Probing Higgs-sector Cp Violation at a Photon Collider

The two-photon collision option of the International Linear ee Collider (ILC), a photon linear collider (PLC), is using back-scattered laser photons off the incident electrons and/or positrons. 1,2 A photon collider provides unique capabilities for probing new physics appearing in neutral spin-zero particles such as neutral Higgs bosons in the Standard Model (SM) and its Minimal Supersymmetric extension, MSSM. 3 The fact that the two-photon energy can reach about 80 % of the ee center-of-mass (c.m.) energy and the two-photon luminosity can be comparable to or even larger than that of ee collisions makes it easy to produce Higgs bosons copiously as s-channels resonances at the PLC. Especially, a capability of controlling polarizations of the colliding photons makes the PLC an ideal machine to probe CP properties of the Higgs bosons. In this review, to demonstrate physics potential of the PLC, we investigate the neutral Higgs-boson sector of the MSSM in which CP-violating mixing among scalar and pseudo-scalar states could be radiatively induced. This review is organized as follows. Section 2 describes a basic mechanism of generating polarized back-scattered laser photons at the ILC. We explicitly show the helicity-dependent γγ luminosity as well as the mean polarization of two colliding photon beams. In Sec. 3, we briefly review the Higgs-sector CP violation in the MSSM induced by the CP-violating phases of the soft SUSY breaking terms. Section 4 and Section 5 are devoted to CP violation in s-channel production of Higgs bosons and in the process γγ → f f̄ , respectively. Section 6 summarizes our conclusions.