Recently, the research team led by Dr. Chen Xurong at the Quark Matter Research Center of the Institute of Modern Physics, Chinese Academy of Sciences, has made significant progress in the study of the origin of proton mass. The team delves into the impact of strange quarks within the proton on its mass, starting from experimental investigations. The relevant research was published on February 27th in Physical Review D.
Nucleons (collectively referring to protons and neutrons) constitute over 99% of the mass of visible matter in the universe. The origin of nucleon mass is closely linked to phenomena such as trace anomalies, color confinement, and dynamic chiral symmetry breaking. Understanding the origin of nucleon mass is a crucial and profound question in nuclear physics research.
Traditionally, it has been believed that the mass of quarks inside the proton primarily arises from its three constituent quarks: two up quarks and one down quark, with the contributions of other quarks considered negligible. Recent studies, however, suggest the possible existence of heavier quarks within the proton, such as strange quarks and charm quarks. Nevertheless, there is a lack of direct experimental evidence regarding whether heavy quarks like strange and charm quarks significantly contribute to the proton's mass.
By establishing the relationship between the quantum anomaly energy of the proton and the total sigma term (including contributions from both light and heavy quarks to the proton mass), the research team at the Institute of Modern Physics extracted the value of the sigma term from experimental data on the photoproduction differential cross-section of vector mesons near the mass threshold. The results indicate that the sigma term value for heavy quarks is larger than previously expected, approximately 337 mega-electron volts (dipole fit) and 455 mega-electron volts (exponential fit), constituting approximately 36~48% of the entire proton mass (938 mega-electron volts). The significance of the non-zero result in the exponential fit reaches approximately 7 standard deviations. Additionally, the team utilized the Kolmogorov–Smirnov test to confirm the compatibility of the sigma term values extracted from two different data sets.
In contrast to previous models or lattice calculations, this study marks the first instance of directly providing the range of sigma term values using experimental data. This work further deepens our understanding of quantum chromodynamics theory and the origin of mass, providing theoretical support for domestic and international electron-ion collider projects and offering new perspectives for future research on the origin of proton mass.
Link to the related paper: https://doi.org/10.1103/PhysRevD.109.036034
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The value of the charm quark sigma term has been extracted experimentally using methods based on the dipole form (top figure) and the exponential form (bottom figure). Image courtesy of the Institute of Physics.
