Although the existence of Dark Matter (DM) has been suggested by various astrophysical observations, it has not yet been discovered today. We can assume a scenario in which the particles that account for the DM can interact with the Standard Model particles only through their couplings with the Higgs sector, the so-called Higgs-portal model. This model can be investigated by collider experiment using the invisible decay of Higgs boson. In this study, we evaluate the search ability of International Linear Collider (ILC) for invisible decay of the Higgs using International Large Detector (ILD) full detector simulation. We estimate 95% C.L. upper limit (UL) on the branching ratio of invisible Higgs decays and compare them between two center-of-mass energy conditions: $\sqrt{s} = 250$ GeV and 500 GeV. In addition, we describe the complementarity of lepton collider experiment to the direct detection experiment about DM search ability.
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 Standard Model (SM) predicts a branching ratio of the Higgs boson decaying to invisible particles of $\mathcal{O}$(0.001), though current measurements have only set upper limits on this value. The small SM-allowed rate can be enhanced if the Higgs boson decays into new particles such as dark matter. Upper limits have been placed on BR(H$\rightarrow$inv.) by ATLAS and CMS at $\mathcal{O}$(0.1), but the hadron environment limits precision. The ILC `Higgs factory' will provide unprecedented precision of this electroweak measurement. Studies of the search for H$\rightarrow$invisible processes …
The Randall-Sundrum model with all Standard Model (SM) fields in the bulk, including the Higgs, can be probed by precision measurements at the ILC. In particular, the couplings of the Higgs to the gauge bosons of the SM can be determined with high accuracy at the ILC. Here we examine the deviations in these couplings from their SM values within the framework of the Universal Randall-Sundrum Model (URSM) as well as the corresponding couplings of the first Higgs Kaluza-Klein excitation.
The violation of CP symmetry is one of Sakharov's conditions for the matter-antimatter asymmetry of the Universe. Currently known sources of CP violation in the quark and neutrino sectors are insufficient to account for this. Is CP also violated in the Higgs sector? Could the SM-like Higgs boson be a mixture of even and odd CP states of an extended Higgs sector? With what precision could such effects be measured at future electron-positron colliders? These questions will be discussed in the light of the latest and ongoing studies at ILC.
We consider the Higgs-radion mixing in the context of warped space extra dimensional models with custodial symmetry and investigate the prospects of detecting the mixed radion. Custodial symmetries allow the Kaluza-Klein excitations to be lighter, and protect Zbb to be in agreement with experimental constraints. We perform a complementary study of discovery reaches of the Higgs-radion mixed state at the 13 and 14 TeV LHC and at the 500 and 1000 GeV ILC. We carry out a comprehensive analysis of the most significant production and decay modes of the mixed radion in the 80 GeV - 1 TeV mass range,…
Interpretations of searches for the Higgs boson are governed by model-dependent combinations of Higgs production cross sections and Higgs branching ratios. Mixing of the Higgs doublet with a hidden sector captures modifications from the Standard Model Higgs phenomenology in the standard search channels in a representative way, in particular because invisible Higgs decay modes open up. As a consequence, LHC exclusion bounds, which disfavor a heavy Standard Model Higgs can be consistently understood in terms of a standard-hidden mixed Higgs system. Shedding light on the possible existence of suc…
We study the possibility of fingerprinting a strongly interacting $W$ boson sector which is consistent with present day LHC searches at the ILC with longitudinal as well as transversely polarized electron and positron beams. We account for the final state interaction using a suitable Omnès formalism in terms of a plausible resonance description, and carry out thorough analyses of cross sections, asymmetries and angular distributions of the $W's$. We carry out a comparison with other extensions of the Standard Model, where heavy additional $Z'$ bosons arise naturally. We also consider the effec…
We explore the room for possible deviations from the Standard Model (SM) Higgs boson coupling structure in a systematic study of Higgs coupling scale factor benchmark scenarios using the latest signal rate measurements from the Tevatron and LHC experiments. We employ chi-squared fits performed with HiggsSignals, which takes into account detailed information on signal efficiencies and major correlations of theoretical and experimental uncertainties. All considered scenarios allow for additional non-standard Higgs boson decay modes, and various assumptions for constraining the total decay width …
We consider a gauged B$-$L (Baryon number minus Lepton number) extension of the Standard Model (SM), which is anomaly free in the presence of three SM singlet Right Handed Neutrinos (RHNs). Associated with the $U(1)_{\rm B-L}$ gauge symmetry breaking, the RHNs acquire Majorana masses and then with the electroweak symmetry breaking, tiny Majorana masses for the SM(-like) neutrinos are naturally generated by the seesaw mechanism. As a result of the seesaw mechanism, the heavy mass eigenstates which are mainly composed of the SM-singlet RHNs obtain suppressed electroweak interactions through smal…
We consider a gauged U(1)$_{B-L}$ (Baryon-minus-Lepton number) extension of the Standard Model (SM), which is anomaly-free in the presence of three Right-Handed Neutrinos (RHNs). Associated with the U(1)$_{B-L}$ symmetry breaking the RHNs acquire their Majorana masses and then play the crucial role to generate the neutrino mass matrix by the seesaw mechanism. Towards the experimental confirmation of the seesaw mechanism, we investigate a RHN pair production through the U(1)$_{B-L}$ gauge boson ($Z^\prime$) at the 250 GeV International Linear Collider (ILC). The $Z^\prime$ gauge boson has been …
In this paper we study the direct production of the diphoton resonance $X$ which has been suggested by 2015 data at the LHC, in $e^+e^-\to Xγ/XZ$ processes at the ILC. We derive an analytic expression for the scattering amplitudes of these processes, and present a comprehensive analysis for determining the properties of $X$ at the ILC. A realistic simulation study for $e^+e^-\to Xγ$ is performed based on the full detector simulation to demonstrate the capabilities of the ILC experiment. Complementary to the searches at the LHC, prospects of the measurement of the absolute values of production …
We employ the low energy Minimal Supersymmetric Standard Model (MSSM) to explore the parameter space associated with $Z$-pole and Higgs-pole solutions. Such parameter spaces can not only saturate the cold dark matter relic density bound within 5$σ$ set by the Planck 2018, but also satisfy the other standard collider mass bounds and B-physics bounds. In particular, we show that the right-handed selectron can be light. Thus, we propose a search for the relatively heavier right-handed selectron at the future lepton colliders with the center-of-mass energy $\sqrt{s}=240$ GeV and integrated luminos…
For the first time, the production of a massive dark photon (DP) in the $γe^-$ scattering at the future lepton colliders ILC, CLIC, and CEPC is examined. The invisible decay mode of the DP is addressed. We have studied both the unpolarized scattering and the collision of the Compton backscattered photons with the polarized electron. The missing energy distributions are shown. We have considered the wide range 10 GeV -- 1000 GeV of the DP mass $m_{A'}$. The excluded regions at the 95\% C.L. in the plane $(\varepsilon, m_{A'})$, where $\varepsilon$ is the kinetic mixing parameter, are obtained.
New physics coupled to the Higgs boson may hide it in the standard decay channels to be investigated at LHC. We consider the models where new invisible dominant decay modes of the Higgs boson are responsible for this hiding. We propose to study at LHC the weak boson production associated with heavy quarks: our analysis revealed that boson pair invariant mass distribution is sensitive to both mass and width of the invisible Higgs boson, if it is not too far from the weak boson pair threshold. We present tree-level results for the most relevant cases of top quarks and of bottom quarks in Standar…
We demonstrate by performing a global fit on Higgs signal strength data that large invisible branching ratios Br_{inv} for a Standard Model (SM) Higgs particle are currently consistent with the experimental hints of a scalar resonance at the mass scale m_h ~ 124 GeV. For this mass scale, we find Br_{inv} < 0.64 (95 % CL) from a global fit to individual channel signal strengths supplied by ATLAS, CMS and the Tevatron collaborations. Novel tests that can be used to improve the prospects of experimentally discovering the existence of a Br_{inv} with future data are proposed. These tests are based…
A U(1)' or Z' is generic in many scenarios of physics beyond the Standard Model, such as string theory compactifications, GUTs, extra-dimensions, compositeness, dynamical electroweak symmetry breaking, dark-sector models, etc. We study the potential of probing a TeV-scale Z' with electroweak couplings in future experiments. In particular, we focus on two scenarios: (1) If a Z' is discovered at the LHC, what is the potential of measuring its mass and width and to distinguish between benchmark models utilizing various observables, especially asymmetries, at a high luminosity LHC and the ILC. (2)…
Electron beam fixed target experiments such as NA64 and LDMX use missing energy-momentum to detect the production of dark matter and other long-lived states. The most studied production mechanism is dark Bremsstrahlung through a vector mediator. In this work, we explore a complementary source of missing energy-momentum signals: Bremsstrahlung photons can convert to hard vector mesons in exclusive photoproduction processes, which then decay to dark matter or other invisible particles, such as neutrinos. We find that existing NA64 data can improve the leading constraints on invisible light vecto…
We study the expected sensitivity to measure the branching ratio of Higgs boson decays to invisible particles at a future circular \epem collider (FCC-ee) in the process $e^+e^-\to HZ$ with $Z\to \ell^+\ell^-$ ($\ell=e$ or $μ$) using an integrated luminosity of 3.5 ab$^{-1}$ at a center-of-mass energy $\sqrt{s}=240$ GeV. The impact of the energy spread of the FCC-ee beam and of the resolution in the reconstruction of the leptons is discussed. %Two different detector concepts are considered: a detector corresponding to the CMS reconstruction performances and the expected design of the ILC detec…
Investigating vector-like leptons decaying into an electron and missing transverse energy in e$^{+}$ e$^{-}$ collisions with $\sqrt{s} = 500$ GeV at the ILC
We investigate the potential of the International Linear Collider (ILC) to probe the mechanisms of neutrino mass generation and leptogenesis within the minimal seesaw model. Our results can also be used as an estimate for the potential of a Compact Linear Collider (CLIC). We find that heavy sterile neutrinos that simultaneously explain both, the observed light neutrino oscillations and the baryon asymmetry of the universe, can be found in displaced vertex searches at ILC. We further study the precision at which the flavour-dependent active-sterile mixing angles can be measured. The measurement…
We explore new physics (NP) effects in Higgs-Vector boson associated production at a future International Linear Collider (ILC) via e+e- -> Zh,Zhh, using effective field theory (EFT) techniques. In particular, we focus on a certain class of dimension 6 operators, which are generated by tree-level exchanges of a new heavy vector field in the underlying theory. These operators induce new contact terms of the form ψψϕDϕ, involving the Standard Model (SM) fermions (ψ), gauge-bosons (D is the covariant derivative) and the SM Higgs field (ϕ). We investigate the high-energy behaviour of these new eff…
The first limit on the branching ratio of the electric charge-nonconserving invisible muon decay $Br(μ^+ \to invisible) < 5.2 \times 10^{-3}$ is obtained from the recently reported results on new determination of the Fermi constant from muon decays. The results of a feasibility study of a new proposed experiment for a sensitive search for this decay mode at the level of a few parts in 10^{11} are presented. Constrains on the $τ\to invisible$ decay rate are discussed. These leptonic charge-nonconserving processes may hold in four-dimensional world in models with infinite extra dimensions, thus …
We study exotic Higgs decays $h \to Z X$, with $X$ an invisible beyond the Standard Model (SM) particle, resulting in a semi-dark final state. Such exotic Higgs decays may occur in theories of axion-like-particles (ALPs), dark photons or pseudoscalar mediators between the SM and dark matter. The SM process $h\to Zν\barν$ represents an irreducible "neutrino floor" background to these new physics searches, providing also a target experimental sensitivity for them. We analyze $h \to Z + \text{invisible}$ searches at the LHC and a future ILC, showing that these exotic Higgs decays can yield sensit…
We investigate the scenario where the dark matter only interacts with the charged leptons in the standard model via a neutral vector mediator $Z'$. Such a scenario with a 430 GeV dark matter can fit the recent positron fluxes observed by the AMS-02 Collaborations, with the reasonable boost factors. We study the possibility of searching such leptophilic $Z'$ via its lepton final states and invisible decay modes at the future electron-positron colliders, such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). We find that for the benchmark models with $Z'$ mass fr…
The Higgs self-interactions play a crucial role for exploring the underlying mechanism of electroweak symmetry breaking and the nature of the phase transition involved. In this article, we propose to probe the quartic Higgs self-interaction at lepton and hadron colliders, via the di-Higgs productions. We analyze the contributions of the quartic Higgs coupling, including the renormalization of the cubic Higgs coupling and the modification of the $VVhh$ form factor, to the vector-boson-fusion and the vector-boson associated di-Higgs productions at one-loop level. Such an effect is independent of…
Probing Massive Invisible Dark Photons in Higgs Decays via Gluon--Gluon Fusion at ATLAS Run 3
Hubei Polytechnic University（湖北理工学院）；Khalifa University of Science and Technology（哈利法大学）；Federal Urdu University of Arts, Science and Technology（联邦乌尔都艺术、科学与科技大学）
本文重新解释ATLAS Run 3搜索，将希格斯衰变到光子和不可见暗光子的分支比上限推广到大质量暗光子，发现灵敏度随质量增大而下降。
We propose to search the monophoton events at the BESIII detector and future Super Tau Charm Factory to probe the sub-GeV dark photon decay into lighter dark matter. We compute the cross section due to the dark photon associated a standard model photon production, and study the corresponding standard model irreducible/reducible backgrounds. By using the data about 17 fb$^{-1}$ collected at the BESIII detector since 2011, we derive new leading limits of the mixing strength $\varepsilon$, $\varepsilon\lesssim(1.1-1.6)\times 10^{-4}$, in the mass range of 0.04 GeV $\lesssim m_{A^\prime} \lesssim$…
The possibility that the 125 GeV Higgs boson may decay into invisible non-standard-model (non-SM) particles is theoretically and phenomenologically intriguing. In this letter we investigate the sensitivity of the Large Hadron Electron Collider (LHeC) to an invisibly decaying Higgs, in its proposed high luminosity running mode. We focus on the neutral current Higgs production channel which offers more kinematical handles than its charged current counterpart. The signal contains one electron, one jet and large missing energy. With a cut-based parton level analysis, we estimate that if the $hZZ$ …
We study the CP violation in the Higgs boson and toponia production process at the ILC where the toponia are produced near the threshold. With the approximation that the production vertex of the Higgs boson and toponia is contact, and neglecting the P-wave toponia, we analytically calculated the density matrix for the production and decay of the toponia. Under these assumptions, the production spectrum of the toponia is solely determined by the spin quantum number, therefore the toponia can be either singlet or triplet. We find that the production rate of the singlet toponium is highly suppres…
The recently discovered scalar resonance at the LHC is now almost confirmed to be a Higgs Boson, whose CP properties are yet to be established. At the ILC with and without polarized beams, it may be possible to probe these properties at high precision. In this work, we study the possibility of probing departures from the pure CP-even case, by using the decay distributions in the process $e^+ e^- \to t \bar{t} Φ$, with $Φ$ mainly decaying into a $b\bar b$ pair. We have compared the case of a minimal extension of the SM case (Model I) with an additional pseudoscalar degree of freedom, with a mor…
Observation of Higgs pair production is an important long term objective of the LHC physics program as it will shed light on the scalar potential of the Higgs field and the nature of electroweak symmetry breaking. While numerous studies have examined the impact of new physics on di-Higgs production, little attention has been given to the well-motivated possibility of exotic Higgs decays in this channel. Here we investigate the consequences of exotic invisible Higgs decays in di-Higgs production. We outline a search sensitive to such invisible decays in the $b\bar b+{\not \!\! E}_T$ channel. We…
We study an effective theory beyond the standard model (SM) where either of two additional gauge singlets, a Majorana fermion and a real scalar, constitutes all or some fraction of dark matter. In particular, we focus on the masses of the two singlets in the range of O(10) MeV-O(10) GeV, with a neutrino portal interaction which plays important roles not only in particle physics but also in cosmology and astronomy. We point out that the dark matter abundance can be thermally explained with (co)annihilation, where the dark matter with a mass greater than 2 GeV can be tested in future lepton coll…
Probing the Rare Four-Bottom Higgs Decay $H\to b\bar b b\bar b$ at the HL-LHC and ILC
Skobeltsyn Inst. of Nuclear Physics, M.V.Lomonosov Moscow State Univ.（莫斯科国立大学索别尔廷茨因核物理研究所）；Faculty of Physics, Lomonosov Moscow State University（莫斯科国立大学物理系）；Al-Farabi Kazakh National University（哈萨克斯坦国立阿尔法拉比大学）；Scientific Research Institutes of Experimental and theoretical physics（实验与理论物理科学研究院）
The $ZZγ$ triple neutral gauge couplings are absent in the Standard Model (SM) at the tree level. They receive no contributions from dimension-6 effective operators, but can arise from effective operators of dimension-8. We study the scale of new physics associated with such dimension-8 operators that can be probed by measuring the reaction $e^+e^-\to Zγ$, followed by $Z \to \ell\bar{\ell},ν\barν$ decays, at future $e^+e^-$ colliders including the ILC, CEPC, FCC-ee and CLIC. We demonstrate how angular distributions of the final state mono-photon and leptons can play a key role in suppressing S…
We study the production of a Higgs boson recoiling from a massless invisible system in $e^ + e^ -$ collisions. This is a quite distinctive signature that can arise when the Higgs boson is produced in association with a massless dark photon, which can happen in BSM scenarios foreseeing an extra unbroken $U(1)$ gauge group. Dark photons can indeed acquire effective couplings to the Higgs boson as occurs in models recently proposed to generate exponentially-spread Yukawa couplings. We analyze the signal and corresponding backgrounds for $H\to b\bar{b}$, and estimate ILC and FCC-ee sensitivities i…
In the standard model the rate of the $π^0, η, η', K_S, K_L\to ν\overlineν$ decays is predicted to be extremely small. Therefore, observation of any of these mesons ($M^0$) decaying into an invisible final state would signal the presence of new physics. The Bell-Steinberger relation connects CP and CPT violation in the mass matrix to CP and CPT violation in all decay channels of neutral kaons. It is a powerful tool for testing CPT invariance in the $K^0-\overline{K}^0$ system, assuming that there are no significant undiscovered decay modes of either $K_S$ or $K_L$ which could contribute to the…
We investigate the sensitivity to new physics of the process e+e- -> t bar{t} when the top polarization is analyzed using leptonic final states e+e- -> t \bar{t} -> l+l- b \bar{b} nu_l \bar{nu}_l. We first show that the kinematical reconstruction of the complete kinematics is experimentally tractable for this process. Then we apply the matrix element method to study the sensitivity to the Vt\bar{t} coupling (V being a vector gauge boson), at the tree level and in the narrow width approximation. Assuming the ILC baseline configuration, sqrt{S}=500 GeV, and a luminosity of 500 fb^{-1}, we conclu…
Vector bosons, such as Dark Photon $A'$ or $Z'$, can couple to muons and be produced in the bremsstrahlung reaction $μ^- + N \rightarrow μ^- + N + A'(Z')$. Their possible subsequent invisible decay can be detected in fixed target experiments through missing energy/momentum signature. In such experiments, not only is the energy transfer to $A'(Z')$ important, but also the recoil muon angle $ψ_{μ'}$. In this paper, we derive the total and the double differential cross-sections involved in this process using the phase space Weiszäcker-Williams and improved Weiszäcker-Williams approximations, as w…
It has been suggested that the annihilation of Dark Matter particles χwith mass between 0.5 and 20 MeV into e^+e^- pairs could be responsible for the excess flux of 511 keV photons coming from the central region of our galaxy that has been detected by the IINTEGRAL satellite. The simplest way to achieve the required cross section for χpair annihilation while respecting existing constraints is to introduce a new vector boson U with mass M_U below a few hundred MeV. In this Letter we point out that over most of the allowed parameter space, the process e^+e^- to U γ, followed by the decay of U in…
In the minimal supersymmetric standard models, neutralino dark matter with mass of $m_χ\sim m_Z/2\sim 45$ GeV and $m_χ\sim m_h/2\sim 62$ GeV can have the thermal relic abundance $Ω_{χ_1}h^2\simeq 0.120$ via the $Z$- and Higgs-resonant annihilations, respectively, while avoiding all the current constraints. Phenomenology of such scenarios is determined only by three parameters, Bino mass $M_1$, Higgsino mass $μ$, and $\tanβ$, in the limit that all other supersymmetric particles and heavy Higgs bosons are decoupled. In this paper, we comprehensively study the constraints and future prospects of …
Neutral triple gauge couplings (nTGCs) are absent in the standard model effective theory up to dimension-6 operators, but could arise from dimension-8 effective operators. In this work, we study the pure gauge operators of dimension-8 that contribute to nTGCs and are independent of the dimension-8 operator involving the Higgs doublet. We show that the pure gauge operators generate both $ZγZ^*$ and $Zγγ^*$ vertices with rapid energy dependence $\propto E^5$, which can be probed sensitively via the reaction $e^+e^- \to Zγ$. We demonstrate that measuring the nTGCs via the reaction $e^+e^- \to Zγ$…
Rare kaon decays are excellent probes of light, new weakly-coupled particles. If such particles $X$ couple preferentially to muons, they can be produced in $K\to μνX$ decays. In this letter we evaluate the future sensitivity for this process at NA62 assuming $X$ decays either invisibly or to di-muons. Our main physics target is the parameter space that resolves the $(g-2)_μ$ anomaly, where $X$ is a gauged $L_μ-L_τ$ vector or a muon-philic scalar. The same parameter space can also accommodate dark matter freeze out or reduce the tension between cosmological and local measurements of $H_0$ if th…
Probing Neutral Triple Gauge Couplings via $ZZ$ Production at $e^+e^-$ Colliders with Machine Learning
Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交大李政道研究所）；Department of Physics, King’s College London（伦敦国王学院物理系）；Theoretical Physics Department, CERN（欧洲核子研究中心理论物理部）；School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）；Department of Physics, Tsinghua University（清华大学物理系）；Center for High Energy Physics, Peking University（北京大学高能物理中心）；School of Physics, Peking University（北京大学物理学院）
Spin-0 singlets arise in well-motivated extensions of the Standard Model. Their lifetime determines the best search strategies at hadron and lepton colliders. To cover a large range of singlet decay lengths, we investigate bounds from Higgs decays into a pair of singlets, considering signatures of invisible decays, displaced and delayed jets, and coupling fits of untagged decays. We examine the generic scalar singlet and the relaxion, and derive a matching as well as qualitative differences between them. For each model, we discuss its natural parameter space and the searches probing it.
We introduce the mono-neutrino signal at neutrino detectors as a smoking gun of sub-GeV scale dark matter candidates that mainly interact with standard model neutrinos. In a mono-neutrino process, invisible particles, either dark matter themselves or the mediator particle, are radiated off a neutrino when it undergoes the charged-current weak interaction. The associated signals include a missing transverse momentum with respect to the incoming neutrino beam direction and the production of wrong-sign charged leptons. We demonstrate the potential leading role of the future DUNE experiment, using…
Probing displaced (dark)photons from low reheating freeze-in at the LHC
Facultad de Ingeniería, Universidad San Sebastián（塞巴斯蒂安大学工程学院）；Millennium Institute for Subatomic Physics at the High-Energy Frontier (SAPHIR)（高能前沿亚原子物理千年研究所（SAPHIR））；Instituto de Física, Pontificia Universidad Católica de Chile（智利天主教大学物理研究所）；Instituto de Física, Facultad de Ciencias, Universidad de la República（共和国大学理学院物理研究所）；Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú（秘鲁天主教大学科学院物理组）
