subatomic particle

The hierarchy of subatomic particles

Subatomic particles are all particles which are "smaller" than atoms. Two great classes of subatomic particles exist: elementary particles, which are particles with no substructure (particles that aren't made of other particles), and composite particles, which are particles with substructure (particles that are made of other particles). Particle physics and nuclear physics are concerned with the study of these particles as well as their interactions.

Elementary subatomic particles are divided in three classes: quarks and leptons (particles of matter), and gauge bosons (force carriers, such as the photon). Composite subatomic particles (such as protons or atomic nuclei) are bound states of 2 or more elementary particles. For example, a proton is made of two up quarks and one down quark, while the atomic nuclei of helium-4 is composed of 2 protons and 2 neutrons.

Elementary particles of the Standard Model include six different types of quark ('up', 'down', 'bottom', 'top', 'strange', and 'charm'), as well as six different leptons ('electron', 'electron neutrino', 'muon', 'muon neutrino', 'tauon', 'tauon neutrino'), four force carriers ('photon', the W and Z bosons, gluons), as well as the Higgs boson.

Composite particles include all hadrons (baryons, such as protons and neutrons, and mesons, such as pions and kaons).

Most of the particles that have been discovered are encountered in cosmic rays interacting with matter and are produced by scattering processes in particle accelerators. There are hundreds of known subatomic particles.^{[citation needed]}

Introduction to particles

In particle physics, the conceptual idea of a particle is one of several concepts inherited from classical physics, the world we experience, that are used to describe how matter and energy behave at the molecular scales of quantum mechanics. For physicists, the meaning of the word "particle" is rather different from the common sense of the term, reflecting the modern understanding of how particles are radically different at the quantum-level.

The idea of a particle is one which had to undergo serious rethinking in light of experiments which showed that particles of light (photons) can exhibit the properties of waves. These results necessitated the new concept of wave-particle duality to reflect that quantum-scale "particles" are understood to behave in a way resembling both particles and waves. Another new concept, the uncertainty principle, determined that analyzing particles at these scales would require a statistical approach. In more recent times, this wave-particle duality has been shown to apply not only to photons, but to increasingly massive particles ^[1]

All of these factors ultimately combined to replace the notion of discrete "particles" with the concept of "wave-packets" of uncertain boundaries, whose properties are only known as probabilities, and whose interactions with other "particles" remain largely a mystery, even 80 years after the establishment of quantum mechanics.

Energy

Energy and matter we have studied from Einstein's hypotheses are analogous: matter can be austerely denoted in terms of energy. Thus, we have only discovered two mechanisms in which energy can be transferred. These are particles and waves. For example, light can be expressed as both particles and waves. This paradox is known as the Wave–particle Duality Paradox. ^[2].

Through the work of Albert Einstein, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature.^[3] This phenomenon has been verified not only for elementary particles, but also for compound particles like atoms and even molecules. In fact, according to traditional formulations of non-relativistic quantum mechanics, wave–particle duality applies to all objects, even macroscopic ones; we can't detect wave properties of macroscopic objects due to their small wavelengths.^[4]

Interactions between particles have been scrutinized for many centuries, and a few simple laws underpin how particles behave in collisions and interactions. The most fundamental of these are the laws of conservation of energy and conservation of momentum, which facilitate us to elucidate calculations between particle interactions on scales of magnitude which diverge between planets and quarks^[5]. These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in Philosophiae Naturalis Principia Mathematica originally published in 1687.

Dividing an atom

G. Johnstone Stoney postulated a fundamental unit of electrical charge in 1874, and in 1891 he suggested the name electron (denoted e⁻) for this quantity.^[6] The electron as a sub-atomic particle was first 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.

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

• Boson
• CPT invariance
• Dark Matter
• Electron
• Fermion
• Hot spot effect in subatomic physics
• List of fictional elements, materials, isotopes and atomic particles
• List of particles
• Neutron
• Particle physics
• Poincare symmetry
• Proton
• Quark model
• Spin statistics theorem
• Standard model
• Ylem

References

1. ^ "Wave-particle duality of C60 molecules". Reference to "Nature, Volume 401, Issue 6754, pp. 680-682". 1999. http://adsabs.harvard.edu/abs/1999Natur.401..680A. Retrieved on 2009-05-04. 
2. ^ Einstein, Albert; Robert W. Lawson (1920). Relativity: The Special & General Theory. New York Henry Holt and Company. ISBN 1-58734-092-5. 
3. ^ Walter Greiner (2001). Quantum Mechanics: An Introduction. Springer. ISBN 3540674586. http://books.google.com/books?id=7qCMUfwoQcAC&pg=PA29&dq=wave-particle+all-particles&as_brr=3&sig=2uPutqrcV_8vPVJwJnw3jstZj-o#PPA30,M1. 
4. ^ R. Eisberg and R. Resnick (1985). Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.). John Wiley & Sons. pp. 59–60. ISBN 047187373X. "For both large and small wavelengths, both matter and radiation have both particle and wave aspects. ... But the wave aspects of their motion become more difficult to observe as their wavelengths become shorter. ... For ordinary macroscopic particles the mass is so large that the momentum is always sufficiently large to make the de Broglie wavelength small enough to be beyond the range of experimental detection, and classical mechanics reigns supreme." 
5. ^ Isaac Newton - Newton's Laws of Motion (Philosophiae Naturalis Principia Mathematica). 1687.
6. ^ Klemperer, Otto (1959). Electron Physics: The Physics of the Free Electron. Academic Press. 

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)