Search

In response to the 2013 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) study was launched, as an international collaboration hosted by CERN. This study covers a highest-luminosity high-energy lepton collider (FCC-ee) and an energy-frontier hadron collider (FCC-hh), which could, successively, be installed in the same 100 km tunnel. The scientific capabilities of the integrated FCC programme would serve the worldwide community throughout the 21st century. The FCC study also investigates an LHC energy upgrade, using FCC-hh technology. This document constitutes the second volume of the FCC Conceptual Design Report, devoted to the electron-positron collider FCC-ee. After summarizing the physics discovery opportunities, it presents the accelerator design, performance reach, a staged operation scenario, the underlying technologies, civil engineering, technical infrastructure, and an implementation plan. FCC-ee can be built with today's technology. Most of the FCC-ee infrastructure could be reused for FCC-hh. Combining concepts from past and present lepton colliders and adding a few novel elements, the FCC-ee design promises outstandingly high luminosity. This will make the FCC-ee a unique precision instrument to study the heaviest known particles (Z, W and H bosons and the top quark), offering great direct and indirect sensitivity to new physics. [ABSTRACT FROM AUTHOR]

Abstract We consider the generation of the new light gauge boson Z μ τ predicted by the U (1) L μ − L τ model via Z h 1 associated production followed by the SM-like Higgs boson h 1 decaying into a Z μ τ pair process at future high-energy e + e − colliders. Taking into account the experimental constraints on the free parameters of the U (1) L μ − L τ model and the beam polarizations P (e − , e +) = (− 0.8 , 0.3) , P (e − , e +) = (0.8 , − 0.3) and P (e − , e +) = (0 , 0) , we calculate the cross sections of Z μ τ production and further investigate the observability of Z μ τ production through the leptonic channel Image 1 and hadronic channel Image 2 at e + e − colliders. We find that the signal significance above 5 σ for Z μ τ detection can be achieved via both these two processes with appropriate parameter values and high integrated luminosity, and that the hadronic channel can offer a more detectable signature compared with the leptonic channel. [ABSTRACT FROM AUTHOR]

Various proposals of electron and muon colliders have been put forward for precision studies of the Higgs properties. It is shown that in both cases of Higgs resonant production through a muon collider, and associated HZ production in a electron–positron collider sizeable QED radiative effects must be carefully taken into account for the precise measurements of the decay channels of the Higgs particle. [ABSTRACT FROM AUTHOR]

We briefly summarize some searches for Higgs bosons with a mass of ≲ 1 1 0  GeV at LEP and the LHC. We discuss a possible signal in the diphoton decay mode at m ϕ ∼ 9 6  GeV as reported by CMS, together with a ∼ 2 σ hint in the b b ̄ final state at LEP. We briefly review possible interpretation of such a new particle in various BSM models. We focus on possible explanations as reported within the NMSSM and the μ ν SSM. Conclusions for future collider projects are briefly outlined. [ABSTRACT FROM AUTHOR]

In this study, triple Higgs boson production channels in a γγ-collider are investigated in the context of the two-Higgs-doublet model (2HDM). This model has three neutral Higgs bosons (h0/H0/A0 ) and two charged Higgs bosons (H±). Between all the possible combinations of the triple Higgs bosons in the final state, only the following two scattering processes are probable at tree-level: γγ→ H+H- h0 and γγ→ H+H- H0 . These two processes are important to determine the triple Higgs self-couplings gH+H-h0 and gH+H-H0 in the 2HDM. The calculation is carried out for two scenarios, which are inspired by recent experimental and theoretical results. The cross-section of these processes is calculated and a comparison is made for these scenarios. The total convoluted cross-section with photon luminosity in an e+e- -collider is presented as a function of the center of mass energy. [ABSTRACT FROM AUTHOR]

Abstract Inspired by the new TOTEM data on elastic pp scattering at 13 TeV, we study the possibility to describe all the diffractive collider data (σ tot , d σ el / d t , ρ ≡ Re A / Im A , σ low M D) in a wide interval of energy (0.0625 to 13 TeV) in the framework of a two-channel eikonal model. We show that a satisfactory description can be achieved without an odd-signature (Odderon) exchange contribution. We consider the possible role of the QCD Odderon which may improve the description of ρ and discuss the importance of the odd-signature term if the amplitude were to exceed the black disc limit. [ABSTRACT FROM AUTHOR]

Abstract The observation of Higgs boson production in association with a top quark pair (t t ¯ H), based on the analysis of proton–proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb−1, and considering Higgs boson decays into b b ¯ , W W ⁎ , τ + τ − , γγ , and Z Z ⁎ , the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the t t ¯ H searches using a dataset corresponding to integrated luminosities of 4.5 fb−1 at 7 TeV and 20.3 fb−1 at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total t t ¯ H production cross section at 13 TeV is measured to be 670 ± 90 (stat.) − 100 + 110 (syst.) fb, in agreement with the Standard Model prediction. [ABSTRACT FROM AUTHOR]

We study the next-to-leading order electroweak corrections to Higgs processes from dimension-six top-quark operators in the Standard Model Effective Field Theory approach. We consider the major production channels, including W H, ZH, and VBF production at the LHC, and ZH, VBF production at future lepton colliders, as well as the major decay channels including H → γγ, γZ, Wlν, Zll, b¯b, μμ, τ τ . The results show that within the current constraints, top-quark operators can shift the signal strength of the loop-induced processes, i.e. H → γγ, γZ, by factors of ∼ O(1)- O(10), and that of the tree-level processes, i.e. all remaining production and decay channels, by ∼ 5 - 10% at the LHC, and up to ∼ 15% at future lepton colliders. This implies that essentially all Higgs channels have started to become sensitive to top-quark couplings, and in particular, Higgs observables at high luminosity LHC as well as future lepton colliders, even below the tt¯ threshold, will improve our knowledge of top-quark couplings. We derive the sensitivities of Higgs measurements to top-quark operators at the high luminosity LHC, using projections for both inclusive and differential measurements. We conclude that treating the dimensionsix top-quark sector and the Higgs/electroweak sector separately may not continue to be a good strategy. A global analysis combining Higgs and top-quark measurements is desirable, and our calculation and implementation provide an automatic and realistic simulation tool for this purpose. [ABSTRACT FROM AUTHOR]

We study the sensitivity to the shape of the Higgs potential of single, double, and triple Higgs production at future e+e- colliders. Physics beyond the Standard Model is parameterised through the inclusion of higher-dimensional operators (Φ†Φ-v2/2)n/Λ(2n-4) with n=3,4, which allows a consistent treatment of independent deviations of the cubic and quartic self couplings beyond the tree level. We calculate the effects induced by a modified potential up to one loop in single and double Higgs production and at the tree level in triple Higgs production, for both Z boson associated and W boson fusion production mechanisms. We consider two different scenarios. First, the dimension six operator provides the dominant contribution (as expected, for instance, in a linear EFT); we find in this case that the corresponding Wilson coefficient can be determined at O(10%) accuracy by just combining accurate measurements of single Higgs cross sections at ŝ--√=240-250 GeV and double Higgs production in W boson fusion at higher energies. Second, both operators of dimension six and eight can give effects of similar order, i.e., independent quartic self coupling deviations are present. Constraints on Wilson coefficients can be best tested by combining measurements from single, double and triple Higgs production. Given that the sensitivity of single Higgs production to the dimension eight operator is presently unknown, we consider double and triple Higgs production and show that combining their information colliders at higher energies will provide first coarse constraints on the corresponding Wilson coefficient. [ABSTRACT FROM AUTHOR]

As a high energy e–p collider, FCC-he, has been recently proposed with sufficient energy options to investigate Higgs couplings. To analyze the sensitivity on Higgs boson couplings, we focus specifically on the CP-even and CP-odd Wilson coefficients with h h Z Z and h h γ γ four-point interactions of Higgs boson with Effective Lagrangian Model through the process e − p → h h j e − . We simulate the related processes in FCC-he, with 60 GeV and 120 GeV e − beams and 50 TeV proton beam collisions. We present the exclusion limits on these couplings both for 68% and 95% C.L. in terms of integrated luminosities. [ABSTRACT FROM AUTHOR]

We study the production sensitivity of Higgs bosons h and H, in relation to the possible existence of Z′ boson and a top quark pair at the energy scales that will be reached in the near future at projected e+e- linear colliders. We focus on the resonance and no-resonance effects of the annihilation processes e+e-→(γ,Z,Z′)→tt-h and e+e-→(γ,Z,Z′)→tt-H. Furthermore, we develop and present novel analytical formulas to assess the total cross section involved in the production of Higgs bosons. We find that the possibility of performing precision measurements for the Higgs bosons h and H and for the Z′ boson is very promising at future e+e- linear colliders. [ABSTRACT FROM AUTHOR]

In the framework of the left-right twin Higgs (LRTH) model, we investigate the single top partner production at lepton colliders. We calculate the production cross-sections of the processes , and at TeV, and display some typical differential distributions of the final state particles. [ABSTRACT FROM AUTHOR]

We evaluate the QED corrections due to initial state radiation (ISR) to associated Higgs boson production in electron–positron ( e + e − ) annihilation at typical energies of interest for the measurement of the Higgs properties at future e + e − colliders, such as CEPC and FCC–ee. We apply the QED Structure Function approach to the four-fermion production process e + e − → μ + μ − b b ¯ , including both signal and background contributions. We emphasize the relevance of the ISR corrections particularly near threshold and show that finite third order collinear contributions are mandatory to meet the expected experimental accuracy. We analyze in turn the rôle played by a full four-fermion calculation and beam energy spread in precision calculations for Higgs physics at future e + e − colliders. [ABSTRACT FROM AUTHOR]

A future Circular Electron Positron Collider (CEPC) has been proposed by China with the main goal of studying the Higgs boson. Its baseline design, chosen on the basis of its performance, is a double ring scheme; an alternative design is a partial double ring scheme which reduces the budget while maintaining an adequate performance. This paper will present the collider ring lattice design for the double ring scheme. The CEPC will also work as a W and a Z factory. For the W and Z modes, except in the RF region, compatible lattices were obtained by scaling down the magnet strength with energy. [ABSTRACT FROM AUTHOR]

We propose a novel idea for probing the Higgs boson couplings through the measurement of hadronic event shape distributions in the decay of the Higgs boson at lepton colliders. The method provides a unique test of the Higgs boson couplings and of QCD effects in the decay of the Higgs boson. It can be used to probe the Yukawa couplings of the light quarks and to further test the mechanism of electroweak symmetry breaking. From a case study for the proposed Circular Electron-Positron Collider, assuming a hypothesis of SM-like theory, light-quark couplings with a strength greater than 9% of the bottom-quark Yukawa coupling in the standard model can be excluded. [ABSTRACT FROM AUTHOR]

In this paper we will give an introduction to the Circular Electron Positron Collider (CEPC). The scientific background, physics goal, the collider design requirements and the conceptual design principle of the CEPC are described. On the CEPC accelerator, the optimization of parameter designs for the CEPC with different energies, machine lengths, single ring and crab-waist collision partial double ring, advanced partial double ring and fully partial double ring options, etc. have been discussed systematically, and compared. The CEPC accelerator baseline and alternative designs have been proposed based on the luminosity potential in relation with the design goals. The CEPC sub-systems, such as the collider main ring, booster, electron positron injector, etc. have also been introduced. The detector and the MAchine-Detector Interface (MDI) design have been briefly mentioned. Finally, the optimization design of the Super Proton-Proton Collider (SppC), its energy and luminosity potentials, in the same tunnel of the CEPC are also discussed. The CEPC-SppC Progress Report (2015-2016) has been published. [ABSTRACT FROM AUTHOR]

In September 2012, Chinese scientists proposed a Circular Electron Positron Collider (CEPC) in China at 240 GeV center-of-mass energy for Higgs studies. The booster provides 120 GeV electron and positron beams to the CEPC collider for top-up injection at 0.1 Hz. The design of the full energy booster ring of the CEPC is a challenge. The ejected beam energy is 120 GeV and the injected beam energy is 6 GeV. In this paper we describe two alternative schemes, the wiggler bend scheme and the normal bend scheme. For the wiggler bend scheme, we propose to operate the booster ring as a large wiggler at low energy and as a normal ring at high energy to avoid the problem of very low dipole magnet fields. For the normal bend scheme, we implement the orbit correction to correct the earth field. [ABSTRACT FROM AUTHOR]

The CEPC is a future Circular Electron and Positron Collider proposed by China to mainly study the Higgs boson. Its baseline design is a double ring scheme and an alternative design is a partial double ring scheme. This paper will present the optics design for the main ring of the double ring scheme. The CEPC will also work as a W and Z factory. Compatible optics designs for a W and a Z modes will be presented as well. [ABSTRACT FROM AUTHOR]

The Circular Electron Positron Collider (CEPC) is proposed as a Higgs boson and/or Z boson factory for high-precision measurements on the Higgs boson. The precision of secondary vertex impact parameter plays an important role in such measurements which typically rely on flavor-tagging. Thus silicon CMOS Pixel Sensors (CPS) are the most promising technology candidate for a CEPC vertex detector, which can most likely feature a high position resolution, a low power consumption and a fast readout simultaneously. For the R&D of the CEPC vertex detector, we have developed a prototype MIC4 in the Towerjazz 180 nm CMOS Image Sensor (CIS) process. We have proposed and implemented a new architecture of asynchronous zero-suppression data-driven readout inside the matrix combined with a binary front-end inside the pixel. The matrix contains 128 rows and 64 columns with a small pixel pitch of 25 m. The readout architecture has implemented the traditional OR-gate chain inside a super pixel combined with a priority arbiter tree between the super pixels, only reading out relevant pixels. The MIC4 architecture will be introduced in more detail in this paper. It will be taped out in May and will be characterized when the chip comes back. [ABSTRACT FROM AUTHOR]

Future Higgs factories can reach impressive precision on Higgs property measurements. In this paper, instead of conventional focus of Higgs precision in certain interaction bases, we explore its sensitivity to new physics models at the electron-positron colliders. In particular, we study two categories of new physics models, Standard Model (SM) with a real scalar singlet extension, and Two Higgs Double Model (2HDM) as examples of weakly-interacting models, Minimal Composite Higgs Model (MCHM) and three typical patterns of the more general operator counting for strong interacting models as examples of strong dynamics. We perform a global fit to various Higgs search channels to obtain the 95% C.L. constraints on the model parameter space. In the SM with a singlet extension, we obtain the limits on the singlet-doublet mixing angle sin θ, as well as the more general Wilson coefficients of the induced higher dimensional operators. In the 2HDM, we analyze tree level effects in tan β vs. cos( β − α) plane, as well as the one-loop contributions from the heavy Higgs bosons in the alignment limit to obtain the constraints on heavy Higgs masses for different types of 2HDM. In strong dynamics models, we obtain lower limits on the strong dynamics scale. In addition, once deviations of Higgs couplings are observed, they can be used to distinguish different models. We also compare the sensitivity of various future Higgs factories, namely Circular Electron Positron Collider (CEPC), Future Circular Collider (FCC)-ee and International Linear Collider (ILC). [ABSTRACT FROM AUTHOR]

The higgsino world is one of the popular natural SUSY scenarios, in which $$|\mu | \ll M_{\mathrm{gauginos}} \ll M_{\mathrm{scalars}}$$ . As such, searching for light degenerate higgsinos becomes an essential task of testing naturalness in supersymmetry (SUSY). In this work, we study the indirect effects of light higgsinos in the process $$e^+e^- \rightarrow W^+W^-$$ at future Higgs factories, such as the Circular Electron Positron Collider (CEPC) in China. We find that the higgsino mass parameter $$\mu \lesssim 210 \, \hbox {GeV}$$ favored by naturalness can be covered if the accuracy of the measurement of $$W^+W^-$$ production can reach 0.1% at 240 GeV CEPC. [ABSTRACT FROM AUTHOR]

A search for heavy resonances decaying to a W or Z boson and a Higgs boson in the q q ¯ ( ′ ) b b ¯ final state is described. The search uses 36.1 fb − 1 of proton–proton collision data at s = 13 TeV collected by the ATLAS detector at the CERN Large Hadron Collider in 2015 and 2016. The data are in agreement with the Standard Model expectations, with the largest excess found at a resonance mass of 3.0 TeV with a local (global) significance of 3.3 (2.1) σ . The results are presented in terms of constraints on a simplified model with a heavy vector triplet. Upper limits are set on the production cross-section times branching ratio for resonances decaying to a W ( Z ) boson and a Higgs boson, itself decaying to b b ¯ , in the mass range between 1.1 and 3.8 TeV at 95% confidence level; the limits range between 83 and 1.6 fb (77 and 1.1 fb) at 95% confidence level. [ABSTRACT FROM AUTHOR]

We describe ColliderBit, a new code for the calculation of high energy collider observables in theories of physics beyond the Standard Model (BSM). ColliderBit features a generic interface to BSM models, a unique parallelised Monte Carlo event generation scheme suitable for large-scale supercomputer applications, and a number of LHC analyses, covering a reasonable range of the BSM signatures currently sought by ATLAS and CMS. ColliderBit also calculates likelihoods for Higgs sector observables, and LEP searches for BSM particles. These features are provided by a combination of new code unique to ColliderBit, and interfaces to existing state-of-the-art public codes. ColliderBit is both an important part of the GAMBIT framework for BSM inference, and a standalone tool for efficiently applying collider constraints to theories of new physics. [ABSTRACT FROM AUTHOR]

While the Higgs characterisation programme is well under way, direct signs for new physics beyond the Standard Model remain elusive. Performing a fit of fully differential Higgs production cross sections at the LHC to a subset of Higgs-relevant effective operators, we discuss the extent to which theoretical uncertainties can limit the sensitivity in such a new physics search programme. Extending the dimension-6 Higgs Effective Field Theory framework by introducing new light degrees of freedom that can contribute to an invisible (or undetectable) Higgs decay width $$h\rightarrow \phi \phi $$ , we show how differential coupling fits can disentangle effects from non-Standard Model couplings and an invisible decay width, as present in many new physics scenarios, such as Higgs-portal dark matter. Including the so-called off-shell measurement that has been advocated as a sensitive determination of the Higgs width in the $$\kappa $$ framework, we show explicitly that this method does not provide complementary sensitivity for scale-separated new physics $$\Lambda \gg m_h \gg m_\phi $$ , which is favoured in beyond the Standard Model scenarios that relate astrophysics and collider phenomenology in the light of non-observation of new physics during run 1 of the LHC. [ABSTRACT FROM AUTHOR]

A new method is proposed to reconstruct the neutrinos in the $$e^+e^-\rightarrow Zh$$ process followed by the $$h\rightarrow \tau \tau $$ decay. With the help of a refined Higgs momentum reconstruction from the recoiling system and the impact parameters, high precision in the determination of the momentum of neutrinos can be achieved. The prospect of measuring the Higgs CP mixing angle with the $$h\rightarrow \tau \tau $$ decay at $$e^+e^-$$ colliders is studied with the new method. The analysis is based on a detailed detector simulation of the signal and backgrounds. The fully reconstructed neutrinos and also other visible products from the tau decay are used to build matrix element (ME)-based CP observables. With 5 $$ab^{-1}$$ of data at $$E_{\text {CM}}=250$$ GeV, a precision of $$2.9^\circ $$ can be achieved for the CP mixing angle with three main one-prong decay modes of the taus. The precision is found to be about 35% better than the other methods. [ABSTRACT FROM AUTHOR]

In this work we investigate the observability of inert doublet model scalars at the LHC operating at the center of mass energy of 14TeV. The signal production process is pp → AH± → ZHW±H leading to two different final states of ℓ+ℓ-H jj H and ℓ+ℓ-Hℓ ± νH based on the hadronic and leptonic decay channels of theWboson. All the relevant background processes are considered and an event selection is designed to distinguish the signal from the large Standard Model background. We found that signals of the selected search channels are well observable at the LHC with an integrated luminosity of 300 fb-1. [ABSTRACT FROM AUTHOR]

In the framework of the littlest Higgs Model with T-parity, we discuss the top partner production at future e+e- collider. We calculate the cross sections of the top partner production processes and associated production processes of Higgs and top partner under current constraints. Then, we investigate the observability of the T-odd top partner pair production through the process e+e-→T-T¯-→tt¯AHAH in the tt¯ dilepton channel for two T-odd top partner masses mT-=603  (708) GeV at s=1.5 TeV. We analyze the signal significance depending on the integrated luminosity and find that this signal is promising at the future high energy e+e- collider. [ABSTRACT FROM AUTHOR]

New physics coupling to the Higgs sector of the Standard Model can lead to dangerously large corrections to the Higgs mass. We investigate this problem in the type II seesaw model for neutrino mass, where a weak scalar triplet is introduced. The interplay of direct and indirect constraints on the type II seesaw model with its contribution to the Higgs mass is analyzed. The focus lies on testable triplet masses and (sub) eV-scale triplet vacuum expectation values. We identify scenarios that are testable in collider and/or lepton flavor violation experiments, while satisfying the Higgs naturalness criterion. [ABSTRACT FROM AUTHOR]

Models with non-minimal Higgs sectors represent a mainstream direction in theoretical exploration of physics opportunities beyond the Standard Model. Extended scalar sectors help alleviate difficulties of the Standard Model and lead to a rich spectrum of characteristic collider signatures and astroparticle consequences. In this review, we introduce the reader to the world of extended Higgs sectors. Not pretending to exhaustively cover the entire body of literature, we walk through a selection of the most popular examples: the two- and multi-Higgs-doublet models, as well as singlet and triplet extensions. We will show how one typically builds models with extended Higgs sectors, describe the main goals and the challenges which arise on the way, and mention some methods to overcome them. We will also describe how such models can be tested, what are the key observables one focuses on, and illustrate the general strategy with a subjective selection of results. [ABSTRACT FROM AUTHOR]

We investigate the prospects for producing new, light, hidden states at a future e e collider in a Higgsed dark U(1) model, which we call the Double Dark Portal model. The simultaneous presence of both vector and scalar portal couplings immediately modifies the Standard Model Higgsstrahlung channel, e e → Zh, at leading order in each coupling. In addition, each portal leads to complementary signals which can be probed at direct and indirect detection dark matter experiments. After accounting for current constraints from LEP and LHC, we demonstrate that a future e e Higgs factory will have unique and leading sensitivity to the two portal couplings by studying a host of new production, decay, and radiative return processes. Besides the possibility of exotic Higgs decays, we highlight the importance of direct dark vector and dark scalar production at e e machines, whose invisible decays can be tagged from the recoil mass method. [ABSTRACT FROM AUTHOR]

We estimate the possible accuracies of measurements at the proposed CLIC e e collider of Higgs and W W production at centre-of-mass energies up to 3 TeV, incorporating also Higgsstrahlung projections at higher energies that had not been consid-ered previously, and use them to explore the prospective CLIC sensitivities to decoupled new physics. We present the resulting constraints on the Wilson coefficients of dimension-6 operators in a model-independent approach based on the Standard Model effective field theory (SM EFT). The higher centre-of-mass energy of CLIC, compared to other projects such as the ILC and CEPC, gives it greater sensitivity to the coefficients of some of the operators we study. We find that CLIC Higgs measurements may be sensitive to new physics scales $$ \Lambda =\mathcal{O}(10) $$ TeV for individual operators, reduced to $$ \mathcal{O}(1) $$ TeV sensitivity for a global fit marginalising over the coefficients of all contributing operators. We give some examples of the corresponding prospective constraints on specific scenarios for physics beyond the SM, including stop quarks and the dilaton/radion. [ABSTRACT FROM AUTHOR]

Collider signals of heavy Majorana neutrino mass origin are studied in the minimal Left-Right symmetric model, where their mass is generated spontaneously together with the breaking of lepton number. The right-handed triplet Higgs boson Δ, responsible for such breaking, can be copiously produced at the LHC through the Higgs portal in the gluon fusion and less so in gauge mediated channels. At Δ masses below the opening of the V V decay channel, the two observable modes are pair-production of heavy neutrinos via the triplet gluon fusion gg → Δ → NN and pair production of triplets from the Higgs h → ΔΔ → 4 N decay. The latter features tri- and quad same-sign lepton final states that break lepton number by four units and have no significant background. In both cases up to four displaced vertices may be present and their displacement may serve as a discriminating variable. The backgrounds at the LHC, including the jet fake rate, are estimated and the resulting sensitivity to the Left-Right breaking scale extends well beyond 10 TeV. In addition, sub-dominant radiative modes are surveyed: the γγ, Zγ and lepton flavour violating ones. Finally, prospects for Δ signals at future e e colliders are presented. [ABSTRACT FROM AUTHOR]

A 'ground-up' Higgs Factory design methodology is described. For concreteness, numerical parameter choices are drawn primarily from CEPC, the Circular Electron Positron Collider. The goals are to find: (i) optimal parameters, (ii) improved understanding , (iii) a tentative lattice design. As illustration of the method, six chromaticity-corrected lattices, with cell lengths ranging from 45 m to 280 m, all with identical mm or mm intersection region optics, are designed and their properties compared. For simplicity only a single 'toy ring,' circumference (76 km), with one interaction point, and a single beam energy (120 GeV) is considered. For the cell-length optimization a figure of merit FOM (essentially integrated luminosity) is maximized consistent with a dimensionless 'fine tuning penalty function' or figure of demerit FOD not being allowed to exceed a conservatively chosen upper limit. The tentative recommendation from this investigation is that the optimal CEPC route is (except for obvious changes) to simply copy LEP: 80 m cell length and two-in-one single-ring operation. The main luminosity-increasing improvements are increased radius and power, top-off-full-energy-injection, noninterleaved sextupoles, more than 100 beam bunch operation, and improved intersection region design. Local chromaticity compensation (with its inevitable intense hard X-rays incident on the detectors) is found to be unnecessary. With these changes luminosity in excess of is projected to be achievable. [ABSTRACT FROM AUTHOR]

In this work, we point out that the apparent diphoton excess initially presented by the ATLAS and CMS collaborations could have originated from a radion in the bulk Higgs scenario within a warped extra dimension. In this scenario, the couplings of the radion to massive gauge bosons are suppressed, allowing it to evade existing searches. In the presence of mixing with the Higgs, due to the strong constraints from diboson searches, only points near what we denominate the alignment region were able to explain the diphoton signal and evade other experimental constraints. In light of the new measurements presented at ICHEP 2016 by both LHC collaborations, which do not confirm the initial diphoton excess, we study the current and future collider constraints on a radion within the bulk-Higgs scenario. We find that searches in the diphoton channel provide the most powerful probe of this scenario and already exclude large regions of parameter space, particularly for smaller warp factors. The radion has a sizeable branching ratio into top pairs and this channel may also give competitive constraints in the future. Finally, di-Higgs searches can provide a complementary probe in the case of nonzero radion-Higgs mixing but strong alignment. [ABSTRACT FROM AUTHOR]

We analyze an extension of the Standard Model with an additional SU(2) hypercolor gauge group keeping the Higgs boson as a fundamental field. Vectorlike interactions of new hyperquarks with the intermediate vector bosons are explicitly constructed. We also consider pseudo-Nambu–Goldstone bosons caused by the symmetry breaking SU(4)→Sp(4). A specific global symmetry of the model with zero hypercharge of the hyperquark doublets ensures the stability of a neutral pseudoscalar field. Some possible manifestations of the lightest states at colliders are also examined. [ABSTRACT FROM AUTHOR]

We study the effects of the initial state radiation on the s -channel Higgs boson resonant production at μ + μ − and e + e − colliders by convoluting with the beam energy spread profile of the collider and the Breit–Wigner resonance profile of the signal. We assess their impact on both the Higgs signal and SM backgrounds for the leading decay channels h → b b ¯ , W W ⁎ . Our study improves the existing analyses of the proposed future resonant Higgs factories and provides further guidance for the accelerator designs with respect to the physical goals. [ABSTRACT FROM AUTHOR]

The next-generation lepton colliders, such as CEPC, FCC-ee, and ILC will make precision measurement of the Higgs boson properties. We first extract the Higgs coupling precision from Higgs observables at CEPC to illustrate the potential of future lepton colliders. Depending on the related event rates, the precision can reach percentage level for most couplings. Then, we try to estimate the new physics scales that can be indirectly probed with Higgs and electroweak precision observables. The Higgs observables, together with the existing electroweak precision observables, can probe new physics up to 10 TeV (40 TeV for the gluon-related operator ) at 95% C.L. Including the mass measurements and -pole observables at CEPC further pushes the limit up to 35 TeV. Although -pole running is originally for the purpose of machine calibration, it can be as important as the Higgs observables for probing the new physics scales indirectly. The indirect probe of new physics scales at lepton colliders can mainly cover the energy range to be explored by the following hadron colliders of (50-100 TeV), such as SPPC and FCC-hh. [ABSTRACT FROM AUTHOR]

I summarize our recent works on using differential observables to explore the physics potential of future colliders in the framework of Higgs effective field theory. This proceeding is based upon Refs. 1 and 2. We study angular observables in the channel at future circular colliders such as CEPC and FCC-ee. Taking into account the impact of realistic cut acceptance and detector effects, we forecast the precision of six angular asymmetries at CEPC (FCC-ee) with center-of-mass energy GeV and 5 (30) ab integrated luminosity. We then determine the projected sensitivity to a range of operators relevant for the Higgsstrahlung process in the dimension-6 Higgs EFT. Our results show that angular observables provide complementary sensitivity to rate measurements when constraining various tensor structures arising from new physics. We further find that angular asymmetries provide a novel means of constraining the 'blind spot' in indirect limits on supersymmetric scalar top partners. We also discuss the possibility of using -fusion at machines at different energies to probe new operators. [ABSTRACT FROM AUTHOR]

In scenarios with sterile (right-handed) neutrinos with an approximate 'lepton-number-like' symmetry, the heavy neutrinos (the mass eigenstates) can have masses around the electroweak scale and couple to the Higgs boson with, in principle, unsuppressed Yukawa couplings, while the smallness of the light neutrinos' masses is guaranteed by the approximate symmetry. The on-shell production of the heavy neutrinos at lepton colliders, together with their subsequent decays into a light neutrino and a Higgs boson, constitutes a resonant contribution to the Higgs production mechanism. This resonant mono-Higgs production mechanism can contribute significantly to the mono-Higgs observables at future lepton colliders. A dedicated search for the heavy neutrinos in this channel exhibits sensitivities for the electron neutrino Yukawa coupling as small as . Furthermore, the sensitivity is enhanced for higher center-of-mass energies, when identical integrated luminosities are considered. [ABSTRACT FROM AUTHOR]

One item on the agenda of future colliders is certain to be the Higgs boson. What is it trying to tell us? The primary objective of any future collider must surely be to identify physics beyond the Standard Model, and supersymmetry is one of the most studied options. Is supersymmetry waiting for us and, if so, can LHC Run 2 find it? The big surprise from the initial 13 TeV LHC data has been the appearance of a possible signal for a new boson with a mass 750 GeV. What are the prospects for future colliders if the X(750) exists? One of the most intriguing possibilities in electroweak physics would be the discovery of nonperturbative phenomena. What are the prospects for observing sphalerons at the LHC or a future collider? [ABSTRACT FROM AUTHOR]