A new way to explore proton’s structure with neutrinos yields first results

Symmetry dimensions in particle physics


By Madeleine O’Keefe

Physicists used MINERvA*, a Fermilab neutrino experiment, to measure the proton’s size and structure using a neutrino-scattering technique.

For the first time, particle physicists have been able to precisely measure the proton’s size and structure using neutrinos. With data gathered from thousands of neutrino-hydrogen scattering events collected by MINERvA, a particle physics experiment at the US Department of Energy’s Fermi National Accelerator Laboratory, physicists have found a new lens for exploring protons. The results were published today in the scientific journal Nature.

This measurement is also important for analyzing data from experiments that aim to measure the properties of neutrinos with great precision, including the future Deep Underground Neutrino Experiment, hosted by Fermilab.

“The MINERvA experiment has found a novel way for us to see and understand proton structure, critical both for our understanding of the building blocks of matter and for our ability to interpret results from the flagship DUNE experiment on the horizon,” says Bonnie Fleming, Fermilab deputy director for science and technology.
“If we were not optimists, we would say it’s impossible.”

Protons and neutrons are the particles that make up the nucleus, or core, of an atom. Understanding their size and structure is essential to understand particle interactions. But it is very difficult to measure things at subatomic scales. Protons—about a femtometer, or 10-15 meters, in diameter—are too small to examine with visible light. Instead, scientists use particles accelerated to high energies. Their wavelengths are capable of probing miniscule scales.

Starting in the 1950s, particle physicists used electrons to measure the size and structure of the proton. Electrons are electrically charged, which means they interact with the electromagnetic force distribution in the proton. By shooting a beam of accelerated electrons at a target containing lots of atoms, physicists can observe how the electrons interact with the protons and thus how the electromagnetic force is distributed in a proton. Performing increasingly more precise experiments, physicists now have measured the proton’s electric charge radius to be 0.877 femtometers.

The MINERvA collaboration achieved its groundbreaking result by using particles called neutrinos in lieu of electrons. Specifically, they used antineutrinos, the antimatter partners of neutrinos. Unlike electrons, neutrinos and antineutrinos have no electric charge; they only interact with other particles via the weak nuclear force**. This makes them sensitive to the “weak charge” distribution inside a proton.

However, neutrinos and antineutrinos rarely interact with protons—hence the name weak force. To collect enough scattering events to make a statistically meaningful measurement, MINERvA scientists needed to smash a lot of antineutrinos into a lot of protons.

Fortunately, Fermilab is home to the world’s most powerful high-energy neutrino and antineutrino beams. And MINERvA contains a lot of protons. Located 100 meters underground at Fermilab’s campus in Batavia, Illinois, MINERvA was designed to perform high-precision measurements of neutrino interactions on a wide variety of materials, including carbon, lead and plastic.

To measure the proton structure with high precision, scientists ideally would send neutrinos or antineutrinos into a very dense target made only of hydrogen, which contains protons but no neutrons. That is experimentally challenging, if not impossible to achieve. Instead, the MINERvA detector contains hydrogen that is closely bonded to carbon in the form of a plastic called polystyrene. But no one had ever tried to separate hydrogen data from carbon data.

“If we were not optimists, we would say it’s impossible,” says Tejin Cai, a postdoctoral researcher at York University and lead author on the Nature paper. Cai performed this research for his doctorate at the University of Rochester.

“The hydrogen and carbon are chemically bonded together, so the detector sees interactions on both at once. But then, I realized that the very nuclear effects that made scattering on carbon complicated also allowed us to select hydrogen and would allow us to subtract off the carbon interactions.”

Cai and Arie Bodek, a professor at the University of Rochester, proposed using MINERvA’s polystyrene target to measure antineutrinos scattering off protons in hydrogen and carbon nuclei to Cai’s PhD advisor, Kevin McFarland. Together, they developed algorithms to subtract the large carbon background by identifying neutrons produced from antineutrinos scattering off carbon atoms.

“When Tejin and Arie first suggested trying this analysis, I thought it would be too difficult, and I wasn’t encouraging. Tejin persevered and proved it could be done,” says McFarland, a professor at the University of Rochester. “One of the best parts of being a teacher is having a student who learns enough to prove you wrong.”

Cai and his collaborators used MINERvA to record more than a million antineutrino interactions over the course of three years. They determined that about 5,000 of these were neutrino-hydrogen scattering events.
With these data, they inferred the size of the proton’s weak charge radius to be 0.73 ± 0.17 femtometers***. It is the first statistically significant measurement of the proton’s radius using neutrinos. Within its uncertainties, the result aligns with the electric charge radius measured with electron scattering.

The result shows that physicists can use this neutrino-scattering technique to see the proton through a new lens. The result also provides a better understanding of the proton’s structure. This can be used to predict the behavior of groups of protons in an atom’s nucleus. If physicists start with a better measurement of neutrino-proton interactions, they can make better models of neutrino-nucleus interactions. This will improve the performance of other neutrino experiments, such as NOvA at Fermilab and T2K in Japan.

For Cai, one of the most exciting things about the result is showing that “even with a general particle detector, we can do things we didn’t imagine we could do.”

Deborah Harris, co-spokesperson for the MINERvA collaboration, says, “When we proposed MINERvA, we never thought we’d be able to extract measurements from the hydrogen in the detector. Making this work required great performance from the detector, creative analysis ideas and years of running in the most intense high-energy neutrino beam on the planet.”

See: https://www.symmetrymagazine.org/article/a-new-way-to-explore-protons-structure-with-neutrinos-yields-first-results

* MINERvA - is a particle physics experiment, located at Fermi National Accelerator Laboratory in Batavia, Illinois. MINERvA was designed to perform high-precision measurements of neutrino interactions on a wide variety of materials, including water, helium, carbon, iron, lead, and plastic.

MINERvA is located 100 meters underground, and sits directly in front of the MINOS near detector. The source of MINERvA’s neutrino beam is the Neutrinos at the Main Injector beamline, or NuMI. NuMI also provides a beam of neutrinos for the MINOS and NOvA near detectors and the MINOS and NOvA far detectors located in the Soudan mine and on the border of Minnesota and Canada, respectively.

The NuMI neutrino beam is created by firing protons from Fermilab’s Main Injector into a carbon target resembling a yardstick , located inside of a magnetic focusing horn. The interaction of protons with the target produces a stream of positively and negatively charged particles. The horn produces a magnetic field used to focus either the positive or negative particles into a 675 meter long decay pipe. There the particles decay in flight to produce muons and muon neutrinos (from positive particles) or anti-muons and muon anti-neutrinos (from negative particles). The approximately 240 meters of rock and muon absorbers between the end of the decay region and the near detector hall absorb all particles except the neutrinos, creating a clean neutrino beam for use by the particle detectors.

The NuMI detector hall at Fermilab is located 105 meters underground, and contains MINERvA, the MINOS near detector, and the NOvA near detector. The center of the neutrino beam passes through MINERvA , followed by the MINOS near detector, before continuing on its journey through the Earth to the Soudan mine.

Neutrinos passing through a particle detector will collide with nuclei in the various materials making up the detector. These interactions produce ionizing radiation and secondary charged particles that leave measurable energy deposits in the detector. These energy deposits are used to identify and study the neutrino interactions.

See: https://minerva.fnal.gov/how-it-works/

** Weak nuclear force - (or just the weak force, or weak interaction) acts inside of individual nucleons, which means that it is even shorter ranged than the strong force. It is the force that allows protons to turn into neutrons and vice versa through beta decay. This keeps the right balance of protons and neutrons in a nucleus. The weak force is very important in the nuclear fusion that happens in the sun.[1] Nuclear fusion has also been created in laboratories, and that process requires the weak force to work too. See size of the universe for a list of visuals demonstrating how short ranged the weak force is.

As the name implies, the weak force is much weaker than the strong force, or the electromagnetic force, but it is quite a bit stronger than the gravitational force.

Modern physics has unified the electromagnetic and weak forces into the electroweak force. There is a continued effort to try to unify all of the forces in a grand unified theory.

Fully understanding the weak force takes many years of study, but some fun places to start include hyperphysics or the blog of Prof. Matt Strassler.

Below is the Scishow's series on fundamental forces part 2, the weak force:

View: https://www.youtube.com/watch?v=cnL_nwmCLpY

  1. Sears and Zemanski's University Physics, 13th edition by Young and Freedman. Addison Wesley, 2010. Chapter 44, pg 1491.
See: https://www.energyeducation.ca/encyclopedia/Weak_nuclear_force

*** Femtometer - is equal to one quadrillionth of a meter. This metric unit of length is also called fermi, the older non–SI measurement unit of length, in honour of Italian physicist Enrico Fermi (29 September 1901 — 28 November 1954), since it is a typical length-scale of, and often referenced in nuclear physics.

1 fm = 3.2808398950131×10-15 ft
1 fm = 1.0936132983377×10-15 yd
1 fm = 1.0×10-15 m
1 fm = 1.0×10-9 µ

See: https://www.aqua-calc.com/what-is/length/femtometer

Wow! Can you imagine working at these femtometer scales? In any case, despite often being upstaged by CERN, FermiLab continues to make valuable contributions to sub-atomic particle physics. The unique use of neutrinos to further probe the proton is just one such example.


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