Particle physics

Lore & Background

The idea that all matter is composed of elementary particles dates from at least the 6th century BC. In the 19th century, John Dalton concluded each element was composed of a unique type of particle, called the atom. Later, physicists discovered atoms are conglomerates of smaller particles, such as the electron. Early 20th-century nuclear and quantum physics led to proofs of nuclear fission in 1939 by Lise Meitner and nuclear fusion by Hans Bethe that same year, also leading to nuclear weapons. Bethe's 1947 calculation of the Lamb shift is credited with opening the way to the modern era of particle physics. Throughout the 1950s and 1960s, a bewildering variety of particles was found in high-energy collisions, referred to informally as the particle zoo. Important discoveries such as CP violation by James Cronin and Val Fitch brought new questions to matter-antimatter imbalance. After the formulation of the Standard Model during the 1970s, physicists clarified the origin of the particle zoo, explaining the large number of particles as combinations of a relatively small number of more fundamental particles, framed in quantum field theories.

Reader's Guide

Particle physics is significant because it provides the foundational understanding of all known matter and forces, except gravity. The Standard Model, which gained widespread acceptance in the mid-1970s after experimental confirmation of quarks, describes the strong, weak, and electromagnetic interactions via gauge bosons and contains 24 fundamental fermions (12 particles and their antiparticles). It also predicted the Higgs boson, which was confirmed experimentally on 4 July 2012 at the Large Hadron Collider. The Standard Model currently has 61 elementary particles and agrees with almost all experimental tests, though most particle physicists believe it is incomplete. Measurements of neutrino mass have provided the first experimental deviations from the Standard Model, as neutrinos do not have mass in it. The field continues to explore unresolved questions, such as the reconciliation of gravity with particle physics, through theories like loop quantum gravity, string theory, and supersymmetry. Experimental particle physics studies particles in radioactive processes and accelerators like the Large Hadron Collider, while theoretical particle physics studies them in the context of cosmology and quantum theory.

Did You Know?

• Ordinary matter is made only from the first fermion generation: up and down quarks, electrons, and electron neutrinos.
• Hadrons containing an odd number of quarks are called baryons; those with an even number are called mesons.
• The antiparticle of the electron is the positron, which has the same mass but opposite electric charge.
• The Higgs boson gives mass to the W and Z bosons via the Higgs mechanism; the gluon and photon are massless.