R-96-05.1, ETH ZÜRICH - PSI - VIRGINIA - ZÜRICH
M. Daum1, M. Janousch2, P.-R. Kettle1, J. Koglin1,3, J. Schottmüller1, and D. Wyler4
| 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 |
An anomaly in the time distribution of neutrinos from a pulsed beam-stop source was reported by the KARMEN collaboration [1], with a speculative explanation that these events could originate from a rare pion decay process,
(1)
where X is a heavy neutral particle with a rest mass
of mX = 33.9049 ± 0.0009 MeV.
In 1995, we measured the momentum spectrum of muons from
p+
-decay in flight and placed an upper
limit on the branching fraction h for this decay at
2.6 · 10-8 with a confidence level
of 95% [2].
As part of a new effort to place a more stringent limit on
h,
an experiment was started using the beam line setup shown in
Fig. 1 (35 kB). We improved
the pion
beam definition and background suppression with a new beam line optics, an
improved vacuum system, and the addition of active and passive collimators.
The coincidence of the S1, S2 and S3 counters in addition to appropriate timing
and veto counter cuts defined our events. The momentum spectrum of muons in the
momentum range of interest is diplayed in
Fig. 2 (21 kB). No peak is
visible around
pm ~
pp ·
mm /
mp,
the momentum at which a muon peak should be observed due to decay (1).
The data were fitted by a
background distribution (hyperbola) plus the expected distribution for muons
from decay (1) which was predicted using direct pion and muon beams, along with
Monte Carlo simulations. From this, we find a preliminary new upper limit of
(c.l. = 95 %).
(2)
[2] M. Daum et al., Phys. Lett. B 361 (1995) 179.
Picture of the pie1 area (120 kB)
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