Stars as Laboratories for Fundamental Physics - MPP Theory Group

374 Chapter 10
The solar neutrino measurements of the chlorine experiment seem
to anticorrelate with solar activity as first suggested by Subramanian
(1979) and Bazilevskaya, Stozhkov, and Charakhch’yan (1982). These
latter authors also noted a correlation with the primary cosmic proton
flux which, however, is known to correlate with the solar magnetic
cycle because the solar magnetic field affects the flux of charged cosmic
rays hitting the Earth. The cosmic-ray flux is not expected to affect
the chlorine experiment directly; the average cosmic-ray induced 37 Ar
production rate is about 0.055 day −1 , compared with an average solar
neutrino signal of 0.48 day −1 . Figure 10.16 shows the temporal variation
from 1970−1989 of the Homestake argon production rate, the integral
number of sunspots, and the neutron counts at the McMurdo station in
Antarctica. The latter are indicative of the primary cosmic-ray proton
flux with energies above 0.4 GeV.
While a certain degree of anticorrelation between the number of
sunspots and the argon production rate is plainly visible in Fig. 10.16,
there are two important questions: How significant is the effect If it
is significant, what are its physical origins
As for the latter question, the only plausible explanation 55 that has
been put forth over the years is the hypothesis that neutrinos possess a
small magnetic moment which allows them to spin-precess into sterile
right-handed states in the presence of magnetic fields (Voloshin and
Vysotskiĭ 1986; Voloshin, Vysotskiĭ, and Okun 1986a,b), and variations
of this scheme which allow for simultaneous spin and flavor oscillations
(Sect. 10.7). In this case one would expect some degree of a semiannual
variation as explained above, but none is found in the Homestake data.
The issue of statistical significance was addressed on the basis of the
1970−1989 data by Filippone and Vogel (1990), Bieber et al. (1990),
and Bahcall and Press (1991). These latter authors argued for rankordering
as a statistical method because of the likely nonlinear relationship
(if any) with indicators of solar activity, and because of the possibly
non-Gaussian nature of the experimental errors. They produced pairs
of an observed argon production rate and the corresponding sunspot
index. Next, the Homestake data and sunspot indices were replaced by
the ordinal rank in their respective data sets, i.e. the smallest value has
rank 1, the second-largest rank 2, and so forth. In order to discover
correlations they applied two standard rank statistical tests, the Spear-
55 The refractive term in the MSW conversion probability from convective currents
could play a role and could be coupled to solar activity (Haxton and Zhang 1991).
However, extreme conditions are required to explain the observed effect.