Protons and neutrons are built from smaller particles called quarks, held together by force-carrying particles called gluons. A proton contains two up quarks and one down quark, while a neutron contains one up quark and two down quarks. But that tidy accounting barely scratches the surface. The interior of a proton or neutron is a roiling environment of gluon fields, virtual quark-antiquark pairs, and pressure gradients that exceed anything found in a neutron star, and the mass you feel when you pick up a brick has almost nothing to do with the quarks themselves.
The Quarks Inside
Quarks come in six types, but only the two lightest, called “up” and “down,” matter for ordinary matter. An up quark carries an electric charge of +2/3, and a down quark carries −1/3. Stack two ups and a down and the charges sum to +1, giving you a proton. Swap one up for a down and the total charge drops to zero, giving you a neutron. These three quarks in each particle are called valence quarks because they determine the particle’s identity and electric charge.
Despite their importance, the valence quarks are not sitting still inside the proton like marbles in a bag. They zip around at speeds close to the speed of light, and their positions can only be described in terms of probabilities. Modern experiments have moved well beyond simple pictures of three dots inside a circle. Physicists now construct detailed three-dimensional maps of where quarks are likely to be found and how fast they are moving, a kind of tomography of the proton’s interior that has advanced dramatically over the past two decades.
1Progress in Particle and Nuclear Physics. Experimental exploration of the 3D nucleon structureGluons and the Strong Force
Quarks interact through something called the strong force, and gluons are the particles that carry it. If you have heard of photons carrying the electromagnetic force between charged particles, gluons play an analogous role between quarks. But gluons are far stranger than photons. Photons are electrically neutral and do not interact with each other. Gluons, by contrast, carry the strong force’s version of charge (called “color charge”) and interact with one another constantly. This self-interaction is what makes the strong force behave so differently from electromagnetism.
One consequence of gluon self-interaction is a phenomenon called confinement. When quarks are very close together, the strong force between them is surprisingly weak, and they move almost freely. But if you try to pull a quark away from its partners, the force does not diminish with distance the way gravity or electromagnetism does. Instead it stays roughly constant or even grows, like stretching a rubber band that never snaps until you have pumped in enough energy to create entirely new particles. This is why no experiment has ever detected a lone quark. Only bound combinations are observed: either a quark paired with an antiquark (called a meson) or three quarks bound together (called a baryon, the family that includes protons and neutrons).
2PubMed Central. Asymptotic freedom: From paradox to paradigmThe property that quarks interact weakly at very short distances is called asymptotic freedom, and its discovery in the 1970s was a turning point in physics. It explained a confusing experimental pattern: when high-energy electrons were fired into protons, the quarks inside scattered as though they were nearly free particles, yet nobody could knock one out. That tension between apparent freedom at short range and absolute imprisonment at long range is resolved by the mathematics of the strong force.
Where the Mass Really Comes From
Here is one of the most counterintuitive facts in physics: the quarks inside a proton account for only a small fraction of its mass. An up quark has a bare mass of roughly 2 MeV and a down quark about 5 MeV, so the three valence quarks together contribute maybe 9 or 10 MeV. The proton weighs about 938 MeV. That means more than 98% of the proton’s mass comes from something other than the quarks themselves.
Most of that mass comes from the energy of the gluon field and the kinetic energy of the quarks whipping around inside. Einstein’s E = mc² works in reverse here: the energy contained in the strong force field manifests as mass. Lattice calculations that simulate the strong force on supercomputers have broken this down into components. One detailed calculation found that quark energy contributes about 32%, gluon field energy about 36%, and the quantum effects associated with the strong force’s special symmetry-breaking properties (the trace anomaly) contribute about 23%, with a smaller piece from quark condensates making up the remainder.
3PubMed. Proton Mass Decomposition from the QCD Energy Momentum TensorA separate calculation using a different theoretical framework found strikingly consistent numbers: about 31.5% from quark energy, 34.7% from gluon field energy, 11.3% from quark condensates, and 22.5% from the trace anomaly.
4arXiv. Proton Gravitational Structure and Mass Decomposition on the Light FrontThe popular-science story that “the Higgs boson gives particles their mass” is technically true for the bare quark masses, but it accounts for less than 2% of the proton’s total mass. The dominant contribution arises from the gluonic structure of the vacuum and the breaking of a mathematical symmetry in the strong force.
5arXiv. Experimental access to the gluonic origin of the proton massThe Sea of Virtual Particles
If you could somehow freeze-frame the inside of a proton, you would not see just three quarks and some gluons. You would see a constantly churning soup. Gluons split into quark-antiquark pairs that pop in and out of existence, and those pairs can themselves radiate more gluons. These transient quark-antiquark pairs are called “sea quarks,” and they include not just up and down varieties but also heavier types like strange quarks, though heavier flavors are rarer.
The sea is not just a theoretical abstraction. High-energy scattering experiments can measure how much of the proton’s momentum is carried by sea quarks versus valence quarks versus gluons. At the energies probed by modern colliders, gluons carry close to half of the proton’s total momentum, with valence quarks carrying most of the rest and sea quarks contributing a smaller but measurable share. The exact breakdown depends on the energy scale at which you probe, because the quantum fuzziness of the interior looks different at different resolutions. That might sound paradoxical, but it is a well-understood feature of quantum field theory: the structure you see depends on how closely you look.
Extreme Pressure at the Core
In 2018, a team extracted the first measurement of the pressure distribution inside a proton, and the numbers are staggering. Near the proton’s center, out to about 0.6 femtometers (a femtometer is a millionth of a billionth of a millimeter), the pressure is strongly repulsive, pushing outward. Beyond that distance, a binding pressure pushes inward, holding the whole structure together. The average peak pressure near the center is around 10³⁵ pascals, which exceeds the estimated pressure inside neutron stars, the densest stable objects in the known universe.
6PubMed. The pressure distribution inside the protonThink about what that means. The proton is a subatomic particle roughly one femtometer across, yet the forces at its core are more intense than those at the heart of a collapsed star. That balance between an outward-pushing repulsive pressure near the center and an inward-pulling confining pressure at the edges is what gives the proton its stability. Subsequent theoretical models have confirmed this general picture of extreme pressures inside nucleons.
7PubMed Central. Pressure Distribution Inside Nucleons in a Tsallis-MIT Bag ModelWhy the Neutron Decays but the Proton Does Not
A free neutron sitting in empty space will decay in about ten minutes, transforming into a proton, an electron, and an antineutrino. At the quark level, what happens is straightforward: one of the neutron’s down quarks converts into an up quark, changing the quark lineup from up-down-down to up-up-down, which is a proton.
8OpenLearn. Particle physics – Section: 7 Beta-decay at the level of quarks and leptonsThis conversion is mediated by the weak nuclear force, not the strong force. The down quark emits a W⁻ boson, which almost instantly decays into an electron and an antineutrino. The neutron is slightly heavier than the proton (by about 1.3 MeV), and that tiny mass difference provides the energy needed for this transformation. Inside an atomic nucleus, neutrons are usually stabilized by the binding energy of the nucleus, which is why atoms with neutrons do not just fall apart.
The proton, on the other hand, appears to be extraordinarily stable. No experiment has ever observed a proton decaying. Some theoretical frameworks beyond the standard model predict that protons should eventually decay, but with lifetimes vastly longer than the current age of the universe. Massive underground detectors have been watching for proton decay for decades without seeing a single event, pushing the lower limit on the proton’s lifetime beyond 10³⁴ years.
The Proton Radius Puzzle
You might think that after all this understanding of the proton’s interior, physicists would at least agree on how big it is. For years, they did. Then, in 2010, a new measurement threw the field into confusion. Researchers measured the size of the proton using muonic hydrogen, an atom where the electron is replaced by its heavier cousin, the muon, which orbits much closer to the proton and is therefore more sensitive to the proton’s size. The radius they extracted disagreed significantly from the values established by ordinary hydrogen spectroscopy and electron-proton scattering experiments.
9arXiv. The Proton Radius PuzzleThe discrepancy was about 4%, which sounds small but was many times larger than the experimental uncertainties claimed by both methods. It was dubbed the “proton radius puzzle” and prompted years of investigation. Was something wrong with the muonic hydrogen measurement? With the electron experiments? Or was there new physics lurking in the difference between how muons and electrons interact with protons? Over the past decade, newer electron-based measurements have shifted toward the smaller muonic value, which has cooled some of the excitement. But the puzzle has not been declared fully resolved, and it continues to motivate precision experiments that probe the proton’s charge distribution at extraordinary accuracy.
How Scientists Peer Inside
You cannot cut a proton open and look inside with a microscope. Everything physicists know about the proton’s interior comes from scattering experiments and theoretical calculations. The basic idea behind scattering experiments goes back to Rutherford’s gold foil experiment, updated with far higher energies. You hurl a particle (an electron, a muon, or another proton) at a target proton at enormous speed and carefully measure what comes out. The angles and energies of the scattered particles reveal the distribution of charge, momentum, and mass inside.
Deep inelastic scattering, the technique that first revealed quarks in the late 1960s, remains the gold standard. More recent experiments use processes like deeply virtual Compton scattering and Drell-Yan scattering to build three-dimensional maps of the proton’s quark and gluon distributions. Planned facilities like the Electron-Ion Collider, being built at Brookhaven National Laboratory in New York, are designed specifically to produce the most detailed images yet of the gluon-dominated interior of the proton.
On the theory side, lattice QCD is the main tool. Physicists discretize space and time into a fine grid, place the equations of the strong force onto that grid, and use supercomputers to calculate what happens. These simulations can predict the proton’s mass, its internal pressure profile, and how its quarks and gluons are distributed, all from the fundamental equations of the strong force without any ad hoc assumptions. Large-scale lattice QCD simulations running on European supercomputing facilities continue to produce increasingly precise results for nucleon structure.
10Procedia Computer Science. Large-scale simulations of lattice QCD for nucleon structure using Nf=2+1+1 flavors of twisted mass fermionsQuark-Gluon Plasma and What Happens When Confinement Breaks
Under normal conditions, quarks and gluons are permanently locked inside protons and neutrons. But theory predicted early on that if you heat matter to trillions of degrees or compress it to extreme densities, the boundaries between individual protons and neutrons should dissolve. Quarks and gluons would roam freely in a new state of matter called quark-gluon plasma.
11ScienceDirect (Elsevier / Physics Reports). De-confinement and clustering of color sources in nuclear collisionsExperiments at the Relativistic Heavy Ion Collider (RHIC) in the United States and the Large Hadron Collider (LHC) at CERN have created quark-gluon plasma by smashing heavy nuclei together at nearly the speed of light. The resulting fireball exists for only a tiny fraction of a second before cooling and re-forming into ordinary particles, but its properties can be measured through the patterns of particles that fly out. Surprisingly, quark-gluon plasma does not behave like a gas of free particles. It flows more like a nearly perfect liquid, with extremely low viscosity. This discovery upended early expectations and remains an active area of research.
Quark-gluon plasma is thought to have filled the entire universe for the first few microseconds after the Big Bang, before temperatures dropped enough for quarks and gluons to condense into protons, neutrons, and other composite particles. Recreating and studying this plasma in the lab is, in a real sense, recreating the conditions of the earliest moments of the universe.
Beyond Protons and Neutrons
Protons and neutrons are the most familiar particles made of quarks, but they are not the only ones. Quarks can also form mesons (a quark paired with an antiquark) and a growing zoo of more exotic combinations. In recent years, experiments at the LHC and other facilities have confirmed the existence of tetraquarks (two quarks and two antiquarks bound together) and pentaquarks (four quarks and an antiquark). Lattice QCD simulations are increasingly being used to predict and understand these exotic states.
12Physics Reports. Tetraquarks and pentaquarks in lattice QCD with light and heavy quarksThese exotic particles are unstable and decay almost instantly, which is why they do not show up in everyday matter. But their existence confirms that the strong force permits a richer variety of bound states than just the simple two-quark and three-quark combinations known for decades. Whether even more exotic configurations exist, like six-quark “dibaryons” or gluon-only particles called “glueballs,” is an open question that theorists and experimenters are actively pursuing. A glueball would be particularly striking because it would be a particle made entirely of force-carriers with no quarks at all, a possibility that the strong force’s unusual self-interacting nature uniquely allows.
Why Quarks Inside Nuclei Behave Differently
You might assume that quarks inside a proton behave the same whether that proton is floating freely or packed into an atomic nucleus alongside dozens of other protons and neutrons. That assumption turns out to be wrong. In the 1980s, the European Muon Collaboration discovered that the momentum distributions of quarks inside nuclei are measurably different from those in free protons. This is called the EMC effect, and it remains one of the more puzzling findings in nuclear physics.
Recent theoretical work has connected the EMC effect to short-range correlations between nucleons, the brief moments when two protons or neutrons inside a nucleus overlap and interact at very close range. These high-momentum interactions appear to modify the internal quark structure of the participating nucleons. Calculations using realistic nuclear forces show that interactions producing more short-range correlations also produce a deeper EMC effect, supporting the idea that the two phenomena are linked.
13Physics Letters B. The EMC effect for few-nucleon bound systems in light-front Hamiltonian dynamicsThe EMC effect matters because it challenges the tidy picture of a nucleus as a collection of independent protons and neutrons whose internal structures do not talk to each other. Understanding precisely how and why nuclear surroundings alter quark behavior is one of the central goals of the Electron-Ion Collider and will likely keep nuclear physicists busy for the next several decades.