Preprint reports a possible η′ mass shift inside carbon nuclei
A carbon-target photon experiment found a lower-tail excess in low-momentum η′→γγ events and inferred a possible 57.5 MeV/c² mass reduction at normal nuclear density, with an uncertainty of +5.7/−27.8 MeV/c².
Phys. Rev. Lett. 137, 051901 (2026)4 min read