A comprehensive analysis from Jefferson Lab explains the Emergence of Hadron Mass (EHM) through 'dressed quarks.
Scientists have released the most detailed analysis yet of how protons and neutrons acquire their mass, solving a decades-long puzzle that the Nobel Prize-winning Higgs mechanism could not explain.The comprehensive study, recently featured on the cover of a special edition of the scientific journal Symmetry, bridges theoretical physics with nearly 30 years of experimental data collected at the US Department of Energy’s Thomas Jefferson National Accelerator Facility .
By spanning from experiments conducted in the 1990s to current high-intensity studies, the findings mark a major milestone in understanding the “Emergence of Hadron Mass” .The missing mass puzzleFor decades, physicists have wrestled with a fundamental accounting problem deep within the heart of matter: the numbers simply do not add up. Protons and neutrons are composed of quarks, the fundamental building blocks of nature, which are bound together by gluons. According to the Standard Model of physics, quarks acquire their “bare” mass through the Higgs mechanism.While the Higgs mechanism was confirmed by the Large Hadron Collider—leading to the 2013 Nobel Prize—it fails to account for the bulk of the universe’s visible mass.“This mechanism contributes to the measured proton and neutron masses at the level of less than 2%,” explained Victor Mokeev, a staff scientist and phenomenologist at Jefferson Lab. “This clearly demonstrates that the dominant part of the mass of real-world matter is generated through another mechanism, not through the Higgs.”How matter gets its “Heft”The new study clarifies that the remaining 98% of visible mass arises from the dynamics of the strong interaction, described by a theory known as Quantum Chromodynamics . A distinctive feature of this force is “gluon self-interaction.”At distances comparable to the size of a hadron , the relevant constituents are no longer the light “bare” quarks. Instead, they become “dressed” quarks. These are bare quarks surrounded by clouds of strongly coupled quarks and gluons that are undergoing continual creation and annihilation.Through this dynamic process, nearly massless bare quarks transition into fully dressed quarks with masses of approximately 400 MeV. It is the strong interaction among these dressed quarks that generates the proton’s mass of about 1 GeV.Synergy of theory and experimentThe breakthrough was achieved by connecting the Continuum Schwinger Method —a QCD-based theoretical approach—with experimental reality. Inside Jefferson Lab’s Experimental Hall B, the three-story-tall CLAS12 detector played a crucial role. This massive device identifies particles produced when high-intensity electron beams scatter off protons.By comparing the experimentally derived information with CSM predictions, scientists confirmed that dressed quarks with dynamically generated masses are the active forces underlying the structure of the proton.Completing the pictureWhile this study represents a significant leap forward, the quest is not over. Experiments from the earlier 6 GeV era of the facility explored the range where roughly 30% of hadron mass is generated. Data from the current 12 GeV era are extending this coverage to about 50%.Looking ahead, researchers aim to utilize future high-energy beam upgrades to map the full range of distances where the dominant portion of the universe’s mass emerges.“This is the payoff from what we’ve been doing at Jefferson Lab for decades,” concluded Daniel Carman, an experimental nuclear physicist at Jefferson Lab. “We still have a lot of work ahead, but this marks a major milestone along the way.”
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