How XENONnT detected the lowest-energy solar neutrinos ever observed

Built to search for WIMPs—hypothetical weakly interacting massive particles that could make up dark matter—the XENONnT experiment has now demonstrated that the same extreme sensitivity can reveal something much closer to home: neutrinos produced in the Sun.

During a seminar at the INFN Laboratori Nazionali del Gran Sasso in Italy, the XENON Collaboration announced the first observation of low-energy solar neutrinos scattering off electrons inside the detector. The measurement extends direct neutrino observations down to energies of about 17 keV, the lowest neutrino-energy threshold reported so far. The signal is dominated by pp neutrinos, created in the proton–proton fusion reactions that power the Sun. 22

A neutrino leaves behind a tiny recoil

Neutrinos almost never interact with matter. But on rare occasions, a solar neutrino scatters elastically from an electron in the liquid xenon, transferring a small amount of energy to it.

That recoil produces two signals in XENONnT:

• a prompt flash of scintillation light from the liquid xenon;
• a second electroluminescence signal when freed electrons are extracted into the gas above the liquid.

Together, the signals allow researchers to reconstruct the energy and three-dimensional position of each event. The goal is not to identify one unmistakable “neutrino hit”. Instead, the collaboration looks for the predicted statistical pattern of many low-energy electronic recoils and compares it with radioactive and detector-related backgrounds.

The approximately 17 keV figure refers to the neutrino energies brought within reach of the measurement. It does not mean that every electron receives 17 keV in a single collision. The reported signal is a low-energy solar-neutrino observation dominated by the pp component. 22

What are pp neutrinos?

The name “pp” comes from the proton–proton reaction that begins the main fusion chain in the Sun:

[
p+p\rightarrow d+e^++\nu_e
]

In this reaction, two protons combine to form deuterium, releasing a positron and an electron neutrino. The process initiates the chain through which hydrogen is gradually converted into helium.

Because the proton–proton chain supplies most of the Sun’s energy, pp neutrinos are direct messengers of the fusion reactions in its core. Light generated in the core takes a very long time to work its way to the solar surface, while neutrinos escape the production region almost immediately and travel onward with little interference. Measuring pp neutrinos therefore provides a more direct test of the fusion that powers the Sun.

Why XENONnT could see such a faint signal

At the centre of XENONnT is a two-phase time-projection chamber containing a 5.9-tonne active target of liquid xenon. Its large mass increases the chance of capturing an exceptionally rare interaction, while the separate scintillation and ionisation signals provide information about the energy and location of each event. 5

The main challenge is not only the rarity of neutrino interactions. It is also the background. Radon and its decay products can produce electronic recoils that look much like those caused by neutrino–electron scattering.

To address this problem, the collaboration developed online cryogenic distillation, which continuously removes radon from the xenon. The activity of radioactive (^{222}\mathrm{Rn}) in the active target was reduced to about (0.90\ \mu\mathrm{Bq/kg}), bringing the radon background to a level comparable with the unavoidable background from solar neutrinos. 1819

XENONnT’s location roughly 1,400 metres underground beneath Italy’s Gran Sasso mountain also suppresses events caused by cosmic rays. In this unusually quiet environment, the solar-neutrino contribution can be extracted through a spectral fit rather than being buried beneath a much larger population of spurious events. 22

How the result builds on the 2024 boron-8 measurement

The new low-energy result uses a different interaction channel from XENONnT’s 2024 solar-neutrino measurement.

In that earlier analysis, the experiment searched for (^{8}\mathrm{B}) neutrinos through coherent elastic neutrino–nucleus scattering, or CEνNS. In this process, the neutrino scatters from an entire xenon nucleus, producing a nuclear recoil. XENONnT reported a 2.73σ indication—not a 5σ discovery—of this signal. 23

The new measurement instead examines neutrino–electron scattering. The two analyses show that the same detector can identify different solar-neutrino signatures:

• electronic recoils, produced when neutrinos scatter from electrons and dominated at low energies by pp neutrinos;
• nuclear recoils, produced through CEνNS, as in the boron-8 analysis.

This distinction matters because each channel probes a different part of the solar-neutrino spectrum and offers a different view of the physics inside the Sun.

The “neutrino fog” facing dark-matter searches

Solar neutrinos are both a scientific opportunity and a problem for direct dark-matter experiments. They provide a genuine signal to study, but the recoils they create can resemble interactions from WIMPs. Unlike radioactive particles, neutrinos cannot simply be blocked with more shielding: they pass through matter almost unhindered.

This irreducible background is often called the neutrino fog. It is not an absolute barrier. Researchers can use the energy spectrum, measurements with different target materials, directional information and improved knowledge of solar-neutrino fluxes to distinguish a possible WIMP signal statistically. But as detectors become more sensitive, those improvements become harder and systematic uncertainties become increasingly important.

XENONnT’s earlier CEνNS result provided one of the first direct examples of this nuclear-recoil neutrino background appearing in a dark-matter detector. 12 The new electronic-recoil measurement shows that the same instrument can also access the much more numerous, lower-energy pp population.

What XLZD could add

The planned XLZD observatory is intended to be the next generation of liquid-xenon rare-event detectors. Its baseline design calls for an active target of about 60 tonnes of liquid xenon, with the possibility of increasing that to 80 tonnes—roughly an order of magnitude more active xenon than XENONnT. 3235

The larger target would not simply produce more events. It could enable more precise measurements of the pp-neutrino flux and energy spectrum, as well as the probability that electron neutrinos remain electron neutrinos while travelling from the Sun to Earth. At the same time, XLZD is being designed to explore WIMP interactions in the region where they approach the neutrino fog. 3239

The broader lesson from XENONnT is that dark-matter detection and solar-neutrino astronomy are no longer separate pursuits. The same purity, low background and sensitive readout required to find an extraordinarily rare WIMP interaction can also reveal the nearly invisible traces of fusion taking place in the Sun’s core.