From “Symmetry”: “Why aren’t neutrinos adding up?”

Symmetry Mag

From “Symmetry”

8.4.22
Mara Johnson-Groh

Physicists take on the mystery of the missing (and extra) neutrinos.

Of all the known elementary particles, neutrinos probably give physicists the most headaches.

These tiny fundamental bits of matter are the second most common particle in the universe yet are anything but ordinary. Since their discovery, they have taunted scientists with bizarre behaviors, some of which physicists have yet to comprehend.

One source of confusion has showed up in the results from short-distance neutrino experiments, in which neutrinos are measured after traveling somewhere between a few meters and a kilometer. When scientists measure neutrinos in these experiments, the results don’t always match their predictions. Sometimes there are too many of certain types of neutrinos, while in others there are too few.

This mismatch between experiments and predictions has opened a whole subfield in the study of neutrinos since it was first identified in the early 2000s.

While the answer to the mystery could provide physicists with a better understanding of neutrinos, it might also reveal new insights into the fundamental workings of the universe.

Short-baseline anomalies

At the heart of the short-distance miscounts are so-called short-baseline neutrino experiments.

Such experiments typically have a well-understood source or a beam of neutrinos in one location and, some distance away, a detector that can identify one or more of the three different known types of neutrinos—electron neutrino, muon neutrino, and tau neutrino. These experiments look to see if what interacts with the detector is what scientists expect, based on what they know about the neutrinos coming from the source.

This should be straightforward, but unlike most other particles, neutrinos are shape-shifters. Instead of being one thing their whole lives, neutrinos change their type—or “flavour,” as physicists say—as they travel. Similar to how photons travel as waves but interact as particles, each neutrino travels as a probabilistic mix of the three different flavours. Only when it interacts does it settle on a single one. Physicists call this “neutrino oscillation”.

“A neutrino particle doesn’t just have one flavour, and the chance you’ll see it as a certain flavour comes down to probability,” says Zara Bagdasarian, an assistant project scientist at the University of California-Berzerkeley. “It is essentially a quantum phenomenon.”

Of the three different neutrinos, each has a different probability of interacting as each of the three flavours. Additionally, each has a unique mass, so it travels at its own speed. In the end, this means each flavour has a greater likelihood of showing up at some distances than others. The theoretical framework that describes neutrino oscillations tells physicists how many neutrinos of each flavour should show up at different distances.

Over long distances, neutrinos have sufficient time to change flavours—and this is well supported by experiments that study neutrinos traveling to Earth from the sun and experiments that analyze neutrino beams sent halfway across a continent. Over short distances, neutrinos don’t have as much time to oscillate and shift to a different flavour.

But time after time in these short-baseline experiments, including experiments at beam lines and at nuclear reactors, predictions seem to be wrong. In some experiments, too many electron neutrinos appear, while in others, too few show up. These counting mismatches are called short-baseline anomalies.

In the two decades since the anomalies were first discovered, scientists have come up with several guesses about what might cause discrepancies. To test the merits of these ideas, they are working on several ongoing and upcoming experiments.

“At this point there’s a plethora of guesses,” says Georgia Karagiorgi, associate professor of physics at Columbia University. “However, there’s not a clear best guess because no single model can explain all anomalies simultaneously.”

See the full article here .


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Symmetry is a joint Fermilab/SLAC publication.