Cross section and Higgs mass measurement with Higgsstrahlung at the CEPC

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 Ge V. The CEPC will accumulate an integrated luminosity of 5 ab-1over 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 m H measurement at the CEPC in the Z →μ~+μ~-channel. The statistical precision of σZH(m_H) 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 m H precision could be improved to 5.4 Me V 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 B(H → inv.) could reach 1.2% at 95% confidence level.     

Measurements of decay branching fractions of H→b■/c■/gg in associated (e~+e~-/μ~+μ~-)H production at the CEPC

The high-precision measurement of Higgs boson properties is one of the primary goals of the Circular Electron Positron Collider(CEPC).The measurements ofH→b■/c■/gg decay branching fraction in the CEPC experiment is presented,considering a scenario of analysing 5000 fb-1 e~+e~-collision data with the center-of-mass energy of 250 Ge V.In this study the Higgs bosons are produced in association with a pair of leptons,dominantly mediated by the ZH production process.The statistical uncertainty of the signal cross section is estimated to be about 1%in theH→b■ final state,and approximately 5%-10%in theH→c■/gg final states.In addition,the main sources of the systematic uncertainties and their impacts to the measurements of branching fractions are discussed.This study demonstrates the potential of precise measurement of the hadronic final states of the Higgs boson decay at the CEPC,and will provide key information to understand the Yukawa couplings between the Higgs boson and quarks,which are predicted to be the origin of quarks’masses in the standard model.     

Mixed electroweak-QCD corrections to e~+e~-→μ~+μ~-H at CEPC with finite-width effect

The associated production of Higgs boson with a muon pair, e~+e~-→μ+μ-H, is one of the golden channels to pin down the properties of the Higgs boson in the prospective Higgs factories exemplified by CEPC.The projected accuracy of the corresponding cross section measurement is about per cent level at CEPC. In this work, we investigate both O(α) weak correction and the O(ααs) mixed electroweak-QCD corrections for this channel,appropriately taking into account the effect of finite Z~0 width. The μ~+μ~-invariant mass spectrum is also predicted.The mixed electroweak-QCD correction turns out to reach 1.5% of the Born-order result, and thereby must be included in future confrontation with the data. We also observe that, after including higher-order corrections, the simplified prediction for the integrated cross section employing the narrow-width-approximation may deviate from our full result by a few per cents.     

Exotic decays of the 125 GeV Higgs boson at future e~+e~- colliders

The discovery of unexpected properties of the Higgs boson would offer an intriguing opportunity to shed light on some of the most profound puzzles in particle physics. Beyond Standard Model(BSM) decays of the Higgs boson could reveal new physics in a direct manner. Future electron-positron lepton colliders operating as Higgs factories, including CEPC, FCC-ee and ILC, with the advantages of a clean collider environment and large statistics,could greatly enhance sensitivity in searching for these BSM decays. In this work, we perform a general study of Higgs exotic decays at future e~+e~-lepton colliders, focusing on the Higgs decays with hadronic final states and/or missing energy, which are very challenging for the High-Luminosity program of the Large Hadron Collider(HL-LHC).We show that with simple selection cuts,O(10~(-3)-10~(-5)) limits on the Higgs exotic decay branching fractions can be achieved using the leptonic decaying spectator Z boson in the associated production mode e~+e~-→ZH. We further discuss the interplay between detector performance and Higgs exotic decays, and other possibilities of exotic decays.Our work is a first step in a comprehensive study of Higgs exotic decays at future lepton colliders, which is a key area of Higgs physics that deserves further investigation.     

Study of diphoton decays of the lightest scalar Higgs boson in the Next-to-Minimal Supersymmetric Standard Model

The CMS and ATLAS experiments at the LHC have announced the discovery of a Higgs boson with mass at approximately 125 GeV/c² in the search for the Standard Model Higgs boson via, notably, the γγ and ZZ to four leptons final states. Considering the recent results of the Higgs boson searches from the LHC, we study the lightest scalar Higgs boson h₁ in the Next-to-Minimal Supersymmetric Standard Model by restricting the next-to-lightest scalar Higgs boson h₂ to be the observed to the 125 GeV/c² state. We perform a scan over the relevant NMSSM parameter space that is favoured by low fine-tuning considerations. Moreover, we also take the experimental constraints from direct searches, B-physics observables, relic density, and anomalous magnetic moment of the muon measurements, as well as the theoretical considerations, into account in our specific scan. We find that the signal rate in the two-photon final state for the NMSSM Higgs boson h₁ with the mass range from about 80 GeV/c² to about 122 GeV/c² can be enhanced by a factor of up to 3.5 when the Higgs boson h₂ is required to be compatible with the excess from latest LHC results. This motivates the extension of the search at the LHC for the Higgs boson h₁ in the diphoton final state down to masses of 80 GeV/c², particularly with the upcoming proton-proton collision data to be taken at center-of-mass energies of 13-14 TeV.     

Charged Higgs Pair Production in a General Two Higgs Doublet Model at e^＋e^- and μ^＋μ^- Linear Colliders

In this paper, charged Higgs pair production through l^＋l^- → H^＋ H^-, where l = e or μ, is studied within the framework of a general Two Higgs Doublet Model （2HDM）. The analysis is relevant to a future e^＋e^- or μ^＋ μ^- collider operating at center of mass energy of √s = 500 GeV. Two different scenarios of small and large a values are studied. Here a is the parameter, which diagonMizes the neutral CP-even Higgs boson mass matrix. Within the Minimal Supersymmetric Standard Model （MSSM）, cross section of this process is almost the same at e＋ e- and #＋#- colliders. It is shown that at e^＋e^- eolliders within a general 2HDM, cross section is not sensitive to the mass of neutral Higgs bosons, however, it can acquire large values up to several picobarn at μ^＋μ^- colliders with the presence of heavy neutral Higgs bosons. A scan over Higgs boson mass parameter space is performed to analyze the effect of large masses of neutral Higgs bosons involved in the s-channel propagator and thus in the total cross section of this process.     

Light A physics in the lepton-specific two-Higgs doublet model

We examine various direct and indirect constraints on the lepton-specific two-Higgs doublet model and discuss its phenomenology at colliders in the allowed parameter space.The constraints we consider come from the precision electroweak data,the direct search for Higgs boson,the muon anomalous magnetic moment,as well as some theoretical consistency requirements.We find that in the allowed parameter space the CP-odd Higgs boson A is rather light (m A 30 GeV with 95% possibility),which is composed dominantly by the leptonic Higgs and decays dominantly into τ + τ;while the SM-like Higgs boson h (responsible largely for electroweak symmetry breaking) decays dominantly in the mode h → AA → 4τ with a large decay width,which will make the Higgs discovery more difficult at the LHC.Whereas,this scenario predicts rare Z decays Z → AAA and Z →τ + τA with their branching ratios ranging from 10-8 to 10-and 10-5 to 10-4 respectively,which may be accessible at the GigaZ option of the ILC.     

Prospects for chargino pair production at the CEPC

The proposed Circular Electron Positron Collider(CEPC)with a center-of-mass energy√s=240GeV will primarily serve as a Higgs factory.Additionally,it will offer good opportunities to search for new physics phenomena at low energies,which can be challenging with hadron colliders;however,these discoveries are highly motivated by theoretical models developed to explain,e.g.,the relic abundance of dark matter.This paper presents sensitivity studies for chargino pair production by considering scenarios for both a Bino-like and a Higgsino-like neutralino as the lightest supersymmetric particle and using a full Monte Carlo(MC)simulation.By assuming systematic uncertainties at the level of 5%,the CEPC has the ability to discover chargino pair production up to the kinematic limit of for√s/2 both scenarios.The results have a minor dependence on the reconstruction model and detector geometry.These results can also be considered as a reference and benchmark for similar searches at other proposed electron-positron colliders,such as the Future Circular Collider ee(FCC-ee)or the International Linear Collider(ILC),given the similar nature of the facilities,detectors,center-of-mass energies,and target luminosities.     

On physics beyond standard model

In this review we do not try to cover all the aspects of physics besnd tile standard model （BSM）, instead our latest understanding on tile BSM will be presented： i） Tile Higgs sector is likely related to BSM, which can be confirmed at current running large hadron collider （LHC） or tile fllture eolliders. Furthermore we pointed out that spontaneous CP violation can be closely related to the lightness of the Higgs boson, ii） Top quark forward-backward asymmetry, which was mea.sured by Tewttron, might be the sign of BSM.2; proposed a new color-octet particle Zcr to account fi）r the observation and Z can be fllrther studied at the LHC. iii） If dark matter （DM） is utilized to accommodate astrophysical obserwtions, it ought to be observed at the high energy LttC and DM produced at colliders should be tile slnoking gun signal, iv） Lithium puzzle might also be the sign of the BSM. We briefly review tile newly proposed solution to Lithium puzzle, i.e.. the existonce of non-thermal component during the big bang nuclei-synthesis （BBN）. The possible origins of the non-thermal coinponent can be dark matter or the new accelerating mechanism of normal particles.     

Precision Higgs physics at the CEPC

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.     

Interference effects on Higgs mass measurement in e~+e~-→H(γγ)Z at CEPC

A high luminosity Circular Electron Positron Collider(CEPC) as a Higgs Factory will be helpful for precision measurements of the Higgs mass. The signal-background interference effect is carefully studied for the Higgs diphoton decay mode in associated Z boson production at future e~+e~-colliders at energy 246 GeV. The mass shifts go up from about 20 MeV to 50 MeV for the experimental mass resolution ranging from 0.8 GeV to 2 GeV.     

Prospects for the measurement of the Higgs boson masswith a linear <Emphasis Type="Italic">e</Emphasis><Superscript> + </Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>-</Superscript> collider

The potential of a linear e ⁺ e^- collider operated at a centre-of-mass energy of 350 GeV is studied for the measurement of the Higgs boson mass. An integrated luminosity of 500 fb^-1 is assumed. For Higgs boson masses of 120, 150 and 180 GeV the uncertainty on the Higgs boson mass measurement is estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related systematics, namely a bias in the beam energy measurement, the beam energy spread and the luminosity spectrum due to beamstrahlung, on the precision of the Higgs boson mass measurement are investigated. In order to keep the systematic uncertainty on the Higgs boson mass well below the level of the statistical error, the beam energy measurement must be controlled with a relative precision better than 10^-4. Received: 30 May 2005, Revised: 6 July 2005, Published online: 6 October 2005     

Study of CEPC performance with different collision energies and geometric layouts

The Circular Electron-Positron Collider(CEPC) is one of the largest projects planned for high energy physics in China.It would serve first as a Higgs factory and then upgrade to a hadron collider.In this paper we give the 50 km and 100 km design for both single ring and double ring schemes,including Z boson,W boson and Higgs boson,by using an optimized method.Also,we give the potential of CEPC running at the Z and W poles.We analyse the relationship of luminosity with circumference and filling factor,which gives a way to evaluate the choice of geometry,and compare the nominal performances of CEPC-SPPC,LHC and FCC.     

Search for Higgs bosons decaying to WW in e+e- collisions at LEP

A search for Higgs bosons produced in association with a fermion pair, and decaying to WW, is performed with the data collected by the ALEPH detector at centre-of-mass energies ranging from 191 to 209 GeV. The data correspond to an integrated luminosity of 453.2 pb^-1. Thirteen exclusive selections are developed according to the different final state topologies. No statistically significant evidence for a Higgs boson decaying into a WW pair has been found. An upper limit is derived, as a function of the Higgs boson mass, on the product of the e⁺e^-→Hff̄ cross section and the H→WW branching ratio. The data on the search for H→WW are combined with previously published ALEPH results on the search for H→γγ, to significantly extend the limits on the mass of a fermiophobic Higgs boson.     

Search for Higgs Bosons at LEP2 and Hadron Colliders

The search for the Higgs boson was one of the most relevant issues of the final years of LEP running at high energies. An excess of 3σ beyond the background expectation has been found, consistent with the production of the Higgs boson with a mass near 115 GeV/c². At the upgraded TeVatron and at LHC the search for the Higgs boson will continue. At TeVatron Higgs bosons can be detected with masses up to 180 GeV with an assumed total integrated luminosity of 20 fb^—1. LHC has the potential to discover the Higgs boson in many different decay channels for Higgs masses up to 1 TeV. It will be possible to measure Higgs boson parameters, such as mass, width, and couplings to fermions and bosons. The results from Higgs searches at LEP2 and the possibilities for searches at hadron colliders will be reviewed.     

Test of Higgs boson nature in e+e−→HHZ

The process e⁺e^?→HHZ is strongly dependent on the Higgs boson couplings HZZ, HHZZ and HHH, and provides therefore a test of the Higgs assignment to gauge group representations. The cross section varies between 5 × 10^?38 and 10^?39 cm² for Higgs masses between 5GeV and 40 GeV in the Weinberg-Salam model.     

Study of beamstrahlung effects at CEPC

The discovery of a 125 GeV Higgs boson at the LHC marked a breakthrough in particle physics. The relative lightness of the new particle has inspired consideration of a high-luminosity Circular Electron Positron Collider(CEPC) as a Higgs Factory to study the particle's properties in an extremely clean environment. Given the high luminosity and high energy of the CEPC, beamstrahlung is one of the most important sources of beaminduced background that might degrade the detector performance. It can introduce even more background to the detector through the consequent electron-positron pair production and hadronic event generation. In this paper,beamstrahlung-induced backgrounds are estimated with both analytical methods and Monte Carlo simulation. Hit density due to detector backgrounds at the first vertex detector layer is found to be~0.2 hits/cm~2 per bunch crossing, resulting in a low detector occupancy below 0.5%. Non-ionizing energy loss(NIEL) and total ionizing dose(TID), representing the radiation damage effects, are estimated to be~10~(11)1 Me V neq/cm~2/yr and~300 k Rad/yr,respectively.     

Two-photon physics at future electron-positron colliders

Two photon collisions offer a variety of physics phenomena that can be studied at future electron-positron colliders. Using the planned CEPC parameters as a benchmark, we consider several topics within two-photon collisions. With the full integrated luminosity, Higgs boson photoproduction can be reliably observed, and large statistics on various quarkonium states can be collected. The LEP results for the photon structure function and tau lepton anomalous magnetic moment can be improved by 1-2 orders of magnitude.