Future Electron Colliders Research Articles

Probing active-sterile neutrino transition magnetic moments at LEP and CEPC

We consider the sterile neutrino, that is also know as heavy neutral lepton (HNL), interacting with the Standard Model (SM) active neutrino and photon via a transition magnetic moment, the so-called dipole portal, which can be induced from the more general dipole couplings which respect the full gauge symmetries of the SM. Depending on the interactions with ${\rm SU}(2)_L$ and ${\rm U(1)}_Y$ field strength tensors ${\cal W}_{\mu \nu}^a$ and $B^{\mu \nu}$, we consider four typical scenarios and probe the constraints on the couplings with photon $d_\gamma$ at LEP using the analyses to search monophoton signature and the measurement of $Z$ decay. We find that in the considered scenarios assuming the coupling with $Z$ boson $d_Z\neq 0$, the measurement of $Z$ decaying into photon plus invisible particles can provide stricter constraints than the monophoton searches at the LEP1. The complementary constraints to existing experiments can be provided by the LEP. We also investigate the sensitivity on the dipole portal coupling $d_\gamma$ from the monophoton searches at future electron colliders, such as CEPC, and find that CEPC can explore the previously unconstrained parameter space by current experiments.

Hunting for scalar lepton partners at future electron colliders

New physics close to the electroweak scale is well motivated by a number of theoretical arguments. However, colliders, most notably the Large Hadron Collider (LHC), have failed to deliver evidence for physics beyond the Standard Model. One possibility for how new electroweak-scale particles could have evaded detection so far is if they carry only electroweak charge, i.e. are color neutral. Future $e^+e^-$ colliders are prime tools to study such new physics. Here, we investigate the sensitivity of $e^+e^-$ colliders to scalar partners of the charged leptons, known as sleptons in supersymmetric extensions of the Standard Model. In order to allow such scalar lepton partners to decay, we consider models with an additional neutral fermion, which in supersymmetric models corresponds to a neutralino. We demonstrate that future $e^+e^-$ colliders would be able to probe most of the kinematically accessible parameter space, i.e. where the mass of the scalar lepton partner is less than half of the collider's center-of-mass energy, with only a few days of data. Besides constraining more general models, this would allow to probe some well motivated dark matter scenarios in the Minimal Supersymmetric Standard Model, in particular the incredible bulk and stau co-annihilation scenarios.

Probing dark matter particles at CEPC

We investigate the capability of the future electron collider CEPC in probing the parameter space of several dark matter models, including millicharged dark matter models, Z′ portal dark matter models, and effective dark matter operators. In our analysis, the monophoton final state is used as the primary channel to detect dark matter models at CEPC. To maximize the signal to background significance, we study the energy and angular distributions of the monophoton channel arising from dark matter models and from the standard model to design a set of detector cuts. For the Z′ portal dark matter, we also analyze the Z′ boson visible decay channel which is found to be complementary to the monophoton channel in certain parameter space. The CEPC reach in the parameter space of dark matter models is also put in comparison with Xenon1T. We find that CEPC has the unprecedented sensitivity to certain parameter space for the dark matter models considered; for example, CEPC can improve the limits on millicharge by one order of magnitude than previous collider experiments for mathcal{O}(1)-100 GeV dark matter.

Collider phenomenology of e−e−→W−W−

Whether in nature exists Majorana neutrino is one of the most fundamental questions in physics. It is directly related to the violation of accidental $U(1)$-lepton number symmetry(LNV). Enormous efforts had been put into such test and among them, one conventional experiment is the neutrinoless double-beta decay ($0\nu\beta\beta$). On the other hand, there have been proposals of future electron-positron colliders such as International Linear Collider (ILC) for precision measurements of Higgs boson or new discovered beyond SM particles and it can potentially be converted into an electron-electron collider. Such collider enables a new probe of Majorana neutrino via the inverse $0\nu\beta\beta$ process($e^{-}e^{-}\to W^- W^-$) as an alternative and complementary test to the conventional neutrinoless double-beta decay experiments. In this paper, we illustrate the searching strategies of $e^{-}e^{-}\to W^- W^-$ at future electron colliders, and find that the channels of pure hadronic, semi-leptonic with muon and pure leptonic with muons have the most discovery potential.