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Bound states of quarks, in which at least one of these constituents is of the strange (s) type. Strange quarks are heavier than the up (u) and down (d) quarks, which form the neutrons and protons in the atomic nucleus. Neutrons (udd) and protons (uud) are the lightest examples of a family of particles composed of three quarks, known as baryons. These and other composite particles which interact dominantly through the strong (nuclear) force are known as hadrons. The first strange hadron discovered (in cosmic rays in 1947) was named the lambda baryon, &Lgr;; it is made of the three-quark combination uds. A baryon containing a strange quark is also called a hyperon. Although strange particles interact through the strong (nuclear) force, the strange quark itself can decay only by conversion to a quark of different type (such as u or d) through the weak interaction. For this reason, strange particles have very long lifetimes, of the order of 10-10 s, compared to the lifetimes of the order of 10-23 s for particles which decay directly through the strong interaction. This long lifetime was the origin of the term strange particles. See Baryon, Hadron, Neutron, Proton, Strong nuclear interactions
In addition to strange baryons, strange mesons occur. The lightest of these are the kaons (K+ = u and K 0 = d) and the antikaons (= s and - = s). Kaons and their antiparticles have been very important in the study of the weak interaction and in the detection of the very weak CP violation, which causes a slow transition between neutral kaons and neutral antikaons. See Elementary particle, Meson, Quarks