Stars as Laboratories for Fundamental Physics - MPP Theory Group

482 Chapter 12
suppression of the flux is compensated by the steep phase-space factor
m 5 ν in the expression for the decay rate.
These bounds are far more restrictive than those from laboratory
experiments (Fig. 12.3). The reason is that a SN explosion is a strong
ν τ source; such a source is difficult to make in the laboratory. This is
the reason why the ν τ has never been directly measured by its chargedcurrent
conversion into τ.
The strong SN 1987A bounds on |U e3 | 2 imply that a heavy ν τ with
only standard-model interactions must be rather long-lived, in fact too
long to be compatible with cosmological limits derived from big-bang
nucleonsynthesis (Sect. 7.1.5). Those bounds together with the present
results imply that a heavy ν τ cannot exist unless it has fast, invisible
decays induced by interactions beyond the standard model. If
this were the case it could well decay before leaving the SN progenitor.
Therefore, the laboratory experiments with their short distance
between source and decay volume remain important for anomalously
short-lived neutrinos.
12.4.8 Heavy, Sterile Neutrinos
The bounds on U e3 from reactors, beam stops, or the Sun were based on
a ν e flux which partially converts into ν 3 ’s which subsequently decay.
Because the SN emits about equal numbers of all ordinary neutrino
flavors, this approach is obsolete with regard to ν 3 . However, one may
still consider hypothetical sterile neutrinos which interact only by virtue
of their mixing with ν e . By assumption these states would not interact
through ordinary weak interactions and so they would not be trapped
in the SN core. Hence the expected ν h flux would emerge from the deep
interior rather than the surface of the core.
In this case, however, the sterile neutrinos would carry away energy
much more efficiently than the ordinary ones and so the requirement
that enough energy was left for the observed ν e ’s from SN 1987A already
gives one the approximate limit |U eh | 2 < ∼ 10 −10 (Sect. 9.6). If
this limit is approximately saturated one expects that about as much
energy is carried away by ν h as by the ordinary flavors. Taking account
of the harder energies of neutrinos emitted from the SN core one still
obtains about the same limit on |U eh | 2 as on |U e3 | 2 before. Put another
way, the GRS observations do not dramatically improve on the
cooling argument of Sect. 9.6, although they range in the same general
magnitude.