Phenomenology of the Georgi-Machacek model at future electron-positron colliders

We study the phenomenology of the exotic Higgs bosons in the Georgi-Machacek model at future electron-positron colliders such as the International Linear Collider (ILC), assuming the collision energies of 500 GeV and 1 TeV. We show that the existence of the neutral and singly charged Higgs bosons in the 5-plet representation under the custodial SU(2)(V) symmetry can be readily identified by studying various energy and invariant mass distributions of the W(+)W(-)Z final state. Moreover, their masses can be determined with sufficiently high precision to test the mass degeneracy, a feature due to the custodial symmetry of the model. A synergy between such searches at the ILC and the doubly charged Higgs search at the LHC will make the 5-plet Higgs boson study more comprehensive.