Astroparticle Physics

2.1 Examples of Interaction Processes 29The parity transformation P is the space inversion of aphysical state. Parity is conserved in strong and electromagneticinteractions, however, in weak interactions it is maximallyviolated. This means that the mirror state of a weakprocess does not correspond to a physical reality. Nature distinguishesbetween the right and left in weak interactions.The operation of charge conjugation C applied to a physicalstate changes all the charges, meaning that particlesand antiparticles are interchanged, whilst leaving quantitieslike momentum or spin untouched. Charge conjugation isalso violated in weak interactions. In β decay, for example,left-handed electrons (negative helicity) and right-handedpositrons (positive helicity) are favoured. Even though thesymmetry operations P and C are not conserved individually,their combination CP, which is the application of spaceinversion (parity operation P ) with subsequent interchangeof particles and antiparticles (charge conjugation C) isawell-respected symmetry. This symmetry, however, is stillbroken in certain decays (K 0 and B 0 decays), but it is a commonbelief that the CPT symmetry (CP symmetry with additionaltime inversion) is conserved under all circumstances.Some particles, like kaons, exhibit very strange behaviour.They are produced copiously, but decay relativelyslowly. These particles are produced in strong interactions,but they decay via weak interactions. This property is accountedfor by introducing the quantum number strangeness,which is conserved in strong interactions, but violated inweak decays. Owing to the conservation of strangeness instrong interactions, only the associate production of strangeparticles, i.e., the combined production of hadrons one ofwhich contains a strange and the other an anti-strange quark,is possible, such asπ − + p → K + + Σ − . (2.5)In this process, the ¯s quark in the K + (= u¯s) receives thestrangeness +1, whilst the s quark in the Σ − (= dds)is assigned the strangeness −1. In the weak decay of theK + → π + π 0 , the strangeness is violated, since pions donot contain strange quarks (s).Certain particles that behave in an identical way understrong interactions, but differ in their charge state, are integratedinto isospin multiplets. Protons and neutrons are nucleonsthat form an isospin doublet of I = 1/2. When thenucleon isospin is projected onto the z axis, the state withI z =+1/2 corresponds to a proton whereas the I z =−1/2parityparity violationcharge conjugationCP conservationin weak interactions?CP violationCPT symmetrystrange particlesstrangenessisospin multipletisospin doublet of nucleons