Image description: The quarks and gluons inside protons are connected by color charge flux tubes. If a tube is broken, new tubes immediately form between the present quarks.
To date, two types of hadrons are known:
• mesons, composed of one quark and one antiquark with complementary color charges (forming a "white" combination),
• baryons, composed of three quarks with red, green, and blue color charges, whose combination is also "white".
This analogy with additive color synthesis inspired the term "color charge" to describe the strong interaction.
Quarks, leptons, and bosons are the elementary particles that make up all known matter. These quantum entities are described by the Standard Model, experimentally validated by the discovery of quarks (1995), the tau neutrino (2000), and the Higgs boson (2012).
Protons and neutrons, constituents of atomic nuclei, are composite (non-elementary) particles. They belong to the hadron family, alongside about a hundred other ephemeral particles such as mesons (pion, kaon, eta, rho, etc.). All mesons are unstable, with lifetimes between 10-8 and 10-23 seconds. Only the proton is stable, with an estimated lifetime of 1036 years. Neutrons, stable within a nucleus, decay in about 880.3 seconds (≈ 15 minutes) when free.
The theory describing the strong interaction, responsible for the cohesion of nuclei and the mass of quarks and gluons, is quantum chromodynamics (QCD), proposed in 1973 by David Gross, Frank Wilczek, and Hugh David Politzer (Nobel Prize 2004).
The mass of a proton (2 up quarks + 1 down quark) or a neutron (1 up quark + 2 down quarks) does not correspond to the sum of the masses of its constituents:
Quarks account for only ~1% of the mass of nucleons!
The missing mass is explained by:
Quark confinement prevents their isolation: they are bound by gluons, electrically neutral particles but carriers of a color charge.
Unlike other fundamental forces (electromagnetic, weak, gravitational), the strong interaction increases with distance between quarks. This phenomenon, called asymptotic freedom, implies that:
Inside hadrons, quark-antiquark pairs appear and disappear constantly, forming a dynamic "sea". If a quark is ejected during a collision, the released energy immediately creates new particles (pions, kaons, etc.), never leaving a quark isolated.
Example: During an energetic collision, a quark pulled from a proton generates a new hadron (like a pion), but the initial quarks remain confined in the original nucleon.
QCD describes this mechanism by:
Thus, a proton or neutron is not a static set of 3 quarks, but a quantum soup where quarks, antiquarks, and gluons interact constantly. The net balance remains constant: 2 up quarks + 1 down for a proton, and 2 down + 1 up for a neutron.
This complex structure allows:
Confinement is a property of particles with a color charge:
This principle underlies the very existence of visible matter.
In the world of particles, some are as stable as immovable rocks, while others disappear in an instant, as fleeting as a flash. For example, a proton can persist for billions of billions of years, while a rho meson exists for an infinitely brief time—so short that a light beam would travel less than the thickness of a hair during its entire lifetime. Where does this radical difference come from? It is explained by the fundamental laws of quantum physics and the specific interactions that determine the behavior of each particle.
In summary: The lifetime of a particle depends on:
Only three particles in the Standard Model are considered absolutely stable (or at least, with a lifetime so long that it exceeds the age of the universe):
| Particle | Type | Estimated Lifetime | Role in the Universe |
|---|---|---|---|
| Electron | Lepton | > 1020 years* | Constituent of atoms, charge carrier |
| Proton | Baryon | > 1036 years | Nucleus of atoms (with neutrons) |
| Photon | Boson | ∞ (stable) | Transport of light and electromagnetic interactions |
| Neutrino | Lepton | ∞ (stable for all 3 types)** | Result from nuclear reactions (Sun, supernovas) |
| * Experimental limit in 2025. No electron decay has been observed. ** Neutrinos are stable in the Standard Model, but theories beyond predict extremely slow decay. | |||
N.B.: Theoretically, the electron could decay into a neutrino and a photon, but this has never been observed. Experimental limits (2025) place its lifetime well beyond 1020 years.
Neutrinos are stable in the Standard Model, but theories beyond (such as Grand Unification) predict extremely slow decay.
Why Are They Stable?
Most particles are ephemeral, with lifetimes ranging from nanoseconds to seconds. Their instability comes from two factors:
| Particle | Type | Average Lifetime | Typical Decay | Analogy |
|---|---|---|---|---|
| Free neutron | Baryon | 880 seconds | → Proton + electron + antineutrino | A tightrope walker who falls after 15 minutes |
| π⁰ meson | Meson | 8.5 × 10-17 s | → 2 photons (γ) | A soap bubble that bursts |
| K⁺ kaon | Meson | 1.2 × 10-8 s | → Muon + neutrino (63%) or pion (21%) | A spark in the night |
| Z boson | Boson | 3 × 10-25 s | → Electron + positron (or quarks) | A lightning bolt during a storm |
| Top quark | Quark | 5 × 10-25 s | → Bottom quark + W boson | A shooting star |
Concrete example: the neutron
Within an atomic nucleus, neutrons are stable thanks to the strong interaction that binds them to protons. But when isolated, a neutron decays in ~15 minutes via the weak force: n → p⁺ + e⁻ + ν̅e (neutron → proton + electron + electron antineutrino). This reaction is the origin of beta radioactivity, used in medicine (PET imaging) or archaeology (carbon-14 dating).
Beyond protons and neutrons, particle accelerators like the LHC (CERN) have revealed exotic hadrons:
Why Are They So Unstable?
These particles are excited states of quarks and gluons. Their high energy makes them "fragile": they quickly decay into lighter hadrons (such as pions or kaons) to reach a minimal energy state, in accordance with the principle of energy stability.
Is the proton truly stable? The Standard Model predicts it is, but some theories (such as Grand Unification or supersymmetry) suggest it could decay into: p⁺ → π⁰ + e⁺ (proton → neutral pion + positron), with a lifetime > 1036 years (1026 times the age of the universe!).
How to Search for It?
Detectors like Super-Kamiokande (50,000 tons of pure water) or Hyper-Kamiokande (under construction in 2025, 10 times more sensitive) monitor billions of protons in search of ultra-rare decay. So far, no evidence has been found, but these experiments continually push the boundaries of our knowledge.
Stakes: If proton decay were observed, it would be a revolution comparable to the discovery of the Higgs boson, proving that the fundamental forces (electromagnetic, strong, weak) were unified in the primordial universe.
The instability of particles is not just an academic subject; it has concrete applications:
Did You Know?
The neutron, unstable on its own, becomes stable in a nucleus thanks to the strong interaction. This property allows the existence of neutron stars: after a supernova collapse, neutrons, compressed to extreme densities, form a stable "quantum soup" for billions of years!
In 2025, several mysteries persist:
Experiment to Watch: The Future Circular Collider (FCC), planned for the 2040s at CERN, could reach energies of 100 TeV (compared to 13 TeV for the LHC), allowing the study of even more unstable particles or phenomena beyond the Standard Model.
In summary, the stability and instability of particles are not whims of nature, but the result of deep laws that balance energy, symmetries, and interactions. Understanding these mechanisms means unlocking the secrets of the universe, from the infinitely small to the infinitely large.
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