Even if new physics beyond the Standard Model (SM) indeed exists, the energy scale of new physics might be beyond the reach at the Large Hadron Collider (LHC) and the LHC could find only the Higgs boson but nothing else.This is the so-called "nightmare scenario". On the other hand, the existence of the dark matter has been established from various observations. One of the promising candidates for thermal relic dark matter is a stable and electric charge-neutral Weakly Interacting Massive Particle (WIMP) with the mass below the TeV scale. In the nightmare scenario, we introduce a WIMP dark matter singlet under the SM gauge group, which only couples to the Higgs doublet at the lowest order, and investigate a possibility that such WIMP dark matter can be a clue to overcome the nightmare scenario via various phenomenological tests such as the dark matter relic abundance, the direct detection experiments for the dark matter particle, and the production of the dark matter particle at the LHC.

We consider a possibility to naturally explain tiny neutrino masses without the lepton number violation. We study a simple model with SU (2) L singlet charged scalars (s ± 1 , s ± 2 ) as well as singlet right-handed neutrino (ν R ). Yukawa interactions for Dirac neutrinos, which are forbidden at the tree level by a softly-broken Z 2 symmetry, are induced at the one-loop level via the soft-breaking term in the scalar potential. Consequently neutrinos obtain small Dirac masses after the electroweak symmetry breaking. It is found that constrains from neutrino oscillation measurements and lepton flavor violation search results (especially for µ → eγ) can be satisfied. We study the decay pattern of the singlet charged scalars, which could be tested at the LHC and the ILC. We discuss possible extensions also, e.g. to introduce dark matter candidate.

We show a TeV-scale seesaw model where Majorana neutrino masses, the dark matter mass, and stability of the dark matter can be all originated from the U (1) B−L gauge symmetry. Dirac mass terms for neutrinos are forbidden at the tree level by U (1) B−L , and they are induced at the one-loop level by spontaneous U (1) B−L breaking. The right-handed neutrinos can be naturally at the TeV-scale or below because of the induced Dirac mass terms with loop suppression. Such righthanded neutrinos would be discovered at the CERN Large Hadron Collider (LHC). On the other hand, stability of the dark matter is guaranteed without introducing an additional Z 2 symmetry by a remaining global U (1) symmetry after the U (1) B−L breaking. A Dirac fermion Ψ 1 or a complex neutral scalar s 0 1 is the dark matter candidate in this model. Since the dark matter (Ψ 1 or s 0 1 ) has its own B−L charge, the invisible decay of the U (1) B−L gauge boson Z ′ is enhanced. Experimental constraints on the model are considered, and the collider phenomenology at the LHC as well as future linear colliders is discussed briefly.

We investigate the possibility of detecting dark matter at TeV scale linear colliders in the scenario where the dark matter is a massive particle weakly interacting only with the Higgs boson h in the low energy effective theory (the Higgs portal dark matter scenario). The dark matter in this scenario would be difficult to be tested at the CERN Large Hadron Collider when the decay of the Higgs boson into a dark matter pair is kinematically forbidden. We study whether even in such a case the dark matter D can be explored or not via the Z boson fusion process at the International Linear Collider and also at a multi TeV lepton collider. It is found that for the collision energy √ s > 1 TeV with the integrated luminosity 1 ab −1 , the signal (e ± e − → e ± e − h * → e ± e − DD)can be seen after appropriate kinematical cuts. In particular, when the dark matter is a fermion or a vector, which is supposed to be singlet under the standard gauge symmetries, the signal with the mass up to about 100 GeV can be tested for the Higgs boson mass to be 120 GeV.

We study the capability of the international linear collider (ILC) to probe extra dimensions via the seesaw mechanism. In the scenario we study, heavy Kaluza-Klein neutrinos generate tiny neutrino masses and, at the same time, have sizable couplings to the standard-model particles. Consequently, a KaluzaKlein tower of heavy neutrinos (N ) can be produced and studied at the ILC through the process: e + e − → νN followed by N → W ℓ decay. We show that the single lepton plus two-jets final states with large missing energy from this signal process will provide a good opportunity to measure the masses and cross sections of Kaluza-Klein neutrinos up to the third level. Furthermore, the neutrino oscillation parameters can be extracted from the flavor dependence of the lowest-mode signals, which give us information about the origin of low-energy neutrino masses.