Neutrinos have been with us since the beginning. They existed alongside prehistoric humans, dinosaurs, and the first scattered crumbs of life on Earth. The birth of the solar system, the formation of the cosmic scaffolding of the universe, the moments after the Big Bang – all of this was awash with the light subatomic particles we call neutrinos.
But it’s only in the last 70 years that we’ve had any certainty that they’re there. In 1956, physicists Clyde Cowan and Frederick Reines revealed the existence of these particles.
And they exist. Nor in sweet penury. It is the most abundant massive particle in the universe, outnumbering protons by about a billion to one. Scientists are still grappling with the meaning of these particles. And neutrinos are not just one type of particle either, but a trio of particles – electron neutrinos, muon neutrinos, and tau neutrinos – and their corresponding antimatter particles, all of which physicists collectively call neutrinos.
Other particles have their unknowns, but “for neutrinos, the list of questions is deeper and more fundamental than for anything else,” says neutrino physicist Diana Parno of Carnegie Mellon University in Pittsburgh. We do not know if the particles are their own antiparticlesor if other types of neutrinos hide. Some scientists wonder if neutrinos could explain why the universe is filled with matter and contains just a small amount of antimatter.
Perhaps the most glaring is that we don’t know the mass of the particles. We know that their masses must be incredibly small, but not zero. This makes them sneakily difficult to measure.
To make things even more complicated, neutrinos have no electrical charge and interact with other matter via a wimpy effect called weak interaction. This forced Reines and Cowan to concoct inventive techniques to spot them. Their work set a precedent: to study the neutrino, ingenuity is essential.
How neutrino hunters succeeded
In 1930, physicist Wolfgang Pauli proposed the existence of neutrinos to explain the energies of electrons emitted during radioactive decays. In these decays, called beta decays, one nucleus transforms into another, emitting an electron. The conversion releases a fixed amount of energy. If only the electron were emitted, a given decay would be expected to produce electrons with a specific energy. Instead, electrons were observed with a range of energies. The situation was so desperate that some physicists considered abandoning the concept of conservation of energy, a fundamental pillar of physics. Instead, Pauli proposed that a particle without an electrical charge would also be released, carrying a varying amount of energy. He was quoted as saying: “I did a terrible thing, I hypothesized about a particle that cannot be detected. »
Pauli was wrong, but the particles eluded detection for a respectable 25 years. The Reines-Cowan experiment took place in a nuclear reactor. Since many radioactive decays occur in nuclear reactors, these would be a powerful source of neutrinos if the particles existed. (More precisely, these particles would be antineutrinos.) The experiment, carried out at the Savannah River plant in South Carolina, was much more practical than the original plan: dropping a detector into an underground shaft padded with feathers and foam rubber while simultaneously setting off a nearby atomic bomb.
The trick to performing the experiment in a reactor was to measure two signals back to back. When an antineutrino interacted with a proton in the detector, it produced a neutron and a positron – the antimatter equivalent of an electron. The positron would quickly annihilate with an electron, releasing high-energy light called gamma rays that could be detected in a liquid called a scintillator, which lights up in response to the radiation. The neutron wandered around for a bit before being captured by a nucleus, which released more gamma rays and caused a delayed flash in the scintillator.
Cowan and Reines’ detector was constructed as a club sandwich, with three layers of liquid scintillator detectors separated by two layers of target material. The target contained water and cadmium chloride, the latter chosen for its ability to capture stray neutrons. A double flash, produced in adjacent layers of the detector, was a characteristic of the antineutrino, the concluding lub-dub of its figurative heartbeat.
Without this heartbeat to filter out spurious events, Reines and Cowan would not have been able to detect antineutrinos in a reactor. This creative solution to a problem once considered intractable earned Reines the Nobel Prize in Physics in 1995 (Cowan died in 1974.)
Since then, scientists have detected neutrinos using Antarctic ice sheetTHE Mediterranean Sea And in-depth experiences. Scientists have spotted neutrinos produced in the sundeep within the Earth, in the atmosphere and in space — including a star exploding in a nearby galaxy. The experiments revealed that particles oscillateor move from one type to another. This phenomenon can only occur if the neutrinos have mass, but this does not reveal their mass.
New techniques aim to reveal the neutrino
The discovery that neutrinos have mass means they conflict with physicists’ theory of particle physics, the Standard Model. The basic theory assumes that neutrinos have no mass. Neutrinos are therefore a confusion.
“Neutrinos bring something else, outside of the Standard Model, and we’re trying to understand what that is,” says physicist Enectali Figueroa-Feliciano of Northwestern University. “We want to measure neutrinos in every way possible, because they don’t always do what we expect them to do.”
Physicists therefore continue to push detectors further. In 2017, scientists spotted neutrinos interact with an entire core at a timerather than an individual proton or neutron, for the first time. Such reactions are more common than interactions with protons or neutrons, but detecting the slight shift of the neutrino nucleus requires very sensitive sensors. The detectors detect flashes of light generated by the recoil of nuclei in crystals. These detections involved laboratory neutrino sources, but scientists also spotted this process initiated by the lower energy antineutrinos from nuclear reactorsresearchers reported last summer in Nature. This opens up the possibility of using detector technology to monitor nuclear reactors for weapons development.
To measure neutrino-nucleus interactions even more precisely, Figueroa-Feliciano aims to use a transition edge sensor – essentially an extremely sensitive thermometer – to detect the heat generated by receding nuclei. If successful, this approach would allow scientists to test the standard model in a new way.
Another team uses transition sensors to try to obtain neutrino masses. The HOLMES experiment in Italy uses transition sensors incorporating the radioactive element holmium-163. When holmium decays, it transforms into another element and emits a neutrino. The escaping neutrino causes the nucleus to recoil. Measuring these recoils can provide insight into the mass of neutrinos. This technique set a ceiling on neutrino massHOLMES researchers reported last fall in Physical Examination Lettersalthough it does not yet outperform other methods of constraining neutrino masses.
Yale physicist David Moore has another plan for measuring recoil, with nanospheres festooned with radioactive elements and levitated using a laser beam. Observing the movement of nanoparticles as they recoil after radioactive decay could reveal whether there are heavier neutrinos lurking and perhaps one day determine the mass of known neutrinos. In 2024, Moore and his colleagues demonstrated a proof of principle by measuring the recoil produced by a radioactive decay that emits an alpha particlethe nucleus of a helium atom. Neutrinos are the next step, Moore says.
The mass of neutrinos is not only of theoretical interest. “It’s important to know the mass for its own sake, but it’s also important because the mass of neutrinos is important for cosmology,” says physicist Matteo Borghesi of the University of Milan-Bicocca, who works on HOLMES. Neutrino masses helped shape the structure of galaxies. Scientists can use this fact to try to determine the maximum possible neutrino mass by observing galaxies in space. But the questions revolve around these figuresAlso. There seems to be a tension between what field experiments discover and what scientists estimate based on the cosmos.
Cunning might seem to be part of the nature of neutrinos. But the difficulty in understanding them is mainly due to their tiny masses and their weak interactions. “It’s not like the neutrino is sitting there thinking, ‘Okay, what can I do next to these physicists?'” Parno says, folding his hands and wiggling his fingers in pretend nefariousness.
But the particles seem destined to get under the skin of physicists. It’s a bit like the Mad Hatter’s riddles. What is crucial to the structure of the universe but also imperceptible? How can we know that a particle has mass without knowing its mass?
How to detect an undetectable particle?
At least Reines and Cowan beat the Mad Hatter on that one.
