Astroparticle Physics

30 2 The Standard Model of Elementary Particlesisospin triplet of pionsbaryon numberconservation lawsof particle physicsstate relates to the neutron. The three pions (π + ,π − ,π 0 )combine to form an isospin triplet with I = 1. In this case,I z =−1 corresponds to the π − , I z =+1istheπ + , whilstI z = 0 relates to the π 0 . The particle multiplicity m in anisospin multiplet is related to the isospin via the equationm = 2I + 1 . (2.6)Finally, the baryon number should be mentioned. Quarksare assigned the baryon number 1/3, and antiquarks aregiven −1/3. All baryons consisting of three quarks aretherefore assigned the baryon number 1, whereas all otherparticles get the baryon number 0.The properties of the conservation laws for the differentinteraction types in elementary particle physics are compiledin Table 2.4.Table 2.4Conservation laws of particlephysics (conserved: +; violated: –)physicalinteractionquantity strong electromagnetic weakmomentum + + +energy (incl. mass) + + +ang. momentum + + +electric charge + + +quark flavour + + –lepton number ∗ ./. + +parity + + –charge conjugation + + –strangeness + + –isospin + – –baryon number + + +∗ the lepton number is not relevant for strong interactions}uudd}ps uW –Fig. 2.11Lambda decay: Λ → p + π −ud} –Unfortunately, there is a small but important complicationin the quark sector. As can be seen from Table 2.1, thereis a complete symmetry between leptons and quarks. Leptons,however, participate in interactions as free particles,whereas quarks do not. Due to quark confinement, spectatorquarks always participate in the interactions in some way.For charged leptons, there is a strict law of lepton-numberconservation: The members of different generations do notmix with each other. For the quarks, it was seen that weakprocesses can change the strangeness. In Λ decay, the squark belonging to the second generation can transform intoa u quark of the first generation. This would otherwise onlybe allowed to happen to a d quark (Fig. 2.11).