Despite protons and neutrons being composed of quarks—elementary particles with extremely small mass—they themselves are tens of times heavier than the sum of their “constituents.” This mystery has long puzzled scientists. It turns out that the Higgs mechanism, which endows quarks with their mass, accounts for less than 2% of the mass of nucleons. The rest literally “comes out of thin air”—from the processes of strong interaction described by Quantum Chromodynamics (QCD). A team of specialists from the Jefferson National Accelerator Laboratory has presented the most complete description to date of the principles by which the mass of hadrons—particles involved in the strong interaction—is generated. The work is published in the journal Symmetry. QCD postulates that mass arises from the energy contained within the fields of quarks and gluons—the so-called phenomenon of hadron mass generation. Inside hadrons, gluons can interact with themselves, which makes the dynamics of strong interaction extremely complex and varying with distance. At short distances, comparable to the size of a proton, quarks and gluons lose the properties of “bare” particles and become “dressed”—surrounded by a cloud of virtual particles that constantly appear and disappear. In this regime, “dressed” quarks acquire a mass of about 400 MeV, and three such interacting quarks form a proton weighing approximately 1 GeV. It is this dynamically generated mass that constitutes almost the entire mass of the proton—and consequently, all the visible matter in the Universe. For nearly 30 years, Jefferson Lab researchers have been accumulating data by studying the structure of the proton and its excited states. Using the Schwinger continuum approach, which determines the dependence of the strong interaction on distance, scientists correlated theoretical calculations with experimental data. The comparison of theory and experience clearly showed that “dressed” quarks with dynamically generated mass are the key components of the structure of the proton and its excited states. This significantly strengthened the theoretical model of hadron mass origin. However, many questions remain unresolved. The studies from the older 6-GeV CEBAF program covered a range of distances where about 30% of the hadron mass is formed. The data from the current 12-GeV program allow a look into the region responsible for approximately half the mass. Future investigations with even higher-energy electron beams will allow examination of the entire spectrum of distances where the main fraction of hadron mass appears. “When we get the full picture, we will be able to finally understand how the mass of the proton and other particles is formed,” noted Viktor Mokhov from Jefferson Lab. “This is one of the most significant goals of modern nuclear physics.”
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