CP violation in η(′) → π+π−μ+μ− decays

Abstract

It has been pointed out recently that a certain set of dimension-6 scalar P- and CP-violating light-quark-muon operators may be tested in η and η′ decays to various final states involving μ+μ− pairs, at a level not yet excluded by constraints from electric dipole moments. We here work out the hadronic matrix elements required for the predictions for the decays η, η′ → π+π−μ+μ−. We relate a new asymmetry in the angular distribution between the dipion and dimuon decay planes to the corresponding Wilson coefficients. Despite the advantage of not requiring the measurement of muon polarization, the projected sensitivities are shown to be moderate due to a combination of small phase space and chiral suppression. Such sensitivity studies are most timely in view of upcoming or planned high-statistics η(′) experiments such as the JLab eta factory or REDTOP.

Precision tests of fundamental physics with $\eta$ and $\eta'$ mesons

“…The analysis of Ref. [45] has since been generalized to 𝜂 → 𝜋 0 𝜇 + 𝜇 − and related decays [47] as well as to 𝜂 (′) → 𝜋 + 𝜋 − 𝜇 + 𝜇 − [48], but so far, 𝜂 → 𝜇 + 𝜇 − seems to remain the most promising channel to observe these operators.…”