Report by the Committee on the Scientific Case of the ILC
Operating at 250 GeV as a Higgs Factory

July 22, 2017

Committee Members:
Shoji Asai1,2,*, Junichi Tanaka2, Yutaka Ushiroda3, Mikihiko Nakao3,
Junping Tian2, Shinya Kanemura4, Shigeki Matsumoto5, Satoshi Shirai5,
Motoi Endo3, Mitsuru Kakizaki6* Chair
1
2
3

The University of Tokyo

ICEPP, The University of Tokyo

High Energy Accelerator Research Organization (KEK)45Osaka University

Kavli IPMU, The University of Tokyo
6

University of Toyama

Commissioned by the Japan Association of High Energy Physicists

1Preface
In July 2012, a Higgs boson with 125 GeV mass was discovered at the LHC. The discovery of new
phenomena and new principles that can (naturally) explain the electroweak symmetry breaking(EWSB) including the existence of this Higgs boson is now the most important and urgent target ofresearch. In order to attain this goal, the LHC is performing direct searches for new phenomena andnew principles with a center-of-mass (CM) energy increased to 13 TeV. So far, there is no evidenceof new physics beyond the Standard Model (SM). The purpose of this committee is to investigate
and compare, under the current circumstances, the capability to determine the energy scale of new
phenomena and new principles and the capability to uncover the origin of matter-antimatterasymmetry for the following three cases: (i) an ILC operating at 250 GeV as a “Higgs Factory”(ILC250); (ii) an ILC operating up to 500 GeV (ILC500); and (iii) the case of no ILC construction.
The committee members consist primarily of members of the ATLAS collaboration, the Belle II
collaboration, and theorists. The committee aimed to give an assessment on the physics case of the
ILC250 in a way that is independent from the ILC community.
This report consists of the following five chapters:
1.
2.
3.
4.
5.

Introduction
Precise measurements of Higgs and other SM processes: Determination of the energy
scale of new phenomena via precise measurements.
EWSB and the origin of matter-antimatter asymmetry.
Direct search for dark matter and new particles based on “Naturalness”.
Summary: Comparison of the ILC operating at 250 GeV and 500 GeV

Different approaches are summarized in Chapters 2 and 3 to probe the next energy scale beyond
EWSB through precise measurements. Chapter 4 discusses searches to elucidate dark matter (DM)and probes to test the idea of “naturalness”.

1. Introduction
For the purpose of this discussion, the following points are assumed for the timeline and theconditions of the ILC operation.
1.
2.

The operation will start around 2028-2030. It will run concurrently with the HighLuminosity LHC (HL-LHC) experiment and produce complementary results.
The CM energy is fixed at 250 GeV. No energy scan is performed. The integrated
luminosity is 200 fb-1 per year, accumulating 2 ab-1 by 2040.
2

3.

Beam polarization is used (30% for positrons, 80% for electrons).

The key is the synergy with other experiments, including the HL-LHC, SuperKEKB, HyperKamiokande, electric dipole moment (EDM) searches, lepton flavor violation (LFV) searches, andsatellite probes to detect gravitational waves (LISA, DECIGO, etc.), as well as theory developmentin Lattice QCD and higher-order corrections. Various implications are considered combining rich
outputs from these experiments with the ILC results, and we elucidate the role of the ILC withrespect to the other experiments.

2. Higgs and Other Standard Model Processes: Determination of the New
Energy Scale via Precision Measurements
2.1. Precise measurements of Higgs couplings
The precise measurements of the couplings between Higgs boson and otherelementary particles can be performed at the 0.6-1.8% level at the ILC250. The
measurement precisions are summarized in Tables 1 and 2. It is the important task of
ILC to measure the total decay width model-independently. The previous strategy is
as follows, the HWW coupling is measured using the vector boson fusion process, anddecay branching fraction Br(H→WW) can be measured precisely. Then total decay canbe determined model-independently. The vector boson fusion process enables theprecise measurement of the HWW coupling at higher energies. At the ILC250,
however, this cross section is small. It was one of motivations for higher center of massat ILC.
As an alternative approach, we can measure Higgs decay branching fraction at the
HL-LHC to examine the symmetry between the HWW and HZZ couplings (custodial
symmetry) at the 2% level. By taking this symmetry as an assumption, the eeàZH
cross section (HZZ coupling) and the HàWW decay branching ratio measurements
can be combined for the model-independent determination of the total decay width.
This idea can be further extended in the framework of effective field theories todetermine the coupling (denoted as g), in a model-independent way (Ref. airXiv
1708.09079).
The estimated precisions of various Higgs boson couplings are shown in Table 1,
combining ILC250 and HL-LHC results. The precisions are at the 10% level with the
HL-LHC alone, less than 1% accuracies can be obtained as shown in Table 1. The
comparison between ILC250 and ILC500 shown in Fig.1 shows that the differences in
the achievable precisions are small for the same total integrated luminosity of 2 ab-1.
This illustrates the importance of the combination of the HL-LHC and the ILC250
results. These combined results are comparable to those at ILC500 (combined with HLLHC). Table 2 summarizes the precision of the coupling ratios from the direct3determination at ILC250. Many experimental systematic uncertainties cancel by taking
the ratios. These ratios are useful for the precise comparison between the SM
predictions and the experimentally observed values.
Table 1: Precision of Higgs boson couplings in the effective field theory framework. Combinationof the ILC250 and the HL-LHC measurementsΔg/gg(HZZ)

g(HWW)

g(Hbb)

g(Hττ)g(Htt)

g(Hµµ)

g(Hcc)

0.63%

0.63%

0.89%

1.0%

7% (LHC)

6.2%

1.8%

Table 2: Precision of Higgs coupling ratios from the direct measurements at the ILC250.
g(HWW)/g(HZZ)

g(Hbb)/g(HWW)

g(Hττ) /g(HWW)

g(Hcc) /g(HWW)

1.9%

0.64%

0.84%

1.7%

Δ

(EFT fit)

10
ILC 250 GeV, 2000 fb ⊕ LHC 3000 fb
9
ILC 500 GeV, 1000 fb⊕250 GeV, 1000 fb ⊕ LHC, 3000 fb
8
7
6
5
4
3
2
1
0 g(HZZ) g(HWW) g(Hbb) g(Hgg) g(H γ γ ) g(H τ τ ) g(Hcc) g(Htt) g(H µ µ ) ΓPrecision of Higgs coupling and witdh [%]

-1

-1

-1

-1

-1

Γ invis(CL95%)

tot

Figure 1: Precision of coupling measurements.
It will be possible to measure the Yukawa couplings of the second-generation leptons and quarks
with about 2-6% precision. This will show that the differences in the Higgs boson couplings giverise to the generations, providing insight into our understanding of generations. Furthermore, themeasurement of the Higgs boson coupling with the Z boson and the differential cross section can be4done model-independently and provides sensitivity to new phenomena with a mass scale of up to
2.5 TeV for CP-even states and 3.9 TeV for CP-odd states using the effective Lagrangian approach.
Among the various new phenomena and new principles, the supersymmetry is the mostpromising theory. There are three approaches to discover supersymmetric particles, as
described below. The capability of these three approaches depends on the model and theparameter space. Thus it is crucial to be able to cover all three approaches.
(a)

(b)

(c)

Direct search for supersymmetric partners with SU(3) color charge, such as thesquarks and the gluino. The HL-LHC has good sensitivities to search for thesquarks and gluino up to about 3 TeV.
Search for supersymmetric partners with SU(2) and U(1) charge, such as theelectroweak gauginos and higgsinos. In contrast to (a), the mass spectrum can benaturally highly compressed. The ILC will play an important role as described in
Chapter 4.
There are at least two Higgs doublets (2HD) in all supersymmetric models, inwhich multiple Higgs bosons exist. These signatures can be accessed even if thesupersymmetric partners as described in (a) and (b) are beyond the reach of theexperiments.

The precise measurement of the Higgs couplings at the ILC250 provides an important input
to the approach (c) above. The Minimal Supersymmetric Standard Model (MSSM) is
considered first among the 2HD models. The HL-LHC has a high discovery potential forparameter regions with large tanβ. In contrast, the deviation of the Higgs boson coupling withthe gauge bosons, g(HZZ) or g(HWW), becomes larger for smaller tanβ, which is favorable
for the ILC. The sensitivity of direct searches at the LHC and the ILC250 sensitivity are thus
complementary. Heavy Higgs bosons (or SUSY breaking scale Λ) can be discovered almost
up to 1.5–2 TeV by combining the ILC250 and the HL-LHC results, even if thesupersymmetric partners are heavy. In the extended models such as the Next-to-Minimal
Supersymmetric Standard Model (NMSSM), in which the relation between the neutral andcharged Higgs bosons become model-dependent, the large tanβ region is covered by neutral
Higgs boson searches at the HL-LHC and the charged Higgs boson searches at Belle II; thesmall tanβ region is covered by the ILC250.
The energy scale of new phenomena can be probed up to Λ~2 TeV in more general 2HDmodels not restricted to supersymmetry, through coupling deviations. The Kaluza-Klein (KK)
gluon can be probed up to a mass of 10–20 TeV (corresponding to a KK scale of 3–7 TeV).
5

Furthermore, a physics model behind the discovered new phenomena at Λ can be identified
through the deviation pattern. These sensitivities are determined by the precision of the Higgs
couplings, and do not depend highly on the CM energy (250 GeV or 500 GeV). It will be
crucial to reduce the systematic uncertainties (coming from experimental uncertainties, anddetermining the quark mass and αs) through the collaboration between the experimental andtheory communities.
2.2. Precision measurement of the Higgs boson properties
The total decay width ΓH can be determined with an accuracy of 2.1% by fitting theboth results at ILC250 and HL-LHC in the effective field theory framework as
mentioned in Section 2.1. This will allow for the search for decays to unknown particles
with decay branching ratios down to 0.3%. Detail is discussed in Chapter 4. The CP
phase in the coupling between the Higgs boson and fermions can be measured to 3.8
degree precision, which provides an important clue to the origin of the matterantimatter asymmetry, whether it is baryogenesis or leptogenesis (See Chapter 3). The
discovery of CP violation in the Higgs sector will be an important achievement, as it
implies that the SM Higgs field (1HD) is not the correct description of nature, and thatthe Higgs sector must be more complicated (such as general 2HD models or additionof singlet fields).
From the angular distribution of the decay particles, the compositeness of the Higgs
boson can be probed up to a scale of 2.2 TeV.
2.3. Precision observables in the Standard Model: MW / Mt / sinθeff
At ILC250, the W boson mass (MW) and weak mixing angle (sinθeff) can be measured
with accuracies of 3 MeV and 3×10-5 (relative precision), respectively. Although the top
quark mass (Mt) cannot be measured directly at the ILC250, the HL-LHC is expected
to determine the top quark mass with an accuracy of 0.2–0.3 GeV. The contribution ofvarious systematic uncertainties such as ΔMZ and Δαs and a top quark mass precisionof 0.3 GeV are roughly equal to check the Standard Model precisely. Thus, from thepoint of view of precise observables in the SM, the HL-LHC precision of 0.3 GeV issufficient. Supposing that the current central values for MW, Mt, and sinθeff remain
fixed, the improved precision from the ILC250 and the HL-LHC will yield a 3–4σdeviation from the SM. This will indicate that new physics such as supersymmetry
exists around the TeV scale. If an excess is seen at the HL-LHC or if deviations of Higgs
couplings are seen at the ILC, it will be crucial to identify the principles behind theseanomalies. This will be one of the important achievements expected from the ILC250.

6The stability of our vacuum can be computed from the Higgs boson mass (Mh) and Mt. An
upper limit on the energy scale of new physics can be also determined with the assumptionthat our vacuum is stable. Combining the ILC precision of ΔMh=14 MeV and the HL-LHC
precision of ΔMt=0.3 GeV will determine that our universe is metastable or that new physics
should exist at a scale below 1012 GeV to make our universe stable, if the central values are
the same as the current values. These results are crucial to understand the early universe,
including implications about the possibility of leptogenesis, as described in Chapter 3.
2.4.

New phenomena can be discovered up to Λ=2–3 TeV with synergy among the ILC250,
HL-LHC and the SuperKEKB. The ILC250 has high sensitivity in the region that cannot
be covered with the HL-LHC (heavy higgs boson in the 2HD models with small tanβ
and the electroweak gaugino). The ILC is therefore complementary to the HL-LHC.
If an excess is found at the HL-LHC, ILC can play an important role to reveal thephysics behind it.
Figure 2 shows a flowchart of overview. If a deviation from the Standard Model
prediction is observed in the Higgs coupling, the EW precise measurements or
searches, the new energy scale Λ for the new phenomena and new principles isdetermined. It also fixes the technology and the CM energy of the next-generation
accelerators, such as Future Circular Colliders (FCC, HE-LHC) and the energy
upgrade of the ILC.

Figure 2: Precision measurements and the energy scale Λ.
7

The right side of Figure 2 illustrates the different approach, in which the probing forthe origin of the matter-antimatter asymmetry will lead to the next new phenomena(where Λ is the new energy scale). As discussed in Chapter 3, it can be determined thatthe origin of matter is either electroweak baryogenesis (Λ=10–1000 TeV) or
leptogenesis (Λ<10 TeV) with the observation of CP violation (in Higgs or neutrino
sectors), the precise measurement of the Higgs boson, and measurements ofgravitational waves in space.
If the results from the ILC250 and other experiments are found to be consistent with
the SM, and no new sources of CP violation are found, it will be determined that theenergy scale for the physics behind the EWSB mechanism is O(10) times higher thanthe EWSB scale itself. It will be found to be at least O(10) times “unnatural” , and theelectroweak phase transition (EWPT) will be an as unnatural phenomenon as darkenergy. The EWPT is related to the vacuum physics, as the same as the inflation anddark energy, whose scales are also not naturally explained. This will lead to aparadigm shift in our research direction, from the traditional bottom-up approach to
the top-down approach.

3. Electroweak Symmetry Breaking and the Origin of Matter-Antimatter
Asymmetry
There are two promising scenarios for the origin of matter-antimatter asymmetry; leptogenesis
and electroweak baryogenesis (EWBG). Figure 3 shows a flowchart for approaching thesescenarios. EWBG can be probed at the ILC250 in two phases as discussed below.

Figure 3: Origin of matter-antimatter asymmetry.
8As described in Section 2.2, the phase of the couplings between the Higgs boson and fermions canbe measured to 3.8-degree precision. Together with the precise measurements of the HZZ coupling
and Yukawa couplings, it can be determined whether or not the Higgs boson is responsible for theorigin of matter-antimatter asymmetry. These results can be cross-checked with experiments
searching for the electric dipole moment of the neutron and the electron. This is the first step toexamine EWBG (left-lower side of Figure 3) .
Furthermore, the electroweak symmetry breaking should be a strong first-order phase transition
and in non-equilibrium in order to retain the asymmetry produced by the Higgs sector. For this tooccur, it is necessary to introduce additional Higgs fields as in 2HD models or an additional singlet.
These new scalar fields result in large deviations (>20%) in the trilinear Higgs coupling and mostprobably a few-percent deviation in Higgs couplings with gauge bosons as well (right-lower side of
Figure.3). The ILC250 can investigate these couplings at sufficient precision. Gravitational waves
are also emitted during the EWPT in Strong 1st order transition. They can be detected at satellite
probes (such as LISA and DECIGO) which are expected to begin operation around 2040. This is thesecond step to examine EWBG. The Higgs potential demands that new phenomena must be present
in the range up to Λ=a few–1000 TeV in the case of EWBG. The energy of the next energy frontier
experiment and its accelerator technology can be also determined in this case (as shown in Fig.2)
The ILC250 can examine EWBG scenarios from many sides. The key to probe EWBG scenarios
is the precise measurement of CP violation, the Higgs couplings with the gauge bosons, and the
Yukawa couplings. The precisions of these measurements are largely similar between the ILC250
and the ILC500 results. Although the Higgs trilinear coupling (HHH coupling) cannot be measured
at the ILC250, the precise measurement of the Higgs boson couplings with the gauge bosons andthe gravitational wave probes can be used to elucidate the origin of matter. The crucial test of
EWBG can be performed at the ILC250.
If the EWBG scenarios are disfavored at the ILC250, or if CP violation is observed in neutrino
sector at the T2K experiment and neutrino-less double-beta decay is discovered, the leptogenesis
scenario becomes favorable. This implys the existence of a right-handed neutrino at a very highenergy scale as well as grand unification (GUT). The most attractive scenario for GUT is
supersymmetry with gauginos and higgsinos under 10 TeV. This scenario can be examined with thesearch for lepton number violation, the search for proton decays at the HyperKamiokande
experiment, and the search for gauginos and higgsinos under 10 TeV at the next hadron collider(FCC, HE-LHC, etc.) or at a higher energy lepton collider. The favorability of the leptogenesis
scenario is important input for the discussion of the accelerator technology and the CM energy ofthe next-generation facility. This is the path labeled “Λ~ a few–1000 TeV” in Figure 2. Since this is9

an important scenario, it is in the interest of Japan’s long-term strategy to construct a linear collider
which can easily accommodate the next-generation technology.

4. Direct Search for Dark Matter and New Particles Based on Naturalness
Naturalness has played an important role in the history of particle physics. The discovery of the
125 GeV Higgs boson has started to cast some doubt to this idea in the current situation that no new
physics is found. The claim is that using supersymmetry to explain a 125 GeV Higgs boson
nominally requires fine-tuning on the order of around O(100)–O(1000). However, there arepossibilities where squarks become naturally heavy like focus point models. The Higgs boson
becomes also naturally heavy in extensions of the MSSM, with additional singlets for example.
Before giving up on the idea of naturalness, these possibilities (Higgsino/Wino/Singlet-like ) must
be probed. They are also scenarios that provide natural candidates of dark matter (DM). Figure 4
summarizes the candidates of WIMP DM and their searches.

Figure 4: WIMP dark matter candidates and searches.
For the electroweak gauginos and higgsinos, the mass differences from the lightest particle among
them (wino, higgsino) are generally small as described in Section 2.1(b). Such compressed spectra
are challenging to search for at the LHC. It is also possible that the existence of a singlet particle
could suppress the couplings with the gauge bosons. The investigation of these two possibilities areimportant. Taking together the HL-LHC searches (having a sensitivity for the bino) and theapproach of 2.1(b) will make the search strategy complete.
It should be noted that the higgsino mass should be naturally around the Higgs mass; thus thesearch for the higgsino is particularly important. The ILC250 can probe higgsinos indirectly up to
about 200 GeV, corresponding to the test of naturalness of about 10%. This is the higgsino path10

shown in Figure 4. Since it is challenging to search for higgsinos at the HL-LHC, the ILC searches
are indispensable.
In the case of a singlet-like DM as in the middle of Figure 4, there are bosonic and fermionic DM(bino-like or singlino-like). A bino-like DM can be covered by gaugino searches at the HL-LHC.
For singlino-like DM or for bosonic DM, the Higgs invisible decays to unknown particles isimportant. The ILC250 is sensitive to Higgs invisible decays with a branching ratio of 0.3%. This
will be a tight constraint for DM candidates lighter than 62 GeV. Such light DM is challenging forunderground direct detection experiments because the recoiling energy is small. The ILC will
provide coverage for such particles, which makes the search strategy for DM complete.
In the case of a mixed bino and higgsino (Figure 4, left), the strategy depends on its mass. For aheavy mixed bino-higgsino, it can be covered through the gaugino search at the HL-LHC as well as
at direct DM detection experiments. If it is light, it will become challenging for the HL-LHC (due tothe compressed spectrum) and direct detection experiments (small recoil energy). The ILC250 isable to cover most of the remaining parameter space for these particles by the direct search.
For DM searches based on the “naturalness”, the ILC250 will be able to cover regions that cannot
be covered by the HL-LHC and direct detection experiments (up to 200 GeV higgsinos and up to
62 GeV singlet-like DM). The ILC250 together with these experiments make the approach of 2.1(b)and the search strategy for WIMP DM complete.
The ILC will play a crucial role in the search for electroweak gauginos and higgsinos and singlet
particles motivated by naturalness and dark matter. It plays a complementary role to the HL-LHC
and the DM direct detection experiments. Combining these three approaches makes the searchstrategy complete. The necessary CM energy of the ILC depends on how much fine-tuning one cantest for the naturalness.

5. Summary
The contributions of each project are summarized in Table 3. As discussed in Chapter 2, theILC250 will be able to explore the phase space of new physics that cannot be covered by the
HL-LHC or the Belle II experiments. It will be able to probe the new phenomena in a robust way.
In particular, the ILC250 has an excellent sensitive to the heavy Higgs bosons in 2HD models,
which is the 3rd approach as mentioned in Section 2.1(c). The ILC250 has also a good sensitivity tosearch for dark matter based on naturalness described in Section 2.1(b). Combining three
approaches ((a)-(c)) by the ILC250 and the HL-LHC will establish a comprehensive search
network, capable of probing the energy scale of new phenomena and new principles (up to Λ~2–
3 TeV). Thus the ILC250 will play an important role.
11Table 3: The role of each project.
ILC

Higgs & other SM precision measurements; electroweak baryogenesis;
2.1(b): higgsinos, and DM ligher than 62 GeV; 2.1(c): small tanβ.

HL-LHC

Higgs couplings; direct search of new phenomena; top quark mass;
2.1(a),(b): bino, wino;

SuperKEKB

2.1(c): large tanβ.

Additional CP violation in quark-sector; bottom quark mass;tau LFV (GUT);

2.1(c): large tanβ.

T2K, HK

CPV in neutrino-sector; leptogenesis; GUT.

LFV

Leptogenesis; right-handed neutrinos; GUT.

EDM

Flavor-conserving additional CP violation; electroweak baryogenesis.

LISA, DECIGOFirst-order phase transition for electroweak baryogenesis: an alternative
to the HHH coupling measurement.

UndergroundDM direct search; 2.1(b): heavy regions.

experiments
The ILC250 will also be able to elucidate the origin of matter. It can perform a crucial test of theelectroweak baryogenesis models, and probe the energy scale of new phenomena and newprinciples (up to Λ~ a few–1000 TeV).
Table 4 summarizes the list of measurements that become challenging by lowering the ILC
starting energy to 250 GeV. As far as the precision measurements of the Higgs and other SM
observables are considered, the ILC250 operating together with the HL-LHC and the SuperKEKB
experiments will be able to play a sufficient role, with precisions not too far from the ILC500.

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Table 4: List of measurements that become challenging
by making the ILC starting energy 250 GeV.
Observable

Solutions with synergy

Higgs Full Width

From HL-LHC, use custodial symmetry (KW/KZ = 1) to replace
ΓHZZ with ΓHWW in Γtotal=ΓHWW／Br(H→WW)
à becomes comparable to ILC500 precision

Self-coupling HHH

Baryon number violation à EWBG or leptogenesis (T2K,(also challenging forneutrino-less double beta decay). EWBG covered by HL-LHC,

ILC500)

ILC250, SuperKEKB, LISA.
Although direct measurement of self-coupling is not possible,
ILC250 can contribute to examine EWBG through CPV in Higgs
sector and the precise measurements of Higgs couplings.

Higgs couplings

HL-LHC (Top Yukawa coupling)
Lattice (mb, mc, αs uncertainty) à comparable to ILC500
SuperKEKB (Lattice examination)

Searches

Electroweak gauginos/higgsinos based on naturalness:
higgsino (< ~200 GeV); dark matter (<62 GeV).

Top mass

HL-LHC (0.2–0.3 GeV) sufficient precision for test of SM;
roughly sufficient for vacuum stability; (if a detailed study ofhigh scale physics becomes necessary, upgrade to 350 GeV)

Some of the main merits of the ILC operating at 350 GeV, 500 GeV, or above are(a) When the energy scale of the new phenomena and new principles is discovered by the
combined results of the ILC250 and HL-LHC, this energy scale becomes the next target
for an energy upgrade of ILC.
(b) Top quark mass precise measurement: The HL-LHC precision of 0.2–0.3 GeV issufficient for the test of the SM and the vacuum stability. If the results from the HLLHC and ILC250 point to physics at very high energy scales such as GUT and thenecessity to study the vacuum stability in further detail, then the ILC350 becomes
important.
(c) When only the electroweak scale seems to exist (the scenario in Figure 2 (right)), it
becomes important to directly study the breaking of the electroweak symmetry and the
Higgs potential in detail. In this case, the measurement of the Higgs self-coupling(HHH) becomes important, irrespective to the indirect measurement by gravitational
waves. The precise measurement at CM energy of 500 GeV (positive interference) and1 TeV (negative interference) will be both important.
13

Future energy upgrade scenarios should be discussed based on the findings of the energy scale of
new phenomena and new principles as in point (a) above, or the CM energy will be upgraded, as
before, upto 350, 500 GeV or 1TeV based on points (b) and (c).

Conclusions
The conclusions of this committee are the following four points:lll

In order to maximally exploit the potential of the HL-LHC measurements, concurrent
running of the ILC250 is crucial.
LHC has not yet discovered new phenomena beyond the Standard Model. The ILC250
operating as a Higgs Factory will play an indispensable role to fully cover new
phenomena up to Λ~2–3 TeV and uncover the origin of matter-antimatter asymmetry,combing all the results of ILC250, HL-LHC, the SuperKEKB, and other experiments.
Synergy is a key.
Given that a new physics scale is yet to be found, ILC250 is expected to deliver
physics outcomes, combined with those at HL-LHC, SuperKEKB and otherexperiments, that are nearly comparable to those previously estimated for ILC500 inprecise examinations of the Higgs boson and the Standard Model.

l

The inherent advantage of a linear collider is its energy upgradability. The ILC250 has
the potential, through an energy upgrade, to reach the energy scale of the new physics
discovered by its own physics program.

Many thanks to T.Tanabe (ICEPP, U.Tokyo), T.Nakada (EPFL) , H.Aihara (U.Tokyo) and
S.Komamiya (U.Tokyo) for useful discussions and suggestions to translate the original document
into English.

14