Stars as Laboratories for Fundamental Physics - MPP Theory Group

388 Chapter 10
of the Sun. 58 The simple estimate Eq. (10.26) remains approximately
valid. A neutrino mass-square difference m 2 2 −m 2 1 below about 10 −5 eV 2
is required because of the small matter densities encountered in the convective
layers.
The bounds on neutrino dipole and transition moments discussed
in Sect. 6.5.6 yield µ < ν ∼ 3×10 −12 µ B so that a magnetic-field strength
B > ∼ 100 kG is required in the solar convection zone. Typical field
strengths measured in sunspots where the flux breaks through the surface
are of a few kG. While this may not be representative of the largescale
toroidal field, several general arguments suggest that 10 kG is a
generous upper limit (Shi et al. 1993). If there were much stronger
fields they would have to be confined to flux ropes which would not
be effective at inducing neutrino spin oscillations. Therefore, the spin
oscillation scenario would require anomalously large magnetic fields, in
conflict with the arguments presented by Shi et al., or dipole moments
in excess of what is allowed by the bounds of Sect. 6.5.6. Even smaller
dipole moments than 3×10 −12 µ B require novel neutrino interactions.
The solar magnetic field is believed to consist of two opposite flux
tori, separated by the solar equatorial plane. In the course of a year the
line of sight from Earth to the solar core varies between ±7 ◦ 15 ′ solar
latitude and so the neutrinos measured here traverse a field configuration
which varies in the course of a year. The predicted semiannual
flux variation (Voloshin, Vysotskiĭ, and Okun 1986a), however, has not
been confirmed by any of the experiments.
In the spin-flavor oscillation scenario involving Majorana neutrinos
the Sun would be a source for antineutrinos, some of which could be
ν e ’s by a combination of spin-flavor (ν e → ν µ ) and flavor (ν µ → ν e )
oscillations. However, the Kamiokande data already yield restrictive
limits on a solar ν e flux (Barbieri et al. 1991; see also Fig. 10.14).
In summary, the magnetic spin oscillation scenario requires new neutrino
interactions to generate large magnetic dipole moments, and new
astrophysics to allow for sufficiently strong magnetic fields in the solar
convection zone. Neither a semiannual variation of the flux nor ν e ’s
have been observed; each would have been a smoking gun for the occurrence
of this effect. Therefore, one is led to disfavor the magnetic spin
oscillation scenario. Then, of course, one is back to a statistical fluctuation
as an explanation for the time structure of the Homestake data.
58 Recent detailed investigations were performed by Akhmedov, Lanza, and Petcov
(1993, 1995), Krastev (1993), Nunokawa and Minakata (1993), Guzzo and Pulido
(1993), and Pulido (1993, 1994). For a review of earlier works see Pulido 1992.