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This volume focuses on theoretical and experimental aspects of the η′ meson. The η′ pseudoscalar meson plays a special role in QCD as its mass is largely due to the explicit breaking of the axial U(1) symmetry by the quantum fluctuations of QCD (axial anomaly). The precise mechanism that gives the η′ its large mass is not yet fully understood, and also the current knowledge of the η′ specific properties is far from complete. This volume addres...