A new CP violation mechanism generated by the standard neutral Higgs boson : the $\eta \to \pi +\pi$ case

Abstract

Strictly within the standard electro-weak interaction, CP violation in the flavour conserving process $\eta \to \pi +\pi$ could originate from the mixing of the $\eta$ meson with the virtual scalar Higgs boson $H^0$ 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 $\eta \to \pi +\pi$ branching ratio is found to be $3.6\cdot 10^{-26}-2.4\cdot 10^{-29}$ , hence CP violation mechanisms beyond the Standard Model are the only ones that could give rise to its observation at existing or near future $\eta$ factories, unless the Higgs mass is improbably as light as 550 $MeV$.

Comments: 9 pages (Latex), 3 figures