8B Solar Neutrinos: Solar Core and Neutrino Properties

• 8B solar neutrinos are high-energy particles produced in the Sun’s core via the pp-III chain, serving as sensitive probes of temperature, density, and electron interactions.
• Detection methods, including water Cherenkov, scintillator, and CEνNS-based detectors, enable precise channel separation and offer multi-method confirmation of flavor transformation.
• Precision measurements of 8B neutrino flux tighten constraints on solar core conditions and neutrino oscillation parameters, opening avenues for testing new physics.

$^{8}\text{B}$ solar neutrinos are high-energy neutrinos produced in the Sun’s core during the $\prescript{7}{}{\text{Be}}(p,\gamma)\prescript{8}{}{\text{B}}$ reaction in the pp-III branch of the hydrogen fusion chain. Their resulting flux at Earth serves as a highly sensitive probe of solar interior temperatures, electron densities, and flavor transformation via the Mikheyev–Smirnov–Wolfenstein (MSW) effect. Because they are the highest-energy neutrinos from steady-state solar processes, $^{8}$B neutrinos underpin precision measurements of both solar astrophysics and neutrino properties, as evidenced by results from the Sudbury Neutrino Observatory (SNO), Super-Kamiokande, Borexino, and recent advances in scintillator, liquid xenon, and paleo-detector technologies.

1. Production of $^{8}$B Solar Neutrinos and Theoretical Scaling

$^{8}$B solar neutrinos arise via the pp-III chain: $\prescript{7}{}{\text{Be}} + p \rightarrow \prescript{8}{}{\text{B}} + \gamma\,, \qquad \prescript{8}{}{\text{B}} \rightarrow \prescript{8}{}{\text{Be}}^* + e^{+} + \nu_e\,.$

The $\prescript{8}{}{\text{B}}$ beta decay has an endpoint at $\sim$16 MeV, making the emitted neutrinos the most energetic among solar neutrino flux components.

The rate of $\prescript{8}{}{\text{B}}$ production in the solar core is extremely sensitive to the local temperature. Empirically, the total $^{8}$B neutrino flux, $\Phi_{\text{tot}}$, scales as $T_c^{\alpha}$, where $T_c$ is the core temperature and $\alpha \simeq 24$ (Zaidel et al., 14 Apr 2025). Even a modest flux uncertainty, $\delta\Phi_{\text{tot}}/\Phi_{\text{tot}}$, translates to much tighter relative constraints on $T_c$: $$\frac{\delta T_c}{T_c} \simeq \frac{1}{\alpha} \frac{\delta\Phi_{\text{tot}}}{\Phi_{\text{tot}}}$$ As a result, flux measurements with several percent precision yield $\sim$0.1–0.5% level constraints on the central temperature of the Sun.

The $\prescript{8}{}{\text{B}}$ production is not pointlike but follows a sharply peaked radial profile proportional to $r^2 T(r)^{\beta}$, with an effective exponent $\beta \sim 17$–$27$ controlling the reaction rate's temperature dependence (Zaidel et al., 14 Apr 2025).

2. Detection Techniques and Channel Separation

High-precision detection of $^{8}$B solar neutrinos has been accomplished via several methods, each exploiting unique interaction channels:

• Water Cherenkov Detectors (e.g., Super-Kamiokande): Measure elastic scattering (ES) of neutrinos off electrons,

$\nu_x + e^- \rightarrow \nu_x' + e^-\,,$

where $\nu_e$ has a cross-section roughly six times larger than $\nu_{\mu,\tau}$. The directionality of relavistic electron recoils is utilized to suppress backgrounds (Haxton et al., 2012).

• SNO (Heavy Water): Simultaneous measurement of three channels (Aharmim et al., 2011, Haxton et al., 2012):
  • Charged-Current (CC):

  $\nu_e + d \rightarrow p + p + e^-$

  (selective to $\nu_e$) - Neutral-Current (NC):

  $\nu_x + d \rightarrow p + n + \nu_x$

  (sensitive to all active flavors, $x = e, \mu, \tau$) - Elastic Scattering (ES): As above.

SNO's Phase-III introduced an array of $^3$He proportional counters (the NCD array) for independent NC neutron detection via the reaction $$n + {}^3\text{He} \rightarrow p + t + 764\,\rm{keV}$$ (Aharmim et al., 2011).

• Scintillator Detectors (Borexino, SNO+, JUNO): ES channel with low thresholds, leveraging superior energy resolution and background suppression. SNO+ and JUNO also enable CC and NC measurements on $^{13}$C (Collaboration et al., 2022, Collaboration et al., 28 Aug 2025).

• Coherent Elastic Neutrino–Nucleus Scattering (CE$\nu$NS, PandaX-4T): First indication of $^{8}$B solar neutrinos detected via nuclear recoils at sub-keV energy thresholds in liquid xenon (Collaboration et al., 15 Jul 2024). The cross-section is

$$\frac{d\sigma}{dE_R} = \frac{G_F^2}{4\pi} Q_W^2 M (1 - \frac{M E_R}{2 E_\nu^2}) F^2(E_R),$$

with $Q_W = N - (1-4 \sin^2\theta_W)Z$ and $F^2$ the nuclear form factor.

• Paleo-detectors: Utilize materials such as sinjarite, in which $^{8}$B neutrinos have left nanometric nuclear recoil tracks accrued over geological timescales (~1 Gyr) (Arellano et al., 2021).

3. Experimental Results and Oscillation Parameter Constraints

SNO Phase-III

The total $^{8}$B active neutrino flux measured by SNO Phase-III via the NC channel is (Aharmim et al., 2011)

$$\phi_{\rm NC} = 5.54^{+0.33}_{-0.31}(\text{stat.})^{+0.36}_{-0.34}(\text{syst.}) \times 10^6\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$

In comparison, the CC (exclusive $\nu_e$) measurement yielded

$$\phi_{\rm CC} = 1.67^{+0.05}_{-0.04} \times 10^6\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$

illustrating flavor conversion.

A global analysis, combining solar and KamLAND reactor data, yields best-fit parameters of

$$\Delta m^2 = 7.59^{+0.19}_{-0.21}\times 10^{-5}\ \text{eV}^2,\quad \theta = 34.4^{+1.3}_{-1.2}~\text{degrees}$$

favoring the MSW Large Mixing Angle (LMA) solution and excluding the vacuum oscillation regime at $99.73\%$ C.L.

Borexino

Borexino’s latest combined Phase I–III analysis gives an HER interaction rate of $0.223^{+0.015}_{-0.016}$ (stat.) $^{+0.006}_{-0.006}$ (syst.) cpd/100 t, translating to a flux (Kumaran et al., 2021)

$$\Phi^{8\mathrm{B}} = (5.68^{+0.39}_{-0.41}) \times 10^6\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$

The measured electron neutrino survival probability in the $^{8}$B energy range is $P_{ee} \sim 0.35$–$0.39$, confirming the MSW effect's matter-enhanced suppression for high-energy solar neutrinos.

PandaX-4T (CE$\nu$NS)

PandaX-4T observed a best-fit $^{8}$B signal at $2.64\sigma$ significance, corresponding to a flux (Collaboration et al., 15 Jul 2024)

$$(8.4 \pm 3.1) \times 10^6\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$

compatible with the standard solar model.

SNO+ and JUNO ($^{13}$C Channel)

SNO+ reports $4.2\sigma$ evidence for $^{8}$B solar neutrino CC interaction on $^{13}$C, measuring a cross section for the ground-state transition (Collaboration et al., 28 Aug 2025): $$\langle\sigma\rangle = (16.1^{+8.5}_{-6.7}\ (\mathrm{stat.})^{+1.6}_{-2.7}\ (\mathrm{syst.})) \times 10^{-43}\,\mathrm{cm}^2$$ JUNO is projected to obtain $\sim5\%$ precision on the total $^{8}$B flux, with model-independent separation of CC ($^{13}$C), NC ($^{13}$C$^*$), and ES channels (Collaboration et al., 2022).

4. Statistical and Systematic Uncertainties

High-precision $^{8}$B neutrino flux measurements require robust control over both statistical and systematic uncertainties:

• Statistical: Determined by event counts in target-specific channels (e.g., finite NC neutron events in SNO NCDs (Aharmim et al., 2011); elastic recoils in Borexino, counting efficiency in PandaX-4T (Collaboration et al., 15 Jul 2024)).

• Systematic:

  • Detector energy scale, resolution, and background modeling (e.g., $\alpha$–decay background rejection in NCDs).
  • Neutron capture efficiency; e.g., SNO used uniform $^{24}$Na sources to calibrate and applied a spatial correction factor $$\epsilon_{\rm sol} = f_{\rm non-unif} f_{\rm edge} \epsilon_{\rm spike}$$.
  • Advanced pulse-shape discrimination, spectral fits, multivariate and likelihood approaches tightly incorporate uncertainties via nuisance parameters (often floated in MCMC/global fits) (Aharmim et al., 2011, Kumaran et al., 2021, Collaboration et al., 2022).

Systematics for leading experiments are typically at (or below) the $6$–$8\%$ level on the $^{8}$B flux, with some (e.g., Borexino, SNO) reaching $4\%$ or lower depending on channel and combination.

5. Solar Interior Constraints and Impact

Owing to the strong $T_c^{24}$ scaling, current $^{8}$B neutrino flux measurements constrain the solar core temperature to much better than $1\%$ (Zaidel et al., 14 Apr 2025). Forward modeling demonstrates that spatial information about the neutrino production zone (e.g., its radial profile and effective width) directly probes the temperature and density structure of the solar interior, complementing and sharpening helioseismic probes.

A maximal-likelihood fit of the effective production profile parameter $\beta$ to Super-K data implies $\rho_{\mathrm{npz}} = 121^{+19}_{-26}$ g/cm$^3$, $T_{\mathrm{npz}} = (1.453 \pm 0.002) \times 10^{7}$ K (Zaidel et al., 14 Apr 2025). These results are in accord with Standard Solar Model (SSM) predictions, and demonstrate that neutrino measurements can independently constrain interior conditions and test solar modeling, especially central metallicity and opacity.

6. Implications for New Physics: Neutrino Properties and Cosmology

$^{8}$B solar neutrinos are uniquely sensitive to neutrino flavor transformation in matter (MSW effect), with the observed CC/NC flux suppression quantitatively matching LMA-MSW predictions (Aharmim et al., 2011, Kumaran et al., 2021, Haxton et al., 2012). Rapid improvement in mixing parameter precision (notably $\Delta m^2$ and $\theta_{12}$) is directly tied to the precision of $^{8}$B flux and spectrum measurements.

Neutrino lifetime limits can also be set with high confidence. The energy-dependent survival probability with decay is $P_{ee} \sim \sin^2\theta_{12} \exp(-d_2 L_\odot/E)$ (for high-energy $^{8}$B, $d_2 = \Gamma_2 m_2$), yielding robust, model-independent lower bounds on $\nu_2$ lifetimes (Berryman et al., 2014).

Additionally, comparison of measured $^{8}$B fluxes and spectral shapes with SSM predictions places limits on exotic processes:

• Solar Relic Neutrino Capture: Any significant population of cosmic relic neutrinos in the Sun would suppress the observable $^{8}$B neutrino yield via capture on $\beta$-unstable nuclei, but the observed agreement with SSM restricts the relic neutrino density to $n_\nu < 3.75 \times 10^{28}$ cm$^{-3}$ (Ruhe et al., 2020).
• Nuclear S-factor Determination: Precision $^{8}$B neutrino fluxes, combined with $^{7}$Be measurements, have been used to determine the $^{7}$Be$(p,\gamma)^{8}$B S-factor independently of nuclear reaction cross section extrapolations, giving $S_{17}(0) = 19.5 \pm 1.9$ eV·b (Takács et al., 2017).

7. Future and Novel Approaches

• Next-generation detectors (JUNO, DUNE/SoLAr): Model-independent separation of CC, NC, and ES channels in large liquid scintillator or argon targets will achieve world-leading precision on $^{8}$B fluxes and neutrino parameters, access hep neutrinos, and probe rare channels (Collaboration et al., 2022, Parsa et al., 2022).
• Coherent scattering and dark matter experiments: PandaX-4T’s $^{8}$B flux is the first such indication via CE$\nu$NS, marking the onset of neutrino “fog” as an irreducible background in low-threshold dark matter detectors (Collaboration et al., 15 Jul 2024).
• Paleo-detectors: Mineral-based records of $^{8}$B neutrino-induced recoil tracks, combined across samples spanning the solar lifetime, provide a time-resolved probe of solar output and constraints on models of stellar evolution and metallicity (Arellano et al., 2021).

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In summary, $^{8}$B solar neutrinos are a uniquely sensitive probe of both solar core astrophysics and neutrino properties. Their flux, measured independently and precisely via CC, NC, ES, and CE$\nu$NS channels, anchors the modern understanding of solar energy generation, provides direct evidence for matter-induced neutrino flavor conversion, constrains new physics in the neutrino sector, and opens further prospects for precision stellar and cosmological studies.