“Ghostly” neutrinos open a new way to study protons

Neutrinos are one of the most abundant particles in our universe, but they are notoriously difficult to detect and study: they have no electrical charge and almost no mass. They are often called “ghost particles” because they rarely interact with atoms.

But because they are so abundant, they play an important role in helping scientists answer fundamental questions about the universe.

In groundbreaking research described in Nature—led by researchers at the University of Rochester—scientists from the international MINERvA collaboration have, for the first time, used a neutrino beam at the Fermi National Accelerator Laboratory, or Fermilab, to study the structure of protons.

MINERvA is an experiment to study neutrinos, and the researchers did not set out to study protons. But their feat, once considered impossible, offers scientists a new way to examine the tiny components of an atom’s nucleus.

“While we were studying neutrinos in the MINERvA experiment, I realized that a technique I was using could be applied to study protons,” says Tejin Cai, the paper’s first author. Cai, who is now a postdoctoral research associate at York University, conducted the research as a Ph.D. student of Kevin McFarland, physics professor of Dr. Steven Chu in Rochester and a key member of the university’s Neutrino group.

“We weren’t sure at first if it would work, but eventually we discovered that we could use neutrinos to measure the size and shape of the protons that make up the nuclei of atoms. It’s like using a ghost ruler to perform a measurement.”

Using particle beams to measure protons

Atoms, along with the protons and neutrons that make up an atom’s nucleus, are so small that researchers have difficulty measuring them directly. Instead, they construct a picture of the shape and structure of an atom’s components by bombarding the atoms with a beam of high-energy particles. They then measure how far and at what angles the particles bounce off the components of the atom.

Imagine, for example, throwing marbles at a box. Marbles bounced off the box at certain angles, allowing you to determine where the box was – and determine its size and shape – even if the box was not visible to you.

“It’s a very indirect way of measuring something, but it allows us to relate the structure of an object – in this case, a proton – to the number of deviations we see from different angles,” McFarland says.

What can neutrino beams tell us?

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

Although the new technique won’t produce a sharper image than the old technique, McFarland says, it can give scientists new insights into how neutrinos and protons interact – insights they currently can only infer. using theoretical calculations or a combination of theory and other measurements.

Comparing the new technique with the old, McFarland likens the process to seeing a flower in normal visible light and then looking at the flower under ultraviolet light.

“You’re looking at the same flower, but you can see different structures under different types of light,” McFarland says. “Our image is not more precise, but the neutrino measurement gives us a different view.”

Specifically, they hope to use the technique to separate neutrino scattering-related effects on protons from neutrino scattering-related effects on atomic nuclei, which are related collections of protons and neutrons.

“Our previous methods for predicting neutrino scattering by protons all used theoretical calculations, but this result directly measures that scattering,” Cai says.

McFarland adds, “By using our new measurement to improve our understanding of these nuclear effects, we will be better able to make future measurements of neutrino properties.”

The technical challenge of experimenting with neutrinos

Neutrinos are created when atomic nuclei come together or separate. The sun is a great source of neutrinos, which are a byproduct of nuclear fusion from the sun. If you stand in sunlight, for example, billions of neutrinos will pass harmlessly through your body every second.

Even though neutrinos are more abundant in the universe than electrons, it is more difficult for scientists to experimentally exploit them in large numbers: neutrinos pass through matter like ghosts, while electrons interact with matter much more frequently .

“Over the course of a year, on average, there would only be interactions between one or two neutrinos out of the trillions that pass through your body every second,” Cai explains. “There’s a huge technical challenge in our experiments in that we have to get enough protons to observe, and we have to figure out how to get enough neutrinos through this large assembly of protons.”

Use of a neutrino detector

The researchers solved this problem in part by using a neutrino detector containing a target of hydrogen and carbon atoms. Typically, researchers only use hydrogen atoms in experiments to measure protons. Not only is hydrogen the most abundant element in the universe, but it is also the simplest, as a hydrogen atom contains only one proton and one electron. But a pure hydrogen target would not be dense enough for enough neutrinos to interact with the atoms.

“We’re doing a ‘chemical trick,’ so to speak, by binding hydrogen into hydrocarbon molecules that make it capable of detecting subatomic particles,” McFarland says.

The MINERvA group conducted its experiments using a high-power, high-energy particle accelerator, located at Fermilab. The accelerator produces the most powerful high-energy neutrino source on the planet.

The researchers hit their detector made up of hydrogen and carbon atoms with the neutrino beam and recorded data for almost nine years of operation.

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

“Hydrogen and carbon are chemically bonded together, so the detector sees interactions on both at once,” Cai explains. “I realized that a technique I used to study carbon interactions could also be used to see hydrogen on its own once you subtracted the carbon interactions. A lot of our work was to subtract the very large background of neutrino scattering off protons in the carbon nucleus.”

According to Deborah Harris, professor at York University and co-spokesperson for MINERvA, “When we proposed MINERvA, we never thought we would be able to extract measurements of hydrogen in the detector. Achieving this work required excellent performance from the detector, creative analyzes from scientists, and years of operation” of the Fermilab accelerator.

The impossible becomes possible

McFarland, too, at first thought it would be nearly impossible to use neutrinos to accurately measure the proton signal.

“When Tejin and our colleague Arie Bodek (Physics Professor George E. Pake in Rochester) first suggested trying this analysis, I thought it would be too difficult,” McFarland said. “But the old proton vision has been explored very extensively, so we decided to try this technique to get a new vision – and it worked.”

The collective expertise of MINERvA scientists and the collaboration within the group was essential to carrying out the research, says Cai.

“The result of the analysis and the new techniques developed highlight the importance of being creative and collaborative in understanding data,” he says. “Although many components of the analysis already existed, putting them together in the right way really made a difference, and that cannot be done without experts with different technical backgrounds sharing their knowledge to make the experiment a success. .”

In addition to providing more information about the common matter that makes up the universe, the research is important for predicting neutrino interactions for other experiments that attempt to measure neutrino properties. These experiments include the Deep Underground Neutrino Experiment (DUNE), the Imaging Cosmic And Rare Underground Signals (ICARUS) neutrino detector, and the T2K neutrino experiments in which McFarland and his group are involved.

“We need detailed information about protons to answer questions like which neutrinos have more mass than others and whether or not there are differences between neutrinos and their antimatter partners,” Cai says. “Our work is a step towards answering the fundamental questions about neutrino physics that are the goal of these big science projects in the near future.”