Neutrinos offer a new way to probe the building blocks of matter

Depictions of the Roman goddess of wisdom Minerva show her in flowing robes, wearing a noble war helmet and holding an owl. In contrast, the MINERvA experiment features a large particle detector with the names of collaborating scientists emblazoned on its front.

While quite different in appearance, this neutrino experiment offers scientists profound wisdom, as represented by its name. Among its many insights, scientists have used MINERvA to better understand the size and structure of protons, one of the building blocks of atoms.

MINERvA is a neutrino scattering experiment at the Department of Energy’s Fermilab. Neutrinos are tiny electrically neutral particles that are extremely abundant. The Sun, other stars, and many different objects produce them as a result of atomic reactions. In fact, there are more neutrinos in the universe than any other particle that has mass.

Despite being ubiquitous, we never notice neutrinos because they hardly react with anything. The study of neutrinos is essential to understanding how our universe formed in the past and how it works now.

To better understand this fundamental particle, scientists study how neutrinos interact with materials on the rare occasions that they actually do. MINERvA’s mission is to capture these interactions.

It uses a beam of high-intensity neutrinos to study how they interact with the nuclei of five different elements. By hitting neutrinos at targets made of different materials—water, helium, carbon, iron, lead, and plastic—scientists can compare reactions. Mapping the various interactions will help scientists analyze the results of other experiments such as the upcoming Deep Underground Neutrino Experiment.

In addition to this goal, scientists from the MINERvA collaboration discovered another clever use for their data – to probe the size and structure of the proton.

Along with neutrons, protons make up the nuclei of the atoms that make up us and everything around us. They are one of the building blocks of matter that we interact with every day.

But the study of subatomic particles is much more complicated than the study of larger objects. Subatomic particles are too small to be studied with ordinary tools such as microscopes. Furthermore, the “size” of a subatomic particle does not have the same meaning as the size of an object you can measure with a ruler. Instead, scientists study the forces that hold the proton together.

In the past, scientists have studied the size of the proton using the electromagnetic force. Electromagnetism is one of the four fundamental forces of the universe. Magnetic fields, electric fields, even light fall under the electromagnetic force. It binds the electrons to the nucleus (consisting of protons and neutrons) in the atom. It is also partly responsible for the structure of the nucleus.

To represent the size of the proton, scientists have usually used the radius of the electric charge. This is the average radius of the electric charge distributed on the proton. To measure this characteristic, scientists aim a beam of electrons at a single energy at a target. Electrons fly away from protons in many different directions and energies, which gives scientists information about the internal structure of protons.

Using this technique, scientists have been able to make a very precise measurement of the size of the average radius of the proton’s electric charge, and thus of the quarks that provide the electric charge.

Led by Tejin Cai (then a PhD student at the University of Rochester), the MINERvA collaboration took a different approach. The idea was to use antineutrinos – the neutrino’s antimatter twin – to study protons.

Because neutrinos (and antineutrinos) have no charge, they will not interact via the electromagnetic force. Instead, neutrinos will interact via the weak force on protons. The weak force and gravity are the only two ways in which neutrinos interact with anything.

Despite its name, the weak force is powerful. Another of those four fundamental forces enables the process by which protons turn into neutrons or vice versa. These processes are what drive the nuclear reactions of the sun and other stars. Neutrinos provide a unique tool to study the weak force.

But the weak force only comes into play when the particles are very, very close together. As neutrinos are flying through space, they usually move through the (relatively) large spaces between the electrons and the nucleus of an atom.

Most of the time, neutrinos are simply not close enough to protons for them to interact via the weak force. To possibly get enough measurements, scientists need to shoot a staggering number of neutrinos or antineutrinos at a target.

MINERvA’s powerful neutrino beam and various targets made this goal possible. In an ideal world, scientists would aim neutrinos at a target made of pure neutrons, or antineutrinos at a target made of pure protons. That way, scientists could get more specific measurements. Unfortunately, this is not a very realistic experimental setup.

But MINERvA already had the next best thing – lots of antineutrinos and a target made of polystyrene. The material that makes up Styrofoam, polystyrene is made from hydrogen bonded to carbon. Using this target, scientists would obtain measurements of how antineutrinos interact with hydrogen and carbon.

To separate the hydrogen from the carbon, the scientists took an approach similar to taking a photo and then erasing the background to let you focus on just a few items. To determine those “background” neutrino-carbon interactions, scientists looked at neutrons.

When antineutrinos interact with protons in carbon or protons themselves in hydrogen, they produce neutrons. By tracking neutrons, scientists can work backwards to identify and remove carbon-antineutrino interactions from hydrogen-antineutrino interactions.

Getting the required number of interactions really tested MINERvA’s capabilities. Over three years, scientists recorded more than a million interactions of antineutrinos with other particles. Only 5,000 of them were hydrogen powered.

These data finally allowed scientists to calculate the size of the proton using neutrinos. Instead of the electric charge radius, they calculated the weak proton charge radius. It was the first time scientists have used neutrinos to make a statistically significant measurement of this characteristic.

Taking into account the uncertainties, the result was very close to previous measurements of the radius of the proton’s electric charge. Since it is essentially measuring the spatial distribution of the quarks and gluons that make up the proton, the value was expected to be similar.

This new technique gives scientists another tool in their toolkit to study the structure of the proton. It’s a testament to the wisdom we can gain when scientists think creatively about using existing experiments to explore new areas of research.