Neutrinos may hold the key to finally answering a conundrum about the origins of our matter-dominated universe, and plans are underway for two huge, multibillion-dollar studies to uncover their secrets.
Now, a group of nuclear physicists has looked to the humble electron for guidance on how to better prepare these experiments to capture vital data. Their research, which was carried out at the Thomas Jefferson National Accelerator Facility of the US Department of Energy and just published in Nature, demonstrates that considerable changes to neutrino models are required for the experiments to yield high-precision results.
Timothy B. Hayward, who got his Ph.D. in physics from William & Mary in 2021, and his doctorate advisor, Keith A. Griffioen, a professor in the university’s Department of Physics, are members of the team and co-authors on the Nature study. Different members of the physics department at William & Mary are involved in a number of other neutrino experiments.
Neutrinos are abundant in our cosmos, as they are produced in large quantities by stars. Despite their widespread presence, these shy particles rarely interact with matter, making research challenging.
“A phenomenon known as neutrino oscillation occurs when neutrinos change from one type to another. Mariana Khachatryan, a co-lead author on the study and a graduate student at Old Dominion University, said, “It’s interesting to study this phenomenon because it’s not well known.”
Building massive, ultra-sensitive detectors to measure neutrinos deep underground is one technique to research neutrino oscillation. Neutrinos are more likely to interact with dense materials with big nuclei, which are common in detectors. As a result of these interactions, the detectors record a cascade of other particles. Physicists, including numerous academic members and students from William & Mary’s physics department, can utilize the data to piece together facts on neutrinos.
“Neutrino physicists do this by measuring all particles that come out of neutrinos’ interactions with nuclei and reconstructing the incoming neutrino energy to learn more about the neutrino, its oscillations, and to measure them very, very precisely,” said Adi Ashkenazi, the study’s contact author and a research scholar at the Massachusetts Institute of Technology. She presently works at Tel Aviv University as a senior lecturer.
“The detectors are made of heavy nuclei, and neutrinos interact with these nuclei in quite intricate ways,” Ashkenazi explained. “Those neutrino energy reconstruction approaches are still difficult, and we’re working to enhance the models that characterize them.”
Modeling the interactions with a theoretical simulation called GENIE, which allows scientists to estimate the energy of the incoming neutrinos, is one of these ways. GENIE is a collection of models that individually aid scientists in reproducing specific elements of neutrino-nuclei interactions. Because so little is known about neutrinos, it’s difficult to test GENIE directly to assure it will deliver accurate and high-precision results from new data provided by future neutrino experiments like DUNE or Hyper-Kamiokande.
To put GENIE to the test, the team used a simple particle that nuclear physicists are far more familiar with: the electron.
“The commonalities between electrons and neutrinos are used here.” “Electron investigations are being used to validate neutrino-nucleus interaction hypotheses,” Khachatryan explained.
Neutrinos and electrons have a lot of characteristics. They’re both leptons, which means they’re elementary particles that aren’t affected by the strong force, which is one of the four known fundamental interactions. The others are electromagnetism, the weak force, and gravity.
The scientists tested the identical incoming energy reconstruction techniques that neutrino researchers would employ in this work using an electron-scattering variant of GENIE, dubbed e-GENIE. They employed new electron data instead of neutrinos.
“Electrons have been researched for years, and electron beams have very accurate energies,” Ashkenazi explained. “We are aware of their energy. And we can compare it to what we know when we’re trying to rebuild that incoming energy. We can test how well our approaches function at different energy, which is something neutrinos can’t do.”
Experiments with the CLAS detector at Jefferson Lab’s Continuous Electron Beam Accelerator Facility, a DOE user facility, provided the study’s input data. CEBAF is the most advanced electron accelerator in the world for studying the nature of matter. The researchers used data that directly replicated the most basic situation addressed in neutrino experiments: interactions between helium, carbon, and iron nuclei that produced an electron and a proton (rather than a muon and a proton). The materials employed in neutrino experiment detectors are similar to these nuclei.
Furthermore, the team attempted to guarantee that the electron version of GENIE was as similar to the neutrino version as possible.
“We utilized the exact same simulation as neutrino experiments, and we used the same adjustments,” said Afroditi Papadopoulou, the study’s co-lead author and a graduate student in Hen’s research group at MIT. “The model will never work for neutrinos if it doesn’t work for electrons, which is the most simplified scenario.”
Even in this simplest scenario, good modeling is essential since raw data from electron-nucleus interactions only half of the time reconstructs to the right incoming electron beam energy. This effect can be accounted for and the data corrected with a decent model.
GENIE, on the other hand, did significantly worse when used to model these data events.
“The neutrino oscillation results may be skewed as a result of this.” Before we can trust our models to be correct in neutrino experiments, they must be able to duplicate our electron data with known beam intensities,” Papadopoulou added.
Khachatryan was in agreement.
“The outcome is that there are components of these energy reconstruction approaches and models that need to be improved,” Khachatryan explained. “It also demonstrates a method for achieving this in future studies.”
The next stage in this research will be to test specific target nuclei that neutrino researchers are interested in, as well as a wider range of incoming electron energies. The ability to compare these precise results will aid neutrino researchers in fine-tuning their models.
The goal, according to the research team, is to create broad agreement between data and models, which will assist DUNE and Hyper-Kamiokande achieve the high-precision results they predict.