Neutrinos are a perplexing blend of three masses and three tastes. Scientists want to count every single one of them.

Travel is definitely life-changing for a neutrino. One of the small particles may arrive in a completely different state from when it began its 500-mile journey from Fermilab’s neutrino source to the NOvA experiment’s detector in Minnesota. The particles, which whiz through most stuff with no interaction, can oscillate from one of three known neutrino kinds to another.

A flying neutrino is in multiple states at the same time due to quantum mechanics. This is due to a property known as mixing, which, while it may sound obscure, is crucial for some of the universe’s most critical reactions—and researching it may hold the key to solving one of particle physics’ biggest mysteries.

Though mixing can occur with a variety of particles, physicists are particularly interested in lepton mixing, which occurs in only one form of lepton, the elusive neutrino. Neutrinos come in three flavors: electron, muon, and tau, as well as three mass kinds, or mass states. However, unlike objects in our everyday lives, where an apple is always heavier than a grape, neutrino mass states and tastes are not proportional.

Kevin McFarland, a physics professor at Rochester University and co-spokesperson for the MINERvA neutrino experiment at the Department of Energy’s Fermilab, explains, “When we say there’s mixing between the masses and the flavors, what we mean is that the electron flavor is not only one mass of neutrino.”

A neutrino is a proportion of all three distinct mass states at any one time, adding up to 1. Some flavors and mass states have more in common than others. Scientists call neutrinos in their mass eigenstates when they are in a condition of defined mass. The term “mixing angle” is used by physicists to characterize this overlap. A modest mixing angle denotes less overlap, whereas a maximum mixing angle denotes the parameters being combined as equally as possible.

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Mixing angles have constant values, and physicists are baffled as to why those values exist in nature.

Patrick Huber, a theoretical physicist at Virginia Tech, adds, “This is given by nature.” “We’d like to know why these figures are what they are. There are hypotheses floating about to try to explain them, but we have no idea where they come from.”

Physicists will need massive experiments in which they can regulate the generation of neutrinos and observe their interactions in a detector to find out. Antineutrinos created by nuclear power plants, which yield tens of megawatts of power in antineutrinos, were studied in the Daya Bay experiment in China in 2011. That’s a huge number; in comparison, neutrino beams made in laboratories are in the kilowatt range. Scientists working there locked down one of the mixing angles, known as theta13, just a year later (pronounced theta one three).

The finding was significant since it confirmed that all mixing angles are greater than zero. This feature is required for physicists to use neutrino mixing as a probe for one of the universe’s greatest mysteries: why is there even any matter at all.

The Standard Model of cosmology predicts that the Big Bang produced equal amounts of matter and antimatter. Because they annihilate each other when they come into touch, the fact that matter exists at all indicates that the balance tipped in favor of matter. This breaks the charge-parity symmetry, or CP symmetry, rule.

Looking for instances where a matter particle behaves differently than its antimatter counterpart is one technique to examine CP violation. In neutrino mixing, physicists are hunting for a precise value in a mixing parameter known as a complex phase that would indicate CP violation in neutrinos. And the outcome at Daya Bay prepared the way.

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The Daya Bay collaboration’s spokeswoman, Kam Biu-Luk, adds, “Now we know, OK, we have a nonzero value for all mixing angles.” “As a result, we know we have a possibility to create a new experiment to investigate CP violation.”

Daya Bay data, as well as data from active neutrino experiments like NOvA at Fermilab and T2K in Japan, will be utilized to help decipher data from the future international Deep Underground Neutrino Experiment (DUNE). The particles will be sent on an 800-mile journey into enormous detectors loaded with 70,000 tons of liquid argon, making this the largest accelerator-based neutrino experiment ever. The experiment’s goal is to produce exact data regarding the complex phase, which will explain the process that permitted matter to flourish.

“Neutrino oscillation is new physics in a sense,” Huber adds, “but now we’re looking for new physics inside of that.” “We’ll be able to test for these extra things beyond oscillations in a precise experiment like DUNE.”

The mixing of particles is not limited to neutrinos. Quarks, which are building blocks, also have this feature.

Physicists aren’t sure if mixing is a quality that all particles have. But, based on what they’ve learned so far, mixing is critical to the universe’s operation.

“There are all kinds of critical processes in the universe that wouldn’t happen without this mixing, without these reactions,” McFarland adds. “It appears that nature enjoys it when that happens. And we have no idea why.”

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