Kaon

Kaons are mesons distinguished by a quantum number known as strangeness. 

Strangeness was discovered by Murray Gell-Mann and others to explain how certain particles, such as kaons, are created easily in particle collisions, and decayed much more slowly than expected. They dubbed this property "Strangeness" which was preserved during created and not conserved during decay. 

In the quark model, they are understood to be a bound state of of a strange quark or antiquark with an up or down antiquark or quark.

They were discovered in cosmic rays in 1947.