Modern physics seeks to answer what the universe is fundamentally made of. While atoms were once thought to be the smallest units, discoveries revealed a deeper reality. An elementary particle is a subatomic particle with no known substructure, meaning it cannot be broken down into smaller components. This distinguishes them from composite particles, such as protons and neutrons, which are made up of smaller constituent parts.
The Standard Model Framework
The Standard Model of particle physics serves as the most complete and accurate theory describing these fundamental building blocks and three of the four known forces of nature. This model acts as a kind of periodic table for the subatomic world, categorizing the particles and explaining their behavior. It successfully accounts for the electromagnetic force, the strong nuclear force, and the weak nuclear force. The particles within this framework fall into two broad groups based on their intrinsic spin property: Fermions and Bosons.
Fermions are the matter particles, characterized by half-integer spin. They obey the Pauli Exclusion Principle, meaning no two identical Fermions can occupy the exact same quantum state simultaneously, which explains the structure and stability of atoms. Bosons, conversely, are the force-carrying particles, possessing an integer spin, and they mediate the interactions between Fermions.
The Fundamental Building Blocks of Matter
The matter particles, or Fermions, are further divided into two families: the quarks and the leptons. Both groups consist of six distinct types, or “flavors,” which are organized into three generations of increasing mass. Only the particles in the first generation—the lightest ones—are stable enough to form the atoms that constitute ordinary, everyday matter.
Quarks
Quarks are the only elementary particles that experience all four fundamental forces. There are six flavors of quarks: up, down, charm, strange, top, and bottom. The up and down quarks are the most familiar because they are the constituents of protons and neutrons in the atomic nucleus. A proton is composed of two up quarks and one down quark, while a neutron consists of one up quark and two down quarks.
Quarks possess “color charge,” which is distinct from electric charge and relates to the strong nuclear force. Quarks are never observed in isolation but are always bound together in composite particles called hadrons. This confinement occurs because the strong force grows stronger as quarks are pulled apart, preventing the direct observation of a single quark. The top quark is the most massive elementary particle, weighing approximately as much as an entire gold atom.
Leptons
Leptons are the other family of matter particles, and they do not participate in the strong nuclear force. This family also contains six members, divided into three charged leptons and three neutral neutrinos. The charged leptons are the electron, the muon, and the tau particle, all of which carry a negative electric charge. The electron is the most well-known, as it is the particle that orbits the nucleus and facilitates all chemical reactions.
The muon and the tau particle are essentially heavier, unstable copies of the electron, rapidly decaying into lighter particles shortly after they are created. Each charged lepton has an electrically neutral partner: the electron neutrino, the muon neutrino, and the tau neutrino. Neutrinos are almost massless and interact with matter only via the weak nuclear force and gravity, allowing billions of them to pass through the Earth every second without detection.
The Force Carriers
Interactions between matter particles are facilitated by the exchange of elementary particles called gauge bosons. These bosons are the quanta of the force fields, mediating the fundamental forces. When two Fermions interact, they exchange these bosons, which transmit momentum and energy between them.
The electromagnetic force, which governs light, electricity, and magnetism, is carried by the massless photon. Photons are exchanged between all particles that possess an electric charge, causing them to attract or repel one another. This force has an infinite range and is what binds electrons to the atomic nucleus. The strong nuclear force, which holds quarks together inside protons and neutrons, is mediated by the gluon.
Gluons are unique because they carry the color charge of the strong force, meaning they interact with each other, complicating the binding of quarks. The weak nuclear force, responsible for radioactive decay and the nuclear fusion that powers stars, is carried by the W and Z bosons. Unlike the photon and gluon, the W and Z bosons are massive, which explains why the weak force has an extremely short range, operating only within the confines of the atomic nucleus.
The Particle That Gives Mass
One particle in the Standard Model stands apart from the matter particles and the traditional force carriers: the Higgs boson. Its existence is an excitation of the Higgs field, a quantum field that is theorized to permeate all of space. The Higgs field was proposed to resolve a serious problem in the Standard Model, which initially predicted that all elementary particles should be massless.
Mass arises from the strength of a particle’s interaction with the Higgs field. Particles that interact strongly with the field, like the top quark, experience resistance to changes in motion, which is perceived as greater mass. Conversely, particles like the photon do not interact with the Higgs field and therefore remain massless.
The discovery of the Higgs boson in 2012 at the Large Hadron Collider confirmed the existence of this pervasive field and completed the particle inventory of the Standard Model. The Higgs mechanism explained why the W and Z bosons are heavy while the photon is not, providing a coherent picture of mass generation for many fundamental particles.
Unanswered Questions in Particle Physics
Despite its success, the Standard Model is not a complete theory of nature and leaves several mysteries unanswered. The most glaring omission is the exclusion of gravity, the fourth fundamental force. The model is incompatible with General Relativity, which describes gravity as the curvature of spacetime, and it does not include a force carrier particle for gravity, often called the graviton.
Furthermore, the Standard Model only accounts for the ordinary matter that makes up stars, planets, and people, which constitutes a mere five percent of the total mass-energy content of the universe. The remaining 95 percent is composed of mysterious substances called dark matter and dark energy, neither of which has any particle component within the Standard Model. Dark matter’s existence is inferred from its gravitational effects on galaxies, while dark energy is invoked to explain the accelerating expansion of the universe.
Another puzzle concerns the neutrino, which the original Standard Model predicted to be massless. Experiments have shown that neutrinos can oscillate, or change from one flavor to another, a feat only possible if they possess a small but non-zero mass. This discovery indicates a flaw in the Standard Model’s assumptions about neutrinos and suggests the presence of new physics beyond its current boundaries.