Particle physics experiments

Electrons set the stage for neutrino experiments

Neutrinos could hold the key to finally solving the mystery of the origins of our matter-dominated universe, and preparations are underway for two major billion-dollar experiments to unlock the secrets of the particles. Now, a team of nuclear physicists has turned to the humble electron to understand how these experiments can better prepare to capture critical information. Their research, which was conducted at the US Department of Energy’s Thomas Jefferson National Accelerator Facility and recently published in Nature, reveals that major updates to neutrino models are needed for experiments to achieve high-precision results.

Neutrinos are ubiquitous, generated in large numbers by stars in our universe. Although widespread, these shy particles rarely interact with matter, making them very difficult to study.

“There is this phenomenon of neutrinos changing from one type to another, and this phenomenon is called neutrino oscillation. It is interesting to study this phenomenon, because it is not well understood,” said said Mariana Khachatryan, the study’s co-lead author who was a graduate student at Old Dominion University in professor and eminent scholar Larry Weinstein’s research group when she contributed to the research. She is now a Postdoctoral Research Associate at Florida International University.

One way to study neutrino oscillation is to build gigantic ultra-sensitive detectors to measure neutrinos at depth. Detectors typically contain dense materials with large nuclei, so neutrinos are more likely to interact with them. Such interactions trigger a cascade of other particles which are registered by the detectors. Physicists can use this data to unravel information about neutrinos.

“The way neutrino physicists do this is to measure all the particles that arise from the interaction of neutrinos with nuclei and reconstruct the incoming energy of the neutrino to learn more about the neutrino, its oscillations and measure them very, very precisely,” said Adi Ashkenaze. Ashkenazi is the study’s contact author who worked on this project as a researcher in Professor Or Hen’s research group at the Massachusetts Institute of Technology. She is now a lecturer at Tel Aviv University.

“The detectors are made of heavy nuclei, and neutrino interactions with those nuclei are actually very complicated interactions,” Ashkenazi said. “These methods of reconstructing neutrino energy are still very difficult, and it’s our job to improve the models we use to describe them.”

These methods include modeling interactions with a theoretical simulation called GENIE, allowing physicists to infer the energies of incoming neutrinos. GENIE is an amalgamation of many models that each help physicists reproduce certain aspects of the interactions between neutrinos and nuclei. Since so little is known about neutrinos, it is difficult to directly test GENIE to ensure that it will produce both accurate and high-precision results from the new data that will be provided by the future neutrino experiments, such as the Deep Underground Neutrino Experiment (DUNE) or Hyper-Kamiokande.

To test GENIE, the team turned to a humble particle that nuclear physicists are much more familiar with: the electron.

“This exploits the similarities between electrons and neutrinos. We use electron studies to validate neutrino-nucleus interaction models,” Khachatryan said.

Neutrinos and electrons have many things in common. They both belong to the family of subatomic particles called leptons, so they are both elementary particles that are not affected by the strong force.

In this study, the team used an electron-scattering version of GENIE, dubbed e-GENIE, to test the same incoming energy reconstruction algorithms that neutrino researchers will use. Instead of using neutrinos, they used recent results on electrons.

“Electrons have been studied for years, and electron beams have very precise energies,” Ashkenazi said. “We know their energies. And when we try to piece together that incoming energy, we can compare it to what we know. We can test how well our methods work for different energies, which you can’t do with neutrinos.”

Input data for the study came from experiments conducted with the CLAS detector at the Jefferson Lab Continuous Electron Beam Accelerator Facility, a DOE user facility. CEBAF is the world’s most advanced electron accelerator for probing the nature of matter. The team used data that directly reflected the simplest case to study in neutrino experiments: interactions producing an electron and a proton (vs. a muon and a proton) from helium nuclei, carbon and iron. These nuclei are similar to the materials used in the detectors of neutrino experiments.

In addition, the group worked to ensure that the electronic version of GENIE was as parallel as possible to the neutrino version.

“We used the exact same simulation that the neutrino experiments used, and we used the same corrections,” explained Afroditi Papadopoulou, co-lead author of the study and a graduate student at MIT who is also part of the Hen’s research group. “If the model doesn’t work for electrons, where we’re talking about the most simplified case, it will never work for neutrinos.”

Even in this simplest case, accurate modeling is crucial, because the raw data of electron-nucleus interactions are usually reconstructed at the correct energy of the incoming electron beam less than half the time. A good model can account for this effect and correct the data.

However, when GENIE was used to model these data events, performance was even worse.

“This can bias neutrino oscillation results. Our simulations need to be able to reproduce our electron data with its known beam energies before we can be sure it will be accurate in neutrino experiments,” said said Papadopoulou.

Khachatryan agreed.

“The result is actually to highlight that there are aspects of these energy reconstruction methods and models that need to be improved,” Khachatryan said. “It also shows a way to achieve this for future experiments.”

The next step in this research is to test specific target nuclei of interest to neutrino researchers and a broader spectrum of incoming electron energies. Having these specific results available for direct comparison will help neutrino researchers refine their models.

According to the study team, the goal is to achieve broad agreement between the data and the models, which will help ensure that DUNE and Hyper-Kamiokande can achieve the expected high-precision results.