Phys Rev Lett. 2026 Jul 31;137(5):051901. doi: 10.1103/x414-n6g9.
ABSTRACT
We measured the invariant mass of the η^{‘}(958) meson through the η^{‘}→γγ decay channel to search for possible character change of η^{‘} in a nuclear medium. The measured invariant-mass spectra were analyzed by fitting realistic spectral functions. An enhancement was observed in the lower tail of the η^{‘} mass peak for low-momentum (P_{γγ}<1 GeV/c) events. Three statistical methods were applied to assess the significance of the enhancement. All methods yield a statistical significance above 3.7σ. The spectral fitting indicates a mass reduction of 57.5_{-27.8}^{+5.7} MeV/c^{2}. Our result provides the first evidence for an in-medium spectral modification of the η^{‘} meson obtained through a direct measurement of its invariant mass.
PMID:42606438 | DOI:10.1103/x414-n6g9
