Quantum Physics

998 Chapter 30 Nuclear Energy and Elementary ParticlesFigure 30.13 The Standard Modelof particle physics.MATTER AND ENERGYFORCESCONSTITUENTSStrongElectromagneticWeakGravityGluonPhotonW and Z bosonsGravitonQuarksu c td s bLeptonse m t emtA view from inside the LargeElectron–Positron (LEP) collider tunnel,which is 27 km in circumference.Courtesy of CERNand are different manifestations of a single unifying electroweak interaction. Thephoton and the three massive bosons (W and Z 0 ) play a key role in the electroweaktheory. The theory makes many concrete predictions, but perhaps themost spectacular is the prediction of the masses of the W and Z particles at about82 GeV/c 2 and 93 GeV/c 2 , respectively. A 1984 Nobel Prize was awarded to CarloRubbia and Simon van der Meer for their work leading to the discovery of theseparticles at just those energies at the CERN Laboratory in Geneva, Switzerland.The combination of the electroweak theory and QCD for the strong interactionform what is referred to in high energy physics as the Standard Model. Although thedetails of the Standard Model are complex, its essential ingredients can be summarizedwith the help of Figure 30.13. The strong force, mediated by gluons, holdsquarks together to form composite particles such as protons, neutrons, and mesons.Leptons participate only in the electromagnetic and weak interactions. The electromagneticforce is mediated by photons, and the weak force is mediated by W and Zbosons. Note that all fundamental forces are mediated by bosons (particles with spin1) whose properties are given, to a large extent, by symmetries involved in the theories.However, the Standard Model does not answer all questions. A major questionis why the photon has no mass while the W and Z bosons do. Because of this massdifference, the electromagnetic and weak forces are quite distinct at low energies,but become similar in nature at very high energies, where the rest energies of theW and Z bosons are insignificant fractions of their total energies. This behaviorduring the transition from high to low energies, called symmetry breaking, doesn’tanswer the question of the origin of particle masses. To resolve that problem, ahypothetical particle called the Higgs boson has been proposed which provides amechanism for breaking the electroweak symmetry and bestowing different particlemasses on different particles. The Standard Model, including the Higgs mechanism,provides a logically consistent explanation of the massive nature of the Wand Z bosons. Unfortunately, the Higgs boson has not yet been found, but physicistsknow that its mass should be less than 1 TeV/c 2 (10 12 eV).In order to determine whether the Higgs boson exists, two quarks of at least 1 TeVof energy must collide, but calculations show that this requires injecting 40 TeV ofenergy within the volume of a proton. Scientists are convinced that because of thelimited energy available in conventional accelerators using fixed targets, it is necessaryto build colliding-beam accelerators called colliders. The concept of a collideris straightforward. In such a device, particles with equal masses and kinetic energies,traveling in opposite directions in an accelerator ring, collide head-on to producethe required reaction and the formation of new particles. Because the totalmomentum of the interacting particles is zero, all of their kinetic energy is availablefor the reaction. The Large Electron–Positron (LEP) collider at CERN, nearGeneva, Switzerland, and the Stanford Linear Collider in California collide bothelectrons and positrons. The Super Proton Synchrotron at CERN accelerates