When quarks misbehave, symmetry breaks down and changes the rules of physics

Yahoo

18-05-2025

For decades, physicists have relied on the principle of symmetry to simplify and understand the complex behaviors of subatomic particles. Symmetry in physics basically means that some rules of nature stay the same even if you change things around.
This idea has served as one of the foundations of nuclear physics, helping scientists build models of how matter behaves at the smallest scales. However, a team of researchers led by Mississippi State University (MSU) professor Dipangkar Dutta has found cracks in this foundation.
Results from the new study suggest that symmetry, once thought to be a constant, can break down under certain conditions. This finding can reshape our understanding of the strong nuclear force, a force that governs everything from the behavior of particles inside atomic nuclei to the formation of matter across the universe.
To test whether certain symmetries in physics really hold up, the researchers conducted an interesting experiment at the Thomas Jefferson National Accelerator Facility in Virginia. They used a high-energy beam of electrons and fired it at protons and deuterons (a hydrogen isotope).
This allowed them to observe how quarks, the tiny building blocks inside protons and neutrons (inside the deuteron), behave when struck. The technique the scientists used is called semi-inclusive deep-inelastic scattering (or SIDIS). In simple terms, it's a way to knock loose a quark and then study what kind of particle it turns into afterward.
The researchers focused on how often quarks turned into positively or negatively charged pions (a type of subatomic particle), depending on whether they came from protons or deuterons. This process, called fragmentation, gives physicists clues about how quarks behave when they're released from the tight grip of the strong nuclear force.
Now here's where the symmetry jumps in. According to a principle called charge symmetry, an up quark in a proton should behave the same way as a down quark in a neutron, once you flip the charge. That's been a helpful assumption for decades because it simplifies calculations.
However, until now, this idea hadn't been tested carefully in the context of fragmentation. When the researchers compared the behavior of these quarks, they found small but clear deviations, especially at lower energy levels.
These deviations caused the symmetry between the behaviors of up and down quarks to break down, suggesting that charge symmetry doesn't always hold, at least not during fragmentation.
The possibility of symmetry failing under certain conditions can lead to many changes in nuclear physics. For instance, by understanding where and why symmetries break down, scientists can reevaluate theoretical models and more accurately explain particle behavior and interactions.
"The assumptions we make based on symmetries greatly simplify our analyses. But they haven't been tested quantitatively with precision until now. Our new results show when the symmetries are valid and when they need certain corrections," said Dutta, in a statement released by MSU.
Hopefully, future studies will also shed light on other scenarios where symmetries break and lead to an improved understanding of nuclear physics.
The study is published in the journal Physics Letters B.