Neutrinos are discovered by observing the particle showers they produce when they collide with nuclei, but recent research utilizing electrons instead of neutrinos demonstrates that the models used to reconstruct the energy of the incoming neutrinos from these showers are frequently incorrect. The work, according to the researchers, highlights well-known gaps in the theory of neutrino-nucleus interactions, and that improving this theory is essential if next-generation neutrino detectors like the Deep Underground Neutrino Experiment (DUNE) in the United States and Hyper-Kamiokande in Japan are to reach their full potential.

The research of neutrino oscillations continues to yield fascinating indications about physics beyond the Standard Model. Although the Standard Model did not predict that neutrinos would have mass at first, both accelerator experiments and astronomical observations have shown that neutrinos can change flavor between electron, muon, and tau neutrinos as they propagate. This neutrino oscillation is only feasible if neutrinos have mass, which means the Standard Model has to be changed.

Characterizing neutrino oscillations is thus one of physics’ top concerns right now. The Massachusetts Institute of Technology’s Or Hen is a good example. “If there is a significant breach of charge-parity symmetry in how neutrinos and antineutrinos interact, then that can explain why we live in a matter-dominated universe under certain assumptions,” he says. “We definitely need to see if neutrinos and antineutrinos oscillate at various frequencies.”

Reconstruction of energy

Because the speeds at which neutrinos oscillate are determined by their energy, it is necessary to understand this in order to describe neutrino oscillations and look for any irregularities. Because of their incredibly weak interaction with matter, neutrinos are famously difficult to detect. Individual experiments vary in detail, but they all use a massive volume of matter surrounded by sensors (Hyper-Kamiokande will use water; DUNE will use liquid argon). The sensors gather up expelled particles and reconstruct the energy of the incident neutrino when a neutrino interacts with a nucleus in matter.

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One issue is that understanding how neutrinos interact with atomic nuclei is required. “It’s like staring at fireworks in the sky and trying to figure out the energy that started the explosion just by looking at all the lovely colors,” Adi Ashkenazi of Tel Aviv University in Israel explains. “There are a lot of open parameters in the simulation.”

Neutrin beams are typically used to calibrate individual detectors. However, because neutrinos are only produced via particle decay, which is essentially random, it is impossible to create a mono-energetic neutrino beam. It is therefore impossible to calibrate the incoming flux at a detector at each energy in the detection bandwidth. The e4v (electrons for neutrinos) team, on the other hand, studied how nuclei interact with electrons in a simple and surprising way. Although the fundamental physics of the electron-nucleus interaction differs from that of the neutrino-nucleus interaction, Hen claims that the major challenges in simulations emerge from interactions between protons and neutrons in the nucleus, which are the same in both cases.

Neutrinos in a simplified form

“In essence, electrons are a simplified version of neutrinos,” Hen explains, “therefore anything you believe you know about neutrinos is false if the same model can’t explain electron evidence.”

The e4 researchers teamed up with the CLAS Collaboration, based in Virginia (both groups had members named Hen and Ashkenazi), to examine scattering data from 1999, in which electrons of known energies were scattered off carbon, helium, or iron objects. They chose a subset of these where the scattering was quite simple, with only one observed electron and one detected proton. The researchers examine electron interactions as if they were neutrinos in a publication published in Nature, using typical simulations to recreate the energy of the incident particle. Only 30-40% of the simulations for carbon estimate the energy to be within 5% of the real beam energy. The proportion of iron is about 20-25 percent.

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“You shouldn’t be shocked that the models don’t match very well with the facts,” says Eric Zimmerman of the University of Colorado Boulder. “There’s been a large variety of neutrino interaction models generated, and their predictions have had quite a bit of fluctuation…

The main benefit of this effort, in my opinion, is that this dataset will almost certainly be used to improve the models, if it hasn’t already.”

“This should come as no surprise to anyone who is paying attention,” says Daniel Cherdack of the University of Houston in Texas. “The real question is who is paying attention and why?” Because model uncertainties have been included into the error calculations, both Zimmerman and Cherdack think that current detector results are reliable. However, according to Cherdack, these uncertainties will have to shrink if larger detectors are to detect smaller effects. “This is an essential paper because it highlights a component of neutrino physics’ ditch-digging that doesn’t get much attention, and the fact that Nature is focusing on it is really significant since this is one of the things we need to figure out to make DUNE a successful experiment.”

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