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In particle physics, a kaon , also called a K meson and denoted ,〔The positively charged kaon used to be called τ+ and θ+, as it was supposed to be two different particles until the 1960s. See the parity violation section.〕 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 also discovered in the kaon decays in the early 2000s. ==Basic properties== 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 . The mass difference is , consistent with zero. The difference in lifetime is . #, 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. 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. One doublet of strangeness +1 contains the and the . The antiparticles form the other doublet (of strangeness −1). }\, | align="center"| | align="center"| | align="center"| 0− | align="center"| ( *) | align="center"| 0 | align="center"| 0 | align="center"| | align="center"| |- | K-Long〔J. Beringer ''et al''. (2012): (Particle listings – )〕 | align="center"| | align="center"| Self | align="center"| | align="center"| | align="center"| | align="center"| 0− | align="center"| ( *) | align="center"| 0 | align="center"| 0 | align="center"| | align="center"| |} () Strong eigenstate. No definite lifetime (see kaon notes below) () Weak eigenstate. Makeup is missing small CP–violating term (see notes on neutral kaons below). () The mass of the and are given as that of the . However, it is known that a difference between the masses of the and on the order of exists.〔 Although the and its antiparticle 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 which have vastly different lifetimes: #The long-lived neutral kaon is called the ("K-long"), decays primarily into three pions, and has a mean lifetime of . #The short-lived neutral kaon is called the ("K-short"), decays primarily into two pions, and has a mean lifetime . (''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 : : Decay modes for the are charge conjugates of the ones above. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Kaon」の詳細全文を読む スポンサード リンク
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