Particle physics

• 1. Introduction

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  Particle physics begins with a simple count: the Standard Model, as currently formulated, has 61 elementary particles. From that small set, hundreds of other species discovered since the 1960s emerge as combinations. Some of these particles are so fleeting that the longest-lived mesons last for only a few hundredths of a microsecond. Others, like the proton and neutron, hold together nearly all the mass of ordinary matter around you.

  The field is also called high-energy physics, and it studies the fundamental particles and forces that constitute matter and radiation. It draws a line at scale. Combinations of elementary particles up to the size of protons and neutrons belong here, while the study of combinations of protons and neutrons becomes nuclear physics.

  How did physicists go from a chaotic spray of particles to a count of exactly 61? Why can a quark never be seen alone? And why, after agreeing with almost every experimental test ever conducted, does this theory still feel unfinished to the people who built it? Those are the questions ahead.

• 2. The Particle Zoo

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  In the 6th century BC, the idea first appeared that all matter is fundamentally composed of elementary particles. Centuries later, in the 19th century, John Dalton worked through stoichiometry and concluded that each element of nature was composed of a single, unique type of particle. The Greek word atomos, meaning indivisible, gave the atom its name. Physicists then discovered the atom was no such thing. It was a conglomerate of smaller pieces, including the electron.

  The early 20th century pushed deeper into nuclear physics and quantum physics. In 1939, Lise Meitner proved nuclear fission, building on experiments by Otto Hahn, and in that same year Hans Bethe described nuclear fusion. Both discoveries also fed the development of nuclear weapons. Bethe returned in 1947 with a calculation of the Lamb shift, credited with having opened the way to the modern era of particle physics.

  Throughout the 1950s and 1960s, beams of ever higher energy threw off a bewildering variety of particles. Physicists called it, informally, the particle zoo. Sharp surprises arrived alongside it, such as the CP violation found by James Cronin and Val Fitch, which raised new questions about the imbalance between matter and antimatter. The zoo had no organizing logic yet, and a framework was needed to tame it.

• 3. Taming The Zoo

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  During the 1970s, the formulation of the Standard Model finally explained where the particle zoo came from. The large number of particles turned out to be combinations of a relatively small number of more fundamental particles, framed within quantum field theories. This reclassification marked the beginning of modern particle physics. The model gained widespread acceptance in the mid-1970s, after experiments confirmed that quarks exist.

  The Standard Model classifies the fundamental particles as fermions, the matter particles, and bosons, the force-carriers. There are three generations of fermions, yet ordinary matter draws only from the first. That first generation holds up and down quarks, which form protons and neutrons, along with electrons and electron neutrinos. The model accounts for the strong, weak, and electromagnetic interactions using mediating gauge bosons.

  The Standard Model also predicted a particle called the Higgs boson. 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 the Higgs boson should. The theory has since agreed with almost every experimental test conducted to date. Still, most particle physicists believe it is an incomplete description of nature, and that a more fundamental theory awaits discovery.

• 4. Quarks And Leptons

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  Quarks carry a fractional elementary electric charge, either minus one third or two thirds, a strange feature with no everyday parallel. The leptons beside them carry whole-numbered charge, zero or minus one. Together quarks and leptons are called fermions. They have half-integer quantum spin, and they obey the Pauli exclusion principle, so no two of them may occupy the same quantum state.

  Three known generations of quarks exist: up and down, strange and charm, top and bottom. The leptons mirror them across three generations too, from the electron and its neutrino to the muon and its neutrino to the tau and its neutrino. There is strong indirect evidence that a fourth generation of fermions does not exist.

  Color charge sets quarks further apart. It is labeled arbitrarily as red, green, and blue, with no correlation to actual light color. Because the interactions between quarks store energy that converts to other particles when the quarks are pulled far enough apart, a quark can never be observed on its own. This is called color confinement, and it is the reason an isolated quark has never been caught.

• 5. Carriers Of Force

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  Bosons are the carriers of the fundamental interactions, and unlike fermions they have integer quantum spin, zero and one, and can share the same quantum state. Electromagnetism is mediated by the photon, the quanta of light. The weak interaction is mediated by the W and Z bosons. The strong interaction is mediated by the gluon, which links quarks together into composite particles.

  The Higgs boson gives mass to the W and Z bosons through the Higgs mechanism. The gluon and the photon, by contrast, are expected to be massless. The species of gauge bosons in the Standard Model are eight gluons, the W and Z bosons, and the photon.

  Gluons share the quarks' affliction. Because of color confinement, they are never observed independently, and a gluon can carry eight color charges, the result of quarks interacting to form composite particles under the gauge symmetry SU(3).

• 6. Matter And Antimatter

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  Every particle has an opposite written into it. Particles have corresponding antiparticles with the same mass but opposite electric charge, and these antiparticles compose antimatter. The antiparticle of the electron is the positron: the electron carries negative charge, the positron positive. Normal particles hold positive lepton or baryon number, while antiparticles hold those numbers negative.

  Notation keeps the two straight. A plus or negative sign in superscript separates a particle from its antiparticle. When the charge is zero and matches its antiparticle, a line above the symbol marks the antiparticle instead, as with an electron neutrino and its antineutrino. When a particle and an antiparticle meet, they annihilate and convert into other particles.

  Not every particle has a partner. Some particles, such as the photon and the gluon, have no antiparticle at all. The photon is its own antiparticle, a quiet exception in a world otherwise built on opposites.

• 7. Building Bigger Particles

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  A proton is three quarks: two up quarks and one down. The neutron flips the recipe, with two down quarks and one up. Both are baryons, particles composed of three quarks, and these two baryons make up most of the mass of ordinary matter. A meson, by contrast, is composed of two quarks, one normal and one anti. Baryons and mesons together are called hadrons.

  Mesons live briefly and violently. They are unstable, the longest-lived surviving only a few hundredths of a microsecond. They appear after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays, and they are also produced in cyclotrons and other particle accelerators.

  Inside any hadron, the bound quarks must keep their color charge neutral, or white, by analogy with mixing the primary colors. Exotic arrangements push the rules further, into tetraquarks and pentaquarks. Atoms themselves can be rebuilt this way: hydrogen-4.1 swaps one of its electrons for a muon, an exotic atom assembled from familiar parts.

• 8. The Unfinished Edges

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  The graviton remains hypothetical. It would mediate the gravitational interaction, but it has not been detected or fully reconciled with current theories. The reconciliation of gravity with particle physics is, in fact, the field's open wound. Many theories have attacked it, among them loop quantum gravity, string theory, and supersymmetry. String theorists try to unite quantum mechanics and general relativity by building from small strings and branes rather than particles, hoping for what they call a Theory of Everything.

  Neutrinos delivered the first crack from the other direction. In recent years, measurements of neutrino mass produced the first experimental deviations from the Standard Model, since neutrinos have no mass within it. Other proposed particles chase other gaps: supersymmetric particles aim at the hierarchy problem, axions at the strong CP problem, and various particles at the origins of dark matter and dark energy.

  The search has a future already mapped. CERN has proposed the Future Circular Collider, while in the United States the Particle Physics Project Prioritization Panel, known as P5, will update the 2014 P5 study that recommended the Deep Underground Neutrino Experiment, among others. The next unexpected particle may already be circulating in a beam, waiting to be counted.