New Measurements Shed Light on How Neutrinos Change Their Flavor

by johnsmith

Physicists from the MicroBooNE Collaboration at Fermilab have performed a first-of-its-kind measurement: a comprehensive set of the energy-dependent neutrino-argon interaction cross sections.

An illustration of the idea that a neutrino (v) and an electron (e) interact with a nuclei in comparable ways. Image credit: Jefferson Lab.

Neutrinos are tiny subatomic particles that are both famously elusive and tremendously abundant.

While they endlessly bombard every inch of Earth’s surface at nearly the speed of light, neutrinos can travel through a lightyear’s worth of lead without ever disturbing a single atom.

Understanding these mysterious particles could unlock some of the biggest secrets of the Universe.

The MicroBooNE experiment has been collecting data on neutrinos since 2015, partially as a testbed for the Deep Underground Neutrino Experiment (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 neutrino signals as the particles pass through frigid liquid argon kept at minus 186 degrees Celsius (minus 303 degrees Fahrenheit).

Now, the MicroBooNE team has further refined those techniques by measuring the neutrino-argon cross section.

“The neutrino-argon cross section represents how argon nuclei respond to an incident neutrino,” said Dr. Xin Qian, a physicist at Brookhaven National Laboratory.

“Our ultimate goal is to study the properties of neutrinos, but first we need to better understand how neutrinos interact with the material in a detector, such as argon atoms.”

One of the most important neutrino properties is how the particles oscillate between three distinct flavors: muon neutrino, tau neutrino, and electron neutrino.

Physicists know that these oscillations depend on neutrinos’ energy, among other parameters, but that energy is very challenging to estimate.

Not only are neutrino interactions extremely complex in nature, but there is also a large energy spread within every neutrino beam.

Determining the detailed energy-dependent cross sections provides physicists with an essential piece of information to study neutrino oscillations.

“Once we know the cross section, we can reverse the calculation to determine the average neutrino energy, flavor, and oscillation properties from a large number of interactions,” said Dr. Wenqiang Gu, also from Brookhaven National Laboratory.

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

“Previous techniques measured the cross section as a function of variables that are easily reconstructed,” said London Cooper-Troendle, a graduate student at Yale University.

“For example, if you are studying a muon neutrino, you generally see a charged muon coming out of the particle interaction, and this charged muon has well-defined properties like its angle and energy.”

“So, one can measure the cross section as a function of the muon angle or energy.”

“But without a model that can accurately account for missing energy, a term we use to describe additional energy in the neutrino interactions that can’t be attributed to the reconstructed variables, this technique would require experiments to act conservatively.”

The researchers sought to validate the neutrino energy reconstruction process with unprecedented precision, improving theoretical modeling of neutrino interactions as needed for DUNE.

To do so, they applied their expertise and lessons learned from previous work on the MicroBooNE experiment, such as their efforts in reconstructing interactions with different neutrino flavors.

“We added a new constraint to significantly improve the mathematical modeling of neutrino energy reconstruction,” said Dr. Hanyu Wei, a physicist at Louisiana State University.

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

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

The findings were published in the journal Physical Review Letters.


P. Abratenko et al. (MicroBooNE Collaboration). 2022. First Measurement of Energy-Dependent Inclusive Muon Neutrino Charged-Current Cross Sections on Argon with the MicroBooNE Detector. Phys. Rev. Lett 128 (15): 151801; doi: 10.1103/PhysRevLett.128.151801

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