Particle physics experiments

Electrons set the stage for neutrino experiments – Solving the mystery of the origins of our matter-dominated universe


Neutrinos interacting with nuclei. Credit: DOE’s Jefferson Lab

Early-career nuclear physicists show that a better understanding of how neutrinos interact with matter is needed to get the most out of future experiments.

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

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

“There’s this phenomenon of neutrinos changing from type to type, and this phenomenon is called neutrino oscillation. It is interesting to study this phenomenon because it is not well understood, ”said Mariana Khachatryan, co-lead author of the study who was a graduate student at Old Dominion University in the professor’s research group and prominent researcher Larry Weinstein when she helped research it. 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 deep underground. Detectors usually contain dense material with large nuclei, so neutrinos are more likely to interact with them. Such interactions trigger a cascade of other particles which are recorded by the detectors. Physicists can use this data to extract information about neutrinos.

“The way neutrino physicists do it is to measure all the particles that arise from the interaction of neutrinos with nuclei and reconstruct the energy of the incoming neutrinos to learn more about the neutrino, its oscillations, and to measure them. very, very precisely, “Adi Ashkenazi explained. 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.

“Detectors are made of heavy nuclei, and the interactions of neutrinos with these nuclei are actually very complicated interactions,” Ashkenazi said. “These methods of energetically reconstructing neutrinos are always very difficult, and it is 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 deduce the energies of incoming neutrinos. GENIE is an amalgamation of many models, each of which helps physicists reproduce certain aspects of the interactions between neutrinos and nuclei. Since so little is known about neutrinos, it is difficult to test GENIE directly to ensure that it will produce both precise and high-precision results from the new data that will be provided by future neutrino experiments. , such as the Deep Underground Neutrino (DUNE) or Hyper-Kamiokande experience.

To test GENIE, the team turned to a humble particle that nuclear physicists know much better: the electron.

“It exploits the similarities between electrons and neutrinos. We are using electron studies to validate models of neutrino-nucleus interaction, ”said Khachatryan.

Neutrinos and electrons have a lot in common. They both belong to the family of subatomic particles called leptons, so they are both elementary particles that are unaffected 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 the neutrino researchers will use. Instead of using neutrinos, they used recent electronic results.

“Electrons have been studied for years, and electron beams have very precise energies,” Ashkenazi said. “We know their energies. And when we try to rebuild this incoming energy, we can compare it to what we know. We can test the effectiveness of our methods for different energies, which you cannot do with neutrinos.

Input data for the study comes 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 the neutrino experiments: interactions that produced an electron and a proton (against a muon and a proton) from helium nuclei, of carbon and iron. These nuclei are similar to materials used in experimental neutrino detectors.

Additionally, 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 was used by the neutrino experiments, and we used the same corrections,” said Afroditi Papadopoulou, co-lead author of the study and graduate student at MIT which is also part of the research group of Hen. “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, precise modeling is crucial, as 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 capture this effect and correct the data.

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

“It can skew the results of the neutrino oscillation. Our simulations need to be able to reproduce our electron data with its known beam energies before we can be confident that they will be accurate in neutrino experiments, ”Papadopoulou said.

Khachatrian accepted.

“The result is actually to highlight that some aspects of these energy reconstruction methods and models 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 at 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.

Reference: “Electron Beam Energy Reconstruction for Neutrino Oscillation Measurements” by M. Khachatryan, A. Papadopoulou, A. Ashkenazi, F. Hauenstein, A. Nambrath, A. Hrnjic, LB Weinstein, O. Hen, E. Piasetzky, M. Betancourt, S. Dytman, K. Mahn, P. Coloma, the CLAS Collaboration and the e4ν Collaboration, November 24, 2021, Nature.
DOI: 10.1038 / s41586-021-04046-5