Scientists announce first-of-its-kind neutrino measurement

For the first time, physicists have extracted the “energy-dependent neutrino-argon interaction cross section”, a key value for studying how neutrinos change flavor

Physicists studying ghost particles called neutrinos from the international MicroBooNE collaboration have reported a one-of-a-kind measurement: a comprehensive set of energy-dependent neutrino-argon interaction cross sections. This step marks an important step towards achieving the scientific goals of the next generation of neutrino experiments, namely the Deep Underground Neutrino Experiment (DUNE).

Neutrinos are tiny subatomic particles that are both elusive and extremely abundant. As they relentlessly bombard every inch of the Earth’s surface at nearly the speed of light, neutrinos can travel a light-year of lead without ever disturbing a single atom. Understanding these mysterious particles could reveal some of the universe’s greatest secrets.

The MicroBooNE experiment, located at the US Department of Energy’s (DOE) Fermi National Accelerator Laboratory, has been collecting neutrino data since 2015, in part as a test bed for DUNE, which is currently under construction. To identify elusive neutrinos, both experiments use a Low-Noise Liquid Argon Time Projection Chamber (LArTPC), a sophisticated detector that captures signals from neutrinos as the particles pass through freezing liquid argon held at -303 degrees. Fahrenheit. MicroBooNE physicists have refined LArTPC techniques for large-scale detectors at DUNE.

Today, a team effort led by scientists at the DOE’s Brookhaven National Laboratory, in collaboration with researchers from Yale University and Louisiana State University, further refined these techniques by measuring the neutrino-argon cross section. Their work was published on April 12, 2022 in Physical examination letters.

Neutrino-argon interaction

A close view of an argon muon neutrino interaction in an event display at MicroBooNE, one of 11,528 events used to extract energy dependent argon neutrino muon interaction cross sections. Credit: Brookhaven National Laboratory

“The neutrino-argon cross section represents how argon nuclei react to an incident neutrino, such as those in the neutrino beam produced by MicroBooNE or DUNE,” said Brookhaven Lab physicist Xin Qian, head of the physics group MicroBooNE from Brookhaven. “Our ultimate goal is to study the properties of neutrinos, but first we need to better understand how neutrinos interact with a detector’s material, such as argon atoms.”

One of the most important properties of neutrinos that DUNE will study is how the particles oscillate between three distinct “flavors”: the muon neutrino, the tau neutrino and the electron neutrino. Scientists know that these oscillations depend on the energy of neutrinos, among other parameters, but this energy is very difficult to estimate. Not only are neutrino interactions extremely complex in nature, but there is also a great spread of energy in each neutrino beam. The determination of detailed energy-dependent cross sections provides physicists with essential information for studying neutrino oscillations.

“Once we know the cross section, we can reverse the calculation to determine the energy, flavor, and average neutrino oscillation properties from a large number of interactions,” said Wenqiang Gu, postdoc at Brookhaven Lab, which led the physical analysis.

To do this, the team developed a new technique to extract the detailed energy-dependent cross section.

“Previous techniques measured cross-section as a function of easily reconstructed variables,” said London Cooper-Troendle, a graduate student from Yale University who is stationed at the Brookhaven lab as part of the student research program. DOE graduates. “For example, if you study a muon neutrino, you usually see a charged muon emerging from the interaction of the particles, and this charged muon has well-defined properties like its angle and its energy. Thus, one can measure the cross section as a function of the angle or the energy of the muon. But without a model that can accurately explain “missing energy,” a term we use to describe the extra energy in neutrino interactions that cannot be attributed to the reconstructed variables, this technique would require experiments to act on conservative way.

The Brookhaven-led research team sought to validate the neutrino energy reconstruction process with unprecedented accuracy, improving the theoretical modeling of neutrino interactions as required by DUNE. To do this, the team applied their expertise and lessons learned from previous work on the MicroBooNE experiment, such as their efforts to reconstruct interactions with different flavors of neutrinos.

“We added a new constraint to significantly improve the mathematical modeling of neutrino energy reconstruction,” said Hanyu Wei, an assistant professor at Louisiana State University, formerly a Goldhaber Fellow at Brookhaven.

The team validated this newly constrained model against experimental data to produce the first detailed measurement of the energy-dependent neutrino-argon cross section.

“The neutrino-argon cross section results from this analysis are able to distinguish between different theoretical models for the first time,” Gu said.

While physicists expect DUNE to produce improved cross-section measurements, the methods developed by the MicroBooNE collaboration provide a foundation for future analyses. The current measurement of the cross section is already defined to guide further developments on the theoretical models.

In the meantime, the MicroBooNE team will focus on improving its cross section measurement. The current measurement was made in one dimension, but future research will address value in multiple dimensions, i.e. as a function of multiple variables, and explore more avenues of underlying physics.

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DOI: 10.1103/PhysRevLett.128.151801

This work was supported by the DOE Office of Science.

Brookhaven National Laboratory is supported by the U.S. Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges of our time.

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