase, which is the current phase, various issues are being tackled, including criticalissues raised by MEXT-established ILC Advisory Panel, the detailed design of theaccelerator and detectors, the site-specific design of the facility, beam acceleration
tests and the development of the construction plan. All of these items are crucial
for the approval of the ILC project and will be completed in 2017 or 2018. The
project will enter the main preparation phase once it has been approved, followed by
the construction phase, which will take about nine years. The ILC will then start
operations thereafter.
The TDR published in 2013 is based on a reference design assuming a centerof-mass energy of 500 GeV which estimated construction and annual operation costs
to be 7.78 billion USD and 390 million USD, respectively. Cost reduction eﬀorts
making use of possible technological innovations and technical design optimizations
are underway.
Now that the Higgs boson has been discovered, the first issue to be targeted by
the ILC is the detailed study of its properties. Therefore, a staging scenario is being
seriously considered which starts with the construction of the accelerator for the17

center-of-mass energy of 250 GeV and is later followed by step-by-step increases inenergy. This staging approach will lead to a significant reduction of the initial costs
and is considered to be crucial for the early realization of the ILC.
The Japan Association of High Energy Physicists (JAHEP) proposed the staging approach to start with a 250 GeV-ILC in October 2012 and recently made this
proposal again July 2017, calling for the early realization of the ILC as a Higgs
factory.
Other Higgs Factory Projects
A center-of-mass energy of 250 GeV is now an important target after the discoveryof the Higgs boson. A shorter version of the collider is being considered for the
early realization of the project. The Compact Linear Collider (CLIC), which is anelectron-positron linear collider at a multi-TeV center-of-mass energy exceeding thatof ILC has been studied within the framework of a worldwide collaboration leadby CERN. As a Higgs factory, conventional normal-conducting X-band accelerator
technology is being considered instead of the two-beam acceleration scheme that isthe current baseline technology for CLIC.
The feasibility of the FCC-ee, an electron-positron circular collider installed in the
100 km FCC tunnel is being studied by CERN. Center-of-mass energies ranging from250 GeV to 350 GeV are envisaged. Similarly, a circular collider using a 54 km tunnel(CEPC-SppC) is being considered in China, either as an electron-positron collider
or a proton collider. Center-of-mass energies of 240 GeV and 100 TeV, respectively,
are envisaged for these options.
Since the energy loss due to synchrotron radiation rapidly increases at higher
beam energies in circular accelerators, it is not possible to significantly increase thecenter-of-mass energy for circular electron-positron colliders. Even if the ILC starts
as a 250 GeV “Higgs Factory”, the study of the Higgs self-coupling and searches forunknown particles and phenomena can be performed by extending the accelerator
and increasing the collision energy. This energy extensibility is a great advantage
over other Higgs factory projects based on circular colliders. In order to retain
its significance as a “Higgs Factory”, it is important to realize the ILC as soon
as possible, while carefully watching the development of circular electron-positron
colliders.
ILC Detector Technology R&D
ILC detector technology R&D is being undertaken by two international collaborations responsible for the designs, ILD and SiD. To improve the hadron jet reconstruction scheme (Particle Flow Algorithm) invented at the LEP experiments, a pixel vertex detector, a large-solid-angle tracking detector, and a fine-segmented calorimeter18

which is able to distinguish each showering particle, are being developed. In Japan,
a version with a vertex detector surrounding the interaction point, a calorimeter,
and a large TPC tracking detector are being developed. In close collaboration withtheory groups, sensitivity studies and optimization of the detector concept are being advanced. Japanese physicists participate mainly in the ILD conceptual design,
leading the ILC physics and detector studies in Asia and also organizing an international collaboration with our European and U.S. counterparts. Japanese detector
operation experience with Belle II, ATLAS and the T2K near detector has provided
useful information used to develop important components of ILC detectors.
ILC Accelerator R&D
In terms of ILC accelerator R&D, important technologies such as polarized electron
sources, positron beams, flat ultra-low emittance beams, high gradient superconducting accelerating cavities, and nano-beam focusing are being developed.
The ILC 1.2 m long accelerating cavities formed from nine superconducting Nb
nine-cell cavities are cooled down to 2 K and operated at 31.5 MV/m. ILC consistsof 1700 cryomodules and 15000 cavities. In order to fabricate such a large number ofcavities within a reasonable time period and at a reasonable cost, all while meeting
the required performance specifications, the manufacturing process has to be tightly
controlled. One of the goals set in 2012 is to achieve a cavity production yield greaterthan 90%, satisfying the 35 MV/m gradient requirement, which has almost alreadybeen met. The same manufacturing technology of superconducting cavity and cryomodule as ILC is being used at a synchrotron light facility now under constructionin Germany (an x-ray free electron laser facility), and an average accelerating gradient of 27.5 MV/m has been achieved, exceeding the required 23.6 MV/m, for its
800 cavities. The main issue going forward is further cost reduction. The industrial
fabrication of ILC components is being intensively advanced by design and optimization of cavity and cryomodule production lines and by constructing a pilot plant for
production system engineering.
KEK operates key test facilities for ILC, including The Super-conducting RF
Test Facility (STF) and the Accelerator Test Facility (ATF). The aim of the STF
is to demonstrate the performance of the ILC’s main linac cryomodule (a cryogenic
chamber containing the super-conducting accelerator cavities). High-power RF tests
were carried out using 12 nine-cell superconducting cavities in a cryomodule, wherean acceleration gradient exceeding the ILC requirement of 31.5 MV/m was achieved
in eight out of twelve cavities. Four of those showed a gradient higher than 35 MV/m.
R&D work into thing such as the optimization of the cryomodule production process
is now underway in order to achieve component stability while meeting performancerequirements.
19

The aim of the ATF is to generate a ultra-low emittance beam using radiation
damping, and to develop and demonstrate the beam diagnostic system with using the beam as well as the final focusing system. The ATF has already achieved
a 0.015 mm·mrad normalized vertical-emittance, surpassing the ILC requirement.
Furthermore, using this ultra-emittance beam a beam size as small as 41 nm andapproaching the target of 37 nm, has already been achieved at the prototype ILC
final focus system (ATF2). Precise control of the beam position is required for the
stable beam collision at ILC. The beam position jitter has been successfully reduced
from 410 nm to 67 nm using a feedback system developed at ATF2. The jitter islimited by the accuracy of the beam position monitor.
A polarized electron source has been developed by SLAC and Nagoya University
and has already achieved 90% polarizations. The issues here are developing anddemonstrating a laser drive system to generate a 1 ms long pulse train. Activities
related to the production of positrons including, research and development towards
a superconducting helical undulator and the prototyping of a generation target, arein progress. Further, an alternative positron source using an electron beam has been
proposed in Japan.

2.2

LHC Upgrade

Introduction
The two large-scale detectors, ATLAS and CMS, were designed and constructed at
two of the collision points of Large-Hadron-Collider (LHC) with the aim of discovering a Higgs boson and searching for physics beyond the standard model. The LHC
accelerator has been running at a center-of-mass energy greater than 7 TeV since2010. The Higgs boson was found in 2012 with a mass of 125 GeV and a spin-parity
state of 0+ . As the beam energy and the instantaneous luminosity of the LHC improves continuously, the reach of new physics searches are being extended and theprecision of coupling constant measurements of the Higgs boson to both gauge bosons
and fermions are also improving. The LHC has been developing the energy frontier
since the beginning of its operations. The final focusing magnet of the LHC wasdesigned and constructed by a collaboration of KEK (Japan) and Fermilab (USA)and it played a key role in achieving an instantaneous luminosity of 1.4×1034 cm−2 s−1
in 2016, which is 40% higher than the design value. The performance and stabilityof the final focusing magnet is one of the most important components for success.
There are 16 Japanese universities and one laboratory in the ATLAS collaborationand they have been making significant contributions to the design, construction,commissioning, and operation of various systems including the solenoid magnet, the20

silicon-strip detector, and the muon-trigger system. In recent years, grid computing
systems have become a vital tool for processing large scale data and obtaining physics
results. Japan also makes significant contributions to the grid computing system. On
top of these contributions, Japanese ATLAS collaborators are achieving outstanding
physics results, an example of which is the discovery of the Higgs boson decaying
to two photons. Further, several important physics topics have have been exploredby young Japanese researchers and students, including contributions to searches for
SUSY particles and precision measurements of the standard model particles.
Physics researches in the LHC upgrade project
After the discovery of the Higgs boson, a new era in the exploration of physics beyond
the standard model has begun. Direct observations of new particles as well as precise
measurements of the Higgs boson are considered to be eﬀective approaches. In terms
of precision measurements of the Higgs boson, its couplings to the W/Z-boson, top
quark, bottom quark, τ and µ will be measured with a precision of 1 to 10% withan integrated luminosity of 3000 fb−1 . It is crucial to test whether the couplings ofthe Higgs boson with other elementary particles are consistent with the predictions
given by the standard model or there are diﬀerent rules governing the coupling tobosons and fermions, up-type and down-type fermions, or the second and the third
generation fermions.
Direct searches for new particles is another main subject of the LHC upgrade
program and the strategy is to extend their reach by increasing the instantaneous
luminosity significantly. The targets of new physics searches are SUSY particles
produced by the strong interaction, third-generation squarks, which are expected
to be relatively light due to naturalness considerations, SUSY particles producedby the electroweak interaction, extra dimensions, and Z ′ . They may be observed asresonances or broad excesses over the standard model background. Taking the search
for the SUSY gluino as an example, the mass region up to 2 TeV has been excluded(13 TeV , 10 fb−1 ). By increasing the integrated luminosity to 300 fb−1 with 14 TeV,
the reach will be extended to 2.5 TeV, and then to 3.0 TeV with 3000 fb−1 .
The development of the High-Luminosity LHC
To realize the physics program with the High-Luminosity LHC (HL-LHC), the accelerator complex will be upgraded and operated according to the following schedule.
An integrated luminosity of greater than 100 fb−1 will be accumulated with a centerof-mass energy of 13 TeV by the end of 2018 (Run-2). During the subsequent two-year
shutdown (1) a new LINAC (LINAC4) will be introduced (2) the PS-Booster energy
will be increased from 500 MeV to 2 GeV for better emittance (3) the SPS will be
upgraded to increase the RF power and mitigate the electron-cloud problem, and21

then the bunch current will be increased by a factor of two. In the three years from2021 to 2023 (Run-3), the peak instantaneous luminosity will be increased to greaterthan 2.0×1034 cm−2 s−1 and an integrated luminosity to 300 fb−1 at a center-of-massenergy of 14 TeV will be collected. During a two year shutdown thereafter, various
major upgrades of the accelerator components, including the replacement of the final focusing magnets and the beam-separation dipole magnets, the upgrade of themain beam collimators, and the installation of new RF crab cavities, are scheduled
to kick oﬀ the High-Luminosity LHC project (HL-LHC) by the year 2026. In termsof machine operation, the instantaneous luminosity is controlled to 5.0×1034 cm−2 s−1
by introducing the luminosity leveling techniques to maximize the integrated luminosity with longer beam lifetimes. The plan is to accumulate 3000 fb−1 in ten years.
The HL-LHC project has been oﬃcially approved by the CERN council held in June
2016.
The key components for the success of the HL-LHC project are the performanceof the magnets placed around the beam collision points, notably the final focusing
quadrupole magnets and the beam separation dipole magnets, and the crab RF cavities. To increase the luminosity significantly, the beam will be defocused before
the collision point and then strongly squeezed (β ∗ = 0.40 m , the 2016 value, will bechanged to 0.15 m). To cope with this beam handling, the bore size of the final
focusing magnet needs to be enlarged and at the same time the field strength needs
to be increased from 8.6 Tesla to 11.4 Tesla. This will be done using a new superconducting material, Nb3 Sn, which will be applied as an accelerator component for the
first time. A small scale prototype magnet with a length is 25% of the final model
has been developed by a collaboration between CERN and and the U.S. that has
successfully conducted a 18 kA electric current. This current corresponds to 108%of the 16.47 kA operation current and meets the criteria for acceptance. Concerning
the beam separation dipole magnets, KEK has constructed a prototype magnet with
33% of the final design length and an operational 5.6 Tesla magnetic field has alreadybeen achieved. Crab RF cavities are used to rotate beam bunches and keep geometrical loss factors as low as possible even when the beam-crossing angle is large. Thisis the first time a crab RF cavity will be used in a proton machine. Based on the
success on the crab cavity at the KEKB factory, further contributions from Japanare highly anticipated.
Detector developments for HL-LHC
There are two major upgrade tasks for the ATLAS spectrometer, one is to replace
all the inner trackers and the other is to revise all the trigger and DAQ systems.
Aiming to keep the performance as high as possible in a high radiation background,five layers of pixel detectors and four layers of strip detectors with highly radiation
22

tolerant silicon sensors will be installed. Since the Japanese group has experience
assembling the ATLAS silicon detector as well as the fact there is a major siliconsensor vendor in Japan, further contributions from Japan are highly anticipated.
Accordingly, the Japanese group is currently focused on the pixel detector and isintensively developing the silicon sensor. Those eﬀorts will be followed by the detector
construction, installation and operation.
In terms of the triggering and DAQ, in order to eﬃciently record the target
physics objects under high luminosity conditions, the system will be designed withthe longer trigger latency, 10 µs, as well as an increased data storage rate of 1 MHz.
The current numbers are 2.5 µs and 100 kHz, respectively. Due to the longer latency,
sophisticated trigger algorithms that are usually used in the oﬄine analysis will be
introduced to the first stage hardware trigger. The ATLAS Japan group is focusedon these subjects and is intensively working on the development of the silicon sensors,
the triggers and the readout system.
The plan to upgrade the LHC beam energy
Technology developments for the HL-LHC project may open the door to upgrade the
LHC beam energy. There is a plan to construct the High-Energy LHC (HE-LHC)
with a center-of-mass energy of 33 TeV sometime after 2030 or preparing a large scaletunnel and increase the center-of-mass energy of 100 TeV (FCC). The key technology
is strong magnetic fields. Improving the critical current density performance of thesuperconductor, Nb3 Sn, to enable field strengths of 16 Tesla is required all while
enabling the magnets are robust enough for mass production. In parallel to thedevelopment of Nb3 Sn alternative materials such as ReBCO should be studied. The
development of the superconducting magnets for the HL-LHC project is considered
an important milestone for future energy-frontier accelerators. It is further important
to study various kinds of basic technologies related to accelerator magnets to makefurther improvements.

2.3

Future Accelerators and Novel Acceleration Techniques

Compact LInear Collider (CLIC)The Compact Linear Collider (CLIC) is a potential future linear collider which aims
to collide electrons and positrons at energies of several TeV. It has been proposed anddeveloped as an international collaboration between CERN and other laboratories in
the world. In order to produce an acceleration gradient of 100 MV/m, CLIC adopts
a two-beam concept, which utilizes the microwave produced by the deceleration ofa low energy drive beam instead of using conventional microwave sources such as
23

klystrons. The total length of CLIC is planned to be about 50 km and the linac
frequency will be 12 GHz (X-Band) in order to realize this compact size.
The technical challenges of CLIC are pulse compression of the drive beam, transmission of the microwave with high eﬃciency, fabrication of an accelerator tube whichis free from discharge even in the 100 MV/m, and alignment of the acceleration tube
to 10 µm precision over long distances among others. A number of cutting-edge
R&D eﬀorts are being pursued after the first conceptual design report (CDR) was
published in 2012.
Muon Collider
A muon collider is another candidate for a future energy-frontier collider. The highest
energy realizable by an electron-positron circular collider is limited by energy losses
from synchrotron radiation. However, this problem can be mitigated by using muons,
which are 200 times heavier mass than electrons, without losing the benefit of theclean collision leptons provide. On the other hand, there are still many technicaldiﬃculties to overcome in the production, cooling, acceleration, and collision of large
numbers of muons before they decay.
Laser-plasma accelerator, novel accelerator technologies
A laser-plasma accelerator has a potential to realize much higher accelerating gradients than accelerators based on conventional techniques. Recently gradients of afew GV/m have been achieved. Plasma acceleration driven by protons seems to bea promising approach because of high eﬃciency of the beam. However, in order toapply such techniques to future high energy physics experiments, many technical obstacles need to be overcome in the production of high-current beams with multiple
acceleration stages.
Advances in superconducting technologies have been remarkable in recent years.
New multi-layer materials which enable high-gradient cavities or superconducting
materials with higher transition temperature (Tc ) than niobium have been proposed
recently.
High energy physics experiments can reach much higher energy scales if thesetechnologies become practical reality.

24

3

Neutrino Oscillation and Proton Decay

Overview
One goal of neutrino physics is to reveal the full picture of neutrino mass and flavor
mixing, and then getting insight into physics at ultra-high energy scales well beyond
the standard model. In particular, measurements of CP violation in the lepton sector,
when compared with that of quarks and the CP symmetry of QCD will provide
insight into the underlying symmetry mechanisms and allow for the formulation of
a complete picture of the physics at these scales. Non-zero values of three neutrino
mixing angles have already been measured, opening the door to measure the CP
phase in neutrino oscillations. Theoretical studies suggest that CP violation in theneutrino sector may hold clues to understanding the origin of the matter-antimatterasymmetry in the present universe. The verification (rejection) of grand unified
theories (GUT) and baryon number violating processes are also important subjects,
which will be investigated by proton decay searches at large-scale neutrino detectors
with sensitivities far beyond current limits given by the Super-Kamiokande detector.

3.1

StatusNeutrino Oscillation
After the discovery of neutrino oscillations there has been significant progress in our
understanding of the properties and a full picture of neutrino flavor mixing is being
revealed. The squared mass diﬀerences of the three neutrino masses (m1 , m2 , m3 )have been measured to be |∆m232 | ∼ 2.4 × 10−3 eV2 and ∆m221 ∼ 7.5 × 10−5 eV2 , andtwo among three neutrino mixing angles were first measured to be θ23 ∼ 45◦ andθ12 ∼ 34◦ . The last mixing angle, θ13 , was measured to be ∼ 9◦ in 2012 by T2K
and the reactor experiments, Double Chooz, Daya Bay, and RENO, and now all
the mixing angles are known to be finite. On the other hand, CP violation in theneutrino sector is caused by an as-yet unmeasured CP phase (δCP ) in three neutrino
mixing, although hints have been reported by T2K. If future projects find that θ23
is close to 45◦ and δCP is close 90◦ , or that unitarity is violated in neutrino mixing,
these results could be taken as a hint of new physics at an ultra-high energy scale. In
addition, the sign of the mass squared diﬀerence ∆m232 = m23 − m22 is still unknown,that is it is not yet known whether the mass ordering is m3 ≫ m2 > m1 (normal
hierarchy) or m2 > m1 ≫ m3 (inverted hierarchy). The ordering of the neutrino
masses is an important ingredient to understand the origin of particle masses whentaken in comparison with quarks and charged leptons.

25Proton Decay
Searches for proton decay are currently dominated by the Super-Kamiokande detector, which has placed 90% C.L. limits on the proton lifetime as 1.6 × 1034 years
for p → e+ + π 0 and 5.9 × 1033 years for p → ν̄ + K + . Some GUT models predictp → e+ + π 0 as the dominant decay mode, while p → ν̄ + K + is considered as a major
decay mode in supersymmetric GUT models. These models predict proton lifetimes
from 1034 to 1035 years.

3.2

Future Plans

Based on the measured values of the three neutrino mixing angles, the largest possible CP asymmetry observable in long-baseline neutrino experiments is expected
to be 27%. 1 There is currently a tremendous opportunity to study neutrino CP
violation over a wide range of δCP with the combination of a large water Cherenkov
detector, Hyper-Kamiokande, and and upgraded J-PARC neutrino beam. Therefore
these two future projects should be advanced. A plan to upgrade the beam powerof J-PARC to 1.3 MW has been proposed and has been identified as the highest
priority project in the KEK Project Implementation Plan (KEK-PIP). The detector
design proposed by the Hyper-Kamiokande (HK) international proto-collaboration
has excellent sensitivity to proton decay and can thereby provide direct evidence of
the GUT models. The next-generation neutrino detector, DUNE in the U.S., is inprogress and as international competition is foreseen, the early realization of the HK
project is extremely important.
CP Violation Measurement
The CP-violating phase in the neutrino mixing matrix, δCP , can be determined bymeasuring the asymmetry between the νµ → νe appearance probabilities of neutrinos and antineutrinos. As the eﬀect of δCP appears in a small diﬀerence on thethe probability of νe appearance, the νµ → νe channel needs to be measured withlarge statistics. The T2K experiment has reported a hint of CP violation from themeasurement of neutrino and antineutrino oscillation probabilities using the J-PARC
neutrino beam. It is necessary to upgrade the J-PARC neutrino beam and extend
T2K’s operation period in order to confirm the existence of CP violation with higher
significance (T2K-II). T2K-II has the opportunity to measure CP violation at 3 σsignificance depending on the true value of δCP . In the event that CP violation is notobserved by T2K-II, the highest priority for the next-generation neutrino experiment1

This assumes three neutrino mixing as expressed by PMNS matrix. The asymmetry could belarger with new physics.

26

will be the its discovery. On the other hand, if CP violation is confirmed by T2K-II,
the task of next-generation experiments will be to precisely measure the mixing parameters including δCP and investigate existence of any deviation from the standard
three-neutrino mixing framework. In light of these prospects, Japan’s future plans
are unified in pursuit of the large water Cherenkov detector, Hyper-Kamiokande,
and various preparations are proceeding for its construction.
J-PARC Accelerator and Neutrino Beam
The neutrino beam at J-PARC is produced from 30 GeV protons accelerated by its
Main Ring (MR). The operation of the accelerator complex, including the neutrino
beam facility, started in 2009 and it has been providing stable neutrino beams for theT2K experiment since January 2010. In early 2011 operations were interrupted dueto the Great East Japan Earthquake but were resumed in December 2011. By 2016
the beam power reached ∼ 500 kW. The KEK-PIP has attached the highest priority
to the J-PARC accelerator upgrade plan, and consequently the upgrade is progressing
toward a beam power of 1.3 MW for the MR, which exceeds its original design valueof 750 kW. As a first step, the repetition rate will be increased by almost a factor of
two from the current value of 2.5 sec with an upgrade of the power supplies for themain magnets and the introduction of higher gradient RF cavities. These upgrades
will result in a beam power greater than 750 kW. Further upgrades of the RF power
supplies, the Fast-Extraction Kicker and the beam monitors are scheduled and theproton beam will be further optimized and stabilized. The number of protons per
spill will be increased and the beam power is expected to reach 1.3 MW by around2026. Irreplaceable components of the J-PARC neutrino beam facility, such as the
decay volume and beam dump, are designed to withstand beam powers greater than
3 MW, while the cooling capability of the target horns and the radioactive waste
water handling system need to be upgraded to realize a 1.3 MW beam power.
For the long-term future, upgrades towards the realization of a few MW accelerator are being considered. One possibility is to construct a new 8 GeV booster ring
between the current Rapid Cycling Synchrotron (RCS) and MR to allow more protons to be injected with smaller emittance. As a possible operational improvement,
the parallel operation of the fast extraction beam for neutrino experiments and slow
extraction beam for hadron experiments can be realized if a stretcher ring for theslow extraction is newly constructed as an addition to the current MR accelerator.
There is also an idea to arrange linac accelerators in the KEKB tunnel to make a
10 MW-scale proton driver after the completion of the SuperKEKB project.
Hyper-Kamiokande
Hyper-Kamiokande (HK) is a next-generation large-scale water Cherenkov detector
27

based on the technology established in Super-Kamiokande (SK) and represents andupgrade the detector scale and its performance. The physics capabilities of HK
cover a broad range of topics such as the measurement of δCP using the J-PARC
neutrino beam, searches for proton decay, the determination of the mass hierarchy
with atmospheric neutrinos, and the observation of astrophysical neutrinos.
The detector is designed as a cylindrical structure with 74 m diameter and 60 mheight, with a fiducial volume of about 200 kiloton (approximately 10 times larger
than SK). It is estimated the construction of HK will be completed within 10 yearsof budget approval. On longer timescales, there is a staging plan to construct asecond detector and carry out physics research with a detector approximately 20
times larger than SK. The HK proto-collaboration successfully developed a new
photosensor with both improved photon detection performance and water pressure
resistance in cooperation with Hamamatsu Photonics K.K. The detector structure
was re-examined after taking into account the benefits of the new photosensor andthe new design has enabled a significant cost reduction at no expense to the physics
sensitivity.
HK will be able to precisely measure neutrino oscillation parameters using the
J-PARC neutrino beam and has extremely high sensitivity to the measurement of
δCP . Accumulating data for 10 years with the 1.3MW J-PARC neutrino beam, theδCP measurement precision is estimated to be better than 21◦ (7◦ for δCP = 0).
Determination of neutrino mass hierarchy is one of major physics targets of HK,
not only because it may give hints toward the origin of the extremely light neutrino
mass, but also it is also related to studies of Majorana neutrinos being explored by theneutrinoless double beta decay searches. For example, if the neutrino mass orderingis the inverted hierarchy, whether neutrinos are Majorana or Dirac fermions, may
be determined on short time scales with improved searches for neutrinoless double
beta decay. According to its projected sensitivities, HK will be able to determine
the mass hierarchy at a significance of 3σ or more with observations of atmospheric
neutrinos and J-PARC beam neutrinos in about three years of operations.
HK also unprecedented sensitivity to proton decays. In addition to the increased
fiducial volume, atmospheric neutrino backgrounds will be largely suppressed by the
improved performance of the detector. The 3σ discovery potential reaches to 1×1035
years for the p → e+ + π 0 and 4 × 1034 for p → ν̄ + K + decay modes, approximately
one order better than the current limits, and can therefore be used to test various
GUT models.
Because of its huge fiducial volume, HK’s observations of astrophysical neutrinos,such as solar neutrinos, supernova neutrinos, as well as searches for dark matter
can lead to new discoveries. For instance, solar neutrino observation will enable28

matter eﬀect measurements with high precision via the variation of their oscillation
probability. This will subsequently allow tests of models beyond standard neutrino
oscillations that produce the observed deficit of solar neutrinos. In terms of thesupernova neutrino observations, HK will not only be able to observe neutrinos fromsupernova bursts within the galaxy or its near surroundings, but is also expected tomake the observation of as-yet unmeasured supernova relic neutrinos. As the flux ofthe supernova relic neutrinos depends on the frequency of supernova bursts in early
universe, its observation may advance the study of the evolution of the universe.
The Hyper-K design report has been submitted by the Hyper-Kamiokande protocollaboration and the project is ready for construction once its budget is approved.
Hyper-K is the most pragmatic future project in neutrino research because its detector design is based on existing technologies, it has realistic cost estimates, it has
an international collaboration, and it has suﬃcient potential for many discoveries,
including the discovery of neutrino CP violation and proton decay.
Sterile Neutrino Search
The LSND experiment operated in the 1990’s in U.S. reported the appearance ofνe from νµ after traveling a distance of 30 m. If this flavor transition is interpreted
as neutrino oscillation, the mass squared diﬀerence, which corresponds to oscillation
length, should be of order 1 eV2 . This, together with the other measurements ofneutrino oscillations, requires the existence of three diﬀerent neutrino mass diﬀerence
scales, which can not be explained by a three-flavor neutrino framework. For thisreason, it suggests the existence of a new fermion which does not couple to the weakinteraction nor the strong or electromagnetic interaction, known as a sterile neutrino,
as the fourth neutrino. The MiniBooNE experiment also reported results whichindicate a possible neutrino transition though the existence of a sterile neutrino.
Since both experiments lack near detectors it has been pointed out that the excess
may have been produced background.
In order to validate the results reported by LSND, several neutrino experiments
are being planned throughout the world. One such experiment planned in Japan is
JSNS2 , which aims to start data taking in 2018 at the J-PARC MLF facility to test
the oscillation parameter regions suggested by LSND. Further, though the NuPRISM
water Cherenkov detector is proposed as a near detector for T2K and HK to suppress
systematic uncertainties in their long-baseline neutrino measurements, it will also becapable of searching for sterile neutrinos.
Overseas Future Projects
In order to discover neutrino CP violation and to determine the neutrino mass hierarchy, neutrino oscillation experiments using accelerator neutrino beams are of key
29

importance and next-generation projects are being pursued in the world. The Deep
Underground Neutrino Experiment, DUNE, is in progress in the U.S.. DUNE will
use neutrino beam produced from a 1 MW or greater proton beam at Fermilab andobserve neutrinos at the Sanford underground laboratory, located 1300 km away. A
liquid argon TPC is employed as the far neutrino detector and an R&D program isongoing to enlarge the size of the detector. As θ13 turned out to be relatively large,
β beams or neutrino factories are less urgent than previously thought and the planof LAGUNA project in Europe is being re-examined. Under these world conditions,future projects for the measurement of δCP have converged on Hyper-Kamiokande(HK) and DUNE. In case both experiments start simultaneously, HK has superiorsensitivity to neutrino CP violation as well as to the proton decay mode, p → e+ +π 0 .
DUNE is currently proceeding with R&D to establish the detector design (especiallyto enlarge the detector size), while a part of the budget for the facility has alreadybeen approved. Excavation is going to start in 2017. In order to achieve important
physics goals in this situation, the early realization of HK is extremely important. A
study devoted to building a second HK detector in Korea is currently ongoing. The
detector observes the J-PARC neutrino beam at a diﬀerent distance from the HK
detector in Japan and is expected to provide improved sensitivity, especially to themass hierarchy.
Various projects are moving forward to determine the neutrino mass hierarchy. In
China the JUNO project aims to determine the mass hierarchy by measuring reactor
neutrino oscillations with high precision, and is pursuing R&D in parallel with thedetector construction. PINGU is a project which plans to improve the precisionof IceCube by increasing the photosensor density in the ice. It will be sensitive tothe neutrino mass hierarchy by observing atmospheric neutrinos in a same way as
HK. DUNE in U.S. has a long baseline in which the neutrino oscillation probabilitychanges due to matter eﬀects and is therefore sensitive to the mass hierarchy. Thoughthese projects, with the exception of HK, rely on new detector technologies meaning
their sensitivities are still uncertain, it is thought that depending on the progress ofthe detector R&D campaigns the prospects for a measurement will be clear within
ten years.
Concerning sterile neutrino searches, SOX, which uses an artificial neutrino source
at Gran Sasso in Italy, and SBN, which uses accelerator neutrinos at Fermilab in the
U.S., are both in preparation and are aiming for commissioning in 2017 and 2018,
respectively. They will be international competitors with the JSNS2 project at JPARC in Japan.

30

4Flavor Physics

Overview
After the discovery of the Higgs boson and subsequent completion of the standard
model (SM) the focus of particle physics has shifted to attempting to provide ultimate solutions to the mysteries of the universe that cannot be explained by the SM,
including as grand unification, the hierarchy problem, and the nature of dark matter.
While energy-frontier experiments seek for such solutions directly, flavor-physics experiments take indirect, but complementary approach. In flavor-physics experiments,
the eﬀect from the existence of a new particle on a certain decay process manifests
as a discrepancy between measured physics parameters and the prediction from theSM. It is possible in these experiments to discover the eﬀects of new physics eventhough they run at accelerator energies lower than the mass of the new particles.
This is their main advantage over direct-searches at energy-frontier experiments.
In order to discover new physics at flavor-physics experiments it is essential thatrare decay processes with suﬃciently small backgrounds from SM processes are used,that the physics parameters relevant those decays are precisely estimated from thetheoretical calculations, and that systematic uncertainties are well controlled andminimal. Further it is important that the accelerator luminosity and beam intensity on the target area as high as possible in order to find the rare decay event
signal. For these reasons, flavor-physics experiments are often called luminosityfrontier experiments. The search for new physics is being pursued while opening theluminosity frontier through parallel eﬀorts to develop both accelerators with high intensity beams and experiments that can record rare decay events under the diﬃcult
experimental conditions posed by those beams.
Japan is leading the world in the field of flavor experimentation, as represented
by the KEKB factory and it is important that Japan maintains this position over thelong term. Several 2-4σ discrepancies from the theoretical prediction have alreadybeen observed in measurements of the muon anomalous magnetic moment (g−2) anda variety of measurements at the B-factories and the LHCb experiment. Whether
these discrepancies can be interpreted as an indication of a new physics is still underdiscussion.
Concluding this discussion by improving the precision of both theoretical calculations and of experimental measurements is critical going forward. In addition, it willbe crucial to measure size of such discrepancies in several physics processes as well.
With knowledge of which discrepancies are enhanced and which are suppressed relative to the expectation, it is possible to identify the framework of any new physics.

31

This method is a particular advantage of flavor-physics experiments. Accordingly, it
is important to maintain a basic strategy of covering many physics processes withdiverse set of projects in future.
The following sections describe the physics targets for experiments using B mesons,
D mesons, τ leptons, muons, kaons, and neutrons.

4.1

SuperKEKB / Belle-II Experiment

Since the discovery of the CP violation in the B meson system in 2001 by the
Belle and BaBar experiments, a number of achievements in flavor physics have beendelivered by B-factory experiments at high-luminosity lepton colliders. In 2010,
the LHCb experiment, which analyzes large numbers of b-quark events produced inproton collisions, began to report several results in flavor physics, triggering intensecompetition in the search for new physics throughout the world.
The operation of the KEKB accelerator was completed in 2010 after delivering an
integrated luminosity of more than 1 ab−1 . Since then the upgrade of the accelerator
to the SuperKEKB, whose luminosity will be 40 times higher than KEKB, is being
carried out through the installation of new final focusing magnets, strengthening
the injector-linac, and reducing the beam size to O(1 nm). SuperKEKB successfully
circulating and storing beams in the electron and positron rings in February 2016, andthe phase-1 commissioning of the accelerator was finished by June of the same year.
Physics data taking will start by March 2018 after rolling the Belle-II detector into thecollision point, activating the solenoid and superconducting quadrupole magnets, andfinishing the phase-2 commissioning using colliding electrons and positrons. The final
commissioning of the detector will take place in 2018 by including silicon detectors
located near the collision point, and physics data taking with complete detector will
start by March 2019.
The Belle-II experiment will collect 50 ab−1 of data in about 10 years. It aims
for the radical development of flavor physics by precisely analyzing the decays ofb-quarks, c-quarks, and τ leptons with the high statistics data. Sensitivity to new
physics up to O(10) TeV is expected due to quantum eﬀects in Belle-II’s indirect
searches. The experiment will look for such quantum eﬀects, which potentially originate from as-yet undiscovered charged Higgs boson(H± ) decaying into the b-quarkand τ lepton, both third generation particles, or from other new particles. Belle-II
will open up searches for new physics to a new level in the next few years with its
ultra-precise measurements. It will also make a definitive measurement of the lepton universality violating process b → sℓ+ ℓ− as suggested by the LHCb experiment.
Several anomalies observed in Belle will be resolved by Belle-II with the clean ex32

perimental environment aﬀorded by its e+ -e− collisions. The CP -violating phasesin the quark sector will be determined with precision measurements of the CKM
matrix elements to the O(1%) level and enable the detection of possible contributions from the new physics as well as the estimation of the relevant energy scales.
Furthermore, contributions from new physics, such as a charged Higgs boson, SUSY
particles, or right-handed currents, will be probed through precision measurementsof the B meson decays to D(∗) τ ν, τ ν, KS0 π 0 γ, and K (∗) νν, the lepton flavor violationin τ decays to ℓγ, 3ℓ, and the branching fraction of B → Xs γ. Such probes will
provide a foothold to elucidate the structure of the new physics behind the observed
parameters. The integrated luminosity of SuperKEKB will reach 50 ab−1 in about
10 years after the start of the experiment. Belle-II is expected to discover several new
phenomena and provide limits on several physics parameters. It will be important
to develop the successor to Belle-II while watching the evolution of flavor physics.

4.2

Kaon

Kaon experiments have also been playing an important role in establishing the standard model of particle physics, including the first observation of CP violation whichsuggested the existence of the third quark generation. The Vus element of the CKM
matrix is determined most precisely from kaon decays and the parameter εK in K 0 –K 0 mixing constrains the quark unitarity triangle. Taken together with B Factorymeasurements, kaon experiments have provided many important results in the quark
sector.
Flavor changing neutral current (FCNC) processes are sensitive proves to explore
new physics since they are strongly suppressed in the standard model. Among quark
FCNC processes, the s → d transition is suppressed the most strongly by CKM
matrix elements and therefore the kaon decays could be the most powerful tool for
exploring new physics with generic quark flavor-transitions.
Measurements of the branching ratios of the decays KL → π 0 νν and K + →π + νν are among the most important in the search for new physics with rare kaon
decays. Since those branching ratios are extremely small and precisely predicted in
the standard model, if the eﬀects of new physics are comparatively large they could
be easy to observe experimentally. Depending on the model of the new physics, such
as SUSY or Z’, the kaon decay processes are sensitive to energy scales between 10
and 100 TeV and could be detected as a change in these branching ratios of a factor
of a few to one order of magnitude.
In Japan, the KOTO experiment is currently searching for the decay KL → π 0 ννwith the high-intensity proton beam from J-PARC. The experiment started the33physics running in 2013 and set an upper limit of 5×10−8 on this process that iscomparable to the current upper limit 2.6 × 10−8 set by the KEK-PS E391a experiment. With the large amount of data taken in 2015, the KOTO experiment willapproach the sensitivity of the Grossman-Nir bound (1.5×10−9 ), which is an indirect
limit based on the measured branching ratio of K + → π + νν. KOTO will further
increase the sensitivity via background suppression with improvements in the analysis, upgrades to the detector, and by using a more intense primary proton beamwith smaller intensity fluctuations. Sensitivity to the standard model branching ratio
3 × 10−11 will be reached at the beginning of the 2020s.
Internationally, the NA62 experiment at CERN is studying the branching ratioof the decay K + → π + νν and aims to observe O(100)-events. The experiment isboth complementary to the KOTO experiment and a competitor as well, since it willplace an indirect limit on the branching ratio of KL → π 0 νν using the Grossman-Nirbound.
To go beyond the goals of the current KOTO experiment and explore the eﬀects
of new physics at the level of 10% deviations from the standard model prediction,
the detector needs to be significantly upgraded. Additionally, a new beam line must
be built, which requires an extension of the hadron experimental facility at J-PARC.
Competition is further expected with another experiment that is currently underconsideration at CERN that will measure the branching ratio of the same decay
process starting in 2026. The extension plan of the hadron experimental facility at JPARC includes an experiment to search for violations of the time-reversal symmetry
with K + decays as well as various experiments designed for hadron and nuclear
physics. This plan is supported by the KEK Project Implementation Plan andshould be realized in cooperation with the nuclear physics community.

4.3

Muon

Lepton flavor may also be violated in the charged lepton sector and is being studied
seriously after the discovery of neutrino oscillation. Changed lepton flavor violatingprocesses are suppressed at the level of 10−40 −10−50 even taking into account neutrino
oscillation. However if there is a contribution to such processes from new physics,
it might become realistic to observe this phenomena by experiments currently underpreparation. Discovery of charged lepton flavor violation would be a clear signatureof new physics and such experiments should be advanced intensively.
Flavor violation in the τ lepton decay would be seen as mixing between the third
and the first or the second generation fermions, while the decay process µ → eγ
and µ − e conversion represent mixing between the second generation and the first34

generations. As a result the two are complementary. The MEG experiment reported
the final upper limit on the branching ratio of the µ → eγ process at 4.2 × 10−13 ,which is 30 times more stringent than the previous limit from MEGA experiment.
An upgraded experiment, MEG II, will be ten times more sensitive and is expected
to start soon.
The double ratio of the branching ratios of the two processes, µ → eγ and µ − econversion, is estimated to be O(102 ) in various physics models. However it is dependent upon the framework of the new physics and the individual measurements
are therefore complementary. Preparations for experiments searching for µ − e conversion at a sensitivity of 10−14 ∼ 10−17 are underway both in Japan and in the U.S..
The DeeMe experiment at the MLF facility in J-PARC is aiming at the early realization of O(10−14 ) sensitivities and beamline construction is on-going. The COMET
experiment at J-PARC and the Mu2e experiment at Fermilab are now in the R&D
phase and they are aiming for sensitivities of O(10−17 ). This may be enough to reachthe predicted level of the µ − e conversion process in various physics models beyond
the standard model. COMET, a flagship experiment of J-PARC, employs a staging
approach and the first phase of the experiment, which is aiming for a sensitivity ofO(10−15 ), is currently under construction
The muon anomalous magnetic moment, (g − 2), was measured by the BNLE821 experiment with a precision of 0.54 ppm and deviates from the standard model
prediction by 3.3σ. It is essential that this discrepancy be verified with more sensitive
experiments together with a better evaluation of the hadronic contribution to thevacuum polarization of the muon (g − 2). A planned experiment aiming at 0.1
ppm precision will transport the muon storage ring from the BNL-E821 experiment
to Fermilab. Another experiment, which will measure g − 2 and the electric dipole
moment simultaneously, has been proposed at J-PARC. It employs a completely new
method which uses an ultra-cold muon beam. It is technically possible to achieve a
precision of 0.4 ppm and the project is aiming for a final precision of 0.1 ppm. Since
two independent measurements with completely diﬀerent systematics are of great
importance, both experiments should be advanced toward their early realization.

4.4

NeutronThe combination of an accelerator-driven high intensity neutron source and highperformance neutron optics enables various neutron physics experiments with far
superior statistics than previous measurements. As a result a new era of particle
physics with neutrons is dawning.
An upper limit on the neutron electric dipole moment (EDM) of |dn | = 3 ×35

10−26 e·cm, which is close to theoretical predictions of various models, e.g. for SUSY
: 10−27 – 10−28 e·cm, SM : 10−32 e·cm, was obtained by experiments using ultra-cold
neutrons (UCNs). In order to suppress systematic uncertainties, an extremely uniform magnetic field over the neutron storage cell is required to measure their spin
precession. It is highly important to increase the spatial density of UCNs to improvestatistics in the limited storage area and to thereby improve the precision of the EDM
measurement. The development of high-density UCN sources to study the neutron
EDM of 10−27 to 10−28 e·cm are becoming a ground for international competition.
An experiment to measure the EDM with UCNs produced by solid deuterium converter has started at PSI. Even though the intensity of the UCNs was less than thedesign value due to technical problems, the statistical sensitivity of 10−27 e·cm wasachieved in 2017. There are plans to upgrade the apparatus and restart EDM measurements in 2020. At TRIUMF, the construction and commissioning of a beamline
to produce UCNs in 2017 and then measure the neutron EDM in 2018 is underway.
A collaboration between the Japanese institutes KEK and RCNP is in charge of thedevelopment of a superfluid helium converter for high density UCNs at TRIUMF.
Though the average intensity of the proton beam at J-PARC is low, the instantaneous pulse intensity is higher than other facilities. In order to take this advantageof this feature research and development into a new method of transporting UCNs
produced in a solid deuterium converter while maintaining their density is underway.
It is essential to quickly achieve the design value of the UCN density as well as tounderstand and suppress systematic uncertainties in the EDM measurements for the
discovery of new physics.
There are various other experiments using intense neutron beams that are either
planned or underway. The neutron lifetime is an important parameter in big bang
nucleosynthesis and calculations of the CKM matrix elements, but at present themeasured values diﬀer widely depending on the experimental method. Experiments
aiming for better than one second precision measurements of the neutron lifetime
are being pursued at J-PARC and throughout the world. Following the theoreticalsuggestion that there is a possibility of enhanced CP violation in resonant neutronnuclei capture process, a feasibility study has been carried out for a high sensitivity
search for new physics using the J-PARC beam. Using the fact that the neutron is amassive neutral particle, there are various experiments being considered that would
be sensitive to exotic interactions coupled to mass.
Innovative neutron sources for the next generation of experiments. In Europe,
construction of the European Spallation Source (ESS) is in progress and is planned
to start operation in year 2020. Long-baseline neutron beam would allow for searches
for neutron-antineutron oscillation, which is a test of baryon number violation. In
36

Japan, the discussion of a second target station at J-PARC has started. A neutron
source with an order of magnitude higher flux will open up new possibilities forresearch. Key technologies for such an innovation may be the development of bothnew neutron moderators dedicated to very cold neutrons and high-eﬃciency reflectors
to surround the source.

37

5

Non-accelerator Particle Physics

5.1

Particle Physics in Underground Laboratories

Overview
Rare events predicted by new physics are currently being searched for in undergroundlaboratories. Underground environments are beneficial for these experiments because
they provide shelter from cosmic rays, low radioactive backgrounds, and stable experimental conditions. Underground laboratories are important for exploring new
physics at extremely high energy scales, which cannot be accessed with accelerators
but may be revealed through naturally occurring rare events. In particular, experimental searches for dark matter particles and neutrinoless double beta (0νββ) decay
have become highly competitive fields. Domestic activities stemming from neutrino
experiments have concentrated in underground laboratories in the Kamioka mine,
forming a research community of variously sized projects. Although the site is only
moderately deep, the projects proposed by that community will realize world-leading
sensitivities at low cost and over short time periods and can be performed using existing research facilities. A new academic field that aims to accelerate cooperative
work across diﬀerent projects is emerging is being advance and will prosper undercontinuous promotional support.
At present, projects involving 10 – 50 researchers are developing unique experimental approaches in the Kamioka mine. While it is important to recognize theimportance of such diversity, it is necessary to identify common priorities for the
direction of the field so that eﬀort can be concentration on a larger project when
an important discovery is made. Researchers working in underground laboratories
belong to various fields spanning high energy, cosmic ray, and nuclear physics. Communication and cooperation among these fields will ensure that research is properly
evaluated and strongly promoted.
Dark Matter Searches
Recent cosmological observations have established the existence of dark matter thatcannot be explained by the currently known particles. The mass density of dark
matter has been evaluated as 0.3 GeV/cm3 in the vicinity of the Earth. However,
the masses and interaction strengths of dark matter particles as predicted by various
theories range over tens of orders of magnitude. Supersymmetric theories predict
the existence of weakly interacting massive particles (WIMPs) which emerges intheories describing phase transitions in the electroweak interactions. Direct detectionof WIMP dark matter is the goal of many experimental eﬀorts around the world.
Using NaI (Tl) inorganic scintillators the experimental group DAMA/LIBRA
38

detected an annually modulated signal caused by the rotation of the Earth around theSun. However, this signal is absent in liquid xenon experiments such as XMASS. A
recent result from the XENON1T experiment excluded WIMP models with a nucleon
interaction cross section greater than 7.7 × 10−47 cm2 at 35 GeV/c2 , far smaller thanthe cross section that naturally explains the DAMA/LIBRA result. Furthermore,
no WIMP-like signal has been observed in accelerators or in indirect searches withcosmic ray data. Once the direct-detection sensitivity to the cross section improves
to 10−48 cm2 , most of the parameter region predicted by supersymmetric theories will
be covered. Although solar neutrinos will begin contributing to the background whenthat happens, their eﬀects could be removed by studying the diﬀerences in the energy
spectrum for heavy-mass WIMP searches. Under these conditions, the sensitivity ofexperiments must be steadily improved beyond 10−47 cm2 while enhancing sensitivity
to smaller masses and it is important to extend experimental capabilities beyond
WIMPs to other types of dark matter.
Direct detection of dark particles is a top priority for future projects and the construction of a large-scale detector with improved sensitivity is the most important.
The XENONnT and LZ experiments, scheduled to start in 2019 and 2020 respectively, are the representative large-scale dark matter experiments. Each experimentwill require almost 10 tons of liquid xenon, and will measure cross sections down to
3 × 10−48 cm2 for 50 GeV/c2 WIMPs. Larger detectors with ultimate sensitivities
exceeding 10−48 cm2 are also within reach. One example is the DARWIN experiment,
which plans for a 50 ton liquid xenon target. For detectors of this size, atmospheric
neutrinos start to contribute to the background and are diﬃcult to distinguish froma heavy WIMP signal. However, if most of the dark matter consists of Wino particles, the neutralinos predicted in supersymmetric theories, a discovery is expected. A
global cooperative eﬀort toward realizing a large-scale detector is necessary in future.
Once a dark matter signal is observed, the underlying physics of the dark matterparticles must be further investigated and confirmed. To that end, various measurements using diﬀerent target materials and technologies, those using directionsensitive detectors, and those studying inelastic scattering channels will become important.
The PICO-LON group aims to confirm the modulated signal obtained by the
DAMA/LIBRA experiment. The group has developed ultra-low-background NaI(Tl)crystals and achieved the second-lowest background rate in the world. To surpass thesensitivity of the DAMA/LIBRA experiment, they are planning for larger crystals
with even lower background. The ANKOK group is developing gas-liquid dual-phaseargon detectors to search for low mass WIMPs. The group is planning to start datacollection in an underground laboratory within the next few years, and is expected39

to test the DAMA/LIBRA results using an argon-based detector for the first time.
Research into developing a liquid helium detector to search for low mass WIMPs is
also underway.
Directional dark matter measurements particles would provide “smoking gun”
evidence of their existence, as well as important information on the physics of theirmotion and scattering. The NEWAGE and NEWSdm groups are actively developing
direction-sensitive detectors using TPC technology and emulsion methods, respectively. Direction-sensitive scintillators are also being developed. New technologicalapproaches are expected to be further developed for these purposes.
The experiments above are devoted to WIMPs with masses ranging from 10
GeV/c2 to a few TeV/c2 . Other experiments are searching for non-WIMP dark matter candidates such as axions. For instance, the CARRACK experiment is planningto search for axions with masses of 50 – 100 µeV/c2 . Recent technologies will enablesensitive searches for axion-like particles or hidden photon dark matter. Searches forunknown particle interacting with electrons can (and should) be pursued with existing large-scale dark matter detectors. For example, super-WIMPs can be searched forin the low-background data of the XMASS-I detector. It is also argued that WIMPs
with masses below 10 GeV/c2 can scatter electrons and be identified through theelectron recoils the produce. By reducing major background sources such as radon,
various types of dark matter will be explored at current and planned experiments.
0νββ decay
At present, the Majorana properties of neutrinos can only be investigated by observing 0νββ decay. Irrespective of the underlying physics, the observation of thisprocess would provide evidence that the neutrino is a Majorana particle and that
lepton number is violated. Since the decay rate should be proportional to the squareof the eﬀective Majorana mass of the neutrinos, the absolute masses of the neutrinos
can be determined as well.
The absolute masses of the neutrinos will be explored in 0νββ decay experiments, which target masses greater than 200 meV. Cosmological measurements are
sensitive to masses exceeding 50 meV at present, and are predicted to improve to
20 meV in future. Neutrino oscillation experiments have determined the range of the
Majorana mass as >50 meV for a quasi-degenerate mass ordering, 20 – 50 meV for
an inverted ordering, and <20 meV for a normal ordering of the neutrino masses.
Experimental studies at these mass scales represent milestones for the developmentof 0νββ decay experiments. Owing to recent rapid improvements in the sensitivityof the KamLAND experiment, the lower bound of Majorana mass has placed at 60 –170 meV, which almost excludes quasi-degenerate ordering. The uncertainty on thelower bound is dominated by theoretical uncertainties and is expected to be improved40

in future. Though an experimental observation of 0νββ decays in 76 Ge corresponding to an eﬀective Majorana mass of approximately 320 meV has been reported (theKKDC claim), this mass range has been already excluded by the KamLAND-Zenresults with 136 Xe and the GERDA results with 76 Ge.
After excluding quasi-degenerate mass ordering, the KamLAND-Zen experiment
aims to probe the inverted mass ordering. For this purpose, the detector sensitivity
will be improved to 20 meV. If the inverted mass-ordering is confirmed in neutrino
oscillation experiments but no positive signal appears in 0νββ decay experimentswith this sensitivity, it is possible to exclude the Majorana neutrino hypothesis. To
improve sensitivities to below 20 meV, two neutrino double beta decays must be
discriminated from the target signal and therefore both the size and performance ofdetector must be markedly improved.
To cover the inverted ordering mass range, an eﬀective mass of one ton of thetarget material must be procured and must also have low radioactive backgrounds.
As the major background originates from two neutrino double beta decay events,
suﬃciently high energy resolution to distinguish these from the signal is required.
The next phase of the KamLAND-Zen experiment, KamLAND2-Zen, is aiming to
explore the inverted ordering by improving the energy resolution. To this end, thegroup will replace their photomultiplier tubes with higher sensitivity ones, implement
light-collection mirrors, and improve their liquid scintillator.
On the other hand, due to uncertainties in the nuclear matrix elements anddiﬃculty in reducing the unpredictable background, it is essential to establish theexistence of 0νββ decay using a variety of nuclei and experimental methods to understand the underlying physics. Several ongoing projects, including some in theJapanese community, have adopted diﬀerent detection principles and target nuclei.
The CANDLES experiment takes advantage of the high Q value of 48 Ca to realize
a low-background experiment. However, enriching the 48 Ca isotope is indispensable for this project due to its low natural abundance (0.2%). Recent studies havedemonstrated the principle of isotope-enrichment methods, but further optimizationis required for mass production. The energy resolution of the detector needs to be
improved, and research and development on the use of low-temperature detectors is
starting.
In parallel with the above achievements, detectors with particle tracking capability for measuring angular distributions are being developed. However, particletracking detectors are much less sensitive than large-scale calorimetric detectors atpresent. Scaling up the size of these detectors and improving their performance are
their top priorities.
Progress more than ten years in the future is diﬃcult to envision for these projects.
41However, early-stage conceptual designs are necessary for realizing large-scale detectors with the 100 tons of target mass, which will enable investigation of boththe inverted and normal mass orderings. When the Hyper-Kamiokande experimentis realized, Super-Kamiokande might be employed as a dark matter or 0νββ decay experiment. For example, combining Super-Kamiokande with the techniques of
KamLAND-Zen might achieve the sensitivity required to probe 0νββ decay under
the normal ordering.

5.2

Cosmological Observations

Introduction
In recent years, the importance of cosmological observations for elementary particle
physics has increased dramatically. While harmonizing with accelerator experiments
and in cooperation with neighboring fields, the active promotion of projects using
space as a laboratory is expected. In particular, the exploration of inflation anddark energy is an important topic for which cosmological observations are the primaryexperimental method, and observations of the CMB polarization and researches with
the Subaru telescope are underway. Both of these observations have high sensitivity
to the sum of the neutrino masses and the number of neutrino generations, and it
is expected that each observation can determine the former an accuracy of 100 meVor better. This means that it is possible to explore down to the lower limit of
the inverted neutrino mass hierarchy and therefore these observations carry greatsignificance. By measuring the number of neutrino generations to an accuracy of afew percent, it is possible to measure the abundance of light relativistic particles,
such as hot axions, generated in the early universe.
Verifying inflation using Cosmic Microwave Background radiation (CMB)
Observations of the B-mode in the CMB polarization will provide direct evidence for
inflation and further inform about its energy scale. The search for this polarization
is essential as it is possible to probe ultrahigh energy physics that can not be reacheddirectly using accelerators. The intensity of the B-mode polarization is proportional
to the tensor-to-scalar ratio (r), which represents the intensity of primordial gravitational waves generated by space-time quantum fluctuations at the time of inflation.
The value of r is directly related to the distance (∆ϕ) traversed by the inflaton,
which is a hypothetical field responsible for inflation. If r is greater than 0.002,∆ϕ becomes larger than the Planck mass and no field theory is able to calculate
inflaton’s potential from first principles 2 . In this case, a new physics framework
2

Assuming single-field inflation, to be precise.

42

describing energy scales higher than the Planck mass, such as quantum gravity or
string theory, will be necessary, making the observation results gravely important to
the determination of the direction needed to explore the fundamental physical law.
The ground-based CMB polarization observation group in Japan participates asa core in POLARBEAR, which is an international experiment. POLARBEAR set up
a telescope in Atacama, Chile and started observations in 2012. Projects competingwith POLARBEAR include ACTpol in Atacama, Chile, and BICEP2/KECK and
SPTpol, which are conducted at the South Pole. These ground-based experiments
have polarization sensitivity beyond Planck in a limited area of the sky and arecapable of detecting the B-mode polarization caused by gravitational lensing andforeground radiation (dust). On the other hand, the B-mode polarization originatingfrom the primordial gravitational waves has not yet been observed. An upper limit
on r has been established by Planck-BICEP2-KECK as r < 0.07 at 95 % C.L. If thetrue value of r is close to this upper limit, the discovery of the B-mode polarization
originating from primordial gravitational waves can be expected within the next 5
years or so.
The KEK group is advancing the POLARBEAR-2 project, which aims to discoverprimordial gravitational waves with a sensitivity about 6 times higher than that of thecurrent POLARBEAR. POLARBEAR-2 plans to see its first light during the 2017
fiscal year. With the construction of two more POLARBEAR-2 telescopes by fiscal
year 2018, Simons Array plans to achieve 20 times the sensitivity of POLARBEAR
with a total of three telescopes. It is expected that the error σ(r = 0.1) = 6 × 10−3of r be obtained by separating
foregrounds using the results of Planck and C-BASS,∑
and also the error σ( mν ) =40 meV in the sum of neutrino masses by combining the
DESI 3 results of baryon acoustic oscillations. Meanwhile, Japan’s unique technology development is progressing. The GroundBIRD experiment is scheduled to begin
observation in the near future and will demonstrate proprietary technologies including a high-speed rotation scanning method for measuring large angular scales andthe superconducting MKIDs developed in Japan. Simons Array and ACTpol groups
have joined forces to start Simons Observatory, which has already been funded in the
U.S. and its telescope design has begun. Japanese researchers are also participating in the Simons Observatory. As a plan for the further future, the CMB Stage-4
project is being studied by the U.S. DOE and aims to start observation in∑
the middle of 2020s with expected measurement precisions of σ(r) = 0.001 and σ( mν ) =20 meV. Japanese researchers are also considering participating in the project.
The scientific satellite program LiteBIRD aims for launch in the middle of the3

DESI as an observational experiment scheduled to start in 2019 for the purpose of making a 3
dimensional map of galaxies in the universe.

43decade beginning in 2020 and is being studied in Japan in order to further improve its
sensitivity on the large angular scales. LiteBIRD will observe the whole sky without
being influenced by the atmosphere, achieve σ(r) < 0.001, and examine the main
inflation models in further detail. The high precision observation of the B-mode
polarization with the large angular scales aﬀorded by LiteBIRD is considered to beimportant to narrow down the cosmological model, even in the event of an earlier
discovery by ground-based observations or others. Further, it is possible to precisely
measure the optical depth of the CMB by measuring the E-mode polarization withthe ultimate sensitivity, and combining them with results from DESI, the precisionon∑the sum of the neutrino mass measurements is expected to be reduced down toσ( mν ) =15 meV.
The LiteBIRD program has been developed mainly as an international collaboration between Japan and the U.S. The Japanese group applied for JAXA’s strategic
mid-size scientific satellite program in 2015 and was selected as one of the candidates.
In September 2016, the conceptual design phase (Phase-A1) of the JAXA Instituteof Space and Astronautical Science began. LiteBIRD is the only CMB B-mode polarization satellite program in the world that has progressed to the conceptual designphase at the present time (March 2017). The U.S. group has obtained funding fromNASA for technology development for LiteBIRD. The promotion of LiteBIRD is
expected to proceed while balancing the independence of Japanese community. LiteBIRD has been adopted as a priority project of the Science Council Master Plan
2014 and of the Master Plan 2017. It has also been selected as one of ten plans on
the MEXT Road Map 2014, and one of the seven plans on the Road Map 2017.
Exploration of dark energy 4
The observation at the end of the 20th century of non-inflationary accelerated expansion of the universe as suggested by supernova data marked an important discovery.
The physical model behind this accelerated expansion is unknown, and the energy
responsible is termed “dark energy”. In order to understand the essence of darkenergy, it is important to investigate how the cosmic expansion has changed with thepassage of time, using cosmological observations to constrain its equation of state.
There are mainly two methods to achieve this: a method of measuring the cosmic
expansion directly and a method to investigate evolutionary history of structure formation in the universe born from the competition between the cosmic expansion andgravitational attraction. Regarding the former, methods based on type Ia supernova
and baryon acoustic oscillations are being studied, while studies for the latter use
4

Space observation - Exploration of dark energy: contribution from Kavli IPMU Prof. Masahiro
Takada.

44

methods based on clustering statistics of the galaxy and weak gravitational lensing
eﬀects.
The next-generation project of Japanese dark energy exploration is the SuMIRe
project, which upgrades the Subaru telescope to simultaneously perform deep space
imaging and spectroscopy over a broad area. The Hyper Suprime-Cam (HSC)
project, which measures weak gravitational lensing eﬀects from imaging data, waslaunched in 2014. It carries a wide field of view using a focal plane consisting ofa CCD with about 900 million pixels and observes billions of galaxies over about
1400 square degrees. Based on these measurements and the analysis of gravitational
lensing eﬀects, the time evolution of the distribution of dark matter will be revealed
and the eﬀects of dark energy will be investigated.
Another observational instrument is a new spectroscopic device called Prime Focus Spectrograph (PFS), which is scheduled to be installed around 2019. It will
start scientific observations from 2020 and measure the red shift of more than 4 million galaxies in the HSC observation area in order to uncover the three-dimensional
spatial distribution of galaxies. Since HSC will reveal the dark matter distribution
and PFS will measure the detailed galaxy distribution, both observations will reduce uncertainties in the galaxy bias, thus providing a substantial synergistic eﬀect
to the exploration of dark energy’s properties. Other capabilities include the measurement of neutrino mass, verification of inflation theory using measurements of
non-Guassianity, verification of general relativity at cosmological distances, and verification of the properties of dark matter. Scenarios in which primordial black holes
generated in the early universe can also be explored.
HSC will help monitor stars in Andromeda galaxy in the time domain with highdensity, and explore gravitational micro-lensing eﬀects due to primordial black holes.
With these observations, stringent limits can be placed on the number of existing primordial black holes generated in the early universe at thermal temperatures greaterthan TeV scales and with masses of about 10−10 M⊙ . Furthermore, the combinationof small angular scale observations of the CMB and baryon oscillations observed
by galaxy survey experiments can provide measurements of the abundance of light
relativistic particles that were thermally generated in the early universe in units of
the number of neutrino generations with a few percent precision. This can help determine whether or not hot axions frozen following the reheating just after inflation
exist.
A wide range cosmic surveys such as these are planned and ongoing all over theworld. The European Space Agency’s Euclid satellite is scheduled for launch around2021 and will perform wide area imaging and spectroscopic surveys for about six
years. Around the same time, LSST, a ground-based telescope that will undertake45

the ultimate visible light survey, is being lead by the U.S. and will start operations
and conduct wide sky area imaging surveys for about ten years. Further, the U.S.
National Aeronautics and Space Administration (NASA) is planning to launch the
WFIRST-AFTA satellite to perform wide-area imaging and spectroscopic surveys
with high sensitivity and a wide field of view in around 2025. There is no doubt thatthese wide area space surveys will advance fundamental physics research from 2020 toearly 2030. The SuMIRe project aims to obtain results ahead of other observational
projects and is highly competitive internationally.

46

6

Human Resource and Technology Development

High energy physics research aims to achieve the world’s best results by combining
various cutting-edge technologies. Experiments are consequently full of forefront
technologies and are thus a fascinating research field for young researchers especiallyas regards technology. Continuous development of technology and training of young
researchers are essential for high energy experiments as they create positive feedback
loops for further development of the field. However, several concerns about humanresources and technological developments have arisen recently.
• Span of experiments: the size of high energy physics experiments is growing
larger and are taking longer to complete. It is diﬃcult for even a competentstudent or young researcher to experience the entirety of an experiment fromits project design, the R&D and construction of detectors, commissioning andoperation, to the eventual data collection and physics analysis. More and moreyoung researchers miss opportunities to work on technological developments
and it may prevent smooth transfer of technology between generations.
• Specialization and subdivision of technology: there are many driving-force technologies in high energy physics, such as accelerator technology, detector technology, analog and digital electronics, software and large-scale distributed dataprocessing systems, superconducting technology, and mechanical engineering
among others. These cutting-edge technologies have become more and more
specialized and subdivided in recent years, and it is diﬃcult for individuals
to obtain broad experience in the various technologies used in present highenergy physics experiments. This also causes diﬃculties in the exchange ofhuman resources among subdivided fields of expertise.
• Lack of resources: Due to the decline of the economy and heavy pressure
for short-term outcomes, budgets for basic research are suﬀering significant
reductions. This leads to the closure of electronics workshops at universities,
a decrease of technical support staﬀ at universities and laboratories, and otherproblems which results in the deterioration of the research environment for thedevelopment of cutting-edge technologies. Further, the mass retirement of thebaby-boomer generation disrupts the transfer of technological inheritance torising generations.
The active exchange of human resources at various levels is important to themitigation of these problems. Further eﬀorts to increase the exchanges of personnel
between projects and institutes will be necessary. Providing young researchers with47

opportunities to work on small- or medium-size experiments at the beginning of
their careers, and employing young researchers inclined toward challenging cuttingedge technologies may be eﬀective on this front. Also necessary measures should
be seriously considered and undertaken to help to continuously develop forefront
technologies and train young researchers who will support those technologies in the
future. Future large-scale projects cannot be realized without further technologicaldevelopment and the existence of suﬃcient human resources.

48