Standard Model
In 1928, Paul Dirac introduced the Dirac equation. This mathematical work implied the existence of antimatter for the first time in history. Scientists spent decades building upon this foundation to understand subatomic forces. Yang Chen-Ning and Robert Mills extended gauge theory concepts in 1954. They applied these ideas to nonabelian groups to explain strong interactions. Chien-Shiung Wu demonstrated that parity was not conserved during weak interaction experiments in 1957. Sheldon Glashow combined electromagnetic and weak interactions into a single framework by 1961. Murray Gell-Mann and George Zweig introduced quarks in 1964. That same year Oscar W. Greenberg implicitly introduced color charge of quarks. Steven Weinberg and Abdus Salam incorporated the Higgs mechanism into Glashow's electroweak interaction later in 1964. The term Standard Model appeared in print when Abraham Pais and Sam Treiman used it in 1975. Steven Weinberg claimed he chose the phrase out of modesty during a talk in Aix-en-Provence in France back in 1973.
The Standard Model includes twelve elementary particles known as fermions. These particles respect the Pauli exclusion principle which prevents two identical fermions from occupying the same quantum state simultaneously. Each fermion has a corresponding antiparticle with opposite charges. Quarks carry color charge and interact via the strong interaction. Six quarks exist: up, down, charm, strange, top, and bottom. Hadrons form when quarks bind together due to color confinement. Protons and neutrons are the lightest baryons composed of three quarks. Leptons do not carry color charge or respond to strong interactions. The six leptons consist of electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino. Charged leptons carry an electric charge of negative one e while neutrinos carry zero electric charge. Four kinds of gauge bosons mediate fundamental forces. Photons mediate electromagnetic force between charged particles. Gluons mediate strong interactions binding quarks together. W and Z bosons mediate weak interactions responsible for radioactivity. The Higgs boson is a massive scalar particle with spin zero discovered experimentally in 2012.
Electromagnetism remains the only long-range force within the Standard Model framework. It couples to electric charge through photon exchange. Quantum electrodynamics describes these electromagnetic interactions mathematically. Atomic electron shell structure and chemical bonds arise from this force. The weak interaction causes various forms of particle decay like beta decay. W bosons have electric charge and mediate interactions changing particle type. Z bosons are neutral and mediate neutral current interactions without flavor change. Parity violation occurs exclusively in charged current interactions involving left-handed fermions. The weak force becomes united with electromagnetism at high energies into electroweak theory. Experiments confirmed W plus and Z zero bosons existed in 1983. Their mass ratio matched Standard Model predictions precisely. Neutral weak currents caused by Z boson exchange were discovered at CERN in 1973. This discovery led to the 1979 Nobel Prize shared by Glashow, Salam, and Weinberg.
Quantum field theory provides the mathematical framework for the entire Standard Model. A Lagrangian controls dynamics and kinematics of the system. Each particle kind is described as a dynamical field pervading spacetime. Symmetries define the model before writing down the most general renormalizable Lagrangian. Global Poincaré symmetry applies to all relativistic quantum field theories. Local SU(3) times SU(2) times U(1) gauge symmetry defines internal structure. Three factors of gauge symmetry give rise to three fundamental interactions. Dynamics depend on nineteen parameters whose values come from experiment. Electron mass measures 0.511 MeV while muon mass reaches 105.7 MeV. Up quark mass equals 1.9 MeV at specific energy scales. Top quark mass sits around 173.5 GeV on shell scheme. The Higgs vacuum expectation value sets electroweak physics scale near 246 GeV. These numerical constants appear unrelated and arbitrary to many physicists. Some consider the model inelegant due to these nineteen free parameters.
The Standard Model predicted existence of W and Z bosons before observation. It also predicted gluon, top quark, and charm quark existence. Experiments confirmed these predictions with good precision over decades. Sheldon Glashow, John Iliopoulos, and Luciano Maiani introduced GIM mechanism in 1970 predicting charm quark. Martin Perl discovered tau lepton at SLAC in 1976. A team led by Leon Lederman found bottom quark at Fermilab in 1977. Mathematical consistency required mechanisms generating masses visible above certain energies. Large Hadron Collider experiments began searching for Higgs boson in early 2010. Two independent experiments reported finding a new particle with mass about 125 GeV on the 4th of July 2012. Confirmation arrived the 13th of March 2013 that this was indeed the searched-for Higgs boson. Neutrino oscillation data later challenged classic Standard Model assumptions regarding zero neutrino mass.
Gravity remains unexplained within the Standard Model framework despite being familiar. Contradictions arise when combining general relativity with quantum mechanics. The graviton is postulated but never proved to exist experimentally. Quantum field theories of gravity generally break down before reaching Planck scale. No reliable theory exists for the very early universe currently. Some physicists consider the model ad hoc requiring nineteen numerical constants. It does not explain observed amount of cold dark matter or contributions to dark energy. Matter-antimatter asymmetry presents another difficult problem for theorists. Neutrinos have mass which classic Standard Model did not allow originally. Explaining neutrino mass requires additional seven or eight arbitrary parameters. The hierarchy problem emerges if new physics couples to Higgs at high scales. Severe fine tuning becomes required unless scenarios include quantum gravity. No proposed theory of everything has been widely accepted or verified yet.
Common questions
When was the Standard Model term first used in print?
The term Standard Model appeared in print when Abraham Pais and Sam Treiman used it in 1975. Steven Weinberg claimed he chose the phrase out of modesty during a talk in Aix-en-Provence in France back in 1973.
What particles make up the twelve fermions in the Standard Model?
The Standard Model includes six quarks known as up, down, charm, strange, top, and bottom. It also contains six leptons consisting of electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino.
Which bosons mediate the four fundamental forces in the Standard Model?
Photons mediate electromagnetic force between charged particles while gluons mediate strong interactions binding quarks together. W and Z bosons mediate weak interactions responsible for radioactivity and the Higgs boson is a massive scalar particle with spin zero discovered experimentally in 2012.
How many free parameters does the Standard Model require from experimental data?
Dynamics depend on nineteen parameters whose values come from experiment including electron mass measuring 0.511 MeV and muon mass reaching 105.7 MeV. Top quark mass sits around 173.5 GeV on shell scheme while the Higgs vacuum expectation value sets electroweak physics scale near 246 GeV.
When was the Higgs boson officially confirmed by experiments at CERN?
Two independent experiments reported finding a new particle with mass about 125 GeV on the 4th of July 2012. Confirmation arrived the 13th of March 2013 that this was indeed the searched-for Higgs boson.