CP VIOLATION IN THE DECAY η → π⁺ π^- γ

Abstract

We discuss π 0-η mixing and its implication for ε ′ /ε to next-to-leading order in the low-energy expansion. The big effect due to η-η ′ mixing is shown to be largely cancelled by other contributions occurring at the same order in the chiral expansion.

Measurements of branching ratios for η decays into charged particles

“…The process has been observed by several experiments [15,19,41,42], but the only measurement of the branching ratio with statistical significance more than 3σ is a recent result from the KLOE Collaboration with 1555 ± 52 events leading to a branching ratio of (2.68 ± 0.09 stat ± 0.07 sys ) × 10 −4 [15]. The channel is also interesting due to searches for a possible CP-violation mechanism outside of the standard model [43,44]. It has been shown that a contribution to the decay amplitude from the CP-violating electric transition would result in a linear polarization to the virtual photon.…”

Physics and outlook for rare, all-neutral Eta decays

“…Nevertheless, other beyond SM calculations may predict bigger BRs, for example by invoking the large ss content of the . [9] We will lower the BR upper limit for → 2 0 by over an order of magnitude.…”

Perspectives for the studies of the η and η[sup ʹ] decays with KLOE-2 at DAφNE

“…At KLOE-2 we can probe to 0.5% accuracy the azimuthal symmetry of the process, that could be broken by CP-violating processes not constrained by the experimental limits onrj ^ nn or on electric dipole moment of the neutron [25]. KLOE is working on the analysis of the ^ -^ K^K^e^e^ sample, looking also at the angular distribution of the secondaries, on the basis of about 3,100 selected events.…”