Subatomic particle

A subatomic particle is a particle smaller than an atom: it may be elementary or composite. Particle physics and nuclear physics concern themselves with the study of these particles and their interactions.

There are just two basic types of fundamental particles with quite different properties, the fermions and the bosons. It is helpful to think of the fermions as the durable 'pixels of matter' and the bosons as 'pixels of force.

These particles include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are composite particles, made up of quarks), as well as other particles such as photons and neutrinos which are produced copiously in the sun. However, most of the particles that have been discovered and studied are not encountered under normal earth conditions; they are produced in cosmic rays and during scattering processes in particle accelerators.

Helium atom (schematic)
Showing two protons (red), two neutrons (green) and two electrons (yellow).

Dividing an atom

The study of electrochemistry led G. Johnstone Stoney to postulate the existence of the electron (denoted e⁻) in 1874 as a constituent of the atom. It was observed in 1897 by J. J. Thomson. Subsequent speculation about the structure of atoms was severely constrained by the 1907 experiment of Ernest Rutherford which showed that the atom was mostly empty space, and almost all its mass was concentrated into the (relatively) tiny atomic nucleus. The development of the quantum theory led to the understanding of chemistry in terms of the arrangement of electrons in the mostly empty volume of atoms. Protons (p⁺) were known to be the nucleus of the hydrogen atom. Neutrons (n) were postulated by Rutherford and discovered by James Chadwick in 1932. The word nucleon denotes both the neutron and the proton.

Electrons, which are negatively charged, have a mass of 1/1836 of a hydrogen atom, the remainder of the atom's mass coming from the positively charged proton. The atomic number of an element counts the number of protons. Neutrons are neutral particles with a mass almost equal to that of the proton. Different isotopes of the same nucleus contain the same number of protons but differing numbers of neutrons. The mass number of a nucleus counts the total number of nucleons.

Chemistry concerns itself with the arrangement of electrons in atoms and molecules, and nuclear physics with the arrangement of protons and neutrons in a nucleus. The study of subatomic particles, atoms and molecules, their structure and interactions, involves quantum mechanics and quantum field theory (when dealing with processes that change the number of particles). The study of subatomic particles per se is called particle physics. Since many particles need to be created in high energy particle accelerators or cosmic rays, sometimes particle physics is also called high energy physics.

Classification of subatomic particles

Symmetries play quite the important role in the physics of subatomic particles by providing intrinsic quantum numbers which are used to classify particles. Poincaré symmetry, which is the full symmetry of special relativity, is enjoyed by any Hamiltonian which describes these particles. Hence all particles have the following quantum numbers —

• the mass (m) of the particle,
• its spin (J): all particles with integer values of spin are called bosons, those with half-integer spins are called fermions.
• its intrinsic parity (P), which is a multiplicative quantum number.

In addition, some particles may have a definite C-parity (C). Particles may also carry other quantum numbers related to internal symmetries, such as charges and flavour quantum numbers.

Corresponding to every particle there exists an antiparticle. Every additive quantum number of a particle is reversed in sign for the antiparticle. Equality of the masses and lifetimes of particle and antiparticle follows in local quantum field theories through CPT symmetry, and hence tests of these equalities constitute important tests of this symmetry.

Elementary particles

A full classification of subatomic particles involves understanding the fundamental forces that they are subject to: the electromagnetic, weak and strong forces. In the modern unified quantum field theory of these three forces, called the standard model, the elementary particles are

• spin J  =  1 particles called gauge bosons. These include
  • photons, which are carriers of the electromagnetic force,
  • W bosons and Z bosons which mediate the weak forces, and
  • gluons, which carry the strong force.
• spin J  =  1/2 fermions which constitute all matter in the universe and come in two varieties—
  • leptons such as the electron, muon, tau lepton, the three corresponding neutrinos (these are called six flavours of leptons), and their antiparticles. These are affected essentially only by the weak and electromagnetic forces. The former allow flavour changes (for example, from a muon to an electron)
  • quarks which come in six other flavours, and are affected by all three forces unified into the standard model. The weak interactions cause flavour changes.
• spin J  =  0 (and P  =  +1) Higgs boson which is responsible for the masses of the quarks, leptons, W and Z bosons. This remains to be actually seen in experiments; a major purpose of the Large Hadron Collider (LHC) is to search for this particle.

Conjectures and predictions

Further structures beyond the standard model are often invoked. In particular, there is a search for a theory that unifies the standard model with gravity. There is strong evidence that when such a theory is found it will include gravitons (constrained to have spin J = 2), to mediate this fourth fundamental interaction. A further structure called supersymmetry is often invoked, although direct experimental evidence for it is lacking. Supersymmetric extensions of the standard model would contain a bosonic partner for each of the fermions described above (called selectrons, smuons, staus, sneutrinos, squarks), and a fermionic partner for each boson (called gauginos and Higgsinos). Supersymmetric extensions which include a theory of gravity (called supergravity) also involve a partner of the graviton, called the gravitino, which has spin J = 3/2. In many versions of these theories there are extra bosons called axions with J = 0 and P = −1. Relic particles are postulated to be remnants of the early cosmological expansion of the Big Bang.

There were attempts to build theories which posited that the elementary particles in the standard model are actually composites built out of really elementary particles variously called preons, rishons or quinks. However, these theories are so strongly constrained by experimental data now that they are almost ruled out. Extended supersymmetric theories have also been postulated; these allow particles such as leptoquarks, which transmute leptons into quarks.

Composite particles

All observed subatomic composite particles are called hadrons. All bosonic hadrons are called mesons and all fermionic hadrons are baryons. The most well-known baryons are the constituents of atomic nuclei called protons and neutrons, and collectively named nucleons. The quark model of hadrons states that mesons are built out of a quark and an antiquark, whereas a baryon is made up of three quarks. As of 2005, searches for exotic hadrons are currently under way.

History

J. J. Thomson discovered electrons in 1897. In 1905 Albert Einstein demonstrated the physical reality of the photons which were postulated by Max Planck in order to solve the problem of black body radiation in thermodynamics. Ernest Rutherford discovered in 1907 in the gold foil experiment that the atom is mainly empty space, and that it contains a heavy but small atomic nucleus. The early successes of the quantum theory involved explaining properties of atoms in terms of their electronic structure. The proton was soon identified as the nucleus of hydrogen. The neutron was postulated by Rutherford following his discovery of the nucleus, but was discovered by James Chadwick much later, in 1932. Neutrinos were postulated in 1931 by Wolfgang Pauli (and named by Enrico Fermi) to be produced in beta decays (the weak interaction) of neutrons, but were not discovered till 1956. Pions were postulated by Hideki Yukawa as mediators of the strong force which binds the nucleus together. The muon was discovered in 1936 by Carl D. Anderson, and initially mistaken for the pion. In the 1950s the first kaons were discovered in cosmic rays.

The development of new particle accelerators and particle detectors in the 1950s led to the discovery of a huge variety of hadrons, prompting Wolfgang Pauli's remark: "Had I foreseen this, I would have gone into botany". The classification of hadrons through the quark model in 1961 was the beginning of the golden age of modern particle physics, which culminated in the completion of the unified theory called the standard model in the 1970s. The discovery of the weak gauge bosons through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics. Among the standard model particles the existence of the Higgs boson remains to be verified— this is seen as the primary physics goal of the accelerator called the Large Hadron Collider in CERN. All currently known particles fit into the standard model.

See also

• Poincare symmetry, CPT invariance, spin statistics theorem, bosons and fermions.
• Particle physics, list of particles, the quark model and the standard model.

External links

• particleadventure.org: The Standard Model
• particleadventure.org: Particle chart
• University of California: Particle Data Group
• Annotated Physics Encyclopædia: Quantum Field Theory
• Jose Galvez: Chapter 1 Electrodynamics (pdf)

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