Astroparticle Physics

2 The Standard Model of Elementary Particles 25are compared in Table 2.3. It is apparent that gravitation canbe completely neglected in the microscopic domain, becauseits strength in relation to strong interactions is only 10 −40 .weakness of gravitationinteraction→ gravitation electroweak strongpropertyelectromagneticweak↓acts on mass–energy flavouraffectedparticlesexchangeparticlerelativestrengthallquarks,leptonselectricchargeallchargedparticlescolourchargequarks,gluonsgraviton G W + , W − , Z γ gluons g10 −40 10 −5 10 −2 1range ∞ ≈ 10 −3 fm ∞ ≈ 1fmexamplesystemEarth–Moonβ decayatomicbindingnuclearbindingTable 2.3Properties of interactionsIn the primitive quark model, all strongly interactingparticles (hadrons) are composed of valence quarks. Abaryon is a three-quark system, whereas a meson consistsof a quark and an antiquark. Examples of baryons includethe proton, which is a uud system, and the neutron is a uddcomposite. Correspondingly, an example of a meson is thepositively charged pion, which is a u ¯d system. The existenceof baryons consisting of three identical quarks with parallelspin (Ω − = (sss),spin 3 2 ¯h) indicates that quarks must havea hidden quantum number, otherwise the Pauli exclusionprinciple would be violated. This hidden quantum number iscalled colour. Electron–positron interactions show that thereare exactly three different colours. Each quark thereforecomes in three colours, however all observed hadrons haveneutral colour. If the three degrees of freedom in colourare denoted by red (r), green (g), and blue (b), the protonis a composite object made up from u red u green d blue . Inaddition to valence quarks, there is also a sea of virtualquark–antiquark pairs in hadrons.valence quarkshidden quantum numberscolour of quarkssea quarks