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Determination of the η mass with the Crystal Ball at MAMI-B

A new precise determination of the η meson mass is presented. It is based on a measurement of the threshold for the γp -> pη reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous η mass measurement at MAMI-B. The result mη = (547.851 +- 0.031 stat. +- 0.062 syst.) MeV agrees very well with the precise values of the NA48, KLOE and CLEO collaborations and deviates by about 5σ from the smaller, but also very precise value obtained by the GEM collaboration at COSY.