The interaction of the electron and neutrino is said to occur via the weak nuclear force. The Z boson travels from one particle to the other over a short period of time. The electron scatters to the left and the neutrino scatters to the right. The electron exchanges a Z boson (charge zero). The electron moves toward positive values of x (to the right) and collides with an electron neutrino moving to the left. As an example, consider the scattering of an electron and electron-neutrino ( Figure 11.12). Particle interactions in one dimension are shown as a time-position graph (not a position-time graph). Different symbols are used for different particles. A Feynman diagram, invented by American physicist Richard Feynman (1918–1988), is a space-time diagram that describes how particles move and interact. However, the essence of the Standard Model can be captured using Feynman diagram s. The equations are complex and are usually covered in a more advanced course in modern physics. The Standard Model can be expressed in terms of equations and diagrams. Table 11.6 Four Forces and the Standard Model The gravitational force, mediated by the exchange of massless gravitations, is added in this table for completeness but is not part of the Standard Model. A summary of forces as described by the Standard Model is given in Table 11.6. Leptons scatter off other leptons (or decay into lighter particles) through the exchange of massive W and Z bosons. Similarly, quarks bind together through the exchange of massless gluons. The result of these interactions is Coulomb repulsion (or attraction). This can occur at a theoretical infinite range. In the Standard Model, particle interactions occur through the exchange of bosons, the “force carriers.” For example, the electrostatic force is communicated between two positively charged particles by sending and receiving massless photons. The Standard Model combines the theory of relativity and quantum mechanics. Electroweak theory unifies the theory of quantum electrodynamics (QED), the modern equivalent of classical electromagnetism, and the theory of weak nuclear interactions. The Standard Model of particle interactions contains two ideas: electroweak theory and quantum chromodynamics (QCD) (the force acting between color charges). At the end of this section, we review unification theories in particle physics. We describe the Standard Model in detail in terms of electromagnetic, weak nuclear, and strong forces. In this section, we introduce the Standard Model, which is the best current model of particle interactions. The equation itself might be complex, but many scientists suspect the idea behind the equation will make us exclaim: “How could we have missed it? It was so obvious!” The ultimate goal of physics is a unified “theory of everything” that describes all particle interactions in terms of a single elegant equation and a picture. This work builds directly on work done on gravity and electromagnetism in the seventeenth, eighteenth, and nineteenth centuries. A particle physicist seeks to develop models of particle interactions. The chief intellectual activity of any scientist is the development and revision of scientific models. Explain the rationale behind grand unification theories.Use Heisenberg’s uncertainty principle to determine the range of forces described by the Standard Model.Draw a Feynman diagram for a simple particle interaction.Describe the Standard Model in terms of the four fundamental forces and exchange particles.By the end of this section, you will be able to:
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