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Heavier Quarks Impact Proton Mass Significantly

A research initiative spearheaded by Prof. Xurong Chen at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) has yielded fresh perspectives on the origins of proton mass.

Through their experimental endeavors, the researchers proposed that the impact of heavy quarks on proton mass might be more significant than previously presumed. These findings were published in Physical Review D on February 27th, 2024.

Nucleons, consisting of protons and neutrons, constitute over 99 % of the observable mass of the universe. The mechanisms governing nucleon mass are intricately associated with phenomena like quantum trace anomaly, color confinement, and dynamical chiral symmetry breaking.

Consequently, investigating the origins of nucleon mass stands as a crucial research avenue within the realms of nucleon structure and quantum chromodynamics studies.

Previous studies have posited that the mass of quarks within protons primarily arises from its constituent quarks: two up quarks and one down quark, with contributions from other quark types deemed negligible.

Recent inquiries have hinted at the potential existence of heavier quark species within protons. However, there is currently insufficient direct experimental evidence to validate the significant influence of heavy quarks (such as strange, charm, and beauty quarks) on proton mass.

Researchers at IMP established a connection between the quantum anomaly energy of protons and the total sigma term, encompassing contributions from light and heavy quarks to proton mass. They extracted the sigma term by utilizing experimental data involving vector meson near-threshold photo-production.

The study unveiled a sigma term of heavy quarks larger than initially expected, measuring approximately 337 MeV (dipole fit) and 455 MeV (exponential fit), representing 36-48 % of the total proton mass (938 MeV).

The statistical significance of the non-zero value (exponential fit) reaches approximately seven standard deviations, corresponding to a probability of 99.999999999744 %.

Employing data from two experimental groups and utilizing the Kolmogorov–Smirnov test method, this study confirmed the compatibility of the sigma term extracted from both datasets.

These findings provide fresh perspectives for future inquiries into the origins of proton mass and introduce new observables for research on upcoming electron-ion colliders (EICs).

Journal Reference:

Kou, W. & Chen, X. (2024). Unraveling proton strangeness: Determination of the strangeness sigma term with statistical significance. Physical Review D.


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