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 can be helpful to think of the fermions as the 'pixels of matter' and the bosons as 'pixels of force.'

Everyday matter is made up of two types of fermions—quarks and electrons—and two types of boson—photons and gluons. Each of these has a history that is determined, over time, by a quantum wavefunction.

The fermions are essentially eternal while the bosons are ephemeral and flit between the fermions. The consequences of this exchange of bosons is what classical science calls 'forces.' The exchange of photons is called the 'electromagnetic force' while the exchange of gluons is called the 'color force.'

Quarks and electrons are much smaller than even a proton, they have a size that is 1/10,000,000,000,000 the size of an atom; the size of the proton is that of the intense gluon field filling the proton-wavefunction, while the size of the atom is that of the much less intense photon field between the confined quarks and electrons filling the atom-wavefunction or orbital.

The quarks are capable of creating and absorbing both photons and gluons. The proton and neuton are composite particles containing three quarks. The exchange of gluons is very vigorous and confines the quarks to a tiny volume 1/10,000th the size of an atom. The energy of this 'color force field' is large, and is responsible (by Einstein's equivilaence of energy and mass) for 99.9% of the mass of the proton/neutron and hence is responsible for the mass of all material objects. The mass of the bare quarks and electrons together make up the other 0.01%.

The electrons are only capable of coupling with photons. The energy in the photon field of the atom is 1/1,000,000,000 of that in the gluon field and contributes little to the atom's mass.

While not involved in the structure of everyday matter, there are other fundamental fermions and bosons that are all related in a simple hierarchy.

There are three 'families' of fermions: The simplest is the electron neutrino, which is just a quantum spin in spacetime and little else. Next is the electron which has spin and 'electric charge' (couples with photons). It can be conviently thought of as a neutrino with a twist to it. Last is the quark which has spin, electric charge and 'color charge' (couples with gluons) and can be thought of as an electron with an extra twist that can go in two directions, the U and D quarks.

All of these are moving in the 'normal' direction of time. When they go in the opposite direction they are called antiparticles, such as the anti-neutrino, the anti-electron (or positron) anf the anti-quarks. While the physical universe is all matter, antimatter can be readily created in the lab. When matter and antimatter fermionsmeet each other they 'untwist' each other and their energy is liberated as high-energy photons (gamma rays).

There are three 'generations' of fermions that can be thought of as non-oriented twists (see Moebius strip) in spacetime.

The 1st generation is based on the electron neutrino which can be thought of as a quantum twist/spin in 1 spatial dimension. The electron, and U / D quarks are derived from this as above.

The 2nd generation is based on the muon neutrino which can be thought of as a twist/spin in 2 dimensions. The muon and S/C quarks are founded on this as in the 1st generation. All the members of this generation have more mass/energy than the first generation.

The 3nd generation is based on the tauon neutrino which can be thought of as a twist/spin in all 3 dimensions. The tauon and B/T quarks are founded on this. All the members of this generation have more mass/energy than the second generation.

There are three families of bosons that can be thought of as simple waves in spacetime:

The photon which is a (complex) wave in 1 dimension.

The 'weak bosons' which are (complex) waves in 2 dimensions.

The gluons which are (complex) waves in all 3 dimensions.

There is also the (theoretical) Higgs Boson which can best be thought of as a wave or excitation in 0 dimensions. Gravity can be described either as a global phenomenon involving the Higgs and the bending of spacetime, or as a local phenomenon involving the graviton or 'spin-2' boson.

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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