Y. Matsumura, N. Muramatsu, T. A. Hashimoto, M. Miyabe, H. Shimizu, A. O. Tokiyasu, J. K. Ahn, W. C. Chang, J. Y. Chen, M. L. Chu, S. Daté, T. Gogami, H. Hamano, Q. H. He, K. Hicks, T. Hiraiwa, Y. Honda, T. Hotta, Y. Inoue, T. Ishikawa, I. Jaegle, Y. Kasamatsu, H. Katsuragawa, S. Kido, R. Kobayakawa, Y. Kon, S. Masumoto, K. Mizutani, T. Z. Mo, T. Nakamura, T. Nakano, T. Nam, M. Niiyama, Y. Nozawa, Y. Ohashi, H. Ohnishi, T. Ohta, M. Okabe, K. Ozawa, C. Rangacharyulu, S. Y. Ryu, Y. Sada, T. Shibukawa, R. Shirai, K. Shiraishi, E. A. Strokovsky, Y. Sugaya, M. Sumihama, S. Suzuki, S. Tanaka, Y. Taniguchi, N. Tomida, Y. Tsuchikawa, T. Ueda, T. F. Wang, H. Yamazaki, R. Yamazaki, Y. Yanai, T. Yorita, C. Yoshida, M. Yosoi
We measured the invariant mass of the $η^{\prime}(958)$ meson through the $η^{\prime}\toγγ$ decay channel to search for possible character change of $η^{\prime}$ 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 $η^{\prime}$ mass peak for low-momentum ($P_{γγ} < 1$~$\mathrm{GeV/c}$) events. Three statistical methods were applied to assess the significance of the enhancement. All methods yield a statistical significance above 3.7$σ$. The spectral fitting indicates a mass reduction of $57.5^{+~5.7}_{-27.8}$~$\mathrm{MeV/c}^{2}$. Our result provides the first evidence for an in-medium spectral modification of the $η^{\prime}$ meson obtained through a direct measurement of its invariant mass.