The largest, at the moment, statistics of 7×106η→3π0 decays, based on 6.2×107η mesons produced in the γp→ηp reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the η→3π0 dynamics beyond its conventional parametrization with just the quadratic slope parameter α and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center.
Bibliographical noteFunding Information:
The authors acknowledge the excellent support of the accelerator group and operators of MAMI. We thank H. Leutwyler, G. Colangelo, and B. Kubis for fruitful discussions and constant interest in our work. This work was supported by the Deutsche Forschungsgemeinschaft (SFB443, SFB/TR16, and SFB1044), DFG-RFBR (Grant No. 09-02-91330), the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” program (Hadron Physics, Contract No. RII3-CT-2004-506078), Schweizerischer Nationalfonds (Contracts No. 200020-156983, No. 132799, No. 121781, No. 117601, No. 113511), the U.K. Science and Technology Facilities Council (STFC 57071/1, 50727/1), the U.S. Department of Energy (Offices of Science and Nuclear Physics, Awards No. DE-FG02-99-ER41110, No. DE-FG02-88ER40415, No. DE-FG02-01-ER41194) and National Science Foundation (Grants No. PHY-1039130, No. IIA-1358175), INFN (Italy), and NSERC of Canada (Grant No. FRN-SAPPJ-2015-00023). A. Fix acknowledges additional support from the Tomsk Polytechnic University competitiveness enhancement program. We thank the undergraduate students from Department of Physics of Mount Allison University and from Institute for Nuclear Studies of The George Washington University for their assistance.
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