The most prevalent issue in high energy physics, today, is to devise the best strategy to go beyond the current knowledge frontier. What is the most appropriate collider technology for the next step? e+e- or pp?, linear or circular? It is well known that high precision Higgs and top quark measurements far beyond what LHC can deliver are necessary and a strong motivation for an e+e- collider. However, it is essential to investigate in details what such a collider would provide under any of the possible three following scenarios:
- LHC discovers no new particles, beyond the Higgs or
- LHC discovers some new low-mass states or
- LHC discovers new heavy particles.
These 3 inclusive scenarios have been bluntly addressed in a recent arXiv:1702.05333 paper: “The Potential of the ILC for Discovering New Particles“. The executive summary kindly prepared by the authors gives clear cut statements related to the 3 scenarios. It is reproduced in the following with a couple of highlighted figures
In principle, most of the points are also relevant for other e+e- collider projects like CepC, FCCee or CLIC as long as they cover the same energy range or better, provide polarized beams for the same integrated luminosity or better. It must be noted that the circular colliders CepC and FCCee do not provide polarized beams and CepC (70 Km) does not reach the top pair production energy. CLIC in its baseline project does not have polarized positron beams.
“We give the main conclusions of this report on the potential of the ILC for the discovery of new phenomena. Throughout this report, numerical estimates of precision or reach are based on the 20-year plan for ILC operation presented in , including 4000 fb−1 of luminosity at 500 GeV.
The ILC discovery program exploits the joint power of direct searches with the potential to produce new particles and precision measurements which are able to detect virtual effects
of new particles at higher mass scales. The latter will shed light on the structure of physics arising from beyond the Standard Model
even if the associated new particles are too heavy to be produced directly at LHC or ILC. All these measurements will rely on the clean operating environment, low backgrounds, and adjustable beam energy and polarization provided by the ILC. We will discuss the discovery of new interactions in the following programs:
1. New properties of the Higgs boson:
The ILC will be a Higgs boson factory which offers absolute, model-independent measurements of the Higgs boson couplings to Standard Model (SM) fermions and gauge bosons, most of them to better than 1% precision. These measurements would probe for modifications of the Higgs interactions arising from composite structure of the Higgs or from mixings with new particles, including new, heavier Higgs bosons. In addition, the self-coupling
of the Higgs boson can be measured to an accuracy of 27% via reactions such as e+ e− → ZHH , improving to 10% via ν̄νHH at 1 TeV. This measurement is a critical test of the theory of electroweak baryogenesis, a leading contender for explaining the cosmic matter-antimatter asymmetry.
2. New properties of the Top quark:
The ILC will be a precision top quark factory. Scans of the production threshold of e+ e− → t̄t can determine the top quark mass (mt
) to a precision of 50 MeV or better, including all theoretical uncertainties. A precision determination of mt
plays a central role in global electroweak fits which are indirectly sensitive to new particles and new interactions. Using polarized beams, the ILC can determine separately the top-quark left- and right-handed couplings to gauge bosons to the sub-percent level. Such measurements offer a huge discovery potential for a variety of composite Higgs or extra-dimensional new physics
models, even if the scale of the new physics is in the tens of TeV range. Predictions of several models (references see the full paper) on the deviations of the left- and right-handed couplings of the t quark to the Z0 boson. The ellipse in the frame in the upper right corner indicates the precision that can be expected for the ILC at √s = 500 GeV with L = 500 fb−1 of integrated luminosity shared equally between the beam polarizations Pe−, Pe+ =±0.8, ∓0.3 .
3. New force carriers:
By measuring distributions in e+ e − → f̄f production (where f stands for different SM fermions), the ILC is sensitive to new force particles Z0 with masses as high as 12 TeV. Via fermion pair production, either directly or by virtual effects, the ILC can also explore for other new physics resonances that occur in composite Higgs or extra-dimensional models. These measurements are not unlike the first indirect observation of the Z boson at Petra and Tristan via virtual effects in fermion pair production.
Mass limit of the carrier (Z’) of a new force reached by the ILC compared to LHC for various BSM models (see the original paper for details)
4. Additional Higgs bosons:
In addition to the possibility to discover relatives of the Higgs boson via studying the properties of the 125-GeV particle, the ILC offers unique opportunities to discover additional lighter Higgs bosons – or, more generally, any weakly interacting light scalar or pseudo-scalar particle – by their direct production.
5. Supersymmetric sisters of the Higgs boson:
If supersymmetry (SUSY) is the way nature has chosen to generate the symmetry-breaking potential of the Higgs boson, then light sister particles of the Higgs boson – higgsinos
are required. It is very possible, even theoretically preferred, that these particles have masses in the range of ∼ 100-300 GeV (the lighter the better), while all other supersymmetric particles are heavier. Such light higgsinos are difficult, perhaps impossible, to ob- serve at LHC, but their discovery would be straightforward at ILC. In that case, ILC would be a higgsino factory , providing quantitative tests of the hypothesis of supersymmetry and its implications for unification of forces.
6. SUSY without loop-holes:
A central prediction of supersymmetry is that sparticles couple with the same strength as their SM partners. Thus, the rates for production of SUSY particles are well predicted as a function of mass. Then the clean and well-defined conditions at the ILC guarantee either discovery or exclusion. This is not true at the LHC, where many scenarios with light SUSY particles can evade detection.
7. Discovering dark matter particles:
It is possible that particles of dark matter are being produced copiously at accelerators but are invisible to their detectors. To search for pair-production of invisible particles, one must hunt for an associated photon or gluon from initial-state radiation. Such searches at the ILC are complementary to those at the LHC, since they probe dark matter couplings to leptons rather than quarks. Because of the simplicity and calculability of background
reactions at the ILC, the ILC mass reach for discovery of dark matter particles is similar to that of LHC despite the difference in center of mass energy.
8. Identifying the nature of dark matter:
The precision capabilities of the ILC are optimal to uncover which mechanisms are responsible for generating dark matter in the early universe: Is it thermal or non-thermal? Does entropy-dilution play a role? Are there super-WIMPs? Is there a WIMP-axion admixture? Dark matter production in the early universe may be much more intricate than the simple thermal WIMP miracle scenario and ILC can play a key role in gaining a more complete understanding.
Though some models for neutrino mass invoke particles with masses well beyond the energy of any realistic accelerator, other models generate neutrino masses through new physics effects at energies that ILC and LHC can access. We give examples of models in which the ILC will discover new particles and measure properties that are simply related to the neutrino mixing angles.
We conclude that the physics case for the ILC is very strong, independently of future findings at the LHC. We illustrate this with a detailed discussion of consequences for ILC physics 1. if LHC discovers no new particles, 2. if LHC discovers some new low-mass states and 3. if LHC discovers new heavy particles.
Under each scenario, ILC will play a critical role in the discovery of new phenomena. ILC will push forward our knowledge of high energy physics and our understanding of the universe.”
 T. Barklow, J. Brau, K. Fujii, J. Gao, J. List, N. Walker and K. Yokoya, “ILC Operating Scenarios,” arXiv:1506.07830 [hep-ex]
 F. Richard, “Present and future constraints on top EW couplings,” arXiv:1403.2893 [hep-ph]