Astronomers can't see dark matter directly, but know it's there: its gravity shapes galaxies and the large-scale structure of the cosmos. In an effort to uncover the composition of this hidden mass, a team at Lawrence Livermore National Laboratory (LLNL) is pursuing evidence of these particles that exist beyond the standard model of physics.
In a new experimental campaign called Magnetometry for Neutrino physics (Magneto-ν), scientists at LLNL are searching for the sterile neutrino using nuclear beta decays of plutonium-241. This hypothetical neutrino species is significantly heavier than ordinary neutrino particles and is considered a leading candidate for warm-type dark matter. Initial findings of their study were recently published by the American Physical Society.
When a plutonium-241 atom undergoes beta decay, it transforms into an americium-241 atom, releasing an electron and an anti-neutrino. Different masses of neutrinos can be created in this process; one might have a mass in the kiloelectronvolt range - the scale relevant for warm dark matter.
Detecting these neutrino particles directly is extremely difficult since neutrinos rarely interact with matter. The Magneto-ν experiment takes a different approach: rather than trying to observe the escaping antineutrino itself, researchers precisely measure the energies of the other two decay products - the electron and the recoiling americium-241 atom. Because the total energy released in the decay is known, the energy carried away by the unseen antineutrino can be reconstructed.
The technique allows the team to measure subtle signatures that could indicate the presence of sterile-neutrino dark matter. If a sterile neutrino is emitted during beta decay, it would carry away a specific amount of energy to form its mass, leaving behind a tiny distortion in the measured beta-decay spectrum or in the reconstructed anti-neutrino energy spectrum. Magneto-ν uses a highly sensitive magnetic microcalorimeter to detect this kink in the beta-decay spectrum of plutonium-241.
"If successful, the experiment could help us answer two of science's biggest questions: what dark matter is and how neutrinos have mass," said Geon-Bo Kim, LLNL staff physicist.
Plutonium-241 provides an ideal source because its beta‑decay spectrum aligns with the expected mass range of sterile neutrino as warm dark matter, with an average emitted beta energy of about 5 keV. Magneto-ν pairs this isotope with ultra‑sensitive magnetic microcalorimeters (MMCs) to capture the full decay energy, minus the anti-neutrinos, of plutonium-241 beta decays.
"Since plutonium is a highly controlled nuclear material, few research environments can safely prepare, handle and measure it for fundamental physics," said Kim. "LLNL is uniquely positioned for this work, combining expertise in nuclear materials, radiochemistry, detector development and precision measurement to use plutonium-241 as a tool for exploring frontier physics."
In the initial campaign, energy released from a nuclear beta decay of a plutonium-241 atom will increase the temperature of the MMC detector, which is detected by a superconducting quantum interference device (SQUID). This enables researchers to reconstruct the beta-decay spectrum with exceptional precision and accuracy.
"The detector captures nearly all of the energy released in each decay above threshold with nearly uniform detection efficiency and strong energy linearity, minimizing spectral distortion," said Chang Lee, LLNL staff physicist.
While the initial 10-day experiment did not find evidence of a sterile neutrino, the findings demonstrated the efficacy of the method for fundamental neutrino and dark-matter research using plutonium and reported the most accurate beta-decay energy of plutonium-241 to date. The approach also mitigates key systematic uncertainties that can arise from the detector, the source or the way the spectrum is modeled.
With future large-scale experiments including additional detectors, pixelated sensor arrays and longer measurement periods, Magneto-ν could search for sterile-neutrino signatures in the plutonium-241 beta spectrum more precisely, testing cosmological models that predicted sterile neutrino dark matter.
"A confirmed discovery of this kind would be regarded as a landmark result in modern physics," said Lee.
Other LLNL authors include Xianyi Zhang, Alexander Kavner, Tashi Parsons-Davis, Dongwon Lee, Nathan Hines and Ryan Wood. This work was supported by LLNL's Laboratory Directed Research and Development program.
