Measurement of solar $pp$ neutrino flux with the new PandaX-4T data

Abstract: We report a new measurement of the solar proton--proton ($pp$) neutrino flux via neutrino--electron elastic scattering using the PandaX-4T Run 2 data set collected between 2024 and 2026, corresponding to an exposure of 1.9 tonne$\cdot$yr. Before Run 2 data taking, the detector underwent a series of upgrades to improve its response and background conditions. Time variations of radioactive noble-gas impurities are constrained using the physics data themselves, complemented by measurements from the gas-assay system. The analysis introduced improvements in the data processing chain, detector response characterization, and background models. A blind spectral analysis was then performed on the electronic-recoil data across a wide energy range from 20 to 1000 keV. In combination with the Run 0 data published earlier, the fitted $pp$ flux is $(8.5 \pm 3.5)\times 10^{10}$ $\mathrm{cm^{-2}s^{-1}}$, consistent with the prediction of the Standard Solar Model. With a statistical significance of $2.2σ$ above background, this marks the first positive indication of solar $pp$ neutrino--electron scattering below an electronic-recoil energy of 165 keV.

• The paper demonstrates the first positive indication of solar pp neutrino–electron scattering below 165 keV using an upgraded liquid xenon TPC.
• It employs a unified spectral fit combining high- and low-argon background data with precise calibration and robust background modeling.
• Results align with Standard Solar Model predictions, establishing LXe TPCs as effective tools for low-energy neutrino detection and new physics searches.

Measurement of Solar $pp$ Neutrino Flux with PandaX-4T: A Technical Assessment

Introduction

The measurement of solar $pp$ neutrino flux provides a precise probe of solar fusion processes and constitutes a stringent test of the Standard Solar Model (SSM). The $pp$ branch neutrinos are the dominant solar neutrino component, with spectral endpoints well below 420 keV, in a region where experimental backgrounds and detector systematics are challenging. This essay examines the methodology, technical advancements, results, and implications of the PandaX-4T Run 2 solar $pp$ neutrino measurement, which delivers the first indication of solar $pp$ neutrino–electron scattering below a 165 keV electronic-recoil threshold, and demonstrates the potential of liquid xenon (LXe) time projection chambers (TPCs) in low-energy rare event searches (2607.02405).

Detector Upgrades and Experimental Approach

The PandaX-4T detector, situated at China Jinping Underground Laboratory (CJPL-II), implements a dual-phase LXe TPC with a fiducial mass of 1.6 tonnes. Major hardware upgrades before Run 2 included a new cathode, improved linearity of photomultiplier tube (PMT) bases (extending to 40,000 PE), and enhanced PTFE field-cage reflectivity. The digitizer system was upgraded for 14-bit resolution and 500 MS/s sampling, and a large water Cherenkov veto was installed.

The liquid xenon was continuously purified, achieving an electron lifetime of $1.08 \pm 0.05$ ms, required for full drift-length charge collection. A notable effort was invested in reducing radioactive noble-gas backgrounds: Rn and Ar levels were actively monitored and dramatically reduced via a reversed-mode distillation system, minimizing radioactive backgrounds in the ER (electronic recoil) region of interest.

A significant revision of the data processing chain unified the previously split low- and high-energy pipelines into a single analysis with improved low-energy $S1$ resolution and $S2$ pileup rejection. The event selection efficiency was calibrated using $^{232}$Th sources and detailed Geant4-based simulation, yielding a total efficiency uncertainty of 6.7%.

The reconstructed energy response was calibrated using monoenergetic peaks across the full energy range, producing a nearly linear response up to 2.6 MeV and supporting robust background and signal modeling.

Figure 1: Reconstructed energy linearity and relative deviation across calibration energies in PandaX-4T Run 2, confirming sub-percent nonlinearity throughout the relevant range.

Background Control and Signal Extraction

Low-energy radioactive backgrounds dominate, arising primarily from detector materials, noble-gas impurities ($^{39}$Ar, $pp$0Kr, $pp$1Rn), and xenon isotopes ($pp$2Xe, $pp$3Xe, etc). Rigorous offline and real-time gas-assay supported precise modeling of these backgrounds, segregating the data into High-Ar and Low-Ar periods based on argon activity.

Spatial event distributions were tightly controlled using 3D position reconstruction, with a fiducialization strategy that minimized external and boundary-origin backgrounds.

Figure 2: Projected spatial distributions (Y–X and Z–$pp$4) for the selected physics data, defining the 1.6-tonne fiducial volume.

Solar neutrino spectra ($pp$5 and $pp$6Be) were modeled with up-to-date nucleon–electron cross sections, including atomic binding and many-body corrections. The background model involved both external constraints (HPGe radioassay, delayed-coincidence tagging for $pp$7Kr, RGA for $pp$8Ar) and floating parameters for dominant internal sources.

A blind, joint spectral fit was performed on the High-Ar and Low-Ar data sets, exploiting a binned likelihood approach with detailed nuisance parameter treatment. The final region of interest was 20–1000 keV over a fiducial mass of $pp$9 tonnes, yielding $pp$0 events post-selection.

Figure 3: Post-unblinding spectral fits for High-Ar (top) and Low-Ar (bottom) datasets across the 20–1000 keV region, showing data, background, and signal model agreement.

Results

The joint fit yielded a solar $pp$1+$pp$2Be normalization of $pp$3 (relative to the SSM prediction). Isolating the $pp$4 flux yields

$pp$5

for Run 2 and

$pp$6

when combined with Run 0. These results are consistent with SSM expectations within $pp$7 [e.g., Vinyoles et al. 2017].

The statistical significance for non-zero $pp$8 flux is $pp$9, providing the first positive indication for solar $pp$0 neutrino–electron scattering below 165 keV ER energy. Uncertainties are dominated by degeneracies with $pp$1Ar and $pp$2Kr backgrounds, reflecting substantial systematic overlap in the low-energy, featureless part of the spectrum.

Figure 4: Profile likelihood scan ($pp$3NLL) versus $pp$4 neutrino flux, showing best-fit and uncertainty envelope with comparison to SSM and other experimental results.

Discussion and Implications

These results demonstrate the viability of LXe TPCs, originally optimized for dark matter searches, as competitive probes for low-energy solar neutrinos. The achieved energy linearity, background reduction, and analysis methodology establish a path toward precision neutrino measurements in a multi-purpose detector environment.

The primary practical implication is that multi-ton LXe TPCs can complement liquid scintillator and gallium-based measurements, providing independent and cross-checking results with distinct dominant systematics. The achieved $pp$5 flux precision is systematics-limited, with $pp$6Ar and $pp$7Kr degenerate with the signal in spectral fits. Future progress will hinge on further reduction and real-time monitoring of these isotopic backgrounds; distillation, improved gas-assay, and possibly external tagging (as with the active water Cherenkov system) will be pivotal.

Theoretically, an accurate, independent $pp$8 flux measurement provides input for the SSM solar luminosity constraint and new physics searches (e.g., non-standard neutrino–electron interactions, anomalous electromagnetic moments, sterile admixtures). Exploring even lower energies—beneath existing liquid scintillator thresholds—would allow tests of neutrino oscillation transitions in the vacuum–matter interface region and probe exotic interactions with atomic electrons.

The roadmap outlined for the next-generation PandaX-xT project [PANDA-X:2024dlo] anticipates larger active masses, further background abatement, and increased exposure, promising further reductions in $pp$9 flux uncertainties and new reach in other rare-event channels.

Conclusion

The PandaX-4T Run 2 solar $pp$0 neutrino analysis establishes low-background, multi-ton LXe detectors as precision instruments for solar neutrino physics. The reported $pp$1 flux of $pp$2 matches SSM predictions and is statistically significant at the $pp$3 level. Future expansions in target mass and background control will enhance the sensitivity of LXe TPCs, supporting both neutrino physics and searches for physics beyond the Standard Model.