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The discovery of the Higgs boson with its mass around 125 GeV by the ATLAS and CMS Collaborations marked the beginning of a new era in high energy physics. The Higgs boson will be the subject of extensive studies of the ongoing LHC program. At the same time, lepton collider based Higgs factories have been proposed as a possible next step beyond the LHC, with its main goal to precisely measure the properties of the Higgs boson and probe potential new physics associated with the Higgs boson. The Circular Electron Positron Collider~(CEPC) is one of such proposed Higgs factories. The CEPC is an $e^+e^-$ circular collider proposed by and to be hosted in China. Located in a tunnel of approximately 100~km in circumference, it will operate at a center-of-mass energy of 240~GeV as the Higgs factory. In this paper, we present the first estimates on the precision of the Higgs boson property measurements achievable at the CEPC and discuss implications of these measurements.
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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$^{rm{-1}}$ in ten years 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 dedicated fast simulation tool, we evaluated the statistical precisions of the Higgstrahlung cross section $sigma_{ZH}$ and the Higgs mass $m_{H}$ measurement at the CEPC in the $Zrightarrowmu^+mu^-$ channel. The statistical precision of $sigma_{ZH}$ ($m_{H}$) measurement could reach 0.97% (6.9 MeV) in the model-independent analysis which uses only the information of Z boson decay. For the standard model Higgs boson, the $m_{H}$ precision could be improved to 5.4 MeV by including the information of Higgs decays. Impact of the TPC size to 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 ${cal B}(Hrightarrow rm{inv.})$ could reach 1.2% at 95% confidence level.
The existence of dark matter has been established in astrophysics. However, there is no candidate for DM in the Stand Model (SM). In SM, the Higgs boson can only decay invisibly via $Hrightarrow ZZ^ast rightarrow ubar{ u} ubar{ u}$ or DM, so any evidence of invisible Higgs decay that exceeds BR (H$rightarrow$inv.) will immediately point to a phenomenon that is beyond the standard model (BSM). In this paper, we report on the upper limit of BR (H$rightarrow$invisible) estimated for three channels, including two leptonic channels and one hadronic channel, under the assumption predicted by SM. With the SM ZH production rate, the upper limit of BR (H$rightarrow$inv.) could reach 0.24% at the 95% confidence level.
The proposed Circular Electron Positron Collider (CEPC), with a center-of-mass energy $sqrt{s} = 240$ GeV, will serve as a Higgs factory. At the same time, it can offer good opportunity for searches for new physics phenomena at low energy: these are challenging in hadron colliders, but well motivated by some theory models developed to explain, e.g., the relic abundance of dark matter. This paper presents sensitivity studies of chargino pair production, considering scenarios for both a Bino-like and a Higgsino-like neutralino as lightest supersymmetric particle, using full Monte Carlo (MC) simulation. With the assumption of systematic uncertainties at the level of 5%, the CEPC has the ability to discover chargino pair production up to the kinematic limit of $sqrt{s}/2$ for both considered cases. Thanks to the conservative assumptions on the systematic uncertainties and the low dependence on the reconstruction model and detector geometry considered, the results of this study can be considered as a reference and benchmark also for similar searches in other similar electron positron colliders, such as Future Circular Collider $e^{+}e^{-}$ (FCC-ee) and the International Linear Collider (ILC).
The precise determination of the $B_c to tau u_tau$ branching ratio provides an advantageous opportunity for understanding the electroweak structure of the Standard Model, measuring the CKM matrix element $|V_{cb}|$ and probing new physics models. In this paper, we discuss the potential of measuring the processes of $B_c to tau u_tau$ with $tau$ decaying leptonically at the proposed Circular Electron Positron Collider (CEPC). We conclude that during the $Z$ pole operation, the channel signal can achieve five $sigma$ significance with $sim 10^9$ $Z$ decays, and the signal strength accuracies for $B_c to tau u_tau$ can reach around 1% level at the nominal CEPC $Z$ pole statistics of one trillion $Z$ decays assuming the total $B_c to tau u_tau$ yield is $3.6 times 10^6$. Our theoretical analysis indicates the accuracy could provide a strong constraint on the general effective Hamiltonian for the $b to ctau u$ transition. If the total $B_c$ yield can be determined to $mathcal{O}(1%)$ level of accuracy in the future, these results also imply $|V_{cb}|$ could be measured up to $mathcal{O}(1%)$ level of accuracy.
A concise review of precision measurements in the Higgs sector of the Standard Model (SM) of particle physics is given using ATLAS and CMS data. The results are based on LHC Run-2 data, taken between 2015 and 2018. Impressive progress has been made since the discovery of the Higgs boson in 2012 for measuring all major production and decay modes. Good agreement with the SM predictions was observed in all measurements.
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