FCTUC scientists record first measurement of a nuclear recoil signal from solar neutrinos

The XENONnT international collaboration, including four scientists from the UC Laboratory for Instrumentation, Biomedical Engineering, and Radiation Physics (LIBPhys - FCTUC), has recorded the first-ever measurement of low-energy nuclear recoils from neutrinos produced in nuclear reactions inside the Sun. This historic result was obtained using XENONnT, a system designed to detect dark matter with unprecedented sensitivity.

Alongside hypothetical dark matter particles, neutrinos from the Sun have long been predicted to be observable in detectors built to search for dark matter nuclear recoil signals when these detectors reach sufficient “exposure” and “sensitivity.” Observing such feeble signals requires excellent detector performance and sophisticated signal-to-background discrimination methods.

XENONnT, located deep underground at the INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy, is one of the most advanced underground research facilities worldwide for particle physics and astrophysics, providing a unique environment that significantly reduces cosmic radiation. The results announced are based on the analysis of data collected over two years, between July 2021 and August 2023.

XENONnT’s system uses six tonnes of ultra-pure liquid xenon as an active target, and its low-energy detection capabilities and ultra-low background environment have enabled this first measurement. The analysis used data collected over two years, between July 2021 and August 2023.

José Matias-Lopes, a researcher at FCTUC's LIBPhys and coordinator of the Portuguese team, says: "To measure events as rare as neutrinos and dark matter, the most important requirement is that the target has the lowest possible background radiation levels so that you can distinguish what you want to measure; it's much more difficult than finding a needle not in one, but in a thousand haystacks."

Neutrinos from the Sun can interact with the nuclei of the xenon atoms in the XENONnT target via coherent elastic neutrino-nucleus scattering (CEvNS). This Standard Model process, first predicted in 1974, has been challenging to observe due to the very low energy recoils involved and the elusive nature of neutrinos. Only in 2017, the COHERENT experiment reported the first observations of CEvNS with higher energy neutrinos from the Spallation Neutron Source in Oak Ridge, Tennessee (USA).

Now, XENONnT is the first experiment to measure CEvNS from neutrinos produced in the Sun's core and to measure the CEvNS process with the element xenon. XENONnT thus joins the list of famous solar neutrino experiments, which typically require 10-500 times larger detector masses. An excess of low-energy nuclear recoil events over the expected background was measured, compatible with a signal from solar boron-8 neutrino interactions, with a statistical significance of 2.7 sigma.

This marks the first measurement of CEvNS from an astrophysical neutrino source. Moreover, such a significant result opens a new chapter in the direct dark matter detection field: XENONnT has started exploring the so-called neutrino fog, where neutrino interactions are becoming a background that can mimic dark matter signals. As it continues to gather more data, the collaboration looks forward to exciting discoveries in the fields of astroparticle and nuclear physics.

For more information on this experience, visit the official website or the Portuguese team's website.