‘Ghostly’ neutrinos provide 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 referred to as “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 pioneering research described in NatureScientists from the international MINERvA collaboration, led by researchers at the University of Rochester, have used, for the first time, a neutrino beam at the Fermi National Accelerator Laboratory, or Fermilab, to investigate 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 thought impossible, offers scientists a new way to look at the tiny components of an atom’s nucleus.

“While we were studying neutrinos as part of the MINERvA experiment, I realized that a technique I was using could be applied to investigate 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 PhD. student of Kevin McFarland, Dr. Steven Chu Professor of Physics at Rochester and key member of the University’s Neutrino Group.

“At first we weren’t sure if it would work, but we eventually found 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 make a measurement.”

Using particle beams to measure protons

Atoms, and the protons and neutrons that make up the nucleus of an atom, are so small that researchers have difficulty measuring them directly. Instead, they build a picture of the shape and structure of an atom’s components by bombarding 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 into a box. The 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.

“This is a very indirect way of measuring something, but it allows us to relate the structure of an object, in this case, a proton, to how many deflections we see at different angles,” says McFarland.

What can neutrino beams tell us?

Researchers first measured the size of protons in the 1950s, using an electron beam accelerator 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.

While the new technique doesn’t produce a sharper image than the older technique, McFarland says, it may give scientists new information about how neutrinos and protons interact, information they can currently only infer through theoretical calculations or a combination of theory and science. other measurements.

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

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

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

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

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

The technical challenge of experimenting with neutrinos

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

Although neutrinos are more abundant in the universe than electrons, it is more difficult for scientists to harness them experimentally 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 says. “There is a huge technical challenge in our experiments where we have to get enough protons to look, and we have to figure out how to get enough neutrinos through that big pool of protons.”

Using 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. Hydrogen is not only the most abundant element in the universe, it is also the simplest, since 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 performing a ‘chemical trick’, if you will, by binding the hydrogen into hydrocarbon molecules that make it capable of detecting subatomic particles,” says McFarland.

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

The researchers hit their detector made 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 had to subtract the background “noise” from the carbon atoms.

“Hydrogen and carbon are chemically bound, so the detector sees interactions in both at once,” Cai says. “I realized that a technique I was using to study the interactions on carbon could also be used to see hydrogen itself once the carbon interactions are subtracted. A lot of our work involved subtracting the background very large part of the neutrinos that scatter into the protons in the carbon nucleus”.

Says Deborah Harris, a professor at York University and co-spokesperson for MINERvA: “When we proposed MINERvA, we never thought we would be able to extract measurements of the hydrogen in the detector. Doing this job required great detector performance, creative analysis from scientists and years of operation” of the accelerator at Fermilab.

The impossible becomes possible

McFarland also initially thought that it would be nearly impossible to use neutrinos to accurately measure the signal from the protons.

“When Tejin and our colleague Arie Bodek (the George E. Pake Professor of Physics at Rochester) suggested trying this analysis, I thought it would be too difficult,” says McFarland. “But the old view of protons has been explored very thoroughly, so we decided to try this technique to get a new view, and it worked.”

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

“The result of the analysis and the new techniques developed highlight the importance of being creative and collaborative in understanding the data,” he says. “While many of the components for the analysis already existed, putting them together in the right way really made a difference, and this cannot be done without experts from 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 in 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 are more massive than others and whether or not there are differences between them. neutrinos and their antimatter partners,” says Cai. “Our work is a step forward in answering the fundamental questions about neutrino physics that are the focus of these big science projects in the near future.”