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.     

Measurement of H→μ~+μ~- production in association with a Z boson at the CEPC

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 Ge V 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.     

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.     

HZ associated production with decay in the Alternative Left-Right Model at CEPC and future linear colliders

In this study, Higgs and Z boson associated production with subsequent decay is attempted in the framework of alternative left-right model, which is motivated by superstring-inspired E_6 model at CEPC and future linear colliders. We systematically analyze each decay channel of Higgs with theoretical constraints and latest experimental methods. Due to the mixing of scalars in the Higgs sector, charged Higgs bosons can play an essential role in the phenomenological analysis of this process. Even though the predictions of this model for the signal strengths of this process are close to the standard model expectations, it can be distinct under high luminosity.     

Associated ZH and WH Production in Left-Right Twin Higgs Model at LHC

At the CERN large hadron collider (LHC), production of the Higgs boson in association with Z or W bosons provides a dramatic experimental signal for detecting the standard model (SM) Higgs boson. In this paper, we consider the contributions of the left-right twin Higgs (LRTH) model to the processes q\bar{q}→Z(W)H. Our numerical results show that, in the favorable parameter spaces, the cross sections deviate distinctly from the predictions
of the SM. The possible signals of the LRTH model can be detected via these processes at the LHC experiments.     

Reconstruction of physics objects at the Circular Electron Positron Collider with Arbor

After the Higgs discovery, precise measurements of the Higgs properties and the electroweak observables become vital for the experimental particle physics. A powerful Higgs/Z factory, the Circular Electron Positron Collider (CEPC) is proposed. The Particle Flow oriented detector design is proposed to the CEPC and a Particle Flow algorithm, Arbor is optimized accordingly. We summarize the physics object reconstruction performance of the Particle Flow oriented detector design with Arbor algorithm and conclude that this combination fulfills the physics requirement of CEPC.     

Improved search for a Higgs boson produced in association with Z → l+ l- in pp collisions sqrt[s] = 1.96 TeV

We search for the standard model Higgs boson produced with a Z boson in 4.1 fb(-1) of integrated luminosity collected with the CDF II detector at the Tevatron. In events consistent with the decay of the Higgs boson to a bottom-quark pair and the Z boson to electrons or muons, we set 95% credibility level upper limits on the ZH production cross section multiplied by the H → bb branching ratio. Improved analysis methods enhance signal sensitivity by 20% relative to previous searches. At a Higgs boson mass of 115 GeV/c2 we set a limit of 5.9 times the standard model cross section.     

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.     

LHT model and Higgs boson production in association with a weak gauge boson at the LHC

Considering the process pp → VH+X(V = W or Z) is a significant channel for searching for a light Higgs boson,we calculate the contributions of the littlest Higgs model with T-parity(called LHT model) to its production cross section.We find that,in most of the parameter space,the value of the relative correction parameter R is very small.However,with reasonable values of the free parameters,its value can be significantly larger.     

Light minimal supersymmetric standard model Higgs boson scenario and its test at hadron colliders

We show that, in the minimal supersymmetric standard model, the possibility for the lightest CP-even Higgs boson to be lighter than Z boson (as low as about 60 GeV) is, contrary to the usual belief, not yet excluded by the CERN LEP2 Higgs search nor any direct searches for supersymmetric particles at high energy colliders. The characteristic of the light Higgs boson scenario (LHS) is that the ZZh coupling and the decay branching ratio Br(h/A-->bb) are simultaneously suppressed as a result of generic supersymmetric loop corrections. Consequently, the W(+/-)H(-/+)h coupling has to be large due to the sum rule of Higgs couplings to weak gauge bosons. We discuss the potential of the Fermilab Tevatron and B factories to test the LHS, and show that the associated neutral and charged Higgs boson production process, pp-->H(+/-)h(A), can completely probe the LHS at the CERN Large Hadron Collider.     

Differential cross sections for Higgs boson production at tevatron collider energies

The transverse momentum Q(T) distribution is computed for inclusive Higgs boson production at sqrt[S]=1.96 TeV. We include all-orders resummation of large logarithms associated with emission of soft gluons at small Q(T). We provide results for Higgs boson and Z* masses from M(Z) to 200 GeV. The relatively hard transverse momentum distribution for Higgs boson production suggests possibilities for improvement of the signal to background ratio.     

Signatures of right-handed gauge bosons through their decays into W and Z bosons in high-energy pp collisions

We study the decay of right-handed gauge bosons W′ and Z′, produced in high-energy pp collisions, into pairs of ordinary gauge bosons and into a gauge boson and a Higgs scalar. The branching ratios are found to be comparable to those into fermion pairs in the case of the Z′. For the W′ they depend on the specific symmetry breaking pattern. We also discuss the background associated to these signals.     

Search for Higgs bosons decaying into bb and produced in association with a vector boson in pp collisions at square root of s = 1.8 TeV

We present a new search for H0V production, where H0 is a scalar Higgs boson decaying into bb with branching ratio beta, and V is a Z0 boson decaying into e+e-, mu+mu-, or nunu. This search is then combined with previous searches for H0V where V is a W+/- boson or a hadronically decaying Z0. The data sample consists of 106 +/- 4 pb(-1) of pp collisions at square root of s = 1.8 TeV accumulated by the Collider Detector at Fermilab. Observing no evidence of a signal, we set 95% Bayesian credibility level upper limits on sigma(pp --> H0V) x beta. For H0 masses of 90, 110, and 130 GeV/c2, the limits are 7.8, 7.2, and 6.6 pb, respectively.     

Six-quark decays of the Higgs boson in supersymmetry with R-parity violation

Both electroweak precision measurements and simple supersymmetric extensions of the standard model prefer a mass of the Higgs boson less than the experimental lower limit (on a standard-model-like Higgs boson) of 114 GeV. We show that supersymmetric models with R parity violation and baryon-number violation have a significant range of parameter space in which the Higgs boson dominantly decays to six jets. These decays are much more weakly constrained by current CERN LEP analyses and would allow for a Higgs boson mass near that of the Z. In general, lighter scalar quark and other superpartner masses are allowed. The Higgs boson would potentially be discovered at hadron colliders via the appearance of new displaced vertices.     

Sensitivity study of anomalous HZZ couplings at a future Higgs factory

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 g, which will be a clear window to test the Standard Model and look for new physics signals     

Role of h-->etaeta in intermediate-mass Higgs boson searches at the Large Hadron Collider

The dominance of the h-->etaeta decay mode for the intermediate-mass Higgs boson is highly motivated to solve the little hierarchy problem and to ease the tension with the precision data. However, the discovery modes for m(h) approximately <150 GeV, h-->gammagamma, and W/Zh-->(lnu/ll)(bb), will be substantially affected. In this Letter, we show that h-->etaeta-->4b is complementary and we can use this decay mode to detect the intermediate Higgs boson at the LHC, via Wh and Zh production. Requiring at least one charged lepton and 4B tags in the final state, we can identify a clean Higgs boson signal for m(h) approximately <150 GeV with a high significance and with a full Higgs mass reconstruction. We use the next-to-minimal supersymmetric standard model and the simplest little Higgs model for illustration.     

Complete two loop bosonic contributions to the muon lifetime in the standard model

The last missing correction to the muon lifetime in the standard model at O(alpha(2)) coming from gauge and Higgs boson loops is presented. The associated contribution to the parameter Deltar in the on-shell scheme ranges from 6x10(-5) to -4x10(-5) for Higgs boson masses from 100 GeV to 1 TeV. This result translates into a shift of the W boson mass which does not exceed +/-1 MeV in the same range and amounts, in particular, to approximately -0.8 MeV for a 115 GeV Higgs boson.     

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.     

Search for invisibly decaying Higgs bosons with large decay width using the OPAL detector at LEP

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e⁺e^-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb^-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (M_H=60 GeV) to 0.57 pb (M_H=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.