Proton is one of the basic particles that make up the material world. It has a complex internal structure, consisting of charged quarks and uncharged gluons. The radius of charge distribution inside protons is also often used to measure the size of protons.
In 20 10, physicists accurately measured the Lamb shift of hydrogen atoms (that is, the electrons in hydrogen atoms were replaced by muons), and captured the tiny influence of the internal charge distribution of protons on the atomic energy level of muons, thus determining the charge distribution radius. Lamb shift is the energy level difference between 2S( 1/2) and 2P( 1/2) of hydrogen atom measured by physicists Lamb and Reserford in 1947.
Although the accuracy of Miu hydrogen spectrum experiment is much higher than other experiments, the resulting charge distribution radius is 5 standard deviations from the previous global experimental average, which is the so-called proton size mystery. 20 19' s latest electron-proton scattering and hydrogen atomic spectroscopy experiments are consistent with the experimental results of Miu hydrogen, which shows that the mystery of proton size is gradually being solved and the differences in experiments are gradually narrowing.
So far, the experiment of MuH spectrum is still the most accurate experimental means to obtain the proton charge radius. The high-precision measurement of spectroscopy makes the contribution of QCD more important in the comparison between theory and experiment. In fact, the main theoretical error of extracting charge distribution radius from Muhlam displacement comes from the two-photon exchange Feynman diagram dominated by non-perturbation QCD.
This time, the research team of the Institute of Theoretical Physics of Peking University Institute of Physics cooperated with Assistant Professor Kim of the University of Connecticut in the United States to solve the infrared divergence problem of the two-photon spectrum, developed a brand-new long-range subtraction scheme to reduce statistical errors, and realized the grid calculation of the two-photon spectrum for the first time based on the supercomputer "Tianhe No.3" of China Supercomputer Tianjin Center. On this basis, the team plans to further carry out more systematic and accurate calculations to finally solve the basic scientific problem of "how big is the proton"
Previous studies have shown that the lattice method can also be used to study other important spectral physical quantities, such as hyperfine spectra. One of the key points of the grid team of Peking University in the future is to extend the research of grid QCD to atomic spectroscopy, so as to build an interdisciplinary bridge between quark and gluon scale high-energy physics research and extremely high-precision atomic spectroscopy research.
The first author of this paper is Fu Yang, a doctoral student in Peking University Institute of Physics, and Lu, an undergraduate, participates in some calculations and data analysis. The above research work is supported by the National Natural Science Foundation, the National Key R&D Program, the Collaborative Innovation Center of Quantum Matter Science, the Peking University High Energy Physics Research Center and the National Supercomputing Tianjin Center.
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