Measurement of the eta-Meson Mass using psi(2S) --> eta J/psi

TL;DR

This paper reports a precise measurement of the eta meson mass using psi(2S) decays at the CLEO-c detector, achieving an uncertainty comparable to the best previous results and resolving discrepancies among earlier measurements.

Contribution

The study provides a new, highly precise measurement of the eta meson mass using multiple decay modes in psi(2S) decays, improving the accuracy and consistency of previous data.

Findings

Measured eta mass: 547.785 MeV with combined uncertainty of 0.061 MeV.

Result agrees with NA48 measurement, but conflicts with GEM measurement at 6.5 sigma.

Utilized four decay modes to enhance measurement reliability.

Abstract

We measure the mass of the eta meson using psi(2S) --> eta J/psi events acquired with the CLEO-c detector operating at the CESR e+e- collider. Using the four decay modes eta --> gamma gamma, 3pi0, pi+pi-pi0, and pi+pi-gamma, we find M(eta)=547.785 +- 0.017 +- 0.057 MeV, in which the first uncertainty is statistical and the second systematic. This result has an uncertainty comparable to the two most precise previous measurements and is consistent with that of NA48, but is inconsistent at the level of 6.5sigma with the much smaller mass obtained by GEM.