Future Higgs Factory Research Articles

Probing the Higgs boson through Yukawa force

The ATLAS and CMS collaborations of the LHC have observed that the Higgs boson decays into the bottom quark-antiquark pair, and have also established that the Higgs coupling with the top quark-antiquark pair is instrumental in one of the modes for Higgs production. This underlines the discovery of the Yukawa force at the LHC. We demonstrate the impact of this discovery on the Higgs properties that are related to the dynamics of electroweak symmetry breaking. We show that these measurements have considerably squeezed the allowed window for new physics contributing to the Higgs couplings with the weak gauge bosons and the third generation quarks. The expected constraints at the HL-LHC and future Higgs factories are also shown. We project these constraints on the parameter space of a few motivated scenarios beyond the Standard Model. We pick them under two broad categories, namely, the composite Higgs and its RS dual, as well as various types of multi-Higgs models. The latter category includes models with singlet scalars, Type I, II and BGL-type two-Higgs doublet models, and models with scalar triplets à la Georgi and Machacek.

Exotic Higgs boson decays and the electroweak phase transition

Light new physics weakly coupled to the Higgs can induce a strong first-order electroweak phase transition (EWPT). Here, we argue that scenarios in which the EWPT is driven first-order by a light scalar with mass between $\sim 10$ GeV - $m_h/2$ and small mixing with the Higgs will be conclusively probed by the high-luminosity LHC and future Higgs factories. Our arguments are based on analytic and numerical studies of the finite-temperature effective potential and provide a well-motivated target for exotic Higgs decay searches at the LHC and future lepton colliders.

Synchrotron radiation effects on the dynamic aperture of CEPC

In the tracking of the future circular Higgs factories, significant reduction of dynamic aperture due to the synchrotron radiation has been observed. This paper will analyze the synchrotron radiation effects including the closed orbit and optics distortion, radiation damping and quantum excitation for the lattice of CEPC.

BSM: Bundled Software Manager and the application for CEPC

The Circular Electron Positron Collider (CEPC) is designed as a future Higgs Factory. Like most high energy physics experiments, the offline software consists of many packages. BSM (Bundled Software Manager) is thus created in order to simplify the deployment and usage of software which has many packages and dependencies. BSM utilizes Git as the software release repository. The details of software are defined in the Git repository including installation instructions, environment, dependencies, etc. Commands are supported for various shells. Json output and python API are also available for advanced development. The installation of each package could be configured separately and extended with customized handlers. BSM manages the environment variables and the version switching is easy. It also has fine environment control on a single package. Users can also define their own packages easily and these packages will be managed by BSM with simple configuration. CEPC software has already set up the deployment procedure with BSM. And BSM is also designed with flexibility to create different applications other than CEPC software. It is suitable for the projects including a lot of packages and it is safe for different BSM applications to coexist with each other under proper configuration.

Sensitivity study of anomalous HZZ couplings at a future Higgs factory * * This work was supported in part by National Natural Science Foundation of China under Grant No. 11521505 and 11621131001

We study the sensitivity of constraining the model independent HZZ coupling based on the effective theory up to dimension-6 operators at a future Higgs factory. Using the current conceptual design parameters of the Circular Electron Positron Collider, we give the experimental limits for the model independent operators given by the total Higgsstrahlung cross-section and the angular distribution of Z boson decays. In particular, we give the very small sensitivity limit for the CP violation parameter , which will be a clear window to test the Standard Model and look for new physics signals.

Higgs boson decay into four bottom quarks in the SM and beyond

We present predictions for the Higgs boson decay into four bottom quarks in the standard model and via light exotic scalars retaining full bottom-quark mass dependence. In the SM the decay can be induced either by the Yukawa couplings of bottom quarks and top quarks or the electroweak couplings. We calculate the partial decay width and various differential distributions up to next-to-leading order in QCD. We find large QCD corrections for decay via Yukawa couplings, as large as 90% for the partial decay width, and reduced scale variations. The results of this paper are therefore helpful for the measurement of this multi-jets final state at future Higgs factory of electron-positron colliders. We also propose several observables that can differentiate the SM decay channel and the exotic decay channel and compare their next-to-leading order predictions.

New physics implication of Higgs precision measurements

Studying the properties of the Higgs boson can be an important window to explore the physics beyond the Standard Model (SM). In this work, we present studies on the implications of the Higgs precision measurements at future Higgs Factories. We perform a global fit to various Higgs search channels to obtain the 95% C.L. constraints on the model parameter spaces of two-Higgs Doublet Model (2HDM) and Minimal Supersymmetric Standard Model (MSSM). In the 2HDM, we analyze tree-level effects as well as one-loop contributions from the heavy Higgs bosons. The strong constraints on [Formula: see text], heavy Higgs masses and their mass splitting are complementary to direct search of the LHC as well as possible future [Formula: see text] pole precision measurements. For the MSSM, we study both the Higgs couplings and mass precisions. The constraints on the CP-odd Higgs mass [Formula: see text] and stop mass scale [Formula: see text] can be complementary to the direct search of HL-LHC. 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).

The energy sawtooth effects in the partial double ring scheme of CEPC

The energy sawtooth will be significant in the future circular Higgs factory with beam energy as high as 120 GeV. For the partial double ring scheme, unlike the double ring scheme, the effects of energy sawtooth cannot be corrected by tapering the magnet strength with beam energy along the beamline. In this paper, the energy sawtooth effects in the partial double ring scheme and its mitigation methods are presented. As an example, the study of the energy sawtooth effects of the CEPC advanced partial double ring scheme is presented.

Probing CP-violating Higgs sectors via the precision measurement of coupling constants

We study how effects of the CP violation can be observed indirectly by precision measurements of Higgs boson couplings at a future Higgs factory such as the international linear collider. We consider two Higgs doublet models with the softly broken discrete symmetry. We find that by measuring the Higgs boson couplings very precisely we are able to distinguish the two Higgs doublet model with CP violation from the CP conserving one.

Measurement of H → μ+ μ− production in association with a Z boson at the CEPC**Supported by National Natural Science Foundation of China (11475190, 11575005), CAS Center for Excellence in Particle Physics (CCEPP) and CAS Hundred Talent Program (Y3515540U1)

The Circular Electron-Positron Collider (CEPC) is a future Higgs factory proposed by the Chinese high energy physics community. It is planned to operate at a center-of-mass energy of 240–250 GeV and is expected to accumulate an integrated luminosity of 5 ab−1 over ten years of operation. At the CEPC, Higgs bosons will be dominantly produced from the ZH associated process. The vast number of Higgs events collected will enable precise studies of its properties, including Yukawa couplings to massive particles. With GEANT4-based simulation of detector effects, we study the feasibility of measuring the Higgs boson decaying into a pair of muons at the CEPC. The results with and without information from the Z boson decay products are provided, showing that a signal significance of over 10 standard deviations can be achieved and the H-μ-μ coupling can be measured within 10% accuracy.

R&D of the CEPC scintillator-tungsten ECAL

The circular electron and positron collider (CEPC) was proposed as a future Higgs factory. To meet the physics requirements, a particle flow algorithm-oriented calorimeter system with high energy resolution and precise reconstruction is considered. A sampling calorimeter with scintillator-tungsten sandwich structure is selected as one of the electromagnetic calorimeter (ECAL) options due to its good performance and relatively low cost. We present the design, the test and the optimization of the scintillator module read out by silicon photomultiplier (SiPM), including the design and the development of the electronics. To estimate the performance of the scintillator and SiPM module for particles with different energy, the beam test of a mini detector prototype without tungsten shower material was performed at the E3 beams in Institute of High Energy Physics (IHEP). The results are consistent with the expectation. These studies provide a reference and promote the development of particle flow electromagnetic calorimeter for the CEPC.

ISR corrections to associated HZ production at future Higgs factories

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−bb¯, 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.

H → e+e− at the CEPC: initial state radiation effect with MadGraph

The Circular Electron–Positron Collider (CEPC) is a future Higgs factory proposed by the Chinese high energy physics community. It will operate at a center-of-mass energy of 240–250 GeV. The CEPC will accumulate an integrated luminosity of 5 ab in ten years’ operation. With GEANT4-based full simulation samples for the CEPC, the Higgs boson decaying into an electron pair is studied at the CEPC. The upper limit of could reach 0.024% at 95% confidence level. The signal process is generated by MadGraph, with initial state radiation implemented, as a first step to adjusting MadGraph for an electron–positron collider.

Learning from Higgs physics at future Higgs factories

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).

An indirect probe of the higgsino world at the CEPC

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.

Cross section and Higgs mass measurement with Higgsstrahlung at the CEPC **Supported by the Joint Funds of the NSFC (U1232105) and CAS Hundred Talent Program (Y3515540U1)

The Circular Electron Positron Collider (CEPC) is a future Higgs factory proposed by the Chinese high energy physics community. It will operate at a center-of-mass energy of 240–250 GeV. The CEPC will accumulate an integrated luminosity of 5 ab−1 over ten years of operation, producing one million Higgs bosons via the Higgsstrahlung and vector boson fusion processes. This sample allows a percent or even sub-percent level determination of the Higgs boson couplings. With GEANT4-based full simulation and a dedicated fast simulation tool, we have evaluated the statistical precisions of the Higgstrahlung cross section σZH and the Higgs mass mH measurement at the CEPC in the Z → μ+μ− channel. The statistical precision of σZH (mH) measurement could reach 0.97% (6.9 MeV) in the model-independent analysis which uses only the information from Z boson decays. For the standard model Higgs boson, the mH precision could be improved to 5.4 MeV by including the information from Higgs decays. The impact of the TPC size on these measurements is investigated. In addition, we studied the prospect of measuring the Higgs boson decaying into invisible final states at the CEPC. With the Standard Model ZH production rate, the upper limit of could reach 1.2% at 95% confidence level.

Higgs-strahlung production process e+e− → hZ at the future Higgs factory in the SM extension with color-octet scalars

We investigate the Higgs-strahlung production process e+e− → hZ at the future Higgs factory in the Manohar-Wise model, which extends the SM by a family of color-octet and isospin-doublet scalars. We scan over the parameter space of the Manohar-Wise model considering the theoretical and experimental constraints from unitarity, electroweak data and LHC searches for the color-octet scalars and perform fits with the Higgs data from ATLAS and CMS. Then we calculate the cross section σ(e+e− → hZ) at s=240GeV by including the radiative corrections from loops mediated by color-octet scalars. We find that the deviation of the cross section in the Manohar-Wise model from its SM prediction can maximally reach 11%, and the large deviations are mainly due to the potentially large coupling of Higgs boson with CP-odd color-octet scalars SIA (i.e., the coupling hSIA∗SIB).

New physics searches with Higgs-photon associated production at the Higgs factory**Supported by National Natural Science Foundation of China (11275102) and Tsinghua Scholarship for Overseas Graduate Studies

A future Higgs factory is being designed for precise measurement of Higgs characteristics and to search for new physics. In this paper we propose that the Higgs-photon associated production process, e+ e− →γh could be a useful channel for new physics. We express new physics model-independently in the effective Lagrangian approach, and find that the new physics effects of γh have only two degrees of freedom, much fewer than the Higgsstrahlung process. This point could be used to reduce the degeneracies of Wilson coefficients. We also calculate for the first time the 95% confidence level(CL) bounds of γh at the Higgs factory, and prove that γh is more sensitive to some dimension-6 operators than the current experimental data. In the optimistic scenario new physics effects may be observed at the CEPC or FCC-ee after the first couple of years of their run.

Radiative Corrections to e+e− → Zh at Future Higgs Factory in the Minimal Dilaton Model

The minimal dilaton model (MDM) extends the Standard Model by one singlet scalar called dilaton and one top quark partner called t′. In this work we investigate the t′-induced radiative correction to the Higgs-strahlung production process e+e− → Zh at future Higgs factory. We first present the analytical calculations in detail and show how to handle the ultraviolet divergence. Then we calculate the correction numerically by considering the constraints from precision electroweak data. We find that, for sinθL = 0.2 and mt′ = 1200 GeV, the correction is 0.26% and 2.1% for , 1 TeV respectively, and a larger value can be achieved as sin θL increases.

Higgs-strahlung production process e + e − → Zh at the future Higgs factory in the Minimal Dilaton Model

We investigate the Higgs-strahlung production process e^+ e^- \to Z h at the future Higgs factory such as TLEP by including radiative corrections in the Minimal Dilaton Model (MDM), which extends the SM by one singlet scalar called dilaton. We consider various theoretical and experimental constraints on the model, and perform fits to the Higgs data taken from ATLAS, CMS and CDF+D0. Then for the 1\sigma surviving samples, we calculate the MDM predictions on the inclusive production rate \sigma(e^+e^-\to Zh) at the 240-GeV Higgs factory, and also the signal rates of e^+e^-\to Zh with the Higgs boson decaying to b\bar b and \gamma\gamma. We have following observations: (1) In the heavy dilaton scenario, the deviation of \sigma(e^+e^-\to Zh) from its SM prediction can vary from -15\% to 85\%, which mainly arises from the modification of the tree-level hZZ coupling and also the radiative correction induced by possibly large Higgs self-couplings. (2) The processes e^+e^-\to Zh at the Higgs factory and pp\to hh at 14-TeV LHC are complementary in limiting the MDM parameter space. Requiring the deviation of \sigma(e^+e^-\to Zh) from its SM prediction to be less than 1\% and that of \sigma(p p \to h h) to be less than 50\%, \tan \theta_S in the MDM will be limited to be -0.1<\tan\theta_S<0.3, and the deviations of the signal rates are constrained to be |R_{b\bar b}|<2\% and |R_{\gamma\gamma}|<7\%. Especially, the Higgs self-coupling normalized to its SM prediction is now upper bounded by about 4. (3) In the light dilaton scenario, the deviation of \sigma(e^+e^-\to Zh) may reach -7\%, and requiring its size to be less than 1\% will result in 0<\tan\theta_S<0.1, and -10\% < R_{b\bar b}, R_{\gamma\gamma} < 1\%.