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Direct Measurement of the In-Medium η^{'} Mass Spectrum through the γγ Decay Channel
Y Matsumura1, N Muramatsu2, T A Hashimoto3
1Tohoku University, Research Center for Accelerator and Radioisotope Science, Sendai, Miyagi 982-0826, Japan.
None:
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.
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