A team of theoretical physicists at Johannes Gutenberg University Mainz (JGU) has made remarkable progress in refining their calculations of the electric charge radius of the proton. This achievement, described in three newly published preprints on the arXiv server, represents a significant advancement in our understanding of the proton’s physical characteristics. In addition to updating the proton radius measurement, the researchers have also made groundbreaking strides in predicting the magnetic charge radius. These findings shed light on the enigmatic nature of atomic nuclei and open doors for further exploration into the mysteries of the proton.
The proton, one of the fundamental building blocks of matter, has eluded precise measurement of its radius for many years. In 2010, a breakthrough occurred when laser spectroscopy of muonic hydrogen revealed a strikingly smaller proton radius compared to traditional methods. The question then arose: is this deviation evidence of new physics or inherent to the measuring techniques? Recent evidence suggests that the smaller experimental value reflects the true proton radius, rather than signaling new physics beyond the Standard Model.
In 2021, Prof. Dr. Hartmut Wittig and his team at the Mainz Cluster of Excellence PRISMA+ made significant progress in performing lattice calculations to determine the proton’s radius. This methodology involves using lattice field theory, where quarks are arranged on a discrete space-time lattice similar to a crystal structure. By simulating electromagnetic form factors, which describe the distribution of electric charge and magnetization within the proton, the researchers were able to improve the precision of their calculations.
Building upon the previous breakthrough, doctoral student Miguel Salg and the Mainz research group have achieved new results that not only enhance but also extend the earlier calculations. In a considerable advancement, the team was able to omit experimental data entirely from their calculations and still obtain highly precise results. By including additional calculations, refining statistics, and better constraining systematic errors, they have strengthened the evidence supporting the smaller proton radius.
The Mainz physicists base their calculations on the theory of quantum chromodynamics (QCD), which describes the forces at play within atomic nuclei. The strong interaction binds quarks together, forming protons and neutrons, and is mediated by exchange particles called gluons. Lattice field theory, coupled with QCD, enables the researchers to mathematically evaluate these intricate processes. By utilizing supercomputers, they can determine properties of nucleons, such as the electromagnetic form factors that ultimately provide insights into the proton’s radius.
Beyond the electric charge radius, the proton also possesses a magnetic charge radius, which poses its own puzzle. The Mainz research group tackled this challenge by calculating the magnetic charge radius based on QCD principles. This groundbreaking achievement contributes to our understanding of the proton’s magnetic characteristics and enriches our overall comprehension of this fundamental particle.
The Mainz researchers went even further by deriving the Zemach radius purely from QCD, using the precise knowledge of the electric and magnetic form factors. The Zemach radius serves as a crucial input parameter for experimental measurements on muonic hydrogen. This achievement not only confirms the accuracy of their lattice QCD calculations but also showcases the remarkable progress made in this field.
The recent breakthroughs in theoretical physics at Johannes Gutenberg University Mainz represent exciting advancements in our understanding of the proton. By refining calculations of the electric and magnetic charge radii, researchers have provided further evidence supporting the smaller values obtained through muonic hydrogen spectroscopy. These achievements deepen our understanding of atomic nuclei and raise new questions about the nature of fundamental particles. The ongoing pursuit of unraveling the mysteries of the proton remains a captivating endeavor for the scientific community, as it promises to broaden our knowledge of the fundamental laws governing the universe.