Direct Measurement of the In-Medium η ′ Mass Spectrum through the γ γ Decay Channel

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 (Pγγ<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/c². 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.