Can a single resonant spin-flavor-precession scenario with light sterile neutrinos simultaneously fit all solar neutrino data, and is the low-energy flux time-variable through magnetic effects?

The 2009 analysis of Picariello et al. (arXiv:0907.0637) asked whether resonant spin-flavor precession (RSFP) of solar neutrinos into light sterile states, added to the established MSW-LMA oscillations, is consistent with experiment: fits to all then-available solar data except Borexino preferred a sizable solar internal magnetic field (a tachocline field of order 140 kG or a core field of order MG) with a transition magnetic moment of order $10^{-12}$ Bohr magnetons, while the Borexino fit deteriorated under the same hypothesis, leaving an unresolved tension. The paper also flagged the apparent $2.4\sigma$ epoch-dependent drop of the gallium capture rate between 1991-97 and 1998-2003 as a possible solar-activity-correlated magnetic modulation, and proposed precise pep and CNO flux measurements as the decisive discriminator. Later literature delivered the discriminators - Borexino measured pep, $^7$Be, pp and CNO neutrinos and limited the effective magnetic moment and the solar antineutrino flux; BEST reconfirmed the gallium anomaly. However, no dedicated modern global fit of the LMA+RSFP-to-sterile scenario against the complete dataset exists, and the active antineutrino limits do not bound the sterile channel: the question whether magnetic effects supplement LMA oscillations remains open.

Background

Solar neutrinos are electron neutrinos produced in the Sun's core by the pp chain (proton-proton fusion) and the CNO (carbon-nitrogen-oxygen catalytic) cycle; the low-energy components are pp, $^7$Be (from electron capture on beryllium-7) and pep (from proton-electron-proton fusion) neutrinos, while CNO neutrinos trace the catalytic cycle. Radiochemical experiments such as SAGE, GALLEX and GNO detected solar neutrinos through the reaction $^{71}\mathrm{Ga}(\nu_e,e^-)^{71}\mathrm{Ge}$, with rates quoted in SNU (one SNU = one capture per $10^{36}$ target atoms per second); these experiments are uniquely sensitive to the low-energy pp and $^7$Be fluxes. The long-standing solar neutrino problem was resolved by neutrino oscillations with the MSW-LMA solution: flavor conversion dominated by large mixing between the first two mass eigenstates and enhanced by matter effects (the Mikheev-Smirnov-Wolfenstein mechanism) as neutrinos cross the solar interior.

A hypothesized subdominant addition to LMA is resonant spin-flavor precession (RSFP). A neutrino magnetic dipole moment couples the spin to a magnetic field; for Dirac neutrinos the moment is diagonal in flavor, while for Majorana neutrinos only transition moments between different flavors (or between an active and a sterile state) are allowed. In spin-flavor precession (SFP) the solar magnetic field flips the neutrino helicity while changing its flavor (for example $\nu_e \to \bar{\nu}_\mu$, or $\nu_e \to \nu_s$, a light sterile state that does not interact weakly); when the level-crossing condition between the two states is met inside the Sun, in analogy with the MSW resonance, the conversion is resonantly enhanced. The Standard Model expectation for neutrino magnetic moments is of order $10^{-19}\,(m_\nu/1\,\mathrm{eV})$ Bohr magnetons, far below any current sensitivity, so a measurable effect would be new physics. Whether RSFP occurs, and where its resonances lie, is governed by the neutrino transition magnetic moment $\mu$ and by the internal solar magnetic field profile $B(x)$ as a function of fractional solar radius $x$. Solar physics constrains this profile only loosely: a field peaking at the tachocline (the shear layer at the base of the convection zone, $x$ of about 0.71) is bounded by roughly 300 kG there, while a core-peaked profile could reach a few MG - so constraints on $\mu$ are degenerate with astrophysical assumptions about $B(x)$.

The 2009 status (Picariello et al., arXiv:0907.0637) was: in a scheme with one light sterile neutrino and transition moments of order $10^{-12}\,\mu_B$ ($\mu_B$ denotes the Bohr magneton, the natural unit for neutrino magnetic moments), global fits to all solar neutrino data except Borexino (a liquid-scintillator detector at Gran Sasso measuring low-energy solar neutrinos in real time) improved markedly when a sizable magnetic field was introduced - preferring a tachocline field of about 140 kG or a core field of about 0.75 MG with a sterile mass splitting of order $10^{-8}\,\mathrm{eV}^2$ - but the same field worsened the fit to the first Borexino $^7$Be and $^8$B spectral data. In parallel, the combined gallium rate appeared to drop from $77.8 \pm 5.0$ SNU (1991-97) to $63.3 \pm 3.6$ SNU (1998-2003), a $2.4\sigma$ discrepancy, hinting at a possible solar-activity-correlated time modulation of the low-energy flux that only gallium experiments, Homestake-type chlorine data, and later Borexino could probe. Spin-flavor conversion of the Sun's $^8$B neutrinos into electron antineutrinos is independently constrained by solar antineutrino searches at Borexino, KamLAND, SNO and Super-Kamiokande, but those bounds apply to the active antineutrino channel, not to conversion into sterile states. The source paper argued that precise pep and CNO flux measurements (anticipated from SNO+ and Borexino) would discriminate between standard LMA and LMA+RSFP, since the RSFP scenario predicts a pep rate reduction of roughly 40-60% for tachocline fields of 100-300 kG.

Problem Statement

Determine whether resonant spin-flavor precession (RSFP) of solar neutrinos, added to MSW-LMA oscillations, is consistent with experiment - specifically, whether there exists a single scenario with one fixed nonzero transition magnetic moment $\mu$ (of order allowed by direct limits; the leading direct bound, from the Borexino Phase-II recoil-electron spectrum, is $\mu_\mathrm{eff} < 2.8\times10^{-11}\,\mu_B$ at 90% CL) and one fixed nonzero solar internal magnetic field profile $B(x)$ (within solar-physics bounds: tachocline fields up to roughly 300 kG at the base of the convection zone, or core fields up to a few MG), converting active solar neutrinos predominantly into light sterile states, that simultaneously describes all solar neutrino data: the radiochemical chlorine and gallium solar-neutrino rates, the real-time rates and spectra from Super-Kamiokande, SNO and Borexino, Borexino's precision pp, $^7$Be, pep and CNO measurements, and the solar antineutrino flux limits. The degenerate choices $\mu = 0$ or $B(x) = 0$, and more generally any field or moment small enough, or sterile parameters so far off resonance, that the level crossing lies outside the Sun and the RSFP conversion becomes negligible - so that the scenario again reduces to pure LMA - are not admissible affirmative answers to the existence branch: as in the source's comparison of fits at $B_0 = 0$ against $B_0 \approx 140$ kG (tachocline) or $B_0 \approx 0.75$ MG (core), the question concerns whether a genuine, nonzero-field resonant conversion of non-negligible size, at least at the scale of tens of kG in the tachocline or 0.1 MG in the core and with its level crossing inside the Sun, is compatible with - or preferred by - the data. This resolves the tension identified in 2009 - fits to all data except Borexino preferred a sizable magnetic field while the Borexino fit deteriorated under the same hypothesis - either by producing the allowed region in $(\mu, B(x))$ and sterile parameters with its statistical significance, or by excluding the scenario outright with a quantified confidence level. As an inseparable part of the same question, establish or refute the apparent epoch-dependent time variation of the gallium solar-neutrino capture rate as a genuine solar-activity-correlated modulation of the low-energy neutrino flux caused by such magnetic effects, rather than a statistical fluctuation or experimental systematics; this branch concerns the time dependence of the internal field, so it permits the field magnitude to vary with solar activity (a time-dependent $B(x,t)$), unlike the existence branch above, which is required to use one fixed profile $B(x)$. The single overarching objective is to decide whether magnetic spin-flavor effects supplement LMA oscillations in the solar neutrino data at all; the global-fit consistency and the time-variation reality are jointly needed deliverables, since both constrain the same transition magnetic moment $\mu$ and radial field profile $B(x)$, evaluated at one fixed profile for the existence branch and at an activity-correlated time dependence of that profile for the time-variation branch. Borexino's pep and CNO measurements are subsumed by the global fit and are not a separate deliverable.

References

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