Expected statistical uncertainties at future e+e− colliders

Now that the mass of the Higgs boson is known, circular electron positron colliders, able to measure the properties of these particles with high accuracy, are receiving considerable attention. Design studies have been launched (i) at CERN with the Future Circular Colliders (FCC), of which an e+e- collider is a potential first step (FCC-ee, formerly caller TLEP) and (ii) in China with the Circular Electron Positron Collider (CEPC). Hosted in a tunnel of at least 50 km (CEPC) or 80-100 km (FCC), both projects can deliver very high luminosity from the Z peak to HZ threshold (CEPC) and even to the top pair threshold and above (FCC-ee).At the ZH production optimum, around 240 GeV, the FCC-ee (CEPC) will be able to deliver 10 (5) ab-1 integrated luminosity in 5 (10) years with 4 (2) interaction points: hence to produce millions of Higgs bosons through the Higgsstrahlung process and vector boson fusion processes. This sample opens the possibility of sub-per-cent precision absolute measurements of the Higgs boson couplings to fermions and to gauge-bosons, and of the Higgs boson width. These precision measurements are potentially sensitive to multi-TeV range new physics interacting with the scalar sector. The ZH production mechanism also gives access to the invisible or exotic branching ratios down to the per mil level, and with a more limited precision to the triple Higgs coupling. For the FCC-ee, the luminosity expected at the top pair production threshold (s∼340–350 GeV) further improves some of these accuracies significantly, and is sensitive to the Higgs boson coupling to the top quark.

We investigate the sensitivity of proposed circular electron positron collider (CEPC) and future circular collider in its electron–positron mode (FCC-ee) with a center-of-mass energy of 240 GeV to long-lived dark photons heavier than 2 GeV that are pair produced via the prompt decays of a light scalar mixed with the Standard-Model Higgs boson. We compute the production and decay rates of both the light scalar and the dark photon, and develop two search strategies targeting displaced vertices within the inner tracker of the main detectors. Using Monte Carlo simulations, we evaluate the signal acceptance and projected sensitivity for each strategy. Our results show that, for the scalar-Higgs mixing angle set at 10 − 2 just below the current upper limit, the proposed searches at CEPC and FCC-ee can probe a dark-photon kinetic-mixing parameter several orders of magnitude below existing bounds, for dark photons lighter than half the dark-scalar mass.

We study phenomenological consequences of the strong first-order electroweak phase transition in an extension of the standard model with an inert doublet and vector-like leptons motivated by the muon $g-2$ anomaly and dark matter. We find that a condition for the strong first-order electroweak phase transition inevitably induces a large logarithmic enhancement in $Z$ boson decays, which relegates the explanation of the anomalous muon $g-2$ at below 2$\sigma$ level. Our analysis shows that future lepton collider experiments, especially the Giga-$Z$ at the International Linear Collider and Tera-$Z$ at the Circular Electron Positron Collider as well as Future Circular Collider have great capability to explore the nature of the electroweak phase transition, which is complementary to conventional approaches via measurements of the triple Higgs boson coupling and gravitational waves.

We examine the new physics sensitivity of the rare decay Λb→Λνν¯, which can be accessible at future Z-pole machines like Future Circular Collider and Circular Electron Positron Collider. We find that the longitudinal polarization of Λb baryons produced in Z decays introduces a novel observable, the forward-backward asymmetry AFB↑ in the angle between the outgoing Λ momentum and the Λb spin. We provide Standard Model predictions for the Λb→Λνν¯ branching ratio and AFB↑, and show that future precision measurements of these observables are complementary and probe new physics scales comparable to other b→sνν¯ and b→sℓ+ℓ− processes. We also show that the zero crossing of the forward-backward asymmetry offers a robust test of form factor calculations independent of new physics.

We study displaced vertex signatures of long-lived particles (LLPs) from\nexotic Higgs decays in the context of a Higgs-portal model and a\nneutral-naturalness model at the CEPC and FCC-ee. Such two models feature two\nrepresentative mass ranges for LLPs, which show very different behavior in\ntheir decay signatures. The Higgs-portal model contains a very light sub-GeV\nscalar boson stemming from a singlet scalar field appended to the Standard\nModel. Such a light scalar LLP decays into a pair of muons or pions, giving\nrise to a distinctive signature of collimated muon-jet or pion-jet, thanks to\nthe sub-GeV mass. On the other hand, the neutral-naturalness model, e.g.,\nfolded supersymmetry, predicts the lightest mirror glueball of mass $O(10)$\nGeV, giving rise to long decays with a large transverse impact parameter\nbecause of the relatively large mass. Utilizing such distinct characteristics\nto remove the background, we estimate the sensitivities of searches for light\nscalar bosons and mirror glueballs at the CEPC and FCC-ee. We find either\ncomplementary or stronger coverage compared to the previous results in the\nsimilar contexts.\n

This chapter presents a comprehensive examination of the fundamental technologies underpinning modern particle accelerators, highlighting current challenges and future directions across key subsystems. Core components, such as ultra-high vacuum systems, superconducting magnets, radio frequency (RF) acceleration cavities, cryogenic infrastructure, and beam diagnostics, are analyzed in terms of performance, integration, and operational reliability. Particular attention is given to the interplay between beam dynamics phenomena, such as electron cloud effects, beam-gas interactions, and instabilities, and enabling technologies like surface coatings, magnetic optics, and thermal management strategies. Advances in superconducting radio-frequency (SRF) cavities, high-field Nb3Sn and hybrid high-temperature superconductors (HTS) magnets, and neon-helium refrigeration systems are discussed with respect to luminosity goals, energy efficiency, and system scalability. Drawing on insights from leading collider facilities—mainly including the Large Hadron Collider (LHC), HL-LHC, HE-LHC, Future Circular Collider (FCC)Circular Electron Positron Collider (CEPC), and Super Proton-Proton Collider (SPPC)—the chapter synthesizes cross-disciplinary innovations and outlines critical directions for the next generation of high-energy particle colliders.

Interaction of the beam with the fundamental impedance of the accelerating cavities can limit the performance of high-current accelerators. It can result in a significant variation of bunch-by-bunch parameters (bunch length, synchronous phase, etc.) if filling patterns contain gaps that are not negligible compared with the cavity filling time. In the present work, this limitation is analyzed using the steady-state time-domain approach for the high-current option (Z) of the future circular electron-positron collider and for the future circular hadron-hadron collider. Mitigation of transient beam loading by direct rf feedback is addressed with evaluation of additional required generator power.

Our main aim in this paper is to present detailed studies to probe the top quark flavor changing neutral current (FCNC) interactions at tqg, tqγ, tqH and tqZ(σμν,γμ) vertices in the triple-top signal pp→tt¯t(t¯tt¯) at the high energy proposal of Large Hadron Collider (HE-LHC) and future circular hadron-hadron collider (FCC-hh). To this end, we investigate the production of three top quarks which arises from the FCNC couplings taken into account the fast simulation at s=27 TeV of HE-LHC and 100 TeV of FCC-hh considering the integrated luminosities of 10, 15 and 20 ab−1. All the relevant backgrounds are considered in a cut based analysis to obtain the limits on the anomalous couplings and the corresponding branching ratios. The obtained exclusion limits on the coupling strengths and the branching ratios are summarized and compared in detail with the results in the literature, namely the most recent direct LHC experimental limits and HL-LHC projections as well. We show that, for the higher energy phase of LHC, a dedicated search for the top quark FCNC couplings can achieve much better sensitivities to the triple-top signal than other top quark production scenarios. We found that the limits for the branching ratios of tqg and tqH transitions could reach an impressive sensitivity and the obtained 95% CL limits are at least three orders of magnitude better than the current LHC experimental results as well as the existing projections of HL-LHC.

We study the signatures of the triplet-quadruplet dark matter model at the LHC and future colliders, including the 100 TeV Super Proton-Proton Collider and the 240 GeV Circular Electron Positron Collider. The dark sector in this model contains one fermionic electroweak triplet and two fermionic quadruplets, which have two kinds of Yukawa couplings to the Higgs doublet. Electroweak production signals of the dark sector fermions in the $\text{monojet}+/\!\!\!\!E_\mathrm{T}$, disappearing track, and $\text{multilepton}+/\!\!\!\!E_\mathrm{T}$ channels at the LHC and the Super Proton-Proton Collider are investigated. Moreover, we study the loop effects of this model on the Circular Electron Positron Collider precision measurements of $e^+e^-\to Zh$ and $h\to\gamma\gamma$. We find that most of the parameter regions allowed by the observed dark matter relic density will be well explored by such direct and indirect searches at future colliders.

<p indent="0mm">Recently, the future of particle physics and high energy accelerators aroused extensive attention from society. The view that current accelerators will soon be replaced by those based on the new acceleration mechanism, is widely spread. This view implies that next-generation high energy accelerators shall not be built until the new mechanism is ready. This paper introduces the near and mid-term science goals of particle physics, and the needed accelerators with the energy and luminosity requirements. By analyzing current status and possible future achievements of the new acceleration mechanism, or more precisely, plasma wakefield acceleration mechanism driven by laser or particle beams, we conclude that traditional accelerators will still be the main choice in the next <sc>20–30 years.</sc> First of all, we argue that studies such as neutrino physics, cosmic-rays physics, dark matter searches and dark energy studies may not rely on accelerators, but they do not represent the main-stream of particle physics. After the discovery of the Higgs boson, the particle physics community reached a consensus that our next priority is the study of Higgs couplings with all the other elementary particles. This study will not only test precisely the Standard Model, but also be the best probe to new physics beyond the Standard Model. For this purpose, a very high luminosity Higgs factory, based on the electron-positron collision, is needed. After a short introduction of different types of traditional accelerators and their related technologies, we conclude that linear electron (positron) accelerators are limited by their acceleration capabilities while their luminosity is much smaller than circular electron (positron) accelerators. Circular electron (positron) accelerators are limited by the synchrotron radiation hence their energy cannot reach <sc>300 GeV</sc> or above for a reasonable power consumption. Below this energy limit, circular colliders have advantages on luminosity, power consumption, cost, etc., over linear ones. Circular proton accelerators have very little synchrotron radiation, hence the acceleration goal is easy to achieve but the bending power of the magnet becomes the limit of energy. For linear proton accelerators, they have no advantages at all on energy, luminosity, power consumption, cost, etc., except at very low energies. From the above analysis, it is evident that the plasma-acceleration mechanism can only play a role on linear accelerators to improve acceleration capabilities if luminosity is not highly demanding. Obviously, even if plasma-accelerators are applicable, they are still not suitable for Higgs factories where the luminosity is a key while the energy is in the comfort zone of circular accelerators. Laser-driven accelerators are still far away to be applicable since their average power, power efficiency, beam quality and beam intensity are several orders of magnitude off, while beam-driven accelerators are off by a few factors to one order of magnitude. In addition, issues such as positron and proton acceleration mechanism, high efficiency multi-stage cascade acceleration need to be resolved and experimentally demonstrated. It is a common understanding that there are still <sc>20–30 years</sc> for the new acceleration mechanism to be a viable choice of high energy accelerators. However, plasma-based accelerators may find applications in irradiation, radiation imaging, free-electron laser, etc. in coming years. It may also be combined with traditional accelerators to form an injector. The future of high energy accelerators becomes then very clear: We shall build a circular electron-positron collider as a Higgs factory while actively working on R&amp;D for possible future very high energy (1–10 TeV) e⁺e^– linear colliders, as well as on high <italic>T</italic>_c superconducting technologies, in particular iron-based superconducting materials for future circular proton colliders.

A study of single top quark production via flavor changing neutral current interactions at $tq\gamma$ vertices is performed at future circular hadron electron collider. The signal cross sections for the processes $e^{-}p\to e^{-}W^{\pm}q+X$ and $e^{-}p\to e^{-}W^{\pm}bq+X$ in the collision of electron beam with energy $E_e=$ 60 GeV and proton beam with energy $E_p=$ 50 TeV are calculated. In the analysis, the invariant mass distributions of three jets reconstructing top quark mass, requiring one b-tagged jet and other two jets reconstructing the $W$ mass are used to count signal and background events after all selection cuts. The upper limits on the anomalous flavor changing neutral current $tq\gamma$ couplings are found to be $\lambda_q < $ 0.01 at future circular hadron electron collider for $L_{int}=100$ fb$^{-1}$ with the fast simulation of detector effects. Signal significance depending on the couplings $\lambda_q$ is analyzed and an enhanced sensitivity is found to the branching ratio BR($t\to q\gamma$) at the future circular hadron electron collider when compared to the current experimental results.

With the LHC about to start its last data-taking period before being upgraded to the High-Luminosity LHC, it is time for the international high energy physics community to define the future of collider particle physics. The European Strategy for Particle Physics highlights an electron-positron Higgs boson factory as the main priority and as a first step towards a very high-energy future hadron collider. A staged Future Circular Collider (FCC), consisting of a luminosity-frontier highest-energy electron-positron collider (FCC-ee) followed by an energy-frontier hadron collider (FCC-hh), promises the most far-reaching physics program for the post-LHC era. FCC-ee is a precision instrument to study the Z, W, Higgs and top particles, and offers unprecedented sensitivity to signs of new physics. Most of the FCC-ee infrastructure can later be reused for the subsequent hadron collider, FCC-hh. The FCC-hh provides proton-proton collisions at a centre-of-mass energy of 100 TeV and can directly produce new particles with masses of up to several tens of TeV. This collider will also measure the Higgs self-coupling and explore the dynamics of electroweak symmetry breaking. Thermal dark matter candidates will be either discovered or conclusively ruled out by FCC-hh. Heavy-ion collisions and ep collisions (FCC-eh) further contribute to the breadth of the overall FCC program. The integrated FCC infrastructure will serve the particle physics community through the end of the 21st century. This presentation summarizes the feasibility of such a plan, possible implementation and conceptual designs of FCC-ee and FCC-hh, as well as physics potential.

In 2012, the discovery of a particle matching the Higgs boson at CERNs Large Hadron Collider became a very important milestone in high energy physics history. Research and development towards future high-energy electron-positron colliders is actively explored around the world with the purpose of measuring precisely the properties of this unconfirmed Higgs boson and to discover new physics beyond the Standard Model of particle physics. In this paper, we briefly review the history from the past half century of electron-positron colliders at the high-energy frontier, including the first electron-positron collider AdA, as well as the colliders based AdA, such as ACO, VEPP-II, ADONE, and the later colliders SPEAR, DORIS, CESR, TRISTAN etc., and the last and largest electron-positron collider in the world up to now, LEP. Then, the paper reports several proposals for future electron-positron colliders from around the world such as ILC, CLIC TLEP and so on, in particular Chinas entry, the Circular Electron-Positron Collider.

We study how to unravel the dark matter blind spots by phase transition gravitational waves in synergy with collider signatures at electroweak one-loop level taking the inert doublet model as an example. We perform a detailed Monte Carlo study at the future lepton colliders in the favored parameter space, which is consistent with current dark matter experiments and collider constraints. Our studies demonstrate that the Circular Electron Positron Collider and other future lepton colliders have the potential to explore the dark matter blind spots.

We calculate the next-to-leading order (NLO) quantum chromodynamics (QCD) corrections to the exclusive processes gamma +gamma rightarrow {mathcal {Q}}+{mathcal {Q}}, with {mathcal {Q}}=J/psi , eta _c, Upsilon , or eta _b, in the framework of non-relativistic QCD (NRQCD) factorization formalism. The cross sections at the SuperKEKB electron–positron collider, as well as at the future colliders, like the circular electron positron collider (CEPC) and the international linear collider (ILC), are evaluated. Numerical result indicates that the processes for J/psi -pair production and eta _c-pair production are hopefully observable at the Belle II detector within the next decade.