PSI - ETHZ - VIRGINIA
M. Daum1, R. Frosch1, W. Hajdas1, M. Janousch2, P.-R. Kettle1, St. Ritt3, and Z. G. Zhao2
| 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. |
Recently 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 mass of 33.9 MeV.
The KARMEN collaboration estimated values of the branching fraction
eta of this decay mode down to 10-16,
depending on the lifetime
tX
of such a particle.
The reported mass value is very close to the
mass difference between the charged pion and the muon,
mp -
mm =
33.91157 ± 0.00067 MeV.
For pion decay in flight, the resulting
small Q-value has several advantages: firstly, the velocity
of the muon is very close to the velocity of the original
pion and secondly, the flight direction of the muon
differs only slightly from that of the pion.
We measured the momentum spectrum of muons from
p+
-decay in flight [2] and took advantage of the fact that
a "quadrupole channel", i.e. a beam-line consisting
of dipoles and quadrupoles only, accepts muons preferably
in the forward or backward direction in the pion center-of-mass system.
The momenta of these muons
differ strongly from those of the decay (1), whereas the muons
from the main decay mode which are emitted sideways
are suppressed because of the limited
angular acceptance of the beam-line.
Muons from the decay (1) should produce a peak in the momentum distribution at
pm ~
pp ·
mm /
mp.
The momentum spectrum of muons in the momentum range of interest
is displayed in Fig. 1 (31 kB).
In our data, no peak is visible;
the events in Fig. 1 (31 kB)
are consistent, within fairly large uncertainties,
with Monte-Carlo simulations of muons which were emitted by pions
and subsequently scattered at the apertures of the beam-line elements.
The height of the hypothetical peak predicted for an assumed branching fraction
of 10-7 was
multiplied with a parameter alpha which was varied
in steps of 0.1 between -1 and +1. For each value of alpha the data
were fitted by a background distribution (hyperbola)
plus the expected distribution for muons from the decay (1).
From the summed c2 of the fits we find the branching fraction to be h = (-0.40 ± 0.23) · 10-7; the uncertainty corresponds to one standard deviation. From this, using the "Bayesian approach" we find an upper limit of
(c.l. = 95 %).
(2)
This value is lower than the upper limit of 7 · 10-8 (c.l. = 95 %) found by Bilger et al. [3] who had a significantly smaller total number of useful pions.
[2] M. Daum et al., Phys. Lett. B 361 (1995) 179.
[3] R. Bilger et al., Phys. Lett B 363 (1995) 41.
Picture of the pie1 area (112 kB)
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