The Stranger the Better December 20, 1947 A charged Kaon decaying into a muon and neutrino in this case the neutrino leaves no track A neutral Kaon (leaving no track as it is uncharged) decaying into two pions 1947 The discovery of a strange meson called the Kaon

K-mesons (Kaons): p + p- K0 + L0 K- mesons are produce in certain collisions. They eventually decay but live far too long 10-10s rather than 10-23 s p + p- K0 + L0 B no: 1 1 Check that this reaction conserves baryon number. The K meson is always produced with a (relatively) long lived baryon in this case a Λ0 We have a pair of strange particles. The Kaon was allocated a strangeness number of +1 and the lambda zero was given a strangeness number of -1

K-mesons (Kaons): p + p- K0 + L0 Strangeness number +1 -1 +1 -1 Many other particles were found with this ridiculously long lifetime. (1013 times longer than it should be!) It was found that strangeness number is conserved in any strong interaction

What is strangeness due to? Strange particles have a new type of quark called a strange quark which is very like a down quark with a charge of -1/3 but heavier s d u d ŝ The lambda particle has a strange quark and strangeness -1 The kaon has an antistrange quark and strangeness +1

How Does It Get There? p + π- K0 + Λ0 d u s d u d ŝ û d Notice that the strange and antistrange particle are always produced together. Notice that Strangeness is conserved here

s d u d d ŝ ŝ u ŝ ŝ ŝ s u s d u ŝ ŝ s ŝ s s ŝ s What is the strangeness of each of these mesons? What is the strangeness of each of these baryons?

Strangeness is conserved Particle S K +1 K- -1 S- S+ Strangeness is conserved in all strong interactions Which of these is forbidden? p+π-→ K-+ Σ+ π-+p → K++ Σ- K++ p → π++ Σ+

Some strange particles charge Baryon no S K +1,-1,0 +1 1 -1 X +1,-1 -2

Strange particles can decay but only do so by the weak interaction p + p- S -1 0 0 Not strange particles strange particle This is very much like a negative beta decay. It involves a change of quark flavour! Note that strangeness is not conserved in the weak interaction

L0 p + p- s u d W- π- Λo p π- W- A Feynman Diagram for the weak decay of a strange particle

Strangeness Rules We have a new quark called a strange quark Strangeness is conserved in strong interactions Strangeness is not conserved in weak interactions Strange quarks have a strangeness of -1 Antistrange particles have a strangeness of -1