Newswise — NEWPORT NEWS, VA — Deep in the heart of matter, some numbers don’t add up. For example, while protons and neutrons are made of quarks, nature’s fundamental building blocks bound together by gluons, their masses are much larger than the individual quarks from which they are formed.
This leads to a central puzzle … why? In the theory of the strong interaction, known as quantum chromodynamics or QCD, quarks acquire their bare mass through the Higgs mechanism. The long-hypothesized process was confirmed by experiments at the CERN Large Hadron Collider in Switzerland and led to the Nobel Prize for Peter Higgs in 2013.
Yet the inescapable issue remains that “this mechanism contributes to the measured proton and neutron masses at the level of less than 2%,” said Victor Mokeev, a staff scientist and phenomenologist at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility. “This clearly demonstrates that the dominant part of the mass of real-world matter is generated through another mechanism, not through the Higgs. The rest arises from emergent phenomena.”
The unaccounted mass and how it arises have long been open problems in nuclear physics, but scientists at Jefferson Lab are now getting a more detailed understanding of this mass-generating process than ever before.
So, what gives protons and other strongly interacting particles (together called hadrons) their added “heft”? The answer lies in the dynamics of QCD. Through QCD, the strong interaction generates mass from the energy stored in the fields of the strongly interacting quarks and gluons. This is termed the emergence of hadron mass, or EHM.
Clarity Begins to Emerge
Over the past decade, significant progress has been made in understanding the dominant portion of the universe’s visible mass. This is driven by studies of the distance (or momentum) dependence of the strong interaction within a QCD-based approach known as the continuum Schwinger method (CSM).
Bridging CSM and experiments via phenomenology, physicists analyzed nearly 30 years of data collected at Jefferson Lab. The comprehensive effort has given scientists the most detailed look yet at the mechanisms responsible for EHM.
Spanning from the earliest experiments in the 1990s to the discoveries that would be enabled by a potential energy upgrade of Jefferson Lab’s high-intensity accelerator, this monumental study was recently featured on the cover of a special edition of the journal Symmetry.
“This is more than what you’d see from a single experiment or set of experiments,” said Daniel Carman, an experimental nuclear physicist at Jefferson Lab. “This is the payoff from what we’ve been doing at Jefferson Lab for decades. We still have a lot of work ahead, but this marks a major milestone along the way.”
Deep in the Quest to Understand the Emergence of Mass
QCD describes the dynamics of the most elementary constituents of matter known so far: quarks and gluons. Through QCD processes, all hadronic matter is generated. This includes protons, neutrons, other bound quark-gluon systems and, ultimately, all atomic nuclei. A distinctive feature of the strong force is gluon self-interaction.
“Without gluon self-interaction, the universe would be completely different,” Mokeev said. “It creates beauty through different particle properties and makes real-world hadron phenomena through emergent physics.”
Because of this property, the strong interaction evolves rapidly with distance. This evolution of strong-interaction dynamics is described within the CSM approach. At distances comparable to the size of a hadron, ~10-13 cm, its relevant constituents are no longer the bare quarks and gluons of QCD. Instead, dressed quarks and dressed gluons emerge when bare quarks and gluons are surrounded by clouds of strongly coupled quarks and gluons undergoing continual creation and annihilation.
In this regime, dressed quarks acquire dynamically generated masses that evolve with distance. This provides a natural explanation for EHM: a transition from the nearly massless bare quarks (with masses of only a few MeV) to fully dressed quarks of approximately 400 MeV mass. Strong interactions among the three dressed quarks of the proton generate its mass of about 1 GeV, as well as the masses of its excited states in the range of 1.0-3.0 GeV.
This raises the question: Can EHM be elucidated by mapping the momentum dependence of the dressed-quark mass from experimental studies of the proton and its excited states?
Experiments at Jefferson Lab – Showdown in Connecting to Theory
The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab is a DOE Office of Science user facility that supports one of the nation’s largest communities of scientific users. It delivers high-intensity electron and photon beams, with energies up to 12 GeV, onto nuclear targets in Jefferson Lab’s four experimental halls.
Inside Experimental Hall B sits a three-story-tall detector: the CEBAF Large Acceptance Spectrometer for 12 GeV (CLAS12). Upgraded from its 6 GeV predecessor (CLAS), CLAS12 is unique in its ability to identify the particles produced when electrons scatter off protons while covering a wide range of particle emission angles.
Studies from data collected with CLAS and CLAS12, which probe the proton and lead to the creation of its excited states, allow the structure of these states to be unraveled. This experimentally derived information can be directly compared with predictions of CSM that encompass the distance-dependent evolution of quarks and gluons to test the EHM paradigm in detail.
These investigations, comparing experiment with theory, demonstrate conclusively that dressed quarks with dynamically generated masses are the active degrees of freedom underlying the structure of the proton and its excited states. They also solidify the case that experimental results from Jefferson Lab can be used to assess the mechanisms responsible for EHM.
“This kind of work requires synergy between experiment, phenomenology, and theory,” Carman said. “You need all of these different contributors working together in close collaboration to get at the physics we’re trying to uncover.”
Completing the Picture
“We see much more work ahead,” Mokeev said.
Experiments from the 6 GeV era of CEBAF explored the dressed-quark momentum (or distance) range in which roughly 30% of hadron mass is generated. Data from CEBAF’s current 12 GeV era — still being collected and analyzed — are extending this coverage to about 50%. Future experiments with a higher-energy electron beam will enable full coverage of the distance domain where the dominant portion of hadron mass emerges.
“When we get this information from the data of future experiments, we will be able to map out the full range of distances where the dominant part of hadron mass and structure emerge,” Mokeev said.
These results were recently published in Symmetry 17, 1106 (2025).
— Daniel Carman and Victor Mokeev contributed to this story.
Other Articles of Interest from Jefferson Lab
A New View of the Proton and its Excited States
Charming Experiment Finds Gluon Mass in the Proton
Scientists Locate the Missing Mass Inside the Proton
Determining the gluonic gravitational form factors of the proton
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DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science
