## Definition The **Standard Model of Particle Physics** is the quantum field theory that describes all known elementary particles and three of the four fundamental forces (electromagnetic, weak, and strong). It was assembled during the 1950s–1970s through contributions by Feynman, Gell-Mann, Weinberg, Salam, Glashow, and many others, and has been confirmed by every precision experiment to date, most recently by the detection of the Higgs boson at CERN in 2013. ## Elementary Particles Matter is made of two families of fermions (spin-½ particles): - **Quarks** — six "flavours" (up, down, charm, strange, top, bottom). Quarks bind in triplets to form protons and neutrons (baryons), or in quark-antiquark pairs (mesons). The name "quark" was coined by Murray Gell-Mann, borrowing a nonsense word from James Joyce's *Finnegans Wake*: "Three quarks for Muster Mark!" - **Leptons** — electrons, muons, tau particles, and their associated neutrinos. Forces are mediated by bosons (integer-spin particles): - **Photons** — carry the electromagnetic force. - **Gluons** — carry the strong force that binds quarks inside protons and neutrons. - **W and Z bosons** — carry the weak force (responsible for radioactive decay). - **Higgs boson** — the quantum of the Higgs field, which gives mass to other elementary particles. All told, fewer than a dozen types of elementary particle suffice to build the entire material world. ## Particles as Quanta of Fields In the Standard Model, particles are not tiny billiard balls but *excitations* — quantised vibrations — of underlying quantum fields that pervade all of space. Even in a perfect vacuum, these fields fluctuate, and particles are continuously created and annihilated by these quantum fluctuations. Matter is better described as a ceaseless, restless seething of fields than as a collection of stable objects. ## Renormalisation: The Baroque Subtlety The naive equations of the Standard Model produce infinitely large answers for many quantities. Physicists cure this by a procedure called *renormalisation*: the bare parameters in the equations are themselves taken to be infinite in precisely the right way so that infinities cancel, leaving finite, measurable predictions. The procedure works to extraordinary precision but, as Paul Dirac complained until the end of his life, leaves an unsatisfying logical aftertaste. ## Successes and Experimental Confirmation All experimental predictions of the Standard Model have been confirmed. Carlo Rubbia's discovery of the W and Z bosons at CERN (Nobel Prize 1984) was one landmark. The final piece — the Higgs boson — was detected at the LHC in 2013. ## Known Deficiencies Despite its success, the Standard Model is widely considered incomplete: - **Aesthetic inelegance** — Multiple disconnected sectors (electroweak + QCD) with ~19 free parameters whose values are not predicted by any deeper principle. - **Dark matter** — Galaxies are surrounded by vast halos of matter detectable only through its gravitational effects. This matter interacts with nothing the Standard Model describes. Supersymmetric extensions predict new particles that could be dark-matter candidates, but as of the writing of this source no such particles had been observed. - **Gravity excluded** — The Standard Model does not include gravity (General Relativity). Combining them is the central problem of fundamental physics. See [[The Problem of Quantum Gravity]]. ## Related - [[Quantum Mechanics]] - [[Photon]] - [[The Problem of Quantum Gravity]] - [[General Relativity]] - [[Architecture of the Cosmos]] ## Sources - [[Seven Brief Lessons on Physics (Rovelli 2014)]]