Researchers have made significant progress in understanding the distribution of mass in subatomic particles by investigating how energy and momentum are related in four-dimensional spacetime. At the heart of these studies is a property known as the trace anomaly, which reveals essential information about the mass and internal structure of particles composed of quarks, such as protons, neutrons, and pions.
The trace anomaly is linked to the way that physical measurements from high-energy experiments change with the energy and momentum scales involved. This property plays a critical role in how quarks are bonded within subatomic particles, suggesting that it is essential for the stability of matter at a fundamental level. Scientists are particularly interested in how the trace anomaly influences the binding of quarks by the strong force, which is the fundamental force responsible for holding the components of atomic nuclei together.
In a recent study published in Physical Review D, researchers conducted advanced calculations to examine the trace anomaly for nucleons (which include protons and neutrons) and pions (particles composed of one quark and one antiquark). The results of these calculations revealed intriguing patterns in the mass distribution of these particles. In the case of the pion, the distribution of mass closely mirrors the charge distribution found in neutrons. Conversely, in nucleons, the mass distribution is more aligned with the charge distribution typical of protons. This discovery sheds new light on how different types of hadrons (particles composed of quarks held together by the strong force) exhibit unique internal structures.
One of the central aims of nuclear physics is to uncover the origin of mass in nucleons, which are fundamental building blocks of matter. Much of the mass of nucleons does not come directly from the quarks themselves but rather emerges from the dynamic interactions between quarks and gluons—the particles that mediate the strong force. Understanding this intricate relationship is a major scientific goal, particularly for upcoming experiments at the highly anticipated Electron-Ion Collider (EIC) being developed at Brookhaven National Laboratory.
The EIC is set to revolutionize our understanding of nucleon structure by allowing scientists to probe the inner workings of protons with unprecedented precision. By firing electrons at protons at high energies, researchers can study how these collisions produce heavy states that are sensitive to the distribution of gluons within the proton. This process is akin to how X-ray diffraction enabled scientists to unravel the structure of DNA. By analyzing the scattering data, scientists can map how mass is distributed within the proton, revealing the intricate dance of quarks and gluons that underlies the particle’s structure.
One of the exciting outcomes of this research is the ability to calculate the mass distribution of hadrons directly from first principles using fundamental physical laws. This approach, grounded in the Standard Model of particle physics, allows researchers to connect theoretical predictions with experimental observations. It marks a significant step forward because it means that scientists can derive the distribution of mass within particles like protons and pions using purely mathematical techniques, without relying on approximations or empirical data.
These theoretical calculations are not just abstract exercises—they have practical implications for future experiments. By providing a clearer picture of how mass is distributed within hadrons, the findings offer crucial guidance for interpreting the results of experiments at facilities like the EIC. For instance, understanding the role of gluons in contributing to the mass of nucleons could help clarify why quarks, which are relatively light, combine to form protons and neutrons that are much heavier. This insight is essential for explaining how matter in the universe acquires mass, a fundamental question in physics that goes beyond the Higgs mechanism.
The study also highlights the unique role of the pion in linking two fundamental properties within the Standard Model: the existence of an absolute energy scale and the asymmetry between left-handed and right-handed particles, known as chirality. The pion’s structure appears to bridge these concepts, indicating that it may play a crucial role in understanding the symmetry-breaking mechanisms that govern the behavior of particles at the smallest scales.
As scientists continue to delve into the mysteries of mass and the strong force, these new findings bring us closer to a deeper understanding of the fundamental forces that govern the universe. The work not only answers long-standing questions about the internal structure of subatomic particles but also opens up new avenues for exploration in high-energy physics. With the upcoming experiments at the EIC, researchers are poised to uncover even more about how quarks, gluons, and the forces between them give rise to the mass of the particles that make up our world.
Source: US Department of Energy