SEARCH FOR A NEUTRAL PARTICLE OF MASS 33.9 MeV IN PION DECAY

R-96-05.1, ETH ZÜRICH - PSI - VIRGINIA - ZÜRICH - BEIJING

M. Daum¹, M. Janousch², P.-R. Kettle¹, J. Koglin^1,3, D. Pocanic³, J. Schottmüller¹, C. Wigger¹, D. Wyler⁴, and Z.G. Zhao⁵

1	PSI, Paul-Scherrer-Institut, CH-5232 Villigen-PSI, Switzerland
2	IPP, Institut für Teilchenphysik der ETHZ, CH-5232 Villigen-PSI, Switzerland
3	Physics Department, University of Virginia, Charlottesville, Virginia 22901,USA.
4	Physik-Institut der Universität Zürich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland
5	Institute of High Energy Physics, Chinese Academy of Science, Beijing 100039, The People's Republic of China

In 1995, the KARMEN collaboration [1] reported an anomaly in the time distribution of neutrinos from a pulsed beam-stop source, with a speculative explanation that these events could originate from a rare pion decay process,

p⁺ --> m⁺ + X (1)

where X is a heavy neutral particle with a rest mass of m_X = 33.9049 ± 0.0009 MeV. Also in 1995, we undertook a measurement of the muon momentum spectrum from p⁺ -decay in flight using the beamline itself as a spectrometer and placed an upper limit on the branching fraction h for this decay at 2.6 · 10^-8 of active and passive collimators [3]. The beamline consisted of the pion transport channel, the pion decay region and the muon spectrometer. The coincidence of three beam counters in the muon spectrometer, in addition to appropriate timing and veto counter cuts defined our events. After a detailed analysis of the 1997 data and further simulation studies, we concluded that a significant fraction of our background could be attributed to scattered muons originating from pions decaying through the normal decay mode within the pion decay region and also in the first dipole magnet of the spectrometer (ASL52) which serves to separate out the beam pions from the decay muons.

In 1998, a fullscale experimental run was conducted. We used the same basic 1997 setup with a widened ASL52 pole-gap and the addition of several strategically positioned active veto counters to reduce the possibility of accepting scattered muons. After optimizing our setup, we were able to reduce the background by more then a factor of seven. Our data can be divided into three sets, each of which were fitted by a background distribution plus the expected distribution for muons from the decay (1). The fitted momentum spectrum of muons from the second data set is displayed in Fig. 1  with a peak indicating the signal we would expect for the branching fraction of the previously published upper limit [2]. Combining the preliminary fit results for each data set (see Fig. 2 ), we find a new upper limit of

h  <  2.5 · 10^-9 (c.l. = 95 %). (2)

References

[2] M. Daum et al., Phys. Lett. B 361 (1995) 179.

[3] M. Daum et al., PSI Newsletter (1997).

Pictures of the NEW HEAVN Experiment

The pie1 beam line

visits to this page since 17th December 2001.