P. Goslawski, A. Khoukaz, S. Barsov, I. Burmeister, D. Chiladze, S. Dymov, R. Gebel, M. Hartmann, A. Kacharava, P. Kulessa, A. Lehrach, B. Lorentz, R. Maier, T. Mersmann, M. Mielke, S. Mikirtychiants, H. Ohm, M. Papenbrock, D. Prasuhn, T. Rausmann, V. Serdyuk, H. Stockhorst, H. Stroeher, A. Taeschner, Y. Valdau, C. Wilkin
A value for the mass of the $\eta$ meson has been obtained at the COSY-ANKE facility on the basis of pure kinematics through the measurement of a set of deuteron laboratory beam momenta and associated $^3$He center-of-mass momenta in the $dp\to{}^{3}$He$\,X$ reaction. The $\eta$ was then identified by the missing-mass peak and the production threshold determined. The individual beam momenta were fixed with a relative precision of $3 \times 10^{-5}$ for values around 3 GeV/c by using a polarized deuteron beam and inducing an artificial depolarizing spin resonance, which occurs at a well-defined frequency. The final-state momenta in the two-body $dp\to{}^{3}$He$\,\eta$ reaction were investigated in detail by studying the size of the $^3$He momentum ellipse with the forward detection system of the ANKE spectrometer. Taking advantage of the full geometrical acceptance, the necessary fine calibration of the spectrometer for this high precision experiment was achieved through a comprehensive study of the $^3$He final-state momenta as a function of the center-of-mass angles. The value obtained for the mass, $m_{\eta}=(547.873\pm0.005_{\text{stat}}\pm0.023_{\text{syst}})$ MeV/$c^2$, is consistent and competitive with other recent measurements, where the meson was detected through its decay products.
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http://arxiv.org/abs/1204.3520
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