Fragmentation experiment reveals surprising fractured isospin symmetry

From the powdered wings of a butterfly to the icy spines of a snowflake, symmetry is a common feature in nature. This often even holds true down to the smallest bits of matter, which helps nuclear physicists ensure their measurements of the inhabitants of the subatomic world are accurate. The trick is knowing when something you’re measuring is symmetric and when it is not.

Now, nuclear physicists conducting experiments at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility have found new and unexpected cases of broken isospin symmetry. The discovery upends thoughts on how some particles are produced in experiments and could have implications for future studies of these particles.

The research is published in the journal Physics Letters B.

The physicists made the discovery by applying the rules of symmetry to the strong force that binds small particles called quarks and gluons to form larger particles, including the protons and neutrons at the center of every atom. Although this force holds our universe together, it remains mysterious.

Some experiments seek to learn more about it by exploring a complicated process known as fragmentation. Fragmentation happens when a nucleus is ripped apart, freeing quarks from their protons and neutrons. These solitary but energized quarks never stay single, but they instead transform their extra energy into other types of strongly interacting particles to pair up with.

In isospin symmetry, the different types—”flavors”—of quarks, such as an up quark or a down quark, will fragment into new particles in a symmetric way. However, new results from an experiment found that fragmentation does not always obey this symmetry.

“A lot of the way we understand nature has to do with symmetries and whether they are good symmetries or bad symmetries, and I think the fact that this is not always a good symmetry is a hint to us that the strong interaction is more complicated than previously assumed,” said David Gaskell, a Jefferson Lab staff scientist and a principal investigator on the experiment.

Precision with pions

This symmetry was tested in an experiment carried out at Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF).

CEBAF’s energetic electrons were aimed at hydrogen targets to study the quarks in protons and at deuteron targets to study the quarks in neutrons. The electrons from CEBAF’s beam were able to completely break apart a proton or neutron, which caused the struck quark to undergo fragmentation and form a new particle.

The team studied the pions that were formed by these struck quarks. Pions are a particle made up of two quarks. The researchers were able to precisely detect any created pions with the Super High Momentum Spectrometer. They then worked backwards to find out which type of quark was originally hit.

Dipangkar Dutta, a professor at Mississippi State University and principal investigator of this work, said the detailed data allowed for an exquisite accounting for this process.

“Because Jefferson Lab has a very high-intensity machine and the spectrometers are very well understood, we can select out small regions of the energy and momentum of the pions that are coming out in order to figure out what exactly happened when the quark was hit by an electron and how it turned into a pion,” he said.

Often, when an up quark in a proton is struck, it will turn into a “pi plus,” a positively charged pion that contains an up quark. Similarly, if a down quark in a neutron is struck, it will usually turn into a “pi minus,” a negatively charged pion that contains a down quark.

Isospin symmetry decrees that these two processes, known as the favored fragmentation processes because they are more likely to occur, happen with the same probability. In kind, the unfavored fragmentation processes, in which an up quark turns into a pi minus and a down quark turns into a pi plus, are also projected to occur with the same probability, although at lesser rates than favored processes.

When nuclear physicists use experimental data to extract information about fragmentation using mathematical frameworks called fragmentation functions, the assumptions from isospin symmetry allow them to use the same functions for up quarks and down quarks. That reduces the total number of functions they have to extract.

“The assumptions we make based on symmetries make extracting these functions easier and our analyses simpler,” Dutta said. “But they haven’t been tested quantitatively with precision yet.”

Until now. The researchers looked at the total energy carried by all the outgoing strongly interacting particles. At high energies, they found the symmetry held for both the favored and unfavored fragmentation processes. However, this wasn’t true at low energies, where they found the symmetry broke for the unfavored fragmentation processes: the unfavored processes did not occur with the same probability.

This symmetry violation hints that physicists have more to learn about fragmentation and how to apply it for future experiments.

“The fact that the symmetry seems to be violated is really interesting and has pretty important ramifications for how we treat other experiments that rely on this process,” Gaskell said.

Future fragmentation

These results suggest that experiments conducted at higher energies can safely make assumptions related to isospin symmetry during analysis. Experiments under the threshold, however, must tread more cautiously and may need to make additional corrections.

“Some experiments take this fragmentation stuff as a given and then they use it to extract other information about the 3D structure of protons and neutrons or things like that,” Gaskell said. “But what we’ve shown is that some of those assumptions may not be correct, so we need to be very careful in the interpretation of data from those experiments or an unambiguous interpretation of the data.”

These results will also help physicists learn more about another type of symmetry. In the strong interaction, charge symmetry assumes that the distribution of up quarks in the positively charged proton is the same as the distribution of down quarks in the negatively charged neutron. Researchers will test this symmetry by extracting the distribution of quarks in protons and neutrons from the same experiment, using the data collected on the deuteron target.

“Now that we’ve seen that these fragmentation functions are symmetric at high energies, we can associate any differences in these distributions to a breaking of charge symmetry,” Dutta said.