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ISSN - 15507998, Physical Review D - Particles, Fields, Gravitation and Cosmology, 2008, vol. 77, p. -

Measurement of η meson decays into lepton-antilepton pairs

Berłowski M., Bargholtz Chr, Bashkanov M., Bogoslawsky D., Bondar A., Calén H., Cappellaro F., Clement H., Demirörs L., Ekström C., Fransson K., Gerén L., Gustafsson L., Höistad B., Ivanov G., Jacewicz M., Jiganov E., Johansson T., Keleta S., Koch I., Kullander S., Kupść A., Kuzmin A., Kuznetsov A., Laukhin I. V., Lindberg K., Marciniewski P., Meier R., Morosov B., Oelert W., Pauly C., Pettersson H., Petukhov Y., Povtorejko A., Ruber R. J M Y, Schönning K., Scobel W., Shafigullin R., Shwartz B., Skorodko T., Sopov V., Stepaniak J., Tegnér P. E., Engblom P. Thörngren, Tikhomirov V., Turowiecki A., Wagner G. J., Wolke M., Yamamoto A., Zabierowski J., Zartova I., Złomańczuk J.

A search for rare lepton decays of the η meson was performed using the WASA detector at CELSIUS. Two candidates for double Dalitz decay η→e+e-e+e- events are reported with a background of 1.3±0.2 events. This allows to set an upper limit to the branching ratio of 9.7×10-5 (90% CL). The branching ratio for the decay η→e+e- γ is determined to (7.8±0.5stat±0.8syst)×10-3 in agreement with world average value. An upper limit (90% CL) for the branching ratio for the η→e+e- decay is 2.7×10-5 and a limit for the sum of the η→μ+μ-μ+μ- and η→π+π-μ+μ- decays is 3.6×10-4. © 2008 The American Physical Society.