‘Ghostly’ neutrinos provide new path to study protons

Neutrinos are one of the most abundant particles in the universe, but they are notoriously difficult to detect and study. It has no electric charge and almost no mass. They are often called “ghost particles” because they interact very little with atoms.

But they are so abundant that they play a big role in helping scientists answer fundamental questions about the universe.

In a landmark study described in Nature— Led by researchers at the University of Rochester — International collaboration MINERvA scientists used a beam of neutrinos at the Fermi National Accelerator Laboratory (Fermilab) for the first time to investigate the structure of protons.

MINERvA was an experiment to study neutrinos, researchers did not set out to study protons. But their feat, once thought impossible, gives scientists a new way of looking at the tiny building blocks of the nucleus.

“When I was studying neutrinos as part of the MINERvA experiment, I realized that the techniques I was using could be applied to proton research,” said Tejin Cai, the paper’s first author. I’m here. Kai, now a postdoctoral fellow at the University of York, did his research as a Ph.D. He is a professor of physics to Dr. Stephen Chu in Rochester and a student of Kevin McFarland, a key member of the university’s neutrino group.

“I wasn’t sure if it would work at first, but I eventually discovered that I could use neutrinos to measure the size and shape of the protons that make up the nucleus. It’s like using a ghost ruler to make measurements.”

Proton measurement by particle beam

Atoms, and the protons and neutrons that make up the nucleus, are so tiny that researchers have difficulty measuring them directly. Instead, it creates an image of the shape and structure of the atomic building blocks by bombarding the atoms with a beam of high-energy particles. We then measure the distance and angle at which the particles bounce off the atomic building block.

For example, let’s say you throw a marble into a box. The marbles bounce off the box at certain angles, allowing you to locate where the box was and identify its size and shape, even if you can’t see the box.

“It’s a very indirect way of measuring something, but it allows you to relate the structure of an object (a proton in this case) to the number of deflections seen at different angles,” McFarland says. .

What can neutrino beams tell us?

Researchers first measured the size of protons in the 1950s using an accelerator with an electron beam at the Linear Accelerator Facility at Stanford University. But instead of using beams of accelerated electrons, a new technique developed by Cai, McFarland, and their colleagues uses beams of neutrinos.

Although the new technology doesn’t produce a sharper image than the old one, McFarland says it could give scientists new information about how neutrinos and protons interact. Information that can currently only be inferred using theoretical calculations or a combination of theory and other measurements.

Comparing the new technology to the old one, McFarland likens the process to viewing a flower in normal visible light and then in ultraviolet light.

“You’re looking at the same flower, but you can see different structures under different kinds of light,” McFarland says. It offers us a different perspective.”

Specifically, we would like to use this technique to separate the effects associated with neutrino scattering on protons from the effects associated with neutrino scattering on atomic nuclei, which are the coupled assemblies of protons and neutrons.

“All previous methods for predicting neutrino scattering from protons used theoretical calculations, but this result directly measures that scattering,” says Kai.

“Using the new measurements to improve our understanding of these nuclear effects will allow us to better perform future measurements of neutrino properties,” McFarland adds.

Technical challenges of neutrino experiments

Neutrinos are produced when atomic nuclei combine or split. The sun is a major source of neutrinos, a byproduct of nuclear fusion in the sun. For example, if you are standing in sunlight, trillions of neutrinos pass harmlessly through your body every second.

Neutrinos are more abundant in the universe than electrons, but it is difficult for scientists to exploit neutrinos in large quantities experimentally. Neutrinos pass through matter like ghosts, but electrons interact with matter much more often.

“On average, out of the trillions of neutrinos that pass through the body every second over the course of a year, only one or two neutrinos interact,” says Cai. “Huge technical challenge For our experiments, we need to get enough protons to see, and we have to find a way to get enough neutrinos from a large collection of protons. ”

Use of neutrino detectors

To partially solve this problem, researchers Neutrino detector Includes both hydrogen and carbon atom targets.Researchers usually hydrogen atom In an experiment measuring protons. Hydrogen is not only the most abundant element in the universe, it is also the simplest, as the hydrogen atom contains only one proton and one electron. However, pure hydrogen targets are not dense enough for enough neutrinos to interact with atoms.

“We’re doing a sort of ‘chemical trick’ by bonding hydrogen to hydrocarbon molecules so that subatomic particles can be detected,” McFarland says.

The MINERvA group performed experiments using the high-power, high-energy particle accelerator at Fermilab. Accelerators produce the most powerful source of high-energy neutrinos on Earth.

The researchers directed a beam of neutrinos into a detector made of hydrogen and carbon atoms and recorded operational data for about nine years.

To isolate only the information from the hydrogen atoms, the researchers had to subtract background “noise” from the hydrogen atoms. carbon atom.

“Because hydrogen and carbon are chemically bonded, the detector can see both interactions at the same time,” says Cai. “I realized that the techniques I used to study interactions on carbon could also be used to look at hydrogen itself, with carbon interactions subtracted. It was to subtract a very large background from the neutrinos scattered by the protons, the carbon nuclei.”

Deborah Harris, University of York professor and co-spokesperson for MINERvA, said: , creative analysis by scientists, and the Fermilab accelerator that has been running for years.

impossible becomes possible

McFarland also initially believed that using neutrinos to accurately measure the signal from protons would be nearly impossible.

“When Tejin and our colleague Arie Bodek (Rochester’s George E. Pake physics professor) first suggested trying this analysis, I thought it would be too difficult,” says McFarland. increase. “But the old view of the proton has been explored so thoroughly that I decided to try this technique to get a new view. And it worked.”

The collective expertise and collaboration within the group of MINERvA scientists was essential in achieving the research, says Cai.

“The results of the analysis and the new techniques that have been developed highlight the importance of being creative and collaborative in making sense of the data,” he says. Already existed, but combining them in the right way made a big difference, and this can only happen if experts from different technical backgrounds share their knowledge and successfully experiment. ”

The study not only provides more information about the common matter that makes up the universe, but is also important for predicting neutrino interactions in other experiments seeking to measure neutrino properties. . These experiments include the Deep Underground Neutrino Experiment (DUNE), the Imaging Cosmic And Rare Underground Signals (ICARUS) neutrino detector, and his T2K neutrino experiment involving his McFarland and his group.

“Detailed information about protons is needed to answer questions such as which neutrinos have more mass than others and whether there are differences between them. Neutrino “Our work is a step forward in answering fundamental questions about neutrino physics and is the goal of these major scientific projects in the near future.”