Test of charge conjugation invariance in eta->pi0 e+e- and eta->pi+pi-pi0 decays

TL;DR

This study investigates charge conjugation symmetry in eta meson decays, analyzing data from the WASA-at-COSY detector to search for violations and establish upper limits on rare decay processes.

Contribution

It provides the first detailed Dalitz plot analysis for eta->pi+pi-pi0 and sets a more precise upper limit on eta->pi0 e+e- decay branching ratio, testing C symmetry in these decays.

Findings

No evidence of C violation in eta->pi+pi-pi0 decay within experimental accuracy.

Established an upper limit of BR(eta->pi0 e+e-) < 3.7x10^-5 at 90% confidence level.

Collected around 5x10^7 eta mesons for analysis.

Abstract

Charge conjugation C is one of the fundamental symmetries in nature which transforms particles into antiparticles. This symmetry was studied in weak interaction where it is fully violated, but it is poorly known in the strong and electromagnetic interactions. To this end, in this thesis we investigated eta->pi+pi-pi0 and eta->pi0 e+e- decays, which might violate charge conjugation symmetry. The violation of C symmetry in eta->pi+pi-pi0 process could manifest itself as an asymmetry between energy spectra of charged pions. The eta->pi0 e+e- decay is forbidden by C symmetry in the first order of electromagnetic interaction, with the branching ratio on a level of 10^-8. Therefore, observation of a larger branching ratio could indicate a mechanism involving first order electromagnetic interaction violating charge conjugation. Both decays were investigated by means of the WASA-at-COSY detector operating at the COSY synchrotron at the Forchungszentrum Julich in Germany. The eta meson was produced via pp->pp eta reaction at the proton beam momentum of 2.14GeV/c. In total around 5x10^7 eta mesons were collected. As a result of the analysis conducted in the framework of this thesis a Dalitz Plot distribution for the eta->pi+pi-pi0 decay was obtained. From this distribution we extracted asymmetry parameters sensitive to C symmetry violation for different isospin values of the final state and we have established that all are consistent with zero within the obtained accuracy. For the eta->pi0 e+e- decay we have not observe signal and thus we estimated an upper limit for the branching ratio. The established upper limit amounts to $BR(eta->pi0 e+e-)<3.7x10^-5 at the 90% confidence level. This result is more precise than previously obtained in other experiments.