How the proton gets its spin

Big Bang 8, textbook

74 RG 8.1 / G 8.1 Competence Area Particle Physics Particle Physics and Standard Model 47 What are the smallest, indivisible building blocks of the universe? Democrit's atoms have given way to an unmanageable number of well over 400 particles, which are jokingly referred to as the particle zoo (Fig. 47.1). Most of them, however, are not elementary, so they can be further subdivided. In the sciences, however, a principle of economy has proven its worth: If there are several models for one issue, the simplest is preferred. This is also called Occam's razor. The standard model of particle physics, which works with a handful of elementary particles, has existed since 1978. But you can use it to "assemble" all observed particles and also explain three of the four known forces. With the help of the model, a large number of predictions have been made over the past few decades, which have been brilliantly confirmed in experiments. Most particle physicists consider the Standard Model to be so convincing that they believe it could one day be supplemented or improved, but it is certainly never completely discarded. R ICHARD F EYNMAN (Fig. 47.11, p. 78) once said: “The standard model is working too well”. What he meant was that the standard model worked too well for it to be wrong. Fig. 47.1: The “particle zoo” comprises a few hundred particles. Some examples: Sigma (Σ), Delta (∆), Omega (Ω), Kaon (K), Phi (Φ), Neutron (n), Electron (e -), J / Psi (J / Ψ), Proton (p). Of the particles shown, only the electron is elementary. 47.1 Where our imagination fails Spin, fermions and bosons One of the most important characteristics of a particle is its spin. On the basis of the spin, all particles can generally be divided into two groups. The difference between these two groups determines the structure of the entire universe. What is the angular momentum and what is its unit? Read in chap. 17.4, "Big Bang 6"! What is Planck’s quantum of action, and what unit does it have? Read in chap. 30.1, "Big Bang 7". R ICHARD F EYNMAN, who, to put it casually, received his Nobel Prize for quantum mechanics, is said to have once said: “I assume that nobody understands quantum mechanics”. What did he mean by that? Read up in chap. 33.3, "Big Bang 7". What is the Pauli ban? How does it influence the shell structure of the atoms and the structure of the atomic nuclei? Why does the Pauli ban give the elements their chemical properties? Read in chap. 34.4, "Big Bang 7" and in Chap. 44.3, p. 58! Lightsabers would be a damn cool thing. However, there are major difficulties in implementation. For example, a ray of light doesn't just stop like that. But there is another very big problem. Which one could it be? F1 W1 F2 W1 F3 W1 F4 E1 The smallest objects like those in the particle zoo are called quanta. There is one property of all quanta that the Austrian Nobel Prize winner W OLFGANG P AULI discovered in electrons in 1925: the spin! In general and unfortunately quite unsatisfactory one can say: The spin is a fundamental property of every particle, similar to its mass or its charge. We can measure and prove all of these properties. But nobody can say what charge, mass or spin “really” is. The quantum mechanics unfortunately completely eludes our pictorial representation (F2). The spin of a quantum is given in so many “h across” (ħ), where h is Planck’s quantum of action and ħ has the value h / 2 π. For example, the spin of an electron is ½ ħ. Often one leaves out the unit and says briefly that the spin of an electron is ½. Usually it is explained by the fact that the quantum rotates around its own axis. But you have to be aware that this is just a desperate attempt to visualize something that cannot be visualized. The location of a particle is generally blurred. Quanta can therefore not be small spheres, even if they are shown in this chapter for better clarity. Info: Spin and quantum spin To put it bluntly, the spin tells us how a particle “looks” from different angles. Of course you can't see the spin, but you can describe it mathematically. A particle with spin 0 is similar to a point and looks the same from all directions (Fig. 47.2 a). A particle with spin 1 looks the same after a full turn (b), a particle with spin 2 looks the same after half a turn (c). A particle with spin ½ only looks the same again after two full rotations (d). Here our imagination fails completely! e9hr27 For testing purposes only - property of the publisher öbv

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