Kaon

In particle physics, a kaon, also called a K meson and denoted , is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model they are understood to be bound states of a strange quark (or antiquark) and an up or down antiquark (or quark).

Kaons have proved to be a copious source of information on the nature of fundamental interactions since their discovery by George Rochester and Clifford Butler at the Department of Physics and Astronomy, University of Manchester in cosmic rays in 1947. They were essential in establishing the foundations of the Standard Model of particle physics, such as the quark model of hadrons and the theory of quark mixing (the latter was acknowledged by a Nobel Prize in Physics in 2008). Kaons have played a distinguished role in understanding of fundamental conservation laws. Charge conjugation parity (CP) symmetry violation, a requirement in any theory to explain the observed matter–antimatter asymmetry of the universe, was discovered in the kaon system in 1964 (which was acknowledged by a Nobel Prize in 1980). Moreover, direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 experiment at CERN and the KTeV experiment at Fermilab.

Basic properties

thumb|upright=1.1| The decay of a kaon () into three [[pions (2 , 1 ) is a process that involves both weak and strong interactions.Weak interactions: The strange antiquark () of the kaon transmutes into an up antiquark () by the emission of a boson; the boson subsequently decays into a down antiquark 

() and an up quark ().

Strong interactions: An up quark () emits a gluon (), which decays into a down quark () and a down antiquark ().|left]]The four kaons are:

, negatively charged (containing a strange quark and an up antiquark) has mass and mean lifetime .
(antiparticle of above) positively charged (containing an up quark and a strange antiquark) must (by CPT invariance) have mass and lifetime equal to that of . Experimentally, the mass difference is , consistent with zero; the difference in lifetimes is , also consistent with zero.
, neutrally charged (containing a down quark and a strange antiquark) has mass . It has mean squared charge radius of .
, neutrally charged (antiparticle of above) (containing a strange quark and a down antiquark) has the same mass.

As the quark model shows, assignments that the kaons form two doublets of isospin; that is, they belong to the fundamental representation of SU(2) called the 2. One doublet of strangeness +1 contains the and the . The antiparticles form the other doublet (of strangeness −1).

{| class="wikitable sortable" style="text-align: center;"

|+ Properties of kaons

! class=unsortable|Particle name

! Particle symbol

! Antiparticle symbol

! class=unsortable | Quark content

! Rest mass

! IG

! JPC

! S

! C

! B′

! Mean lifetime

! class=unsortable | Commonly decays to (> 5% of decays)

| Kaon

| style="text-align:center;" |

| style="text-align:left;" |

| style="text-align:center;" |

| style="text-align:center;" | 0−

| style="text-align:center;" | 1

| style="text-align:center;" | 0

| style="text-align:right;" |

| style="text-align:left;" | or or or

| Kaon

| style="text-align:center;" |

| style="text-align:left;" |

| style="text-align:center;" |

| style="text-align:center;" | 0−

| style="text-align:center;" | 1

| style="text-align:center;" | 0

| style="text-align:center;" |

| K-short

| style="text-align:center;" |

| style="text-align:center;" | self

| style="text-align:center;" | <math>\mathrm{\tfrac{d\bar{s} + s\bar{d{\sqrt{2}\,</math>

| style="text-align:center;" |

| style="text-align:center;" |

| style="text-align:center;" | 0−

| style="text-align:center;" | [[List of mesons#Notes on neutral kaons|[*]]]

| style="text-align:center;" | 0

| style="text-align:right;" |

| style="text-align:left;" | or

| K-long

| style="text-align:center;" |

| style="text-align:center;" | self

| style="text-align:center;" | <math>\mathrm{\tfrac{d\bar{s} - s\bar{d{\sqrt{2}\,</math>

| style="text-align:left;" |

| style="text-align:center;" |

| style="text-align:center;" | 0−

| style="text-align:center;" | [[List of mesons#Notes on neutral kaons|[*]]]

| style="text-align:center;" | 0

| style="text-align:right;" |

| style="text-align:left;" | or or or

thumb|150x150px|Quark structure of the kaon (K+).

[*] See Notes on neutral kaons in the article List of mesons, and neutral kaon mixing, below.

[§]Strong eigenstate. No definite lifetime (see neutral kaon mixing).

[†]Weak eigenstate. Makeup is missing small CP–violating term (see neutral kaon mixing).

[‡]The mass of the and are given as that of the . However, it is known that a relatively minute difference between the masses of the and on the order of exists. It was resolved only by the discovery of parity violation in the weak interaction (most significantly, by the Wu experiment). Since the mesons decay through weak interactions, parity is not conserved, and the two decays are actually decays of the same particle, now called the .

History

<blockquote> 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. — Bigi & Sanda (2016)</blockquote>

While looking for the hypothetical nuclear meson, Louis Leprince-Ringuet found evidence for the existence of a positively charged heavier particle in 1944.

In 1947, G.D. Rochester and C.C. Butler of the University of Manchester 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 that appeared to 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.

thumb|The "k track plate" showing the three-pion decay mode of a kaon. The kaon enters from the left, and decays at the point labelled A.

In 1949, Rosemary Brown (later Rosemary Fowler), a research student of Cecil Powell of the University of Bristol, spotted her 'k' track, made by a particle of very similar mass that decayed to three pions. what seemed to be the same particle (now called ) decayed in two different modes, Theta to two pions (parity +1), Tau to three pions (parity −1). The solution to this puzzle turned out to be that weak interactions do not conserve parity.

Since the mass of K2 is just a little larger than the sum of the masses of three pions, this decay proceeds very slowly, about 600 times slower than the decay of K1 into two pions. These two different modes of decay were observed by Leon Lederman and his coworkers in 1956, establishing the existence of the two weak eigenstates (states with definite lifetimes under decays via the weak force) of the neutral kaons.

These two weak eigenstates are called the (K-long, τ) and (K-short, θ). CP symmetry, which was assumed at the time, implies that  = K1 and  = K2.

Oscillation

An initially pure beam of 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 always decayed into a positron, whereas the antiparticle decayed into the electron. The earlier analysis yielded a relation between the rate of electron and positron production from sources of pure 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 and . Since this is due to weak interactions it is very small, 10−15 times the mass of each state, namely .

Regeneration

A beam of neutral kaons decays in flight so that the short-lived disappears, leaving a beam of pure long-lived . If this beam is shot into matter, then the and its antiparticle interact differently with the nuclei. The undergoes quasi-elastic scattering with nucleons, whereas its antiparticle can create hyperons. Quantum coherence between the two particles is lost due to the different interactions that the two components separately engage in. The emerging beam then contains different linear superpositions of the and . Such a superposition is a mixture of and ; the is regenerated by passing a neutral kaon beam through matter. Regeneration was observed by Oreste Piccioni and his collaborators at Lawrence Berkeley National Laboratory. Soon thereafter, Robert Adair and his coworkers reported excess regeneration, thus opening a new chapter in this history.

CP violation

While trying to verify Adair's results, J. Christenson, James Cronin, Val Fitch and Rene Turlay of Princeton University found decays of into two pions (CP = +1)

in an experiment performed in 1964 at the Alternating Gradient Synchrotron at the Brookhaven laboratory. 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 nonlinear quantum mechanics and a new unobserved particle (hyperphoton) 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.

It turns out that although the and 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),

: = K2 + εK1

and similarly for . Thus occasionally the decays as a K1 with CP = +1, and likewise the can decay with CP = −1. This is known as indirect CP violation, CP violation due to mixing of 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. Direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 and KTeV experiments at CERN and Fermilab.

Basic properties

History

Oscillation

Regeneration

CP violation

See also

Footnotes

References

Bibliography

External links