What Are Neutrinos, and How Can Their Mass Be Measured?


Neutrinos may be the oddest of all the primary particles in the cosmos. These strange small packets of energy, sometimes known as "ghost particles," have no electrical charge, nearly no mass, and appear in at least three different kinds. New study is bringing scientists closer than ever to a complete knowledge of neutrinos, from their size to their fundamental features.

Neutrinos are very small particles. They are "hundreds of thousands of times lighter" than the next lightest particle, the electron, with a mass of less than 0.8 electron volt apiece, according to Kathrin Valerius, an astroparticle researcher at Germany's Karlsruhe Institute of Technology.

They're also all over the place. Every second, tens of billions of neutrinos, largely from the sun, flow through your body. They seldom interact with your tissues—or anything else—due to their tiny size and lack of charge. "If one neutrino interacts with you throughout your lifetime, you're lucky," says Los Alamos National Laboratory experimental particle physicist Sowjanya Gollapinni.

Despite the fact that neutrinos have been known for over a century, theoretical physicists still know very little about them. Wolfgang Pauli, a distinguished scientist, was perplexed by an apparently insurmountable predicament in 1930. When watching beta decay, the method by which some radioactive atoms break down, Pauli's colleagues detected an accounting inaccuracy throughout a number of trials. A little part of the energy of the decaying atom had apparently disappeared rather than being released as electrons.

This discovery defied thermodynamics' first rule, which stipulates that energy cannot be generated or destroyed. As a result, Pauli offered a "desperate remedy": a new sort of tiny, chargeless basic particle that was released alongside the electrons and accounted for the energy that was absent. The neutrino concept was born.

In 1956, Pauli's neutral particle was finally proven in an experiment that established its existence but not its size. Neutrinos, according to theory, should have no mass.However, Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen's University in Ontario were awarded the Nobel Prize in Physics in 2015 for their work proving that particles indeed have mass, however it did not specify how much. The Mainz Neutrino Mass Experiment in Germany established the top limit of a neutrino's mass at 2.3 electron volts in the mid-2000s. And data from Germany's Karlsruhe Tritium Neutrino Experiment (KATRIN) in early 2022.

However, even this arrangement is unable to detect the elusive ghost particles directly. Instead, the spectrometer monitors the energy of electrons emitted by radioactive hydrogen as it decays, coupled with neutrinos. These electrons' maximal energy has been thoroughly reported. After the scientists have calculated the entire energy of the experiment, they simply deduct the energy of the electrons; whatever is left goes to the neutrinos.

Researchers are actively working on new experiments to learn more about neutrinos. The Deep Underground Neutrino Experiment, or DUNE, is one of them, and it hopes to learn more about another unknown aspect of neutrinos: how they oscillate or change type.

Neutrinos are classified into three types: electron, muon, and tau. These identities, though, aren't set in stone. "If a neutrino is born with a given flavor, it can transform into other flavors as it travels," says Gollapinni, a member of the DUNE group. "It's like taking on a new persona."

 Some electron neutrinos from the sun, for example, transform into muon and tau neutrinos once they reach Earth. DUNE will track a beam of neutrinos as it travels 800 miles below from the experiment's headquarters at Fermi National Accelerator Laboratory in Batavia, Illinois, to the Sanford Underground Research Laboratory in South Dakota, in order to figure out why and how this transition happens.

Other key cosmological concerns, such as the nature of dark matter (which might just be a fourth, undiscovered flavor of neutrino dubbed a "sterile neutrino"), how black holes arise, or even the genesis of matter itself, are hoped to be addressed by experiments like these. "The KATRIN cooperation has done an excellent job," says Anthony Ezeribe, a particle physicist at the University of Sheffield in England and a DUNE member. "However, there is more work to be done."

Valerius concurs. And, like many other neutrino researchers, she is enthralled by the enormous study possibilities of this little particle. "Our comprehension of the neutrino, or lack thereof," she continues, "is not comprehensive." "We have no idea what we don't know yet."