Recent theoretical and experimental advances provide new insights into how mass is distributed within particles like protons, neutrons, and pions, offering critical guidance for future research.
The two-dimensional spatial distribution of mass due to gluons in the nucleon (left) and the pion (right), simulated at seven valence pion masses with lattice quantum chromodynamics. Image Credit: Bigeng Wang
In this study, scientists have investigated the mass of subatomic particles composed of quarks by examining their energy and momentum in four-dimensional spacetime. A key property, known as the trace anomaly, plays a vital role in this process.
The trace anomaly reflects how physical measurements from high-energy experiments depend on energy and momentum scales. Researchers suggest it is essential for binding quarks within subatomic particles.
In this study, the research team calculated the trace anomaly for nucleons (protons and neutrons) and pions (particles made of a quark-antiquark pair). The results revealed that the mass distribution in pions mirrors the charge distribution of neutrons, while in nucleons, the mass distribution aligns with the charge distribution of protons.
The Impact
Understanding the origin of nucleon mass is a key scientific objective for the Electron-Ion Collider (EIC). Researchers are also investigating how quark and gluon mass is distributed within hadrons—subatomic particles like protons and neutrons that are bound together by the strong force.
Recent calculations show that mass distribution can be determined numerically using first-principle methods, which rely on fundamental physical laws. This approach not only provides deeper insights into particle structure but also supports the interpretation of data from nuclear physics experiments, paving the way for a more comprehensive understanding of the building blocks of matter.
Summary
The scientists are gearing up for further experiments at the EIC at Brookhaven National Laboratory to better understand where nucleon mass comes from. By using electron-proton scattering, they hope to uncover how quarks and gluons distribute mass inside the proton. This process, which creates high-mass states sensitive to the proton's inner workings, is similar to how X-ray diffraction helped reveal the double-helix structure of DNA.
These experiments are guided by theoretical calculations based on the Standard Model, the framework that explains how particles interact at the smallest scales. These calculations shed light on how mass is spread out in particles like pions and nucleons. They also suggest that the pion, in particular, plays a unique role in linking two fundamental ideas: the existence of a universal energy scale and the imbalance between left-handed and right-handed interactions.
This understanding could shape the way scientists design and interpret future experiments, bringing us closer to unraveling the mysteries of the universe’s tiniest building blocks.
Journal References:
Wang, B., et al. (2024) Trace anomaly form factors from lattice QCD. Physical Review D. doi.org/10.1103/physrevd.109.094504.
Liu, K.-F. (2024) Hadrons, superconductor vortices, and cosmological constant. Physics Letters B. doi.org/10.1016/j.physletb.2023.138418.