Prof. Rani Bodnik from the Department of Particle Physics and Astrophysics at the Weizmann Institute of Science is a partner in this research, and among other things built the control and calibration systems and took part in the data analysis
The Xenon top array detector array of the XENONnT partnership. PR photo
The international scientific partnership XENONnT announced today (Wednesday, July 10) the first measurement of low-energy nuclear recoil from neutrinos produced in nuclear reactions inside the Sun, such as those involving the decay of the element Boron 8. The announcement was made at the IDM conference in L'Aquila (Italy).
XENONnT is an international experiment for the direct search for dark matter located in the Earth Condenser at the National Laboratories of Gran Sasso (LNGS) in Italy. It is one of the most advanced research facilities in the world for particle physics and astrophysics, which provides a unique environment that significantly reduces cosmic radiation. The research group of Prof. Rani Bodnik from the Department of Particle Physics and Astrophysics at the Weizmann Institute of Science is a partner in this research, and among other things built the control and calibration systems and took part in the data analysis. The group includes Dr. Lauren Levinson, Dr. Hagar Landsman, Dr. Jacques Pienar, Barbara Petch, Micah Weiss and Yossi Mosbacher.
Apart from hypothetical dark matter particles, according to the long-standing scientific forecast, neutrinos from the sun are also expected to be detectable in detectors built to look for nuclear recoil signals from dark matter, provided that these detectors reach sufficient "exposure" and "sensitivity". Exposure refers to how long we waited and how much material we used to observe the particles. Sensitivity is about how good we are at detecting the smallest particles. Observing this weak signal, with barely detectable energies in a "Time Projection" detector with liquid xenon, such as XENONnT, requires excellent performance of the detector and sophisticated methods for separating the signal from the background. The measurement confirms the understanding of the lowest energy signals in XENONnT.
"This achievement is the result of hard work, perseverance and cooperation of excellent scientists from different groups. Dark matter experiments like XENONnT stretch the limits of human detection ability, and directly contribute to discoveries in tangential fields - in this case of neutrinos from the sun, but of course there were and will be other fields. This makes us very happy," says Prof. Bodnik.
XENONnT's central detector is a time-projection cell (TPC) in two accumulation modes with 5.9 tons of ultrapure liquid xenon as an active target. It is designed to be sensitive to rare interactions of potential dark matter candidates. To achieve advanced performance, the XENONnT experiment uses several advanced subsystems, such as a cryogenics system to maintain the liquid xenon at the required low temperature, an online cryogenic distillation plant for the active removal of radioactive elements dissolved in xenon, and advanced control and data acquisition systems. A 700-ton water tank with active veto systems for neutrons and muons by Cherenkov radiation surrounds the XENONnT TPC to reduce the background.
Neutrinos from the Sun can interact with the nuclei of xenon atoms via neutrino-nucleus coherent elastic scattering (CEvNS). This Standard Model process, first predicted in 1974, has been challenging to observe due to very low-energy recoil and the elusive nature of neutrinos. Just in 2017, the COHERENT experiment reported the first observations of CEvNS with higher-energy neutrinos from the neutrino source in Oak Ridge, Tennessee. Now, XENONnT is the first experiment to measure CEvNS from neutrinos created in the solar core, measuring the CEvNS process with the element xenon. XENONnT joins the list of famous solar neutrino experiments that typically require detector masses 500-10 times larger.
The low energy detection capabilities of XENONnT and the ultra-low background environment enabled this first measurement. The analysis used data collected over two years, from July 7, 2021 to August 8, 2023, for a total of approximately 3.5 ton-years. An excess of low-energy nuclear recoil events over the expected background is measured, consistent with a signal from boron-8 neutrino interactions from the Sun, with a statistical significance of 2.7 sigma, meaning there is about a 0.35% chance that the observed signal is due to background noise. The partnership's graduate students and postdoctoral fellows led the development of the detailed analysis. The result was achieved through blind analysis, meaning the signal area remained hidden from the scientists' eyes until all stages of the analysis were determined to avoid human bias. This is the first measurement of coherent elastic neutrino-nucleus scattering from an astrophysical neutrino source. Furthermore, this significant result opens a new chapter in the field of direct detection of dark matter. XENONnT began investigating the so-called neutrino nebula, where neutrino interactions become a background that can mimic dark matter signals. XENONnT continues to collect data, and the international partnership expects more exciting discoveries in the field of particle physics, nuclear physics and astrophysics.
