In particle physics, a kaon, also called a K meson and denoted K, 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.
Contents
Basic properties
The four kaons are:
K−, negatively charged (containing a strange quark and an up antiquark) has mass 493.677±0.013 MeV/c2 and mean lifetime (1.2380±0.0020)×10−8 s.
K+ (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 K−. Experimentally, the mass difference is 0.032±0.090 MeV/c2, consistent with zero; the difference in lifetimes is (0.11±0.09)×10−8 s, also consistent with zero.
K0, neutrally charged (containing a down quark and a strange antiquark) has mass 497.648±0.022 MeV/c2. It has mean squared charge radius of −0.076±0.01 fm2.
K0, 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 K+ and the K0. The antiparticles form the other doublet (of strangeness −1).
[*] 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 K0L and K0S are given as that of the K0. However, it is known that a relatively minute difference between the masses of the K0L and K0S on the order of 3.5×10−6 eV/c2 exists.
Although the K0 and its antiparticle K0 are usually produced via the strong force, they decay weakly. Thus, once created the two are better thought of as superpositions of two weak eigenstates that have vastly different lifetimes:
Parity violation
Two different decays were found for charged strange mesons into pions:
The intrinsic parity of the pion is P = −1 (since the pion is a bound state of a quark and an antiquark, which have opposite parities, with zero angular momentum), and parity is a multiplicative quantum number. Therefore, assuming the parent particle has zero spin, the two-pion and the three-pion final states have different parities (P = +1 and P = −1, respectively). It was thought that the initial states should also have different parities, and hence be two distinct particles. However, with increasingly precise measurements, no difference was found between the masses and lifetimes of each, respectively, indicating that they are the same particle. This was known as the Theta-Tau puzzle (also written θ − τ puzzle, or with the particle reversed). 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 K+.
History
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)
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.
In 1949, Rosemary Brown (later Rosemary Fowler), a research student of Cecil Powell of the University of Bristol, spotted the track k made by a particle of very similar mass that decayed to three pions.
I knew at once that it was new and would be very important. We were seeing things that hadn't been seen before – that's what research in particle physics was. It was very exciting. — Fowler (2024)
This led to the so-called tau–theta puzzle: what seemed to be the same particle (now called K+) decayed in two different modes, Θ to two pions (parity +1), τ to three pions (parity −1). The solution to this puzzle turned out to be that weak interactions do not conserve parity.
The first breakthrough was obtained at Caltech, 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 1954 the following terminology was being used: L-meson for a particle of mass up to a pion, for the then-called π-meson and μ-meson, and K-meson for a particle of mass between the pion and proton, for the tentative τ± (now decay mode Γ11 of K±), κ (Γ4), χ (Γ9), and θ⁰ (turned lowercase as capitals were reserved for baryons, Γ8 and Γ2 of K0L). Leprince-Ringuet coined the still-used term hyperon to mean any particle heavier than a nucleon. The Leprince-Ringuet particle turned out to be the K+ meson.
CP violation in neutral meson oscillations
Initially it was thought that although parity was violated, CP (charge parity) symmetry was conserved. 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.
Neutral kaon mixing
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 adjacent figure).
These oscillations were first investigated by Murray Gell-Mann and Abraham Pais together. They considered the CP-invariant time evolution of states with opposite strangeness. In matrix notation one can write
ψ
(
t
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=
U
(
t
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ψ
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0
)
=
e
i
H
t
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a
b
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,
H
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CP violation
While trying to verify Adair's results, J. Christenson, James Cronin, Val Fitch and Rene Turlay of Princeton University found decays of KL 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 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),
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, 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. Direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 and KTeV experiments at CERN and Fermilab.