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Spontaneous Symmetry Breaking Bachelor Thesis Spontaneous Symmetry Breaking Johannes Zierenberg Faculty of Physics and Earth Sciences University of Leipzig July 2008 Supervisor: Roland Kirschner Second Referee: Klaus Sibold Contents 1 Introduction 3 2 Preliminaries 5 2.1 Notations....................................5 2.2 Euler-Lagrange Equation and Noether Theorem for Classical Fields.....6 3 Global Symmetry Breaking 10 3.1 Global Symmetries............................... 10 3.2 The Meaning of Vacuum............................ 12 3.3 Goldstone Theorem............................... 13 3.4 Physical Consequences - The Goldstone Bosons................ 18 3.5 Examples.................................... 20 3.5.1 Breaking of a U(1) Symmetry..................... 20 3.5.2 Non-Abelian Example......................... 22 4 Local Gauge Symmetry Breaking 26 4.1 Local Gauge Symmetry............................. 26 4.1.1 General Considerations......................... 26 4.1.2 U(1) Example............................. 30 4.2 The Higgs Mechanism............................. 31 4.3 Non-Abelian Example............................. 34 5 Summary and Physical Importance 41 Bibliography 43 2 1 Introduction In the last century much of physics has been built on the properties of symmetry. For example, Special Relativity, which was built on space-time symmetries, or internal conserved charges discovered due to the existence of internal symmetries. Symmetries are always related to the conservation of generalized charges, which makes them attractive. In Nature however, they are not always observed. Many symmetries prefer massless particles, but we only observe a few, one of them the photon. At the same time, the massive elementary particles with different properties have different masses. The idea that the underlying symmetry, which is important for interactions, must be somehow hidden, gave rise to different possible explanations. One explanation is that the symmetry is simply not observed on the scale of the experiment. An example is the hydrogen atom which appears in low energy physics as an entity. Today, we know that it consists of a proton and an electron. These two are only observed as particles, when we apply enough energy to separate them. Otherwise the symmetry related to charge is not observed at all. The important difference to the following considerations is that we do not experience an asymmetry. However, in this thesis the concept of symmetry breaking is of more importance. When symmetry is broken, it undergoes a transition from a symmetric system to an asymmetric sys- tem. This can occur in two ways, through the explicit or the spontaneous symmetry breaking. Explicit symmetry breaking happens if a symmetry is broken by additional objects. Just as a scar in a man’s face breaks the face’ reflection symmetry explicitly, the small difference in mass between a neutron and a proton breaks their symmetry of exchange explicitly as well. In the case of a symmetric Lagrangian we can add terms that themselves do not obey the symmetry of the Lagrangian. This leaves the new Lagrangian asymmetric. In the case of Spontaneous symmetry breaking the underlying system or Lagrangian remains symmetric but the symmetry is hidden when observing the vacuum state, as will be shown later. An example from biology would be the houses of snails [4]. In nature we find that all houses have a spiral structure which is in general right turning. This observation is called spiral tendency of veg- etation (Goethe). There are few exceptions, snails with left turning houses as a consequence of mutation. However, they are not capable of reproduction with other right turning snails due to important DNA differences. At first sight, it seems to be an asymmetric law of nature that 3 1 Introduction snails always have right turning houses. It is an example of spontaneous symmetry breaking because nature can produce the other turning snails. In principle both house types are possible (symmetric system). We could say that at the beginning there were two small populations of snails with different house types. But in evolution the snails with the one type of houses dom- inated and the other ones did not find appropriate reproduction partners and died out. Which type of house survived was of random choice, it could have been exactly the other way around. This example shows that the underlying system can be symmetric even though reality first ap- pears asymmetric and is described by asymmetric laws. Physical examples of spontaneous symmetry breaking are the ferromagnet and the electroweak interaction and will be explained later on. This bachelor thesis will give an introduction into the field of spontaneous symmetry break- ing. The 2 main chapters each start with a brief introduction to the global or local symmetries, which is based on the books "Fields, Symmetries, and Quarks" [8] and "Quantenmechanik, Symmetrien" [11]. After the introduction of some important terminology, we come to the case of global continuous symmetries leading to the Goldstone [2] theorem. Goldstone and Nambu [9] were the first to find massless particles in specific models. These particles are called the Goldstone- or sometimes Nambu-Goldstone bosons and the proof of their existance was then given by Goldstone, Salam and Weinberg [7]. This proof and the derivation of the number of appearing Goldstone bosons can be found in the books "Quantum Field Theory" [10] and "The Quantum Theory of Fields" [12] and are performed in detail in this thesis. Afterwards the theorem is demonstrated on the U(1) and U(1) ⊗ SU(2) example. Chapter 4 will discuss the spontaneous breakdown of local symmetries, since Goldstone bosons are absent whenever the broken symmetry is local [6],[1],[3]. The phenomenon called Higgs mechanism, will be introduced with the help of a simple U(1) example followed by an application to a U(1) ⊗ SU(2) invariant system. The strong dependence of the Higgs mech- anism on the Goldstone theorem will be shown by argumentation and by comparison of the global and local results. At the end, following a summary of my discussion, I will give some insight into the physical importance of these concepts. 4 2 Preliminaries 2.1 Notations We will shortly introduce the notion used in this thesis. In general the unit convention is used, where we set the speed of light and Planck’s number equal to one, meaning that we choose them as units for velocity and action c = 1 = h¯: (2.1) It follows that length and time have the same dimensions and the relativistic energy - momen- tum relations becomes E2 = p2 + m2: (2.2) This leads immediately to the use of four vectors and four operators since time and space are connected. The four vectors are denoted by a greek index, whereas three-component vectors are denoted by an arrow Am = (A0;−~A): (2.3) Four vectors with subscripts are called covariant vectors, contravariant vectors are four vec- tors with superscripts, defined by m mn A = g An (2.4) mn where g = gmn is the metric tensor, that can be given in the Minkowski space (space and ct-time axes) in Cartesian coordinates 0 1 1 0 0 0 B C B0 −1 0 0 C gmn = B C: (2.5) B0 0 −1 0 C @ A 0 0 0 −1 5 2 Preliminaries The space-time four vector, the four-momentum operator and the four-current are then given by xˆm = (t;~x) = (x0;~x); (2.6) ¶ ¶ pˆ = i¶ m = i = i( ;∇) (2.7) m ¶xm ¶t jm = (r;~j): (2.8) The scalar product of four-vectors is defined by the the scalar product of a covariant and a con- travariant four-vector. It is very important that under exchange of covariant and contravariant vectors this scalar product remains invariant m m n m AB = A Bm = Am B = gmn A B : (2.9) Then the square of four-vectors is invariant under Lorentz transformations 2 m 2 ~ 2 A = Am A = A0 − A : (2.10) 2.2 Euler-Lagrange Equation and Noether Theorem for Classical Fields All of our further considerations are going to be connected to fields, which will be described by m fa (x) with x = x = (t;~x); (2.11) where a labels the different fields appearing in the theory. In classical mechanics we obtained our equation of motion from the Lagrangian via the Ansatz of Hamilton’s principle of least ac- tion. This can be used in full analogy for fields. In this case the field amplitude at a coordinate xm can be considered as the coordinate of the theory. For an adequate discussion of classical field theory see [8] or [5] We will see that for fields fa we obtain an analogous outcome as for generalized variables qi. For the further short derivation of the Euler-Lagrange equation and the Noether Theorem we will drop the index in the field variables. m 0 0 0 0m Let us consider a variation of the Lagrangian L (f;¶m f;x ) ! L (f ;¶m f ;x ) under the following condition that simplify calculations but maintain applicability of the results: 0 0 0 0m 0 0 0m L (f ;¶m f ;x ) = L (f ;¶m f ;x ): 6 2 Preliminaries We consider the following transformations coordinate transformation: xm ! x0m = xm + dxm (2.12) field variable transformation: f(x) ! f 0(x) = f(x) + df(x) (2.13) where the d indicate infinitesimal variations in the coordinates or fields. This restricts the ap- plication of the Noether theorem, we want to derive at this point, to continuous transformations only. Asking for a vanishing variation of the action we obtain Z Z Z 4 0 0 0 0m 4 0 m 4 dS = dL d x = L (f ;¶m f ;x )d x − L (f;¶m f;x )d x = 0: The transformed space volume element d4x0 can be transformed, recalling the transformation of the coordinates, by the corresponding Jacobian 0m 0 ¶x m m m J(x ;x) = det = det(d + ¶ (dx )) = 1 + ¶m (dx ): ¶xl l l Inserting this in the previous expression for the variation of the action we get Z Z 0 0 m 0 m 4 m 4 dS = L (f ;¶f ;x )J(x ;x) − L (f;¶f;x ) d x = dL + L ¶m (dx ) d x with dL = L (f 0;¶f 0;xm ) − L (f;¶f;xm ), L (f 0;¶f 0;xm ) = L and changing the notation f 0 ! f.
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