Stars as Laboratories for Fundamental Physics - MPP Theory Group

256 Chapter 7
ever effect causes the “wrong” deformation of the end-point spectrum.
In spite of apparent systematic problems, the Troitsk group (Belesev
et al. 1994) claims a limit of m νe < 4.5 eV at 95% CL; see also Otten
(1995).
One has attempted to determine m νµ by measuring the muon momentum
from the decay of stopped pions, π + → µ + ν µ , leading to m 2 ν µ
=
m 2 π
+ m 2 + µ − 2m π +(m 2 µ + p 2 µ) 1/2 . A recent p µ measurement (Daum et al.
1991) implied a negative squared mass of m 2 ν µ
= −(0.154±0.045) MeV 2 ,
probably due to large systematic uncertainties in the determination of
m π +. Hence, it seemed that the often-quoted bound of m νµ < 0.27 MeV
did not apply. An older experiment studied the in-flight decay of pions
with a result m 2 ν µ
= −(0.14 ± 0.20) MeV 2 , largely independent of
the pion mass (Anderhub et al. 1982). This implies a 90% CL upper
limit of m νµ < 0.50 MeV. Most recently, the mass of the negative
pion was reconsidered by Jeckelmann, Goudsmit, and Leisi (1994).
Their previous experiment allows for two mass assignments, m π =
139.56782±0.00037 MeV or 139.56995±0.00035 MeV. The larger value
had previously been rejected on the basis of evidence which now appears
questionable. Together with a new p µ measurement (Assamagan et al.
1994) one finds m 2 ν µ
= −0.148 ± 0.024 MeV 2 or −0.022 ± 0.023 MeV 2 .
The first value is negative by 6.2 standard deviations and thus may
be rejected as unphysical. The second solution is compatible with zero
and gives a 90% CL upper limit of m νµ < 0.16 MeV.
For ν τ the best bounds also come from limits on missing energy
in certain reactions, the only form in which ν τ has ever been “observed.”
The ARGUS Collaboration (1988, 1992) studied the decay
τ − → 3π − 2π + ν τ with a total of 20 events with good energy determinations
for all five pions, leading to m ντ < 31 MeV at 95% CL.
A similar experiment by the CLEO Collaboration (1993) based on a
much larger data sample gave m ντ < 32.6 MeV at 95% CL. Most recently,
the ALEPH Collaboration (1995) at CERN has reported a new
Table 7.2. Experimental neutrino mass limits.
Flavor Limit CL Reference
ν e (5 eV) — See Tab. 7.1
ν µ 0.16 MeV 90% Assamagan et al. (1994)
ν τ 23.8 MeV 95% ALEPH Collaboration (1995)