The radius of the proton has long been a subject of fascination and research among scientists. Theoretical physicists at Johannes Gutenberg University Mainz (JGU) have made significant progress in improving their calculations of the electric charge radius of the proton, while also providing stable predictions for the magnetic charge radius. Their findings, published in three preprints on the arXiv server, shed new light on the fundamental properties of the proton and contribute to a better understanding of atomic nuclei.
Atomic nuclei are composed of protons and neutrons, and yet many aspects of these fundamental particles remain unexplained. One such mystery revolves around determining the radius of the proton. In 2010, a revolutionary technique involving laser spectroscopy of muonic hydrogen revealed a smaller value for the proton radius compared to traditional methods. This discrepancy sparked intense debate among physicists, as it raised the question of whether it indicated new physics beyond the Standard Model or was simply a result of systematic uncertainties in measurement methods.
Theoretical calculations play a crucial role in resolving this puzzle definitively. In 2021, the Mainz research group led by Prof. Dr. Hartmut Wittig made significant strides in performing lattice calculations to provide further evidence of the smaller proton radius. However, they have now taken an even greater leap forward. Doctoral student Miguel Salg and the research group have achieved remarkable results that improve and expand upon their previous calculations.
Two years ago, the Mainz physicists calculated the isovector radius, which differs from the proton radius. They derived the proton radius by combining experimental data for the neutron radius. However, their recent calculations eliminate the need for experimental data, allowing them to rely solely on theoretical calculations. By refining their statistical methods and constraining systematic errors, they have enhanced the precision of their calculations. Salg states, “We can now completely dispense with experimental data for the first time.”
The latest calculations conducted by the Mainz research group further support the notion that the smaller experimental value is the correct one for the proton radius puzzle. Based on quantum chromodynamics (QCD), which describes the forces within atomic nuclei, the calculations employ lattice field theory. Quarks, the elementary building blocks of matter, are distributed on a discrete lattice, allowing scientists to mathematically treat the processes involved. Supercomputers then facilitate the calculations of the nucleon’s properties, specifically the electromagnetic form factors that elucidate the distribution of electric charge and magnetization within the proton.
Predicting New Properties
In addition to the electric charge radius, the proton also possesses a magnetic charge radius, which presents its own enigma. The Mainz theorists have successfully calculated this property using QCD, providing a stable prediction purely through theoretical calculations. Hartmut Wittig elucidates, “One could illustrate the different radii in a very simplified way by the expansion of an accumulation of electric or magnetic charge given by the proton, which an incoming electron ‘sees’ in the scattering process.” Moreover, the researchers derived the Zemach radius of the proton purely from QCD, which serves as a crucial input for experimental measurements on muonic hydrogen.
The Mainz research group’s breakthrough calculations highlight the remarkable progress made in lattice QCD calculations. By leveraging supercomputing power and enhancing the precision of their calculations, the scientists have validated their previous results while also providing new insights into the proton’s magnetic charge radius. These advancements underscore the ever-increasing quality and reliability of lattice QCD calculations as a tool for studying fundamental particles.
The ongoing endeavor to determine the radius of the proton serves as a testament to the tenacity and ingenuity of theoretical physicists. The Mainz research group’s latest achievements mark significant milestones in unraveling the mysteries surrounding the proton’s size and properties. While the quest for answers continues, these breakthroughs bring us closer to a comprehensive understanding of atomic nuclei and the fundamental building blocks of matter.
The Mainz research group’s improved calculations, devoid of experimental data, provide further evidence supporting the smaller value for the proton radius. Their theoretical calculations also yield stable predictions for the proton’s magnetic charge radius. By refining their methods and harnessing the power of lattice QCD calculations, they have advanced our knowledge of fundamental particles and atomic nuclei. As the realm of theoretical physics progresses, these breakthroughs offer glimpses into the fascinating world of subatomic particles and the forces that bind them.
Leave a Reply