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The weak interaction (often called the weak force or sometimes the weak nuclear force[1]) is one of the four fundamental interactions of nature. In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them The W and Z bosons are the Elementary particles that mediate the Weak force. Its most familiar effect is beta decay (of electrons in atomic nuclei) and the associated radioactivity. In Nuclear physics, beta decay is a type of Radioactive decay in which a Beta particle (an Electron or a Positron) is emitted The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. The word "weak" derives from the fact that the field strength is some 1013 times less than that of the strong force. In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and

Contents

Properties

The weak interaction affects all left-handed leptons and quarks. A phenomenon is said to be chiral if it is not identical to its Mirror image (see Chirality) Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. It is the only force affecting neutrinos (except for gravitation, which is negligible on laboratory scales). Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Gravitation is a natural Phenomenon by which objects with Mass attract one another The weak interaction is unique in a number of respects:

  1. It is the only interaction capable of changing flavour. In Particle physics, flavour or flavor (see spelling differences) is a Quantum number of Elementary particles related to their
  2. It is the only interaction which violates parity symmetry P (because it only acts on left-handed particles). In Physics, a parity transformation (also called parity inversion) is the flip in the sign of one Spatial Coordinate. In Physics, a parity transformation (also called parity inversion) is the flip in the sign of one Spatial Coordinate. It is also the only one which violates CP (CP Symmetry). In Particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics
  3. It is mediated by heavy gauge bosons. In Particle physics, gauge bosons are Bosonic particles that act as carriers of the fundamental forces of nature This unusual feature is explained in the Standard Model by the Higgs mechanism. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles The Higgs mechanism is Spontaneous symmetry breaking in a Gauge theory.

Due to the large mass of the weak interaction's carrier particles (about 90 GeV/c2), their mean life is limited to about 3×10−27 seconds by the uncertainty principle. Given an assembly of elements the number of which decreases ultimately to zero the lifetime (also called the mean lifetime) is a certain number that characterizes the rate In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain Even at the speed of light this effectively limits the range of the weak interaction to 10−18 meters, about 1000 times smaller than the diameter of an atomic nucleus. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom

The Feynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W- boson
The Feynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W- boson

Since the weak interaction is both very weak and very short range, its most noticeable effect is due to its other unique feature: flavour changing. Motivation and history When calculating Scattering cross sections in Particle physics, the interaction between particles can be described This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost The W and Z bosons are the Elementary particles that mediate the Weak force. In the Standard Model of Particle physics, the Cabibbo-Kobayashi-Maskawa matrix ( CKM matrix, quark mixing matrix, sometimes also called Consider a neutron (quark content: UDD, or one up quark and two down quarks). This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. The up quark is a particle described by the Standard Model theory of Physics. The down quark is a first-generation Quark with a charge of -(1/3 e. Although the neutron is heavier than its sister nucleon, the proton (quark content UUD), it cannot decay into a proton without changing the flavour of one of its down quarks. In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive In Particle physics, flavour or flavor (see spelling differences) is a Quantum number of Elementary particles related to their Neither the strong interaction nor electromagnetism allow flavour changing, so this must proceed by weak decay. In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In this process, a down quark in the neutron changes into an up quark by emitting a W boson, which then breaks up into a high-energy electron and an electron antineutrino. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. The W and Z bosons are the Elementary particles that mediate the Weak force. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Since high-energy electrons are beta radiation, this is called a beta decay. Beta particles are high-energy high-speed Electrons or Positrons emitted by certain types of Radioactive nuclei such as Potassium -40 In Nuclear physics, beta decay is a type of Radioactive decay in which a Beta particle (an Electron or a Positron) is emitted

Due to the weakness of the weak interaction, weak decays are much slower than strong or electromagnetic decays. For example, an electromagnetically decaying neutral pion has a life of about 10−16 seconds; a weakly decaying charged pion lives about 10−8 seconds, a hundred million times longer. In Particle physics, pion (short for pi meson) is the collective name for three Subatomic particles, and. A free neutron lives about 15 minutes, making it the unstable subatomic particle with the longest known mean life. A subatomic particle is an elementary or composite Particle smaller than an Atom. Given an assembly of elements the number of which decreases ultimately to zero the lifetime (also called the mean lifetime) is a certain number that characterizes the rate

Interaction types

There are three basic types of weak interaction vertices (up to charge conjugation and crossing symmetry). In Quantum field theory, a branch of theoretical physics crossing symmetry is a symmetry that relates S-matrix elements Two of them involve charged bosons, intermediate vector bosons, they are called "charged current interactions. A vector boson is a Boson with spin equal to one unit of \hbar ( Planck's constant divided by 2 \pi Charged current is one of the ways in which subatomic particles can interact by means of the Weak force. " The third type is called "neutral current interaction. Weak neutral current interactions are one of the ways in which Subatomic particles can interact by means of the Weak force. "

Two charged-current interactions together are responsible for the beta decay phenomenon. In Nuclear physics, beta decay is a type of Radioactive decay in which a Beta particle (an Electron or a Positron) is emitted The neutral current interaction was first observed in neutrino scattering experiments in 1974 and in collider experiments in 1983.

Violation of symmetry

The laws of nature were long thought to remain the same under mirror reflection, the reversal of all spatial axes. A physical law or scientific law is a Scientific generalization based on empirical Observations of physical behavior (i Reflection is the change in direction of a Wave front at an interface between two different media so that the wave front returns into the medium from which The results of an experiment viewed via a mirror were expected to be identical to the results of a mirror-reflected copy of the experimental apparatus. This so-called law of parity conservation was known to be respected by classical gravitation and electromagnetism; it was assumed to be a universal law. In Physics, a parity transformation (also called parity inversion) is the flip in the sign of one Spatial Coordinate. In Physics, a conservation law states that a particular measurable property of an isolated Physical system does not change as the system evolves Gravitation is a natural Phenomenon by which objects with Mass attract one another Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of However, in the mid-1950's Chen Ning Yang and Tsung-Dao Lee suggested that the weak interaction might violate this law. Chen-Ning Franklin Yang ( (born October 1, 1922) is a Chinese -born American Physicist who worked on Statistical mechanics Tsung-Dao Lee ( TD Lee,) (born November 24, 1926) is a Chinese -born American Physicist, well known for his work Chien Shiung Wu and collaborators in 1957 discovered that the weak interaction in fact maximally violates parity, earning Yang and Lee the 1957 Nobel Prize in Physics. Chien-Shiung Wu ( May 13, 1912 – February 16, 1997) was a Chinese -born American Physicist with an expertise The Nobel Prize in Physics (Nobelpriset i fysik is awarded once a year by the Royal Swedish Academy of Sciences.

Although the weak interaction used to be described by Fermi's theory of a contact four-fermion interaction, the discovery of parity violation and renormalization theory suggested a new approach was needed. In Physics, Fermi's interaction is an old explanation of the Weak force, proposed by Enrico Fermi. In Particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection In 1957, Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a V−A (vector minus axial vector or left-handed) Lagrangian for weak interactions. Ennackal Chandy George Sudarshan (born September 16, 1931, Pallam, in Kottayam district of Kerala, India) also named Richard Phillips Feynman (ˈfaɪnmən May 11 1918 – February 15 1988 was an American Physicist known for the Path integral formulation of quantum Murray Gell-Mann (born September 15, 1929) is an American Physicist who received the 1969 Nobel Prize in physics for his work In Physics and Mathematics, a pseudovector (or axial vector) is a quantity that transforms like a vector under a proper rotation but gains an The Lagrangian, L of a Dynamical system is a function that summarizes the dynamics of the system In this theory, the weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity.

However, this theory allowed a compound symmetry CP to be conserved. In Particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics CP combines parity P (switching left to right) with charge conjugation C (switching particles with antiparticles). Physicists were again surprised when in 1964, James Cronin and Val Fitch provided clear evidence in kaon decays that CP symmetry could be broken too, winning them the 1980 Nobel Prize in Physics. James Watson Cronin (born September 29, 1931) is an American nuclear physicist. Val Logsdon Fitch (born March 10, 1923, Merriman, Nebraska, USA) is an American nuclear physicist. In Particle physics, a kaon (/ˈkeɪɒn/ also called K-meson and denoted) is any one of a group of four Mesons distinguished by the fact that they The Nobel Prize in Physics (Nobelpriset i fysik is awarded once a year by the Royal Swedish Academy of Sciences. Unlike parity violation, CP violation is a very small effect.

Electroweak Theory

The Standard Model of particle physics describes the electromagnetic interaction and the weak interaction as two different aspects of a single electroweak interaction, the theory of which was developed around 1968 by Sheldon Glashow, Abdus Salam and Steven Weinberg (see W and Z bosons). The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles In Physics, the electromagnetic force is the force that the Electromagnetic field exerts on electrically charged particles In Particle physics, the electroweak interaction is the unified description of two of the four Fundamental interactions of nature Electromagnetism and the Sheldon Lee Glashow (born December 5, 1932) is an American physicist. Abdus Salam ( Urdu: محمد عبد السلام) ( January 29, 1926; Jhang Punjab &ndash November 21, Steven Weinberg (born May 3, 1933) is an American Physicist, and Nobel laureate in Physics for his contributions with Abdus Salam The W and Z bosons are the Elementary particles that mediate the Weak force. They were awarded the 1979 Nobel Prize in Physics for their work. The Nobel Prize in Physics (Nobelpriset i fysik is awarded once a year by the Royal Swedish Academy of Sciences.

According to the electroweak theory, at very high energies, the universe has four identical massless gauge bosons similar to the photon and a scalar Higgs field. In Particle physics, gauge bosons are Bosonic particles that act as carriers of the fundamental forces of nature In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena The Higgs Boson is a hypothetical massive scalar Elementary particle predicted to exist by the Standard Model of Particle physics However, at low energies, the symmetry of the Higgs field is spontaneously broken by the Higgs mechanism. In Physics, spontaneous symmetry breaking occurs when a system that is symmetric with respect to some Symmetry group goes into a Vacuum state The Higgs mechanism is Spontaneous symmetry breaking in a Gauge theory. This symmetry breaking produces three massless Goldstone bosons which are "eaten" by three of the photon-like fields, giving them mass. In particle and Condensed matter physics, Goldstone bosons (also known as Nambu -Goldstone bosons) are Bosons that appear in models These three fields become the W and Z bosons of the weak interaction, while the fourth field remains massless and is the photon of electromagnetism. The W and Z bosons are the Elementary particles that mediate the Weak force.

Although this theory has made a number of impressive predictions, including a prediction of the mass of the Z boson before its discovery, the Higgs boson itself has never been observed. The Higgs Boson is a hypothetical massive scalar Elementary particle predicted to exist by the Standard Model of Particle physics Producing Higgs bosons will be a major goal of the Large Hadron Collider being built at CERN. The European Organization for Nuclear Research (Organisation Européenne pour la Recherche Nucléaire known as CERN

References

See also

External links

For a basic description see the article on the Standard Model. In Particle physics, the electroweak interaction is the unified description of two of the four Fundamental interactions of nature Electromagnetism and the The Weakless Universe is a hypothetical Universe that contains no Weak interactions but is otherwise very similar to our own universe In Physics and Cosmology, the anthropic principle states that humans should take into account the constraints that human existence imposes on the kind of theoretical
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