Particle physics

Particle physics—often termed high-energy physics—is the branch of physics concerned with the elementary constituents of matter and radiation, and with the fundamental interactions governing their behaviour. While nuclear physics focuses on assemblies of protons and neutrons, particle physics investigates structures at a deeper scale, studying quarks, leptons, gauge bosons and their associated quantum fields. The discipline operates at the intersection of experimental and theoretical science, employing both particle accelerators and advanced mathematical frameworks to uncover the underlying order of the universe.

Historical Development

Ideas about matter being composed of indivisible units date back to early Greek philosophy, but modern particle physics emerged gradually from nineteenth- and twentieth-century scientific advances. John Dalton’s atomic theory provided the first scientific model of indivisible atomic units, although later research established that atoms themselves contain smaller constituents such as electrons. The discoveries of nuclear fission and fusion, alongside the development of quantum mechanics, reshaped the understanding of atomic structure.
During the mid-twentieth century, high-energy experiments revealed an unexpectedly large variety of subatomic particles—informally dubbed the “particle zoo”. Important observations such as CP violation, discovered by James Cronin and Val Fitch, raised questions about matter–antimatter asymmetry in the universe. The subsequent development of quantum field theory and the Standard Model in the 1970s provided a unifying framework to classify and explain the zoo as composite structures of a smaller number of fundamental particles.

The Standard Model of Particle Physics

The Standard Model remains the central theoretical structure of particle physics. It describes three of the four known fundamental interactions—electromagnetism, the weak interaction and the strong interaction—through mediating gauge bosons. These include eight gluons, the W⁺, W⁻ and Z⁰ bosons, and the photon. The model also includes 24 fundamental fermions, consisting of twelve matter particles and their antiparticles, and predicts the existence of the Higgs boson, discovered experimentally in 2012 at CERN’s Large Hadron Collider.
Although extremely successful, the Standard Model is regarded as incomplete. It does not incorporate gravity, does not account fully for dark matter and dark energy, and requires extensions to explain non-zero neutrino masses. The search for a universal framework—a potential “theory of everything”—motivates research into quantum gravity, string theory and supersymmetry.

Subatomic Particles and Their Properties

Modern particle physics investigates a range of particles and excitations that emerge from underlying quantum fields. These include:

• Electrons, protons and neutrons, the familiar constituents of atoms. Protons and neutrons are baryons, composite particles formed from quarks.
• Leptons, such as electrons, muons, tau particles and their associated neutrinos.
• Photons, neutrinos and muons, which participate in radioactive decay and scattering processes.
• Numerous exotic particles, discovered at high energies, that expand understanding of fundamental symmetries and interactions.

All particle interactions can be described by quantum field theory, with particles represented as quantum states in a Hilbert space. Their behaviour reflects the principles of quantum mechanics, including wave–particle duality.

Quarks, Leptons and Their Organisation

The Standard Model classifies the fermions—particles of half-integer spin—as quarks and leptons. Ordinary matter consists primarily of first-generation particles: the up and down quarks, the electron and the electron neutrino. All fermions obey the Pauli exclusion principle.
Quarks have fractional electric charges (±⅓ or ±⅔) and a property known as colour charge, which drives the strong interaction. Due to colour confinement, isolated quarks cannot be observed; they bind into hadrons:

• Baryons, composed of an odd number of quarks, such as protons and neutrons.
• Mesons, composed of a quark–antiquark pair, which are highly unstable and often produced in cosmic-ray collisions or particle accelerators.

There are three known generations of quarks (up/down, charm/strange, top/bottom) and three generations of leptons (electron, muon, tau and their associated neutrinos), with strong evidence that no fourth generation exists.

Bosons and Fundamental Forces

Bosons—particles with integer spin—mediate the fundamental interactions:

• Photons transmit electromagnetic forces.
• W and Z bosons govern the weak interaction responsible for certain types of radioactive decay.
• Gluons mediate the strong interaction, binding quarks into hadrons.

The Higgs boson is responsible for giving mass to the W and Z bosons and to fermions via the Higgs mechanism. Gluons and photons are massless, enabling long-range or highly constrained interactions depending on the context.
Bosons do not follow the Pauli exclusion principle and may occupy the same quantum state.

Antiparticles and Antimatter

Most fundamental particles have associated antiparticles, which share identical mass but carry opposite electric or quantum charges. For instance, the positron is the antiparticle of the electron, bearing a positive charge. Some particles, such as the photon, are their own antiparticles. Matter and antimatter annihilate on contact, suggesting the possibility of exotic forms of matter composed entirely of antiparticles.

Experimental and Theoretical Particle Physics

Experimental particle physics investigates high-energy interactions via radioactive decay, cosmic rays and particle accelerators such as the Large Hadron Collider. These experiments test theoretical predictions, search for new particles and map the behaviour of fundamental interactions.
Theoretical particle physics explores the mathematical structures underlying particle interactions. It interfaces with cosmology in explaining early-universe conditions, baryon asymmetry and dark matter candidates. Discoveries such as the Higgs boson exemplify the interplay between theory and experiment.