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These particles include atomic constituents such as Electron s, Proton s, and Neutron s (protons and neutrons are actually composite particles, made up of Quark s), as well as other particles such as Photon s and Neutrino s 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 Ray s and during scattering processes in Particle Accelerator s.


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. Proton s ('''p+''') were known to be the nucleus of the Hydrogen atom. Neutron s ('''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 Accelerator s or Cosmic Ray s, sometimes particle physics is also called High Energy Physics .


CLASSIFICATION OF SUBATOMIC PARTICLES

Symmetries play a very important role in the physics of subatomic particles by providing intrinsic Quantum Number s which are used to classify particles. Poincare 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 s, those with half-integer spins are called Fermion s.

  • 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 Charge s 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 Force s. 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

  • --- Photon s, which are carriers of the electromagnetic force,

  • --- W Boson s and Z Boson s which mediate the Weak Force s, and

  • --- Gluon s, which carry the Strong Force .

  • spin J  =  1/2 fermions which constitute all matter in the universe and come in two varieties—

  • --- Lepton s such as the Electron , Muon , Tau Lepton , the three corresponding Neutrino s (these are called six Flavour s of leptons), and their Antiparticle s. These are affected essentially only by the weak and electromagnetic forces. The former allow flavour changes (for example, from a muon to an electron)

  • --- Quark s 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 Graviton s (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 '' Axion s'' 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 '' Leptoquark s'', which transmute leptons into quarks.


Composite particles

All observed subatomic composite particles are called Hadron s. All bosonic hadrons are called Meson s and all fermionic hadrons are Baryon s. The most well-known baryons are the constituents of atomic nuclei called Proton s and Neutron s, and collectively named Nucleon s. The Quark Model of hadrons posits 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

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 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 particles found till now fit into the standard model.


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