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Non-Abelian Gauge Invariance Notes Physics 523, Quantum Field Theory II Presented Monday, 5Th April 2004 Jacob Bourjaily, James Degenhardt, & Matthew Ong

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Non-Abelian Gauge Invariance Notes Physics 523, Quantum Field Theory II Presented Monday, 5Th April 2004 Jacob Bourjaily, James Degenhardt, & Matthew Ong Non-Abelian Gauge Invariance Notes Physics 523, Quantum Field Theory II Presented Monday, 5th April 2004 Jacob Bourjaily, James Degenhardt, & Matthew Ong Introduction and Motivation Until now, all the theories that we have studied have been somewhat restricted to the `canonical.' The Lagrangians of quantum electrodynamics, Á4-theory, Yukawa theory, etc. have been the starting points for our development{while we have learned to interpret them, we have not yet developed adequate methods to build Lagrangians for new theories. On of the most important lessons of modern physics is that microscopic symmetries can be used to determine the form of physical interactions. In particular, the Standard Model of particle physics, SU(3)£SU(2)L £U(1)Y , has a form completely speci¯ed by its symmetries alone. Today, we will discuss some of the foundational principles from which this type of theory can be built. We have seen already that the QED Lagrangian is invariant to a wide class of transformations. Per- haps the most striking of all is its invariance under arbitrary, position dependent phase transformations of the ¯eld Ã. But this should have been obvious. It is not surprising that phase di®erences between space-like separated points should be uncorrelated in the theory. However, a-causal regions become causal in time; therefore, there must be some way to re-correlate the arbitrary phases between points at future times. As you know, it is this argument of local gauge invariance in quantum electrodynamics that requires the existence of the vector ¯eld, A¹, the photon. Clearly, the local U(1) symmetry of the QED Lagrangian proves to be a powerful source of insight. Let us therefore consider promoting the concept of local gauge invariance to the highest level of physical principle. We may ask, for example, whether or not other local symmetries are observed in nature? What form must our Lagrangians take if they are to be manifestly gauge covariant? In our presentation this afternoon, we will attempt to introduce the tools that must be used to deal with gauge symmetries. We will ¯rst review the construction of quantum electrodynamics from a simply U(1) gauge principle and then extend these arguments to arbitrary SU(n) symmetries. Because higher gauge groups are non-abelian, some care must be observed. We will ¯nish our presentation with an in- troduction to the algebraic tools necessary to investigate gauge theories. The material of this discussion can be found in chapter 15 of Peskin and Schroeder. Geometry of Gauge Invariance One of the most familiar Lagrangians should be that of quantum electrodynamics, 1 L = ¡ (F )2 + Ã(i6@ ¡ m)à ¡ eÃγ¹ÃA : QED 4 ¹º ¹ In chapter 4, we found that this Lagrangian is not only invariant under global gauge variations i® i®(x) à ! e Ã, but to the very general, position-dependent transformation à ! e à if the vector ¯eld A¹ 1 transforms simultaneously by A¹ ! A¹ + e @¹®(x). Transformations under which the phase of quantum ¯elds are changed as arbitrary functions of position are called local gauge transformations. When a Lagrangian is invariant under a local gauge transformation, we say that it is gauge covariant. Local gauge invariance turns out to be so powerful that the entire form of the QED Lagrangian is determined by it alone. Because quantum electrodynamics is very familiar to us, it will serve as an ideal place to begin our investigation of the consequences local gauge invariance. Because the local gauge transformation in QED is a scalar function ei®(x), it commutes with everything. By starting with QED, we can gain a good deal of insight about the physical principles at work before rigorously exploring the symmetries of non-commutative gauge transformations. Derivation of LQED from Local U(1) Gauge Invariance Let us consider the consequences of demanding that LQED be invariant under the local gauge trans- formation Ã(x) ! ei®(x)Ã(x) where à is the standard fermion ¯eld. We see that we may already include terms like mÃà because these will be manifestly covariant under any phase transformation. What are we to make of derivative terms? Recall that, in general, the partial derivative of any ¯eld Ã(x) taken in the direction of the vector n¹ can be de¯ned by ¹ 1 n @¹Ã = lim [Ã(x + ²n) ¡ Ã(x)]: ²!0 ² 1 2 JACOB BOURJAILY, JAMES DEGENHARDT, & MATTHEW ONG Note, however, that this derivative cannot be used for the transformed ¯eld. Because Ã(x + ²n) ! ei®(x+²n)Ã(x + ²n) while Ã(x) ! ei®(x), the subtraction{even in the limit of ² ! 0{cannot be de¯ned. This is because we require that our theory accommodate an arbitrary function ®(x) and any subtraction technique will necessarily lose enormous generality. We must therefore develop a consistent way to de¯ne what is meant to take the derivative of a ¯eld. We can do this by introducing a scalar function, U(y; x), that compares two points in space. The scalar function U(y; x) can be used to `make up' for the ambiguity of ®(x) by requiring that it transform according to U(y; x) ! ei®(y)U(y; x)e¡i®(x): Now the subtraction between two di®erent points can be made consistently. This is because the combined entity U(y; x)Ã(x) transforms identically to Ã(y): Ã(y) ¡ Ã(x) ! ei®(y)Ã(y) ¡ ei®(y)U(y; x)Ã(x): Therefore, we may de¯ne the covariant derivative in the limiting procedure: ¹ 1 n D¹Ã = lim [Ã(x + ²n) ¡ U(x + ²n; x)Ã(x)]: ²!0 ² To get an explicit de¯nition of D¹ we expand U(y; x) at in¯nitesimally separated points. ¹ 2 U(x + ²n; x) = 1 ¡ ie²n A¹(x) + O(² ): Here we have extracted A¹ from the Taylor expansion of the comparator function. This new vector ¯eld is called the connection. Putting this expansion into the rule for the transformation of the comparator, ¹ we ¯nd the transformation of A¹. Note that we have used the fact that ²n @¹®(x) » ®(x + ²n) ¡ ®(x). ¹ i®(x+²n) ¹ ¡i®(x) (1 ¡ ie²n A¹(x)) ! e (1 ¡ ie²n A¹(x))e ; ¹ i²n @¹®(x)+i®(x) ¹ ¡i®(x) = e (1 ¡ ie²n A¹)e ; ¹ ¹ = 1 + i²n @¹®(x) ¡ ie²n A¹; 1 ) A (x) ! A (x) ¡ @ ®(x): ¹ ¹ e ¹ Note that the transformation properties we demanded of our Lagrangian determined the form of the interaction ¯eld and the covariant derivative and required the existence of a non-trivial vector ¯eld A¹ . Gauge Invariant Terms in the Lagrangian We now have some of the basic building blocks of our Lagrangian. We may use any combination of à and its covariant derivative to get locally invariant terms. Now we wish to ¯nd the kinetic term of the interaction ¯eld. To construct this term we ¯rst derive its invariance from U(y; x). We expand U(y; x) to O(²3) and assume that it is a pure phase with the restriction (U(x; y))y = U(y; x), thus ² U(x + ²n; x) = exp[¡ie²n¹A (x + n) + O(²3)]: ¹ 2 Now we link together comparisons f the phase direction around a small square in spacetime. This square lies in the (1,2)-plane and is de¯ned by the vectors 1,^ 2^ (see Fig. 15.1 in PS ) . We de¯ne U(x) to be this quantity. U(x) ´ U(x; x + ²2)^ U(x + ²2^; x + ²1^ + ²2)^ £ U(x + ²1^ + ²2^; x + ²1)^ U(x + ²1^; x): U(x) is locally invariant and by substituting in the expansion of U(x + ²n; x) into U(x) we get a locally invariant function of the interaction ¯eld. ² ² ² ² U(x) = expf¡i²e[¡A (x + 2)^ ¡ A (x + 1^ + ²2)^ + A (x + ²1^ + 2)^ + A (x + 1)]^ + O(²3)g 2 2 1 2 2 2 1 2 This reduces when it is expanded in powers of ². 2 3 U(x) = 1 ¡ i² e[@1A2(x) ¡ @2A1(x)] + O(² ): ¹ We can now see a hint of why F¹º is gauge covariant. It is also very important to note that A¹A is not locally invariant. Luckily however, A¹ commutes with A¹ with each other and so we do not have to worry much about this. In the next section though we won't assume that these vector ¯elds commute. This also shows that there is no mass term for the photon. QUANTUM FIELD THEORY II: NON-ABELIAN GAUGE INVARIANCE NOTES 3 Another way to de¯ne the ¯eld strength tensor F¹º and to show its covariance in terms of the commutator of the covariant derivative. The commutator of the covariant derivative will be invariant in QED. 2 [D¹;Dº ]Ã(x) = [@¹;@º ]à + ie([@¹;Aº ] ¡ [@º ;A¹]Ã) ¡ e [A¹;Aº ]à [D¹;Dº ]Ã(x) = ie(@¹Aº ¡ @º A¹)à [D¹;Dº ] = ieF¹º One can visualize the commutator of covariant derivatives as the comparison of comparisons across a small square like we computed above. In reality, the arguments are identical. Building a Lagrangian We now have all of the ingredients that we can use to construct a general local gauge invariant Lagrangian. The most general Lagrangian with operators of dimension 4 is: 1 L = Ã(i@ γ¹ + ieA )à ¡ (F )2 ¡ c²®¯¹º F F ¡ mÃÃ: 4 ¹ ¹ 4 ¹º ®¯ ¹º If we want a theory that is P , T invariant then we must set c to zero since it would violate these discrete symmetries. This Lagrangian contains only two free parameters, e, and m. Higher dimensional Lagrangians are also generally allowed.
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