The neutrino’s quantum ambiguity is beginning to come into focus

“The quantum properties of neutrino matter are a bit of the Wild West at the moment,” says nuclear physicist Kyle Leach of the Colorado School of Mines in Golden. “We’re still trying to figure it out.”

It is impossible to know everything about a quantum particle. Heisenberg’s uncertainty principle famously states that it is futile to try to determine precisely both the momentum of a quantum object and its position (SN: 1/12/22). Now, Leach and colleagues report new details about the size of the neutrino wave packet, which indicates the uncertainty in the particle’s position.

Quantum particles travel as waves, with ripples related to the probability of finding a particle in a given location. A wave packet is the set of waves corresponding to a single particle. The new experiment puts a limit on the wave packet size for neutrinos produced in a particular type of radioactive decay, Leach’s team reports in a paper submitted April 3 to arXiv.org. The particles have a wave packet size of at least 6.2 trillionths of a meter.

The researchers studied the neutrinos produced in the decay of beryllium-7, through a process called electron capture. In this process, a beryllium-7 nucleus absorbs an electron, and the atom turns into lithium-7 and emits a neutrino.

The team implanted beryllium-7 atoms into a highly sensitive device made of five layers of material, including superconducting tantalum, which can transmit electricity without resistance. In the decay, the newly produced lithium-7 is attracted by the neutrino. When cooled to 0.1 degrees above absolute zero (–273.05 ° Celsius), the device allowed the researchers to detect the energy of that return. The energy spread of lithium atoms revealed the minimum size of the neutrino wave packet.

Neutrinos are special in that they interact so rarely with matter that they retain their quantum properties over long distances. Most quantum effects occur on very small scales, but neutrino oscillations occur over thousands of kilometers.

So studying the size of neutrino wave packets could help reveal the connection between the everyday world of classical physics and the weirdness of quantum physics, says Benjamin Jones, a neutrino physicist at the University of Texas at Arlington, who was not included in the experiment. “If you can predict something like this and then measure it, then you really validate some of the ideas that people have about how the classical world emerges from an underlying quantum reality,” he says. “And that’s what really got me excited about it in the first place.”

In another study, submitted April 30 to arXiv.org, Jones and his colleagues theoretically predicted the size of the neutrino wave packet, pegging it at about 2.7 billionths of a meter. Now it’s up to experimental physicists to try to measure it, not just determine its minimum size.

Measuring the size of neutrino wave packets could help resolve discrepancies between past experiments and potentially point the way to new types of subatomic particles that have yet to be discovered. But the size of the neutrino wave packet depends on how the particle is produced. So it’s not clear how the size limit observed in Leach’s study might translate to neutrinos produced by other tools, says neutrino physicist Carlos Argüelles of Harvard University. For example, many experiments observe neutrinos from nuclear reactors, but they are produced by a different type of radioactive decay.

Still, says Argüelles, “the study of the neutrino wave packet has fundamental implications in neutrino quantization, and neutrino quantization is actually what makes neutrinos interesting. It’s the most unique property they have.”