Stars as Laboratories for Fundamental Physics - MPP Theory Group

Radiative Particle Decays 453
corresponding particles and antiparticles are the same. However, the
partial decay rates into specific channels need not be identical, and
indeed, K ◦ and K ◦ show such CP-violating decays: A transformation
under CP leads to a “mirror world” which is different from the one we
live in. Therefore, in the most general case one may only assume that
ν and ν have the same total lifetimes while their radiative decay rates
may be different.
However, it is common to analyze the available data under the assumption
of CP conservation for all neutrino interactions. For the
radiative decay of a polarized ν in its rest frame, the CP-mirrored decay
is one where a polarized ν with the same spin decays into ν ′ and
γ of reversed momenta. 70 Therefore, ν and ν of the same polarization
are characterized by opposite values for α in Eq. (12.2). In the source,
however, the ν’s and ν’s are produced by weak interactions which violate
parity maximally. If they are relativistic they both have negative
(left-handed) chiralities which means that the ν’s have negative and the
ν’s positive helicities. Thus, relative to their momentum left-handed
ν’s and ν’s show the same distribution of decay photons so that in
Eq. (12.5) one must use the same α for both.
12.2.2 Electron Neutrinos from Reactors
Fission reactors are superb neutrino sources. At a thermal power of
2800 MW, for example, one expects about 5×10 20 ν e /s. At a distance
of 30 m this corresponds to a flux of about 4×10 12 cm −2 s −1 , almost a
hundred times larger than the solar ν e flux of about 6.6×10 10 cm −2 s −1 .
Reactor ν e ’s emerge from many weak decays of the products of the
neutron induced fission of 235 U and 239 Pu while the fission of 238 U and
241 Pu contributes less than 10% to the total rate. The spectral distribution
can be inferred from a measurement of the corresponding β spectra
together with the reasonably well-known distribution of the end point
energies of the fission products (von Feilitzsch et al. 1982; Schreckenbach
et al. 1985). The distribution of ν e energies per fission of 235 U and
239 Pu is shown in Fig. 12.2 with a total of about 6 ν e ’s per fission.
An early, relatively crude, but often-quoted (Particle Data Group
1994) limit on the decay ν e → ν ′ γ was derived by Reines, Sobel, and
Gurr (1974) on the basis of the upper limit γ flux in a scintillation detector
near the Savannah River reactor (U.S.A.). A weaker but more reli-
70 A parity transformation P inverts all polar vectors, e.g. momenta, currents,
or electric fields, while it leaves axial vectors unchanged, e.g. angular momenta,
magnetic moments, or magnetic fields.