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.
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:
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 s. 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 s. (See discussion of neutral kaon mixing below.) An experimental observation made in 1964 that K-longs rarely decay into two pions was the discovery of CP violation (see below). Main decay modes for K+:
Decay modes for the K− are charge conjugates of the ones above.
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 τ–θ puzzle (tau–theta puzzle). 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 her 'k' track, made by a particle of very similar mass that decayed to three pions.
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