WJEC Physics for AS: Student Bk

83 Terms & definitions Notice that the charge on the two sides of the reaction is the same, as is the number of u-quarks (4); there are 2 d-quarks on the left and 3 d + 1 anti-d on the right. This is explored further in Section 1.7.6. The quark structure of protons and neutrons is shown schematically in Fig. 1.7.5 and can be summarised by p = uud n = udd Note that, in particle physics, the proton has the symbol p rather than 1 1 H , which is usual in nuclear physics; the neutron is n rather than 1 0 n . Also the order of writing the quarks is arbitrary: p = udu and n = ddu , etc., are perfectly good ways of writing the structure. Protons are the only stable baryons: there are theories which suggest they could be unstable with a half life of around 10 32 years! Mesons are created in copious numbers when baryons are collided at moderate to high energies (more than a few hundred MeV). The first generation mesons are called pions (or pi mesons). Their names and quark structure are given in the definition: A typical meson-generating reaction is p + p  p + n + π + which, at the quark level, can be written: uud + uud  uud + uud + u  d There are only six first-generation baryons. They are summarised in Table 1.7.2: Tab. 1.7.2 1st generation baryons Family baryons Nucleons p ( uud ); n ( udd ) Δ particles  ++ ( uuu );  + ( uud );  0 ( udd );  – ( ddd ) The symbol,  , is the Greek letter capital delta, so the  family is called delta double plus, delta plus, etc. Note that the quark structures of  + and  0 are the same as those of p and n respectively but the mass of the  + is 1232 MeV/ c 2 against 938 MeV/ c 2 for the proton. The  + can be regarded as an excited state of the proton: similarly for  0 and the neutron. This is discussed in the Stretch & challenge. 1.7.6 Interactions (forces) between particles Macroscopic objects are subject to two types of force: gravitational and electromagnetic. Subatomic particles are also affected by two other forces: the strong and the weak interactions. These are not experienced at all on the everyday scale because their range is so small. The four forces are summarised in Table 1.7.3 in order of increasing strength. Tab. 1.7.3 Interactions summarised Interaction Affects Range Comments gravitational all matter infinite negligible for subatomic particles weak all particles ~10 –18 m only significant when e-m and strong interactions not involved electromagnetic (e-m) all charged particles infinite also affects neutral hadrons because quarks have charges strong all quarks ~10 –15 m also affects interactions between hadrons (e.g. nuclear binding) Study point Because mesons are composed of a quark and an antiquark, there is no need to define a separate category of ‘antimeson’. Study point The antiproton and antineutron have the following structures:  p =  u  u  d ;  n =  u  d  d . Pions have the following names and structure: π + (pi plus) = u  d π – (pi minus) = d  u (or u  d !) π 0 (pi zero) = u  u or d  d . A beam of π 0 mesons is composed of a mixture of the two! Study point & S C Stretch & Challenge The additional mass is related to the excitation energy. This effect also occurs in atomic energy levels. The first excitation energy of atomic hydrogen is 10.2 eV . By what fraction does the mass of a hydrogen atom increase if it is put into its first excited state? 1.7.3 Self-test Heavier nuclei need a greater fraction of neutrons to overcome the increased e-m repulsion of the protons. Illustrate this from the fraction of neutrons in the stable nuclei, 12 6 C, 56 26 Fe and, 197 79 Au. Study point The word interaction is often preferred to force because it has a wider implication than just attraction or repulsion. It includes the control of the creation of particles or their decay. Particles and nuclear structure