Elementary particle

• 1. Introduction

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  An elementary particle is a piece of matter with nothing inside it. No smaller parts, no hidden machinery. The Standard Model of particle physics counts seventeen distinct particles of this kind. Once flavor, color and antimatter are accounted for, those seventeen branch into 61 variations. Protons and neutrons are not on the list, because each is built from simpler pieces. So what earns a particle a place on it, and how did physicists decide which things count as truly basic? The answer turns out to depend less on measurement than on theory. It also involves quarks that can never be seen alone, a particle announced with a famous guest in the room, and several rival ideas about what lies underneath it all.

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• 2. Indivisible

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  The Ancient Greek word atomos means indivisible or uncuttable, and it gave the atom its name. For thousands of years people theorized about atoms, yet their factual existence stayed controversial until 1905. That year Albert Einstein published his paper on Brownian motion. It ended the argument for those who had treated molecules as mathematical illusions. Einstein went on to describe matter as various concentrations of energy.

  The atom itself soon lost its claim to being the final building block. Toward the end of the 19th century the electron was identified as a part of it. The proton followed in 1919, the photon in the 1920s, and the neutron in 1932. By then quantum mechanics had changed what the word particle meant, because a particle could now exist at once as a matter wave.

  The Standard Model was codified in the 1970s, and theorists have been building on it and beyond it ever since.

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• 3. Three Generations

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  The 12 fundamental fermions are sorted into 3 generations of 4 particles each. Half of them are leptons. Three leptons carry an electric charge of -1 e, and they are the electron, the muon and the tau. The other three are the neutrinos, the only elementary fermions with neither electric charge nor color charge. The remaining six fermions are quarks. Every one of the 12 also has an antiparticle. The positron, for example, is the electron's mirror image, with a charge of +1 e.

  Measured masses show how uneven the generations are. On a scale of millions of electron-volts, the electron comes in at 0.511 and the muon at 105.7. The tau reaches 1780. Neutrinos of any type sit below 0.8. The top quark is far heavier than all of them, at 172,700, and it is the most accurately known quark mass.

  The other quark figures come with a catch. The up quark is estimated at 1.9 and the down quark at 4.4, and these depend on which version of quantum chromodynamics is used to describe how quarks interact.

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• 4. Quarks in Hiding

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  No one has ever detected an isolated quark or antiquark. Physicists explain this with confinement. Every quark carries one of three color charges of the strong interaction, and antiquarks carry matching anticolors. Color-charged particles interact by exchanging gluons, much as electrically charged ones exchange photons. But gluons carry color too, so the strong force grows stronger as color-charged particles are pulled apart. The electromagnetic force does the opposite and weakens with distance.

  Color-charged particles can still join into color-neutral groups called hadrons. A quark paired with an antiquark of the corresponding anticolor cancels out and forms a meson. Three quarks, one red, one blue and one green, form a baryon. Three antiquarks with antired, antiblue and antigreen make an antibaryon. This color has nothing to do with visual color. It is a label for how the strong interaction behaves.

  Fractional charge is another thing nobody has isolated. Quarks carry charges of either +2/3 e or -1/3 e, but the hadrons that hold them always have whole-number charges.

  This also explains why quark masses cannot be measured directly. Gluons wrap each quark in an envelope and add vastly more mass to the hadron than the quarks themselves contribute.

  The evidence that quarks exist comes from deep inelastic scattering, where electrons are fired at nuclei to map the distribution of charge inside nucleons.

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• 5. Force Carriers

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  Eight variations of the gluon exist, each built from combinations of color and anticolor. Gluons mediate the strong interaction that binds quarks into protons and neutrons and holds those baryons together in the atomic nucleus. They belong to the vector bosons, particles with spin 1. The photon and the W and Z bosons are vector bosons too. Bosons differ from fermions in that multiple bosons can occupy the same quantum state. Bosons can be elementary, like the photon, or composite, like a meson.

  Three weak gauge bosons, W+, W- and Z0, mediate the weak interaction. The W boson is known for its role in nuclear decay. The W- turns a neutron into a proton and then decays into an electron and an electron-antineutrino. The Z0 changes neither flavor nor charge. It changes momentum, and it is the only mechanism for elastically scattering neutrinos. The weak bosons were discovered through that very effect, a change in the momentum of electrons caused by a neutrino-Z exchange.

  The massless photon carries the electromagnetic force. Together these four make up the electroweak interaction.

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• 6. The Higgs Announcement

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  On the 4th of July 2012, the Large Hadron Collider at CERN produced an announcement that the Higgs boson had been observed. Two main experiments, ATLAS and CMS, had reported it as likely detected. Peter Higgs, who first posited the particle's existence, was present in the room. The reported statistical significance was 5 sigma, which implies a certainty of roughly 99.99994%. That is the level particle physics requires before observations can officially be called a discovery. The particle's mass is believed to be approximately 125.

  The Higgs matters because of an old puzzle about forces. At everyday energies the weak and electromagnetic forces look entirely different. Theory says they unify into one electroweak force at high energies, and measurements of high-energy electron-proton scattering at the HERA collider at DESY clearly confirmed that.

  The Higgs mechanism explains the difference. Through spontaneous symmetry breaking, it picks out a special direction in electroweak space. Three electroweak particles become very heavy, and those are the weak bosons. One stays without a defined rest mass because it is always in motion, and that one is the photon.

  Research into the properties of the newly discovered particle continues.

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• 7. What the Universe Is Made Of

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  Big Bang nucleosynthesis models predict that visible matter began as about 75% hydrogen and 25% helium-4 by mass. Neutrons contain one up quark and two down quarks, while protons contain two up and one down. Electrons, neutrinos and weak bosons are so light or so rare next to atomic nuclei that their mass can be neglected. So most of the visible mass of the universe is baryons, and baryons are made of up and down quarks.

  The count of protons in the observable universe has a name, the Eddington number.

  Counting by number rather than mass gives a different picture. Some estimates say that, leaving out dark matter, nearly all the matter in the visible universe is in neutrinos, which make up the majority of its elementary particles. Other estimates say the count is dominated by photons and other massless force carriers.

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• 8. Beyond the Standard Model

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  Super-Kamiokande, a neutrino observatory, has never seen a proton decay, and that silence has cost physics some of its simplest ideas. Grand unified theories try to merge the electroweak and strong interactions into a single force. Their most dramatic prediction is a pair of X and Y bosons that would cause protons to decay. The missing decay rules out the simplest versions, including SU(5) and SO(10).

  The Standard Model leaves open questions. It omits gravitation, and some of its parameters were added without explanation. It also cannot account for the huge gap between the weak force and gravity, an aspect of the hierarchy problem. The graviton is a hypothetical spin-2 particle proposed to carry gravity. It is sometimes listed in tables of elementary particles despite never having been found.

  Supersymmetry would double the roster. Each known particle would have a heavier superpartner, with spin differing from the ordinary particle's. Sparticles include sleptons, squarks, neutralinos and charginos. They could be too heavy for existing colliders, and they remained undiscovered as of 2026.

  String theory replaces particles with vibrating strings in an 11-dimensional universe according to M-theory, or 12 dimensions according to F-theory. The vibration frequency sets mass, electric charge, color charge and spin. In this picture our universe is a 4-brane, and the remaining 7 dimensions are either tiny and curled up or lie in a larger multiverse.

  Technicolor proposes that the Higgs boson is a bound state of techniquarks held together by technigluons. Preon theory goes the other way, suggesting that three to six more fundamental particles lie beneath the Standard Model. Interest in preons faded in the 1980s, when the simplest models were ruled out by experiment.

  The acceleron idea connects the neutrino's newfound mass to dark energy. In it, neutrinos feel a new force from interacting with accelerons, and dark energy arises as the universe tries to pull neutrinos apart.

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