The word atom comes from the Greek atomos, meaning indivisible, yet the very first particle physicists to challenge this definition were not looking for smaller pieces but for the invisible forces holding them together. In the 6th century BC, ancient philosophers first proposed that matter was composed of fundamental units, but it was not until the 19th century that John Dalton used stoichiometry to prove that each element of nature consisted of a single, unique type of particle. By the early 20th century, however, the scientific community realized that atoms were not the fundamental building blocks of the universe but were instead conglomerates of even smaller particles, such as the electron. This realization sparked a century of exploration into nuclear physics and quantum physics, leading to the proof of nuclear fission in 1939 by Lise Meitner based on experiments by Otto Hahn, and nuclear fusion by Hans Bethe in that same year. These discoveries did more than just explain the stars; they paved the way for the development of nuclear weapons and fundamentally altered the course of human history. Bethe's 1947 calculation of the Lamb shift is credited with having opened the way to the modern era of particle physics, shifting the focus from the atom itself to the subatomic particles that compose it.
The Particle Zoo
Throughout the 1950s and 1960s, physicists faced a bewildering variety of particles that seemed to have no order, leading them to informally refer to the collection as the particle zoo. These particles were found in collisions of particles from beams of increasingly high energy, creating a chaotic landscape of discoveries that defied simple classification. It was not until the formulation of the Standard Model during the 1970s that physicists clarified the origin of this particle zoo, explaining the large number of particles as combinations of a relatively small number of more fundamental particles. This reclassification marked the beginning of modern particle physics, transforming chaos into a structured theory. Important discoveries such as the CP violation by James Cronin and Val Fitch brought new questions to matter-antimatter imbalance, challenging the understanding of why the universe is made of matter rather than antimatter. The Standard Model, which gained widespread acceptance in the mid-1970s after experimental confirmation of the existence of quarks, describes the strong, weak, and electromagnetic fundamental interactions using mediating gauge bosons, including eight gluons, W and Z bosons, and the photon.
The Fermions of Matter
Ordinary matter is made from first-generation quarks and leptons, collectively called fermions, which have a quantum spin of half-integers and obey the Pauli exclusion principle where no two particles may occupy the same quantum state. Quarks have fractional elementary electric charge of negative one-third or positive two-thirds, while leptons have whole-numbered electric charge of zero or negative one. There are three known generations of quarks, including up and down, strange and charm, and top and bottom, as well as three generations of leptons, including the electron and its neutrino, muon and its neutrino, and tau and its neutrino. Quarks also have color charge, labeled arbitrarily with no correlation to actual light color as red, green, and blue, which influences the strong interaction. Because the interactions between the quarks store energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently, a phenomenon called color confinement. This confinement ensures that quarks are always found in combinations, such as baryons made of three quarks or mesons made of two quarks.
The Bosons of Force
Bosons are the mediators or carriers of fundamental interactions, such as electromagnetism, the weak interaction, and the strong interaction, and all bosons have an integer quantum spin of zero or one. Electromagnetism is mediated by the photon, the quanta of light, while the weak interaction is mediated by the W and Z bosons. The strong interaction is mediated by the gluon, which can link quarks together to form composite particles, but due to color confinement, gluons are never observed independently. The Higgs boson gives mass to the W and Z bosons via the Higgs mechanism, while the gluon and photon are expected to be massless. The Standard Model also predicted the existence of a type of boson known as the Higgs boson, and on the 4th of July 2012, physicists with the Large Hadron Collider at CERN announced they had found a new particle that behaves similarly to what is expected from the Higgs boson. This discovery confirmed the mechanism by which particles acquire mass, a cornerstone of the Standard Model that had been postulated theoretically for decades before being confirmed by experiments.
Antimatter and Annihilation
Most particles have corresponding antiparticles, which compose antimatter, and normal particles have positive lepton or baryon number while antiparticles have these numbers negative. The electron's antiparticle, the positron, has an opposite charge to the electron, and to differentiate between antiparticles and particles, a plus or negative sign is added in superscript. When a particle and an antiparticle interact with each other, they are annihilated and convert to other particles, releasing energy in the process. Some particles, such as the photon or gluon, have no antiparticles, and in the case that the particle has a charge of zero, the antiparticle is denoted with a line above the symbol. The existence of antiparticles was first theorized and later confirmed, leading to the understanding that antimatter is a real form of matter that can be produced in particle accelerators and cosmic ray collisions. The longest-lived mesons last for only a few hundredths of a microsecond, and they occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays.
The Giants of Science
The world's major particle physics laboratories are distributed across the globe, with the Large Hadron Collider at CERN on the Franco-Swiss border near Geneva, Switzerland, being the world's most energetic collider of protons. The Large Hadron Collider had its first beam circulation on the 10th of September 2008, and it became the most energetic collider of heavy ions after it began colliding lead ions. Earlier facilities include the Large Electron, Positron Collider, which was stopped on the 2nd of November 2000 and then dismantled to give way for the Large Hadron Collider. In the United States, the Fermi National Accelerator Laboratory in Batavia, Illinois, operated the Tevatron until 2011, which collided protons and antiprotons and was the highest-energy particle collider on earth until the Large Hadron Collider surpassed it on the 29th of November 2009. Other major facilities include the Brookhaven National Laboratory in New York, which operates the Relativistic Heavy Ion Collider, and the Budker Institute of Nuclear Physics in Novosibirsk, Russia, which operates the electron-positron colliders VEPP-2000 and VEPP-4.
Beyond the Standard Model
Most particle physicists believe that the Standard Model is an incomplete description of nature and that a more fundamental theory awaits discovery, leading to efforts to look for physics beyond the Standard Model. The graviton is a hypothetical particle that can mediate the gravitational interaction, but it has not been detected or completely reconciled with current theories. Many other hypothetical particles have been proposed to address the limitations of the Standard Model, including supersymmetric particles that aim to solve the hierarchy problem, axions that address the strong CP problem, and various other particles proposed to explain the origins of dark matter and dark energy. String theory attempts to construct a unified description of quantum mechanics and general relativity by building a theory based on small strings and branes rather than particles, and if successful, it may be considered a Theory of Everything. Vanishing-dimensions theory is a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions, offering a new perspective on the structure of the universe.
The Web of Discovery
In practice, even if particle physics is taken to mean only high-energy atom smashers, many technologies have been developed during these pioneering investigations that later find wide uses in society. Particle accelerators are used to produce medical isotopes for research and treatment, such as isotopes used in PET imaging, or used directly in external beam radiotherapy. The development of superconductors has been pushed forward by their use in particle physics, and the World Wide Web and touchscreen technology were initially developed at CERN. Additional applications are found in medicine, national security, industry, computing, science, and workforce development, illustrating a long and growing list of beneficial practical applications with contributions from particle physics. The Future Circular Collider proposed for CERN and the Particle Physics Project Prioritization Panel in the US are major efforts to look for physics beyond the Standard Model, with the Deep Underground Neutrino Experiment recommended by the 2014 P5 study among other experiments.