Baryons are the heavy subatomic particles built from three quarks, and they include the two particles that make up virtually all visible matter: protons and neutrons. Every atom in your body, in the air you breathe, and in every star you can see is overwhelmingly baryonic by mass. Yet baryons are far more than just the building blocks sitting inside atomic nuclei. They come in dozens of varieties, they anchor some of the biggest unsolved problems in physics, and their faint imprint on the structure of the universe serves as a measuring stick for cosmology itself.
What Baryons Are Made Of
A baryon is any particle composed of three quarks bound together by the strong nuclear force. The word comes from the Greek barys, meaning heavy, because baryons are heavier than the other major family of strongly interacting particles, the mesons (which contain one quark and one antiquark). The proton is made of two up quarks and one down quark; the neutron has two down quarks and one up quark. Those two account for all the stable baryonic matter in the universe today, but they are just the lightest members of a much larger family.
What gives baryons most of their mass is not the quarks themselves. The three quarks inside a proton contribute only about one percent of its total mass. The rest comes from the energy of the gluon field that holds them together and from the kinetic energy of the quarks whipping around inside. Recent lattice calculations have started pinning down exactly how this mass breaks down. A 2025 study validated several theoretical frameworks for decomposing hadron mass from first principles, finding that gluonic contributions account for a substantial share of the mass even in particles made of heavier quarks, roughly 15 percent in charmonium states depending on the decomposition scheme used, and that the trace anomaly of quantum chromodynamics plays a measurable role in generating mass.1PubMed. Lattice-QCD Validation of Hadron Mass and Trace-Anomaly Decomposition Sum Rules In lighter baryons like the proton, the gluonic fraction is even larger. The point is that baryonic mass is overwhelmingly a product of the strong force, not of the quarks’ intrinsic mass.
The Baryon Family Tree
Beyond protons and neutrons, there is an entire zoo of baryons. The lightest excited state is the Delta(1232), about 300 MeV heavier than the nucleon. It was one of the first resonances discovered in particle physics and remains the best studied, decaying almost entirely into a nucleon plus a pion. The quark model predicts many more resonances than have actually been observed: below 2 GeV, only 13 nucleon and Delta resonances carry the highest confidence rating from the Particle Data Group, while models expect far more to exist.2arXiv. Baryons as relativistic three-quark bound states – Section: 2.1 The nucleon and its resonances This shortfall, known as the “missing resonances” problem, remains an active area of research. Either the models predict states that do not exist, or experimenters have not yet found them because they are broad, overlapping, or couple weakly to the channels being measured.
Replace one or more of the light quarks with a heavier flavor and you get strange, charmed, or bottom baryons. Lambda baryons contain one strange quark. The Xi (or cascade) baryons contain two strange quarks, and the Omega baryon contains three. Moving to heavier quarks, charmed baryons like the Lambda_c and bottom baryons like the Lambda_b have been catalogued at collider experiments. A comprehensive review of this landscape notes many similarities between the charmed and bottom baryon spectra, as predicted by heavy-quark symmetries, alongside differences in mass splittings that potential models can explain.3Reports on Progress in Physics. 70 years of hyperon spectroscopy: a review of strange Ξ, Ω baryons, and the spectrum of charmed and bottom baryons None of these heavier baryons are stable in isolation. They decay in fractions of a second, usually through the weak force, cascading down to protons and neutrons plus lighter particles.
Pentaquarks and the Boundaries of the Definition
The traditional rule is three quarks for a baryon, but nature turns out to be more flexible. In 2015, the LHCb experiment at CERN reported the discovery of two pentaquark states, dubbed Pc(4380) and Pc(4450), observed as peaks in the decay products of a bottom baryon. These particles contain five quarks rather than three: specifically, a combination that includes a charm quark and a charm antiquark hidden inside, along with three lighter quarks. Theoretical work interprets them as loosely bound molecular states, pairs of conventional hadrons orbiting each other at close range rather than a single compact bag of five quarks.4PubMed. Identifying Exotic Hidden-Charm Pentaquarks
Since 2015, additional pentaquark candidates have been found, including states containing strange quarks. The short-range color force between quarks appears to play a central role in organizing these exotic states, and the same framework that accounts for the observed hidden-charm pentaquarks also predicts analogous states in the hidden-bottom sector, where experimental data are still lacking.5Symmetry. Exotic Heavy Hadrons Whether pentaquarks should really be called “baryons” is a matter of convention. They carry baryon number +1, just like a proton, so in that formal sense they qualify. But their internal structure is so different from a simple three-quark arrangement that many physicists treat them as a separate category of hadron.
Baryon Number and the Stability of Matter
Every baryon carries a quantum number called baryon number, equal to +1. Antibaryons carry −1. In every reaction physicists have ever observed, the total baryon number before and after stays the same. This conservation law is the reason protons do not spontaneously fall apart: there is no lighter baryon for a proton to decay into, and producing anything else would violate baryon number conservation. Your body’s atoms are stable, in other words, because of a bookkeeping rule that nature appears to enforce rigorously.
The catch is that within the Standard Model of particle physics, baryon number conservation is an accidental symmetry rather than a deep fundamental principle. It emerges from the structure of the theory but is not imposed by hand. Many proposed extensions of the Standard Model, including grand unified theories, predict that baryon number should be violated at some incredibly low rate.6Encyclopedia of Particle Physics. Experimental Tests of Baryon and Lepton Number Conservation If protons do decay, the predicted lifetime is so long (well beyond 10^34 years) that catching one in the act requires watching enormous quantities of matter for years. Experiments like Super-Kamiokande have done exactly that, monitoring tens of thousands of tons of ultra-pure water, and have so far seen nothing. The proton’s lifetime, if it is finite at all, is staggeringly long.
Free Neutrons and a Lingering Puzzle
While protons appear to be effectively immortal, neutrons outside an atomic nucleus are not. A free neutron decays through the weak force, emitting an electron and an antineutrino to become a proton, with a half-life of roughly ten minutes. Measuring that lifetime precisely turns out to be surprisingly contentious. Two experimental techniques, one trapping cold neutrons in a bottle and counting survivors, the other passing a beam of neutrons through a detector and counting decay products, have produced results that disagree by about eight seconds. That may sound trivial, but it is several times larger than the uncertainties either method claims.7Physical Review D. Exciting hint toward the solution of the neutron lifetime puzzle
The discrepancy has resisted resolution for over a decade. One possibility is a mundane systematic error in one or both techniques. A more exciting possibility is that the neutron occasionally decays into something the beam experiments cannot detect, like a dark matter particle. Neither explanation has been confirmed, and the puzzle remains one of the more intriguing small-scale mysteries in particle physics.
Why Baryonic Matter Exists at All
If the Big Bang produced equal amounts of matter and antimatter, every baryon should have been annihilated by an antibaryon in the first moments of the universe, leaving behind nothing but radiation. Obviously that did not happen. The fact that baryonic matter exists today means the early universe generated a tiny surplus of matter over antimatter, roughly one extra baryon for every billion baryon-antibaryon pairs. How that asymmetry arose is one of the great open questions in physics.
In the 1960s, Andrei Sakharov identified three conditions any successful explanation must satisfy: there must be processes that violate baryon number conservation, there must be violations of certain symmetries between matter and antimatter (C and CP symmetry), and these processes must occur out of thermal equilibrium.8IOP Publishing (New Journal of Physics). Matter and antimatter in the universe The logic is clean: without baryon number violation, you cannot go from zero net baryons to nonzero. Without CP violation, every process that favors matter has a mirror process favoring antimatter by the same amount. And in thermal equilibrium, all production and destruction rates balance, so no net asymmetry can build up. All three conditions are necessary. The Standard Model technically satisfies all three, but the amount of CP violation it contains is far too small to generate the observed asymmetry. Something beyond the Standard Model must have been at work.
Counting Baryons With the Big Bang
Within the first few minutes after the Big Bang, the universe was hot and dense enough for nuclear fusion to occur on a cosmic scale. Protons and neutrons combined to form the lightest atomic nuclei: deuterium, helium-3, helium-4, and traces of lithium-7. The precise abundances of these elements depend sensitively on the total density of baryonic matter in the universe, because a denser baryon soup fuses more efficiently. By comparing the predicted and observed abundances of these light elements, physicists have determined the baryon density to sit between about 1.7 × 10^−31 and 4.1 × 10^−31 grams per cubic centimeter, which corresponds to roughly 1 to 15 percent of the universe’s critical density.9PubMed. Big-bang nucleosynthesis and the baryon density of the universe
That range has been narrowed considerably since those early estimates. Measurements of deuterium in pristine gas clouds far from any galaxy, combined with the nucleosynthesis calculations, yield a baryon density parameter that agrees precisely with the independent value measured from the cosmic microwave background.10Planetary and Space Science. Deuterium and big-bang nucleosynthesis: implications for the baryon density This agreement is one of the great triumphs of modern cosmology: two completely different methods, one based on nuclear physics in the first minutes and the other based on the pattern of hot and cold spots in microwave radiation from 380,000 years later, arrive at the same number. Baryonic matter makes up about 5 percent of the universe’s total energy budget. Dark matter accounts for roughly 27 percent, and dark energy the remaining 68 percent.
The Missing Baryon Problem
Even within that 5 percent, there has been a nagging accounting problem. When astronomers added up all the baryons they could see in galaxies, clusters of galaxies, and cold intergalactic gas, the total fell well short of what Big Bang nucleosynthesis and the cosmic microwave background said should be there. As much as 30 to 40 percent of the baryons seemed to be missing. The leading theory was that these baryons existed in a warm-hot intergalactic medium, gas heated to temperatures between about 100,000 and 10 million Kelvin, too cool to glow brightly in X-rays but too hot to show up in ultraviolet absorption surveys.
Detecting this diffuse, hot gas has been one of the harder observational challenges in astrophysics. A breakthrough came with the detection of highly ionized oxygen absorbers in the X-ray spectra of distant quasars. Two such absorbers were found in regions of higher-than-average galaxy density, consistent with numerical simulations of the warm-hot intergalactic medium.11Nature. Observations of the MIssing Baryons in the warm-hot intergalactic medium A separate study stacking X-ray data along a different quasar sightline detected an oxygen absorption line at a statistical significance sufficient to call it a definitive detection of the warm-hot intergalactic medium.12The Astrophysical Journal. Detection of the Missing Baryons toward the Sightline of H1821+643
A completely different approach has used fast radio bursts, brief flashes of radio waves from distant galaxies. Because these signals are dispersed by every free electron along their path, they act as a kind of baryon census for the entire line of sight. An analysis of 22 localized fast radio bursts derived a cosmic baryon density consistent with the value from the cosmic microwave background, supporting the conclusion that the “missing” baryons are not actually missing but are spread through the intergalactic medium in a form that is simply hard to see.13PubMed Central. Finding the missing baryons in the intergalactic medium with localized fast radio bursts
Baryon Acoustic Oscillations as a Cosmic Ruler
Before atoms formed, the early universe was a hot plasma in which photons and baryons were tightly coupled. Pressure waves, essentially sound waves, rippled through this plasma at a speed determined by its temperature and composition. When the universe cooled enough for atoms to form (about 380,000 years after the Big Bang), the waves froze in place. The distance each wave had traveled by that point, roughly 500 million light-years in today’s expanded universe, left a characteristic imprint on the distribution of galaxies. Galaxies are slightly more likely to be found separated by that distance than by a somewhat shorter or longer one.
This imprint, called baryon acoustic oscillations, serves as a standard ruler for cosmology. Because the length of the ruler is set by well-understood physics in the early universe, measuring its apparent size at different cosmic epochs reveals how the universe has expanded over time.14PubMed. Signatures of the Primordial Universe from Its Emptiness: Measurement of Baryon Acoustic Oscillations from Minima of the Density Field This makes baryon acoustic oscillations one of the most powerful tools for studying dark energy. The oscillation wavelength in the galaxy power spectrum is highly model-independent, depending on fundamental linear physics rather than complex astrophysical assumptions.15The Astrophysical Journal. Probing Dark Energy Using Baryonic Oscillations in the Galaxy Power Spectrum as a Cosmological Ruler
Large galaxy surveys like the Sloan Digital Sky Survey and the Dark Energy Spectroscopic Instrument have measured baryon acoustic oscillations at multiple redshifts. These measurements, combined with cosmic microwave background data, tightly constrain the cosmological equation of state, the parameter that characterizes whether dark energy is constant or changing over time.16The Astrophysical Journal. Dark Energy Constraints from Baryon Acoustic Oscillations Future gravitational wave observations may provide an independent way to calibrate the sound horizon, offering a model-independent cross-check on these measurements.17PubMed. Model-Independent Test of Prerecombination New Physics: Measuring the Sound Horizon with Gravitational Wave Standard Sirens and the Baryon Acoustic Oscillation Angular Scale
How Baryons Shape Galaxies
On the scale of individual galaxies and galaxy clusters, baryons play a more active role than you might expect from their minority share of the cosmic mass budget. When gas falls into a dark matter halo and condenses to form stars, the concentration of baryonic mass at the center pulls dark matter inward through a process called adiabatic contraction. This steepens the dark matter density profile, making the center of the halo denser than it would be without baryons.
But baryons also fight back. Supernova explosions inject enormous amounts of energy into the surrounding gas, and supermassive black holes at the centers of galaxies launch powerful jets and winds. These feedback processes can blow gas out of galaxies entirely, reducing the baryonic content of a halo and, through the resulting gravitational reshuffling, even flattening the central dark matter profile into a core rather than a cusp. Simulations that include feedback from active galactic nuclei produce dark matter cores of around 10 kiloparsecs in galaxy clusters, compared to the centrally peaked profiles found in simulations without such feedback.18Monthly Notices of the Royal Astronomical Society. The effects of baryon physics, black holes and active galactic nucleus feedback on the mass distribution in clusters of galaxies Observations of some clusters show exactly this kind of flat core, suggesting that baryonic feedback genuinely reshapes the dark matter structures they live in.
The relative importance of supernova feedback versus black hole feedback depends on the mass of the halo. In lower-mass halos, supernovae dominate because the gravitational potential is shallow enough for supernova-driven winds to expel gas. In more massive halos, the deeper gravitational well traps supernova ejecta, and it takes the far more energetic output of an active galactic nucleus to push gas to large distances. Simulations show that AGN feedback dominates over supernova feedback in a broad range of halo masses centered around galaxy-group scales, while at the highest masses the two contribute comparably.19arXiv. Roles of SN and AGN Feedback in Shaping the Baryonic Content in a Wide Range of Dark Matter Halo Mass – Section: III.2 Roles of SN and AGN feedback Getting these feedback recipes right is one of the central challenges in modern galaxy-formation simulations, and the constraints keep tightening as observations of the gas surrounding galaxies improve.20Monthly Notices of the Royal Astronomical Society. The case for large-scale AGN feedback in galaxy formation simulations: insights from XFABLE
Baryons Under Extreme Pressure
Neutron stars represent the most extreme baryonic objects in the universe. When a massive star exhausts its nuclear fuel and its core collapses, the result is a ball of matter roughly the mass of the Sun packed into a sphere about 20 kilometers across. The density at the center reaches several times the density of an atomic nucleus, conditions that cannot be reproduced in any laboratory on Earth.
At these densities, the identity of individual baryons starts to blur. Neutrons are squeezed so tightly that their quark wave functions overlap, and heavier baryons, hyperons containing strange quarks, become energetically favorable to produce. The appearance of hyperons softens the equation of state (the relationship between pressure and density), which should make it harder for the star to support itself against gravitational collapse. This creates what is known as the hyperon puzzle: measured neutron star masses, some approaching or exceeding two solar masses, are difficult to reconcile with theoretical models that include hyperons, because the softened equation of state predicts lower maximum masses than what is observed.21arXiv. The Hyperon Puzzle in Neutron Stars
Possible resolutions include repulsive three-body forces among hyperons that stiffen the equation of state at high density, or a transition to an entirely different phase of matter, a quark-gluon plasma, before hyperons ever become abundant. Modern calculations using nuclear interactions derived from chiral perturbation theory, incorporating the Delta(1232) resonance as an intermediate state, have produced equations of state that can support massive neutron stars while remaining consistent with nuclear physics constraints at lower densities.22Astronomy & Astrophysics. Equation of state of dense nuclear matter and neutron star structure from nuclear chiral interactions Gravitational wave observations from merging neutron stars are now providing direct constraints on the equation of state, because the way two neutron stars deform each other during their final inspiral depends on how stiff or soft their interiors are. This is one of the places where particle physics, nuclear physics, and astrophysics converge on the same question, and baryons sit at the center of it.