A new CP violation mechanism generated by the standard neutral Higgs boson: the η → π + π case

Strictly within the standard electro-weak interaction, CP violation in the flavour conserving process η → π + π could originate from the mixing of the η meson with the virtual scalar Higgs boson H0 via W and top quark exchange. The parity-violation carried by weak gauge bosons makes the mixing possible by quantum effect at two-loop level. Nowhere the Kobayashi-Maskawa (KM) phase mechanism is needed. The phenomenon reveals an unexpected new role of the Higgs boson in the CP symmetry breaking. For the Higgs mass between 100–600 GeV, the η → π + π branching ratio is found to be 3.6 · 10−26 − 2.4 · 10−29, hence CP violation mechanisms beyond the Standard Model are the only ones that could give rise to its observation at existing or near future η factories, unless the Higgs mass is improbably as light as 550 MeV.