July 10, 2024
On Wednesday, July 10, at the IDM conference in L’Aquila, Italy, the XENONnT collaboration announced the first measurement of low-energy nuclear recoils from neutrinos produced by nuclear reactions inside the sun.
Physicists have long predicted that neutrinos from the sun would be observable in detectors designed to search for hypothetical dark matter particles once such detectors reach sufficient exposure (the total amount of particles that a detector is subjected to over a period of time) and sensitivity (the detector’s ability to accurately measure and detect the tiniest particles). Observing such a faint signal, with energies barely detectable in liquid xenon time projection chambers like XENONnT, requires excellent detector performance and sophisticated signal-to-background discrimination methods. The XENONnT collaboration has measured for the first time low-energy nuclear recoils from solar neutrinos—a measurement that confirms the understanding of the lowest-energy signals in XENONnT.
XENONnT is a direct dark matter search experiment located deep underground at the INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy. Representing one of the most advanced underground research facilities worldwide for particle physics and astrophysics, LNGS provides a unique environment that significantly reduces cosmic radiation. Operating the family of increasingly sensitive experiments of the XENON program at LNGS has been critical for the success of the program.
Designed to be sensitive to rare interactions of potential dark matter candidates, XENONnT’s central detector is a dual-phase time projection chamber with 5.9 tonnes of ultra-pure liquid xenon as the active target. To achieve cutting-edge performance, the XENONnT experiment employs several advanced subsystems, such as cryogenic plants to maintain the liquid xenon at the necessary low temperature, an online cryogenic distillation column for the active removal of radioactive elements diluted in the xenon, and advanced slow control and data acquisition systems. A 700-tonne water tank featuring active Cherenkov neutron and muon veto systems surrounds the XENONnT time projection chamber to further reduce the background.
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 did the COHERENT experiment report the first observations of CEvNS with higher energy neutrinos from the Spallation Neutron Source in Oak Ridge, Tennessee.
“My colleague Juan Collar and his collaborators detected for the first time this extremely low-energy process by exposing a small cesium iodide detector to neutrinos produced in the lab,” said Luca Grandi, UChicago physics professor and senior member of the XENONnT Collaboration. “In our case, we are detecting the same process but for neutrinos produced in the sun. This result will help to better constrain CEvNS physics by adding a measurement for xenon nuclei for the first time, as well as provide further inputs to solar physics. Beyond this, the detection of these interactions is extremely important for us because we expect the dark matter particles we’re looking for, WIMPs, to produce similar coherent elastic nuclear weak scatterings in XENONnT.”
XENONnT’s low-energy detection capabilities and ultra-low background environment have enabled this first measurement. The analysis used data collected over two years, from July 7, 2021, to August 8, 2023. The result was obtained through a blinded analysis, for which the signal region remained hidden from the scientists’ view until all analysis steps were completed to avoid human bias.
Grandi’s group contributed to several elements of the XENONnT project: leading the design of the detector and its integration at LNGS, developing the cyber-infrastructure for data processing, and contributing to several aspects of this and other analyses.
“The B8 CEvNS signals we have been searching for are extremely tiny—typically only two scintillation photons with five ionized electrons will be detected,” said Lanqing Yuan, a UChicago graduate student who was charged with developing the detector response model for such low-energy signals.
Another graduate student, David Antòn Martìn, contributed to the analysis: “Discerning the faint neutrino recoil signal from detector backgrounds has been a critical factor in achieving sufficient detector sensitivity for this milestone measurement. At UChicago, we led the development of surface background models, which allowed for the establishment of fiducial volumes in which the search was carried out.”
In addition to being the first measurement of its kind, such a significant result opens a new chapter in the direct dark matter detection field: XENONnT can now explore the so-called neutrino fog, where neutrino interactions create background that can mimic dark matter signals. As XENONnT continues to gather data, the collaboration looks forward to exciting discoveries in the realm of astroparticle and nuclear physics.
Adapted from a press release published on Interactions.org.