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Encyclopedia > Kaon

In particle physics, Kaons (also called K-mesons and denoted K) are a group of four mesons distinguished by the fact that they carry a quantum number called strangeness. In the quark model they are understood to contain a single strange quark (or antiquark). Particles erupt from the collision point of two relativistic (100 GeV) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... A quantum number is any one of a set of numbers used to specify the full quantum state of any system in quantum mechanics. ... In particle physics, strangeness is the number of anti-strange quarks minus the number of strange quarks in a particle. ... The quark model is a classification scheme for hadrons in terms of their valence quarks, ie, the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ...

Contents


Basic properties

The four kaons are

  1. The negatively charged K- (containing a strange quark and an up antiquark) has mass 493.667 ± 0.013 MeV and mean lifetime (1.2384 ± 0.0024) × 10-8 seconds.
  2. Its antiparticle, the positively charged K+ (containing an up quark and a strange antiquark) has mass equal to that of K-. The mass difference is 0.032 ± 0.090 MeV, and hence consistent with zero. The difference in lifetime is (0.11 ± 0.09) × 10-8 seconds. These two numbers are tests of CPT invariance.
  3. The K0 (containing a down quark and a strange antiquark) has mass 497.648 ± 0.022 MeV. It has mean squared charge radius of -0.076 ± 0.018 fm2.
  4. Its antiparticle (containing a strange quark and a down antiquark) has the same mass.

It is clear from the quark model assignments that they form two doublets of isospin, ie, the fundamental representation of SU(2) called the 2. One doublet of strangeness +1 contains the K+ and the K0. The antiparticles form the other doublet. For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... Corresponding to each kind of particle, there is an associated antiparticle with the same mass and spin but with many other quantum numbers flipped in sign. ... CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... The quark model is a classification scheme for hadrons in terms of their valence quarks, ie, the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. ... Isospin (isotopic spin, isobaric spin) is a physical quantity which is mathematically analogous to spin. ... In mathematics, a fundamental representation is a representation of a mathematical structure, such as a group, that satisfies the following condition: All other irreducible representations of the group can be found in the tensor products of the fundamental representation with many copies of itself. ... In mathematics, the special unitary group of degree n is the group of n by n unitary matrices with determinant 1 and entries from the field C of complex numbers, with the group operation that of matrix multiplication. ...


Although the K0 and its antiparticle are usually produced via the strong force, they decay weakly. Thus once created they act as weak eigenstates, which have vastly different lifetimes (see discussion of neutral kaon mixing below): The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In linear algebra, the eigenvectors (from the German eigen meaning inherent, characteristic) of a linear operator are non-zero vectors which, when operated on by the operator, result in a scalar multiple of themselves. ...

  1. The long-lived neutral kaon is called the KL ("K-long") decays primarily into three pions, and has a mean lifetime of 5.18 × 10-8 seconds.
  2. The short-lived neutral kaon is called the KS ("K-short") decays primarily into two pions, and has a mean lifetime 8.958 × 10-11 seconds.

It was in the observation of these particles that CP violation was first detected (see below). In particle physics, pion (short for the Greek pi meson = P middle) is the collective name for three subatomic particles discovered in 1947: π0, π+ and π−. Pions are the lightest mesons. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ...


Strangeness

The discovery of hadrons with the internal quantum number "strangeness" marks the beginning of a most exciting epoch in particle physics that even now, fifty years later, has not yet found its conclusion ... by and large experiments have driven the development, and that major discoveries came unexpectedly or even against expectations expressed by theorists.  —  I.I. Bigi and A.I. Sanda, CP violation, (ISBN 0521443490)

In 1947, G. D. Rochester and C. C. Butler published two cloud chamber photographs of cosmic ray induced events, one showing what appeared to be a neutral particle decaying into two charged pions, and one which appeared to be be a charged particle decaying into a charged pion and something neutral. The estimated mass of the new particles was very rough, about half a proton's mass. More examples of these "V-particles" were slow in coming. 1947 was a common year starting on Wednesday (link will take you to calendar). ... The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ...


The first breakthrough was obtained at Cal Tech, where a cloud chamber was taken up Mount Wilson, for greater cosmic ray exposure. In 1950, 30 charged and 4 neutral V-particles were reported. Inspired by this, numerous mountaintop observations were made over the next several years, and by 1953, the following terminology was adopted: "L-meson" meant muon or pion. "K-meson" meant a particle intermediate in mass between the pion and nucleon. "Hyperon" meant any particle heavier than a nucleon. The California Institute of Technology (commonly known as Caltech) is a private, coeducational university located in Pasadena, California, in the United States. ... Mount Wilson lies in the San Gabriel Mountains in Southern California. ... 1950 was a common year starting on Sunday (link will take you to calendar). ... 1953 is a common year starting on Thursday. ... The moons shadow, as seen in muons 700m below ground at the Soudan 2 detector. ... In particle physics, pion (short for the Greek pi meson = P middle) is the collective name for three subatomic particles discovered in 1947: π0, π+ and π−. Pions are the lightest mesons. ...


The decays were extremely rare: typical lifetimes are of the order of 10-10 seconds. However, production in pion-proton reactions proceeds much faster, with a time scale of 10-23 seconds. The problem of this mismatch was solved by Abraham Pais who postulated the new quantum number called strangeness which is conserved in strong interactions but violated by the weak interactions. Strange particles appear copiously due to associated production of a strange and an antistrange particle together. It was soon shown that this could not be a multiplicative quantum number, because that would allow reactions which were never seen in the new cyclotrons which were commissioned in Brookhaven National Laboratory in 1953 and in the Lawrence Berkeley Laboratory in 1955. Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... Abraham Pais (May 19, 1918 - August 4, 2000) was a Dutch-born physicist. ... The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In quantum field theory, multiplicative quantum numbers are conserved quantum numbers of a special kind. ... 60-inch cyclotron, circa 1939, showing beam of accelerated ions (perhaps protons or deuterons) causing a blue glow, almost certainly the Cherenkov effect. ... Aerial view of Brookhaven National Laboratory. ... 1953 is a common year starting on Thursday. ... The Berkeley Lab is perched on a hill overlooking the Berkeley central campus and San Francisco Bay. ... 1955 is a common year starting on Saturday of the Gregorian calendar. ...


Parity violation: the τ-θ puzzle

Two different decays were found for charged strange mesons

θ+  →  π+  +  π0  and  τ +  →  π+  +  π+  +  π-.

Since the two final states had different parity it was then thought that the initial states should also have different parities, and hence be two distinct particles. However, with increasingly precise measurements, there were found to be no difference between their masses and lifetimes, indicating that they are the same particle. This was known as the τ-θ puzzle. It was resolved only by the discovery of parity violation in weak interactions. Since the mesons decay through weak interactions, parity need not be conserved, and the two decays are of the same particle— now called the K+. In physics, a parity transformation (also called parity) is the simultaneous flip in the sign of all spatial coordinates: A 3×3 matrix representation of P would have determinant equal to -1, and hence cannot reduce to a rotation. ... In physics, a parity transformation (also called parity) is the simultaneous flip in the sign of all spatial coordinates: A 3×3 matrix representation of P would have determinant equal to -1, and hence cannot reduce to a rotation. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...


CP violation in neutral meson oscillations

Initially it was thought that although parity was violated, CP symmetry was conserved (by kaons and everything else). In order to understand the discovery of CP violation, it is necessary to understand the mixing of neutral kaons; this phenomenon does not require CP violation, but it is the context in which CP violation was first observed. Parity is a concept of equality of status or functional equivalence. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ...


Neutral kaon mixing

Two different neutral K mesons, carrying different strangeness, can turn from one into another through the weak interactions, since these interactions do not conserve strangeness. The strange quark in the K0 turns into a down quark by successively emitting two W-bosons of opposite charge. The down antiquark in the K0 turns into a strange antiquark by absorbing them.
Two different neutral K mesons, carrying different strangeness, can turn from one into another through the weak interactions, since these interactions do not conserve strangeness. The strange quark in the K0 turns into a down quark by successively emitting two W-bosons of opposite charge. The down antiquark in the K0 turns into a strange antiquark by absorbing them.

Since neutral kaons carry strangeness, they cannot be their own antiparticles. There must be then two different neutral kaons, differing by two units of strangeness. The question was then how to establish the presence of these two mesons. The solution used a phenomenon called neutral particle oscillations, by which these two kinds of mesons can turn from one into another through the weak interactions, which cause them to decay into pions (see the adjoining figure). Image File history File links K0-K0bar mixing File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links K0-K0bar mixing File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. ... In particle physics neutral particle oscillation is the transmutation of a neutral particle with nonzero internal quantum numbers into its antiparticle. ...


These oscillations were first investigated by Murray Gell-Mann and Abraham Pais together. They considered the CP invariant time evolution of the states with opposite strangeness. In matrix notation one can write Murray Gell-Mann at Harvard University Murray Gell-Mann (born September 15, 1929) is an American physicist who received the 1969 Nobel Prize in physics for his work on the theory of elementary particles. ... Abraham Pais (May 19, 1918 - August 4, 2000) was a Dutch-born physicist. ...

where ψ is a quantum state of the system specified by the amplitudes of being in each of the two basis states (which are a and b at time t=0). The diagonal elements of the Hamiltonian are the strong interaction piece which conserves strangeness. The two diagonal elements have to be equal, since the particle and antiparticle must have equal masses in the absence of the weak interactions. The off-diagonal elements, which mixes the opposite strangeness particles, are due to the weak interactions. CP symmetry requires them to be real. A quantum state is any possible state in which a quantum mechanical system can be. ... Fig. ... The Hamiltonian, denoted H, has two distinct but closely related meanings. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ...


Mixing

The eigenstates are obtained by diagonalizing this matrix. This gives new eigenvectors, which we can call K1 which is the sum of the two states of opposite strangeness, and K2, the difference. They have opposite values of CP, with the K1 having CP=1. Since the two pion final state also has CP=1, only the K1 can decay into this. The K2 must decay into three pions. It turns out that the mass of K2 is just a little larger than the sum of the masses of three pions, so this decay proceeds about 600 times slower. These two different modes of decay were observed by Leon Lederman and his coworkers in 1956, thus establishing the existence of the two weak eigenstates (i.e. states with definite lifetimes under decays via the weak force) of the neutral K mesons. Leon Max Lederman (born July 15, 1922) is an American experimental physicist who was awarded the Nobel Prize in Physics in 1988 for his work on neutrinos. ... 1956 was a leap year starting on Sunday of the Gregorian calendar. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In linear algebra, the eigenvectors (from the German eigen meaning inherent, characteristic) of a linear operator are non-zero vectors which, when operated on by the operator, result in a scalar multiple of themselves. ... 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 of reduction (decay) of the assembly. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...


These two weak eigenstates are called the KL (K-long) and KS (K-short). CP symmetry, which was assumed at the time, implies that KS = K1 and KL = K2. The lifetime of KS is (0.8953 ± 0.0006) × 10-10 seconds and that of the KL is (5.18 ± 0.04) × 10-8 seconds. CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ...


Oscillation

An initially pure beam of K0 will turn into its antiparticle while propagating, which will turn back into the original particle, and so on. This is called particle oscillation. On observing the weak decay into leptons, it was found that a K0 always decayed into an electron, whereas the antiparticle decayed into the positron. The earlier analysis yielded a relation between the rate of electron and positron production from sources of pure K0 and its antiparticle. Analysis of the time dependence of this semileptonic decay showed the phenomenon of oscillation, and allowed the extraction of the mass splitting between the KS and KL. Since this is due to weak interactions it is very small — 10-15 times the mass of each state. In particle physics the semileptonic decay of a hadron referes to a decay through the weak interaction in which one lepton (and the corresponding neutrino) is produced apart from other hadrons. ...


Regeneration

A beam of neutral K mesons decays in flight so that the KS disappears very soon leaving a beam of pure KL. If this is shot into a nucleus, then the K0 and its antiparticle interact differently with the matter. The K0 undergoes quasi-elastic scattering with nucleons, whereas its antiparticle can create hyperons. Due to the different interactions of the two components, quantum coherence between the two particles is lost. The emerging beam then contains different linear superpositions of the K0 and its antiparticle. This can be resolved into a KL and a KS state — thus the latter is regenerated by passing a neutral K beam through matter. Regeneration was observed by Piccioni and his collaborators at LBL. Soon thereafter, Adair and his coworkers reported excess KS regeneration, thus opening a new chapter in this history. In physics a nucleon is a collective name for the two baryons the neutron and the proton. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... Quantum coherence refers to the condition of a quantum system whose constituents are in-phase. ... The Berkeley Lab is perched on a hill overlooking the Berkeley central campus and San Francisco Bay. ... Adair can refer to several places in the United States: Adair in Iowa, Adair in Oklahoma, Adair Village in Oregon, Adair County in Iowa, Adair County in Missouri. ...


CP violation

While trying to verify Adair's results, in 1964 James Cronin and Val Fitch of BNL found decays of KL into two pions (CP = +1). As explained in an earlier section, this required the assumed initial and final states to have different values of CP, and hence immediately suggested CP violation. Alternative explanations such as non-linear quantum mechanics and a new unobserved particle were soon ruled out, leaving CP violation as the only possibility. Cronin and Fitch received the Nobel Prize in Physics for this discovery in 1980. 1964 was a leap year starting on Wednesday (link will take you to calendar). ... James Watson Cronin (born September 29, 1931) is an American nuclear physicist. ... Val Logsdon Fitch (born March 10, 1923) is an American nuclear physicist. ... Aerial view of Brookhaven National Laboratory. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ... List of Nobel Prize laureates in Physics from 1901 to the present day. ... 1980 is a leap year starting on Tuesday. ...


It turns out that although the KL and KS are weak eigenstates (because they have definite lifetimes for decay by way of the weak force), they are not quite CP eigenstates. Instead, for small ε (and up to normalization), The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In quantum mechanics, operators correspond to observable variables, eigenvectors are also called eigenstates, and the eigenvalues of an operator represent those values of the corresponding variable that have non-zero probability of occurring. ... 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 of reduction (decay) of the assembly. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...

KL = K2 + εK1

and similarly for KS. Thus occasionally the KL decays as a K1 with CP = +1, and likewise the KS can decay with CP = -1. This is known as indirect CP violation, i.e. CP violation due to mixing of K0 and its antiparticle. There is also a direct CP violation effect, in which the CP violation occurs during the decay itself. Both are present, because both mixing and decay arise from the same interaction with the W boson and thus have CP violation predicted by the CKM matrix. In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. ... In the standard model of particle physics the Cabibbo Kobayashi Maskawa matrix (CKM matrix, sometimes earlier called KM matrix) is a unitary matrix which contains information on the strength of flavour changing weak decays. ...


See also

In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... Flavour (or flavor) is a quantum number of elementary particles related to their weak interactions. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... The quark model is a classification scheme for hadrons in terms of their valence quarks, ie, the quarks (and antiquarks) which give rise to the quantum numbers of the hadrons. ... In physics, a parity transformation (also called parity) is the simultaneous flip in the sign of all spatial coordinates: A 3×3 matrix representation of P would have determinant equal to -1, and hence cannot reduce to a rotation. ... C-symmetry means the symmetry of physical laws over a charge-inversion transformation. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation. ... CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ...

References and external links

  • Particle data group on strange mesons
  • The quark model, by J.J.J. Kokkedee
  • CP violation, by I.I. Bigi and A.I. Sanda (Cambridge University Press, 2000) ISBN 0521443490
  • Griffiths, David (1987). Introduction to Elementary Particles. New York: John Wiley & Sons. ISBN 0-471-60386-4.


Particles in physics - composite particles
Hadrons: Baryons (list) | Mesons (list)

Baryons: Nucleons | Hyperons | Exotic baryons | Pentaquarks
Mesons: Pions | Kaons | Quarkonium | Exotic mesons
Atomic nuclei | Atoms | Molecules Particles erupt from the collision point of two relativistic (100 GeV) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... Elementary particles An elementary particle is a particle with no measurable internal structure, that is, it is not a composite of other particles. ... In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... A list of baryons. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... A list of mesons. ... In physics a nucleon is a collective name for the two baryons the neutron and the proton. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... Ordinary baryons are bound states of 3 quarks. ... A pentaquark is a subatomic particle consisting of a group of five quarks (compared to three quarks in normal baryons and two in mesons), or more specifically four quarks and one anti-quark. ... In particle physics, pion (short for the Greek pi meson = P middle) is the collective name for three subatomic particles discovered in 1947: π0, π+ and π−. Pions are the lightest mesons. ... In high energy physics, a quarkonium (pl. ... In particle physics, an exotic meson is a meson (a strongly interacting boson) that does not contain exactly one valence quark-antiquark pair. ... A stylized representation of a lithium atom. ... Properties For alternative meanings see atom (disambiguation). ... A molecule is something that Lelea has in her room in Chase. ...


  Results from FactBites:
 
Kaon - Wikipedia, the free encyclopedia (1633 words)
In particle physics, a kaon (also called K-meson and denoted K) is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness.
It is clear from the quark model assignments that the kaons form two doublets of isospin; that is, they belong to the fundamental representation of SU(2) called the 2.
In order to understand the discovery of CP violation, it is necessary to understand the mixing of neutral kaons; this phenomenon does not require CP violation, but it is the context in which CP violation was first observed.
Rare Kaon Decay Observed For Second Time (1071 words)
Since kaons only exist for about 12 billionths of a second, the E787 collaboration then had to build a state-of-the-art particle detector the size of a small house in order to capture these fleeting decays in detail.
Kaon decays in general have proved a rich and often surprising source of information on fundamental questions in particle physics, largely due to the kaon’s “strange” quark, a heavy relative of the quarks that comprise ordinary matter such as atomic nuclei.
KOPIO plans to study the closely related decay of the long-lived neutral kaon into a neutral pi meson and a pair of neutrinos, a process that may offer the single best chance of glimpsing the still-mysterious phenomenon of charge conjugation and parity violation (CP-violation).
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