Direct measurement of the in-medium ${\eta }^{\prime }$ mass spectrum through the $\gamma \gamma$ decay channel

Abstract

We measured the invariant mass of the ${\eta }^{\prime }(958)$ meson through the ${\eta }^{\prime }\to \gamma \gamma$ decay channel to search for possible character change of ${\eta }^{\prime }$ 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 ${\eta }^{\prime }$ mass peak for low-momentum ($P_{\gamma \gamma }<1$~$GeV/c$) events. Three statistical methods were applied to assess the significance of the enhancement. All methods yield a statistical significance above 3.7$\sigma$. 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 ${\eta }^{\prime }$ meson obtained through a direct measurement of its invariant mass.

References in corpus (8)

• Meson-nucleus potentials and the search for meson-nucleus bound states
• η- and η'-mesic nuclei and U_A(1) anomaly at finite density
• Pseudoscalar neutral mesons in hot and dense matter
• Analysis of U${}_A$(1) symmetry-breaking and restoration effects on the scalar-pseudoscalar spectrum
• Determination of the eta'-proton scattering length in free space
• Determination of the real part of the η'-Nb optical potential
• Search for ${\eta }^{\prime }$ bound nuclei in the ${}^{12}\mathrm{C}(\gamma ,p)$ reaction with simultaneous detection of decay products
• Differential cross sections and photon beam asymmetries of $\eta$ photoproduction on the proton at $E_{\gamma }$ = 1.3-2.4 GeV