Super-Kamiokande's 1996-2018 search for periodic modulations of the solar 8B neutrino flux (5804 live days) found only the Earth-orbit modulation and excludes sinusoidal flux modulations only above about 5.1% amplitude (95% CL) for frequencies below 0.15 days−10.15\,\mathrm{days}^{-1}. This leaves unprobed the amplitude range predicted if neutrinos have a magnetic moment interacting with magnetic fields inside the Sun: under resonant spin-flavor precession (RSFP), an average inner-solar field of 105 G10^5\,\mathrm{G} with a neutrino magnetic moment below 10−1110^{-11} Bohr magnetons is expected to modulate the flux by less than about 2%, and a pure RSFP effect by less than about 0.1% - amplitudes the source Letter explicitly states 'are yet to be probed in future experiments'. The open problem is to extend sensitivity to periodic modulation of the solar 8B neutrino flux down to these predicted amplitudes (including frequencies above 0.2 days−10.2\,\mathrm{days}^{-1}, where five-day binning currently destroys sensitivity), establishing either a statistically significant solar-origin modulation or 95% CL amplitude limits at or below the predicted scale, and to translate the result into explicit constraints on the neutrino magnetic moment and the strength or profile of magnetic fields in the solar interior. Independent 2024-2026 re-analyses of the same released dataset confirm the null result at comparable sensitivity but do not reach the predicted amplitude scale, and no published work has performed the magnetic-moment-field mapping.

Background

The Sun produces electron neutrinos (νe\nu_e) in fusion chains; the '8B neutrino' is a high-energy component (endpoint ~15 MeV) produced by beta decay of 8B in the solar core. Super-Kamiokande (SK) is a 50-kiloton water Cherenkov detector in Kamioka, Japan, which detects solar neutrinos via elastic scattering on electrons, recording the time of each event. Across phases SK-I through SK-IV (May 1996 to May 2018, 5804 live days), SK measured the 8B flux in five-day time bins with per-bin uncertainties of roughly 10%. A periodic variation of the measured flux could arise from neutrino oscillation physics or from solar interior dynamics. One particularly motivated mechanism is resonant spin-flavor precession (RSFP): if neutrinos carry a magnetic moment μν\mu_\nu (conventionally quoted in units of the Bohr magneton μB\mu_B) and traverse the Sun's internal magnetic fields, the field can rotate the neutrino spin - for Dirac neutrinos converting active states into sterile ones, and for Majorana neutrinos converting νe\nu_e into antineutrinos of other flavors - in either case reducing the observed flux, with the conversion enhanced resonantly where the effective matter potential matches the precession frequency (the same matter-effect physics that underlies the Mikheyev-Smirnov-Wolfenstein (MSW) oscillation resonance responsible for the observed energy-dependent solar neutrino flavor conversion). Because solar internal magnetic fields, notably near the tachocline (the shear layer at the base of the convective zone where strong fields are believed to form) and in the radiative zone, may vary with time, RSFP would imprint a time-varying 'shadow' on the solar neutrino flux. Theoretical estimates used in the SK analysis, chiefly by Balantekin and Volpe and by Caldwell and Sturrock, indicate that for an average inner-solar field of 10510^5 G and μν<10−11 μB\mu_\nu < 10^{-11}\,\mu_B, the expected flux modulation is below about 2%, while the sole RSFP contribution is below about 0.1%. Earlier searches for such modulations date to the first SK-I analysis (Yoo et al., 2003); claims of flux variability in early datasets (Caldwell and Sturrock; Sturrock and collaborators) did not survive accumulated statistics. The most recent SK Letter (Super-Kamiokande Collaboration, Phys. Rev. Lett. 132, 241803 (2024)) searched the full 22-year dataset for periodic modulations with a maximum-likelihood method on the five-day binned flux, cross-checked with the Lomb-Scargle periodogram (a standard technique for detecting periodic signals in unevenly sampled time series by least-squares fitting of sinusoids). The only significant modulation found is the ~3% annual variation from the ellipticity of Earth's orbit (measured eccentricity (1.53 +/- 0.35)%); no other periodic modulation is present above 5.1% amplitude (95% CL) for frequencies below 0.15 days−10.15\,\mathrm{days}^{-1}. The Letter further notes that for frequencies above 0.2 days−10.2\,\mathrm{days}^{-1}, close to the five-day sampling frequency, amplitude fluctuations average out within the five-day bins, so short-period modulations are entirely unconstrained. Borexino has independently measured Earth's orbital parameters from 7Be solar neutrinos at consistent values but without publishing a periodic-modulation amplitude limit.

Problem Statement

Establish whether the solar 8B neutrino flux is periodically modulated at the amplitudes predicted by neutrino electromagnetic interactions with internal solar magnetic fields, and interpret the result in terms of neutrino and solar-interior properties. Concretely: using solar neutrino data (existing archives or new measurements, in any detection channel), extend the search for sinusoidal and general periodic modulations of the solar 8B neutrino flux to modulation amplitudes at or below the predicted scale - about 2% for magnetic-moment-induced modulation (average inner-solar field 10510^5 G, μν<10−11 μB\mu_\nu < 10^{-11}\,\mu_B) and about 0.1% for a pure resonant spin-flavor precession (RSFP) effect - over the frequency band from 4×10−4 days−14\times10^{-4}\,\mathrm{days}^{-1} up to and beyond 0.2 days−10.2\,\mathrm{days}^{-1}, where five-day binning currently eliminates all sensitivity. The deliverable is either (a) a statistically significant periodic modulation demonstrated to be of solar or neutrino-physics origin rather than Earth-orbit, seasonal, or detector systematic effects, or (b) 95% confidence-level upper limits on the modulation amplitude as a function of frequency that reach at or below both predicted scales (~2% and ~0.1%) across the stated frequency band - and, in either case, the translation of that result into explicit constraints on the product of the neutrino magnetic moment μν\mu_\nu and the strength or radial profile of magnetic fields in the solar radiative zone and tachocline via the RSFP mechanism, with the dependence on the assumed field profile stated.

References

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