Bonn 2001 – scientific programme

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T: Teilchenphysik

T 404: Neutrinos

T 404.5: Talk

Wednesday, March 28, 2001, 15:00–15:15, HS III

Karmen Time Anomaly: Search for a Particle of Mass 33.9 MeV in Pion Decay — •Manfred Daum¹, Markus Janousch¹, Peter-Raymond Kettle¹, Jason Koglin^1,2, Dinko Pocanić², Jörg Schottmüller¹, Claudia Wigger¹, and Zhenguo Zhao³ — ¹PSI, Paul-Scherrer-Institut — ²UVa, University of Virginia, Charlottesville, Va, USA — ³IHEP, Institute of High Energy Physics, Beijing, The People’s Republic of China

We have searched for the pion decay π → µ⁺ + X, where X is a neutral particle of 33.9 MeV. This process was suggested by the KARMEN Collaboration to explain an anomaly in their observed time distribution of neutrino induced reactions. Having measured the momentum spectrum of charged pions decaying in flight, we find no evidence for this process and place an upper limit on the branching fraction η ≤ 6.0 · 10⁻¹⁰ of such a decay at a 95 % confidence level. Furthermore, we show that beam correlated neutron events are not completely suppressed by the KARMEN lower energy cut and could cause the observed deviation of the measured time distribution from an exponential curve with τ = 2.2 µs.