LINEAR e+e- COLLIDERS – FUTURE HIGGS FACTORIES

20th Lomonosov
ConferenceIvanka Bozovic
VINCA Institute of NuclearSciences, University of Belgrade

19.08.-15.08. 2021

OVERVIEW-WHY HIGGS FACTORIES?

-LINEAR COLLIDERS-DETECTOR CONCEPTS & TECHNOLOGIES-HIGGS PHYSICS AT LINEAR COLLIDERS-OUTLOOK

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WHY HIGGSFACTORIES?
--Higgs discovery ended era of reductionism and symmetries in particle physics 1
Never seen before fundamental scalar is discovered, unique (with exception of gravity) in its
self-coupling
Higgs discovery opened several important questions of nature of relativistic vacuum:- How can we accommodate it in energy density of the Universe?
- Why the Higgs is not enormously massive (even Planckian)?

1

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LIMITS, LIMITS,…
-No New Physics discovery at LHC
With the LHC resolution to probe Higgs compositeness, the Higgs could be as elementary as pion.
So, how pointlike is it?
 can be significantly enhanced in EW bariogenesis models.
HL-LHC will probe  with 50% uncertainty 2

1
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WHAT BRINGS US TO THE HIGGS FACTORIES--But, other aspects arealso important:

In the European PP Strategy Update 2020, Higgs factories are the highest priority future initiatives 3 Several projects on the market ( 106 Higgs bosons)All electron-positron colliders- Initial state well-defined- High ( TeV) center-of-mass energies - LCs- Clean environment→ High-precision measurements (dominated by statistical uncertainty)
Linear (ILC, CLIC) vs. circular (CEPC, FCCee)

Extensibility of thephysics span flexibility toaccommodate
other options (pp,hh, ep, gamma
gamma, plasma….)
Flexibility toaccommodatechanges inscenario(i.e. unexpectedHL-LHC discovery)- Technologicalfeasibility and cost

It is clear that it will be a run for statistical precision- Politics(it’s a game ofsupremacy ,unfortunately)

100 x LEP
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LINEAR COLLIDERS---Comes as mature technological options developed for decade(s) – ‘ready to take’Staged, upgradable machines- Various Higgs production mechanisms accessible over the energy scale span
- Less precise determination of an observable at high energy leads to the same
precision on coupling as at low energy
Beam polarization- Chiral nature of charge currents results in significant sensitivity of WW-fusioncross-section on polarization scheme (2L)- Provides new observables sensitive to New Physics- Helps characterization of newly discovered particles
A few technical benefits- Triggerless- Power-pulsing
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Timeline:- Around 2000’s – TESLA, NLC, JLC- (2004) ILC based on ‘cold’ TESLA technology- (2013) Technical Design Report 6- (2020) International Development Team (IDT)- (?) Preparatory lab in Japan- (2035) First collisions 7

- Largest ever accelerator prototype (operating now as E-XFEL)- Full industrialization of RF cavity production

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ILC comes with the collider program and rich auxiliary experiments

Potential ILC site in Kitakami- At the LHC, experiments search for dark particles
produced by pp collisions are placed in existing tunnels
and caverns at CERN (FASER @ATLAS, MilliQan @CMS)- Dark sector (ILC-BDX), fixed-target and beam dump experiments(ILCX)
9

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SiD Detector- 5 T field- More compact- All SiILD Detector- 3.5 T field- Optimized for CM energies 90 GeV - 1 TeV
CMS/40- Si/gaseous tracking- Particle flow calorimetry
CMS/4- Mature design and available technologies
ATLAS/2ATLAS/3

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The only LC to go above 1 TeV- CLIC Test Facility, CTF3, at CERN now the‘CERN Linear Electron Accelerator for Research’facility, CLEAR- Normal conductive high-current drive beam-

8

380 GeV and 1.5 TeV – one drive-beam
3 TeV two drive-beam complexes
100 MV/m gradient in the main-beam cavities

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CLIC det-4 T field
Ultra low-mass VTXAll Si tracking
Particle flow calorimetryTime-stamped readout (10 ns) due to pronounced
Beamstrahlung background at higher energies

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13PHYSICS PROGRAMMEAT A LC
- HIGGS PHYSICS- t-PHYSICSHIGGS COUPLINGS: model-independent measurements
-framework
EFT approach
HIGGS SELF-COUPLINGHIGGS AS A PROBE TO DARK SECTOR AND BSM IN GENERALCPV IN THE HIGGS SECTORHigh E

Low E

High E- BSM
High EDue to staged realization of LCs, these are ideal machines to explore large
physics span, with indirect access to the ~ 100 TeV scale

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HIGGS PRODUCTION MECHANISMS AT LC- Higgsstrahlung (ZH) is a unique feature of particleantiparticle collisions (i.e. e+e- colliders)- It facilitates gHZZ measurement in a modelindependent way * (ZH cross-section)- Higgs invisible width can be determined from therecoil mass
- Most of the Higgs couplings can be determined witha better precision than at HL-LHC only from ZH__________________________________________- Linear colliders foreseen as staged machines
benefit from additional statistics from WW-fusion(clear example is CLIC with ~ 3M Higgs bosons at allstages)- Double Higgs production at higher energies enables
self-coupling measurement* Theory warning: level of accuracy <1% requires incorporation ofloop-corrections  loss of strict model-independence

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High-energy benefits, polarization, combination- Clear advantage from rising cross-section for WW-fusion with energy

access to rare Higgs
decays- ttH production, suitable i.e. for CPV study in the Higgs sector- Multiple-Higgs production  self-coupling measurement- Less precise determination of the observable at high energy leads to the same
precision on coupling as at low energy

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HIGGS PHYSICS
Situation at LHC (HL-LHC, and pp in general)- No absolute measurement of theproduction cross-section (like ZH ate+e- colliders)- Higgs couplings come in combination:- Only ratio of couplings can be directlydetermined (i.e.
))

e+e- colliders- Absolute measurement of the ZH cross-section- Absolute measurement of the Higgs BRs- Nearly model-independent determination of theHiggs total width and couplings- High energy benefits of LCs: , CPV, BSM
extensions of the Higgs sector

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HIGGS COUPLINGS

Similar at circular colliders…

Model independent approach*,
precision better than 1% for most couplings

10

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COMBINATION WITH HL-LHC
To what extent future e+e- experiments are synergistic with the HL- LHC?

Evident synergy

11- An example: ILC250 with 250 fb-1
- Already the single measurement of the HZcross section at ILC 250 yields a very large
improvement of the HL-LHC accuracies

The same holds for CLIC (and FCCee, CEPC)
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HIGGS SELF-COUPLING- High energy (>1 TeV) e+e- collider is superior indetermination of the Higgs self-coupling- High energy (double) Higgs production is the mostsensitive to deviations of the Higgs self-coupling-  is determined from the total rate of HH events (ILD)or template fit of mHH and BDT output (CLICdp)- Polarization (i.e. -80%) almost doubles the HHvv rate
Low energy e+e- colliders (single Higgs production)in combination with HL-LHC:- ILC250 and FCCee365, 35%
- Double-Higgs production:- HL-LHC: ~ 50%
- ILC500 ~  27%- CLIC3000 ~  9%
- FCC-hh ~  5%

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LC BENEFITS: STAGING, COMBINATIONS…

Higgs self-coupling- Intermediate energy (1.4(5) TeV) at CLIC provides
complementarity to 3 TeV option with ZHH
production- Different behavior of ZHH and double-Higgs
production in WW-fusion, for non-SM values of triple
Higgs couplings resolves ambiguity from interference- Statistical uncertainty reduction in combination- Clear gain from high center-of-mass energy

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4

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HIGGS AS A PROBE TO BSM
12- BSM physics can manifest itself in theHiggs sector in several ways:- Contribution from the higher orderoperators (EFT approach)- Higgs compositeness- Extended Higgs sector- DM portal- CPVHigh energy Higgs production is the most sensitive tocontributions from the 6D operators in the EFT approach andthus can probe the highest New Physics scale ILC

12

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HIGGS AS A PROBE TO BSM – EFT INTERPRETATIONS

Above 5 model discrimination already with 250 Gev ILC- Substantial improvement at higher energies (linear e+e- colliders):@ILC a factor 2 in Higgs couplings precision with 500 GeV polarized beams- Complementarity with HL-LHC10

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HIGGS AS A PROBE TO BSM- BSM physics can manifest itself inthe Higgs sector in several ways:- Contribution from the higher
order operators (EFTapproach)- Higgs compositeness- Extended Higgs sector- DM portal
- CPV
11
The scale of compositeness can be probed significantly higher from the highenergy collider kinematic limit
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HIGGS AS A PROBE TO BSM- BSM physics can manifest itself in the Higgs
sector in several ways:- Contribution from the higher orderoperators (EFT approach)- Higgs compositeness- Extended Higgs sector- DM portal- CPV

SM Higgs mixing with a heavy singlet state 11

-In majority of BSM models, SM Higgs comes withadditional Higgses (2HDM, SUSY in general,compositeness,..etc.)- Can be a lighter scalar than SM Higgs – it is important tobe capable of probing such states at future colliders- If SM Higgs is the lightest, other states are nearly massdegenerated

5
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HIGGS TO INVISIBLE

Hinv.

- BSM physics can manifest itself in the Higgs sector inseveral ways:- Contribution from the higher order operators (EFTapproach)- Higgs compositeness- Extended Higgs sector- DM portal
- CPV

13

- Looking at the recoil mass under
the condition that nothingobservable is recoiling against the Zboson (only one Z per event)- Access to DM connected to SM
particles through a specific set ofoperators (portals)<0.5% , 95% CL
12

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CPV IN THE HIGGS SECTOR- More difficult than just a spin/parity
determination: Higgs can be a mixture ofdifferent CP eigenstatesℎ = 𝐻 cos  + 𝐴 sin - Can be measured in Hff and HVV vertices,
both in Higgs production and decays- Hff (HVV) sensitive to CPV contributions
at the tree (loop) levelx

14- Only lose bounds (at present) on aquantum superposition od different CPstates, while experimentally disfavoredhypothesis on purely CP odd state

ILC250 – benefit from polarization & combination (H)1 TeV – optimal for ZZ-fusion

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SUMMARY• All future e+e- projects bring significant added value to theprojected HL-LHC sensitivities in the Higgs sector…
• … enabling discrimination of BSM models inaccessible atHL-LHC• Already lowest energy phases brings sensitivity far beyond
the projected HL-LHC precision• Higher center of mass energies significantly extends
physics span of a LC (Higgs self-coupling, BSM scenarios)
upgrade is important – genuine advantage of a LC• Additional enhancement from polarization (precision,model discrimination)?

READY-TO-WEAR PROJECTS
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20th Lomonosov
Conference
19.08.-15.08. 2021

THANK YOU29

20th Lomonosov
Conference
19.08.-15.08. 2021

References:
1. N. Arkani Hamed, CEPC WS, Beijing, 2019, https://indico.ihep.ac.cn/event/7389/session/0/contribution/18/material/slides/0.pdf
2. Higgs@FC WG
3. European Strategy Briefing Book and arXiv:2001.05278
4. A. Robson, CEPC WS EU edition, Oxford, 2019
5. P. Bambade et al., ILC – A Global Project, arXiv:1903.01629v3
6. ILC TDR, arXiv:1306.6328
7. S. Kawada, ILC Physics potential, EPS-HEP2021
8. A. Robson et al., The Compact Linear e+e− Collider (CLIC): Accelerator and Detector, arXiv:1812.07987
9. S. Kawada, Prospects of measuring Higgs boson decays into muon pairs at the ILC, arXiv:1902.05021
10. J. List, , ECFA HIggs@FutureColliders, 2019 and arXiv:1710.07621
11. The CLIC Potential for New Physics, arXiv:1812.02093
12. M. Perelstein, PHENO-16, Pittsburgh, May 11 2016
13. Yu Kato @ EPS-HEP 201914. D. Jeans et al., Measuring the CP state of tau lepton pairs from Higgs decay at the ILC, arXiv:1804.01241

B1. D. Arominski, A detector for CLIC: main parameters and performance, arXiv:1812.07337
B2. G. Weiglein, Higgs requirements from theory, DESY, Hamburg, May 201930

20th Lomonosov
Conference
19.08.-15.08. 2021

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SIMILAR PERFORMANCE OF LC DETECTORS

B1

B1

Particle Flow is the ‘key word’. Only neutral particles ID (, neutral hadrons) are left to calorimeters.
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THEORY EXPECTATIONS
How well do we need to know Higgs couplings?

The models below are outside the HL-LHC reach- In many BSM models one expects only % level
deviations from the SM couplings for BSM particles in
the TeV range- Higgs to EW bosons couplings are particularly sensitive
to BSM; even more

- Example, 2HDM-type model in decoupling limit B2Percent order accuracy on Higgs couplings offers access to various BSM scenariosFIRST UPCONSULTANTS

LINEAR VS. CIRCULAR-Precision vise, linear and circular colliders’precision is comparable when it comes tothe Higgs couplings
Due to high-energy access of high crosssection Higgs production mechanisms, LCs
are superior in probing of the Higgs selfcoupling---Also, beam polarization brings added value
Extensibility of the physics span (ppcollisions, 100TeVcenter-of mass energy) is agreat advantage of circular colliders

But, + a 100TeVhadron-collider, comes at
the moment with quite a few open issues:-Accelerator & detector technologies-Huge pile-up-Systematics control and theoreticaluncertainties

A word from theory- Precision measurement of couplings at hadron colliders are limited by thesystematic (theoretical) uncertainties- This is also a reason for the fact that the Higgs coupling projections for HELHC show only relatively small improvements over HL-LHC- FCC-hh projections, in particular when taken separately, depend on a drastic
reduction of theory uncertainties B2.
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