### Abstract: Precise Measurement of Short-Range Correlations in Atomic Nuclei by Ye Zhihong's Research Group at Tsinghua University A recent study conducted by Assistant Professor Ye Zhihong of Tsinghua University's Department of Physics, in collaboration with the Tritium experimental group at the Jefferson Lab in the United States, has made significant strides in understanding the short-range correlations (SRCs) within atomic nuclei. The research, published in the prestigious journal *Nature* under the title "Revealing the Short-Range Structure of the Mirror Nuclei 3H and 3He," provides precise measurements of SRCs in three-nucleon systems, specifically focusing on the np-SRC (neutron-proton short-range correlation) and pp-SRC (proton-proton short-range correlation) pairs in helium-3 (3He) and tritium (3H) nuclei. #### Key Elements of the Study **Background and Importance:** The fundamental questions of how protons and neutrons combine to form stable atoms and the nature of nuclear forces have long puzzled physicists. While the Standard Model has successfully explained the structure of visible matter in the universe, the complexity and unknown aspects of the strong interaction between nucleons (protons and neutrons) remain a significant challenge. Nucleons, composed of quarks held together by gluons, are bound within an atomic nucleus due to color confinement. The strong force is much stronger than the electromagnetic force, making it crucial for understanding nuclear stability. However, the exact mechanism by which the strong force manifests between nucleons is not fully explained by quantum chromodynamics (QCD), the theory of strong interactions. Traditional nuclear physics models, such as the shell model, approximate nucleons as point particles interacting within an average field and occasionally exchanging mesons. These models provide a reasonable description of many atomic nuclei but fail to explain certain fundamental properties, such as magic numbers and the dense matter in neutron stars. **Short-Range Correlations (SRCs):** SRCs are extreme ground states within atomic nuclei where the relative momentum between two nucleons is very high, but their total momentum is low, keeping the nucleus in its ground state. When two nucleons come close, their attraction increases, causing partial overlap. As they get even closer, repulsion becomes dominant to prevent complete overlap. SRCs are significant because they involve the internal quark and gluon structure of nucleons, which is essential for a comprehensive understanding of nuclear forces. SRCs also mimic the high-density conditions found in neutron stars, making them valuable for studying both microscopic nuclear interactions and macroscopic astrophysical phenomena. **Experimental Approach:** The team at Jefferson Lab used high-energy electron scattering to probe SRCs in the mirror nuclei 3He and 3H. Mirror nuclei are isospin analogs, meaning they have the same number of nucleons but differ in the number of protons and neutrons (3He has 2 protons and 1 neutron, while 3H has 1 proton and 2 neutrons). This unique property allows for a direct comparison of np-SRC and pp-SRC pairs without the need to measure the high-momentum nucleons directly, which is experimentally challenging and often fraught with large errors. By measuring the total probability of forming any SRC pair in these mirror nuclei, the researchers were able to distinguish and quantify the np-SRC to pp-SRC ratio with unprecedented precision. **Results:** The experimental results showed that the np-SRC to pp-SRC ratio in 3He is 2.17:1, which is significantly higher than the ratio derived from simple combinatorial considerations (2:1). This finding is crucial because it differs markedly from the results obtained in heavier nuclei, where only about 3% of SRC pairs are pp or nn, and the vast majority are np-SRC pairs. The high precision of the measurements, which are 10 times more accurate than previous results, highlights the complexity of nuclear interactions even in the simplest atomic nuclei. The results suggest that there are unknown factors influencing the formation of SRC pairs in lighter nuclei, which need further investigation. **Implications:** The study's findings underscore the need for more precise experiments to explore the probability distribution of SRC pairs in both light and heavy nuclei. This will help uncover richer SRC properties and refine theoretical models to better account for the strong force's role in nuclear structure, including the influence of quark and gluon structures. The insights gained from this research could have far-reaching implications for understanding the internal structure and evolution of neutron stars, as well as the fundamental nature of nuclear forces. **Future Directions:** Building on these results, Ye Zhihong and his collaborators have proposed a second-generation Tritium experiment at Jefferson Lab, which has been approved. This new experiment aims to directly observe the quark structure and its effects within 3He and 3H nuclei, further advancing our understanding of nuclear physics and the strong force. #### Conclusion The precise measurement of SRCs in 3He and 3H nuclei by Ye Zhihong's research group at Tsinghua University represents a significant step forward in the field of nuclear physics. By leveraging the unique properties of mirror nuclei, the team has provided new insights into the complex interactions between nucleons, highlighting the need for more detailed experimental and theoretical work. The results not only enhance our understanding of atomic nuclei but also have implications for astrophysical phenomena, such as the structure of neutron stars. **Reference:** Ye Zhihong et al., "Revealing the Short-Range Structure of the Mirror Nuclei 3H and 3He," *Nature*, 2022. [Link](https://www.nature.com/articles/s41586-022-05007-2)