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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.
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The existence of dark matter has been established in astrophysics. However, there are no dark matter candidates in the Standard Model~(SM). If the dark matter particles or their mediator can not interact with SM fermions or gauge bosons, the Higgs boson is the only portal to the dark matter. We present a simulation study to search for invisible decays of the Higgs boson at the ILC with the ILD detector.
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.
We show that the standard model (SM)-like Higgs boson may decay into neutrinos with a sizable decay branching ratio in one well-known two Higgs doublet model, so-called neutrinophilic Higgs model. This could happen if the mass of the lighter extra neutral Higgs boson is smaller than one half of the SM-like Higgs boson mass. The definite prediction of this scenario is that the rate of the SM-like Higgs boson decay into diphoton normalized by the SM value is about 0.9. In the case that a neutrino is Majorana particle, a displaced vertex of right-handed neutrino decay would be additionally observed. This example indicates that a large invisible Higgs boson decay could be irrelevant to dark matter.
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.
A search is performed for a new sub-GeV vector boson ($A$) mediated production of Dark Matter ($chi$) in the fixed-target experiment, NA64, at the CERN SPS. The $A$, called dark photon, could be generated in the reaction $ e^- Z to e^- Z A$ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay $A to chi overline{chi}$. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to $4.3times10^{10}$ electrons on target no evidence of such a process has been found. New stringent constraints on the $A$ mixing strength with photons, $10^{-5}lesssim epsilon lesssim 10^{-2}$, for the $A$ mass range $m_{A} lesssim 1$ GeV are derived. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the 90% C.L. limits $10^{-11}lesssim y lesssim 10^{-6}$ on the dark-matter parameter $y = epsilon^2 alpha_D (frac{m_chi}{m_{A}})^4 $ are obtained for the dark coupling constant $alpha_D = 0.5$ and dark-matter masses $0.001 lesssim m_chi lesssim 0.5 $ GeV. The lower limits $alpha_D gtrsim 10^{-3} $ for pseudo-Dirac Dark Matter in the mass region $m_chi lesssim 0.05 $ GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using tree level, exact calculations of the $A$ production cross-sections, which turn out to be significantly smaller compared to the one obtained in the Weizs{a}cker-Williams approximation for the mass region $m_{A} gtrsim 0.1$ GeV.
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