Scientists gain new insights into how mass is distributed in hadrons
Nuclear theorists reveal mass distribution within the pion and the proton from first principle numerical calculations.
image: 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.
Credit: Image courtesy of Bigeng Wang
The Science
Scientists can determine the mass of subatomic particles that are built from quarks by looking at the particles’ energy and momentum in four-dimensional spacetime. One of the quantities that encode this information, called the trace anomaly, is linked to the fact that physical observables from high-energy experiments depend on the energy/momentum scales involved. Researchers believe that the trace anomaly is crucial for keeping quarks bonded in subatomic particles. In this study, scientists calculated the trace anomaly for both nucleons (protons or neutrons) and pions (a subatomic particle made of one quark and one antiquark). The calculations show that in the pion, the mass distribution is similar to the charge distribution of the neutron, while in the nucleon, the mass distribution is similar to the charge distribution of the proton.
The Impact
Understanding the origin of the nucleon mass is one of the major scientific goals of the Electron-Ion Collider (EIC). Scientists also want to understand how the mass from quarks and gluons is distributed in hadrons. These are subatomic particles such as protons and neutrons that are made up of quarks and held together by the strong force. The new calculations demonstrate that the distribution of mass can be obtained numerically based on first principle calculations, which start from basic physical laws. Calculations from this new approach will also aid scientists in interpreting data from nuclear physics experiments.
Summary
Experiments exploring the origin of the nucleon mass are planned for the future EIC at Brookhaven National Laboratory. In these experiments, electron-proton scattering can produce heavy states that are sensitive to the inner structure of the proton, particularly the gluons’ distributions. By analyzing the data from the scatterings, scientists can know how the mass of quarks and gluons is distributed within the proton. This is similar to how researchers used X-ray diffraction to discover the iconic double-helix shape of DNA.
Theoretical calculations help scientists understand how mass is distributed among hadrons according to the Standard Model, and they provide direction for upcoming experiments. The findings reveal important aspects of how mass is spread out within particles like the pion and the nucleon. The results suggest that the structure of the pion, in particular, plays a role in connecting two properties of the world that is described by the Standard Model: the existence of an absolute scale, and the asymmetry of left-handed and right-handed quantities.
Funding
This work is partially supported by a Department of Energy (DOE) grant under the framework of the DOE Office of Science topical collaboration in nuclear physics framework, managed by Keith Jankowski and Xiaofeng Guo. Both Bigeng Wang and the PI Keh-Fei Liu are affiliates of Lawrence Berkeley National Laboratory, where the work was carried out. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, a DOE Office of Science user facility. This work also used Stampede time under the Extreme Science and Engineering Discovery Environment, which is supported by National Science Foundation The authors also thank the National Energy Research Scientific Computing Center for providing high performance computing resources. Finally, they acknowledge the role of facilities of the USQCD Collaboration, which are funded by the DOE Office of Science.