Kaon
In particle physics, a kaon (ˈkeɪ.ɒn), also called a K meson and denoted _Kaon, 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 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 our understanding of fundamental conservation laws: CP violation, a phenomenon generating 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. The four kaons are : _Kaon-, negatively charged (containing a strange quark and an up antiquark) has mass 493.677MeV and mean lifetime 1.2380e-8s. _Kaon+ (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 _Kaon-. Experimentally, the mass difference is 0.032MeV, consistent with zero; the difference in lifetimes is 0.11e-8s, also consistent with zero. _Kaon0, neutrally charged (containing a down quark and a strange antiquark) has mass 497.648MeV. It has mean squared charge radius of -0.076fm2. _AntiKaon0, 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 _Kaon+ and the _Kaon0.