## Coulomb Scattering

As an illustration of the usefulness of the two-component theory, we calculate the scattering of a charged spin zero particle from a fixed Coulomb potential which comes from a point charge Ze fixed at the origin. Because of the fact that the two-component KG theory satisfies a first order differential equation, we may use the formalism for time-dependent perturbation theory developed in Sec. 3.1. The first order S-matrix element is compare with Eq. (3.53) Sfi -iJdt f Hi(t) i) , (4.62) where i)...

## Lut pup

The 4E2 factor in front is canceled by the (1 2E)2 factor in ffi 2. The final steps are the same as for the KG case, Sec. 4.7, giving .vhere v2 p2 E2. This famous result is the Mott cross section for the scattering of a spin particle. Comparing it with the KG result, Eq. (4.68), we see that it differs by the factor For large energy, v 1, and the cross section goes to zero in the backward direction. (See Fig. 5.2.) This difference in the backward direction is due to magnetic scattering the...

## Bound States And Unitarity

In the previous chapters it has been implicitly assumed that perturbation theory is adequate and that we can obtain a reasonable estimate of the scattering amplitude by calculating a few Feynman diagrams of lowest order. However, there are many problems for which the calculation of a few Feynman diagrams is inadequate. The study of bound states is one of these problems. A bound state produces a pole in the scattering matrix in the channel in which it appears. If the bound state is truly...

## [rihVri

These give Hamiltonian densities differing from each other only by a constant, so the physics is the same, but the first choice is more convenient, as we will see below. Using this Hamiltonian, the time translation operator to second order was given in Eq. (9.7), We now reduce this expression for the time translation operator using Wick's theorem. First, note that the instantaneous term can be written ij dt J inst( ) -i y J n,t)J r2,t) where the insertion of the T product in the second line has...

## M V 1STd gx

ig 4> 75 Ti( ) -ip -I- ig 4> l > 75 + gn mtfati - W'Mi + g*4npK - g* - 2V'facfia + ig r tpa iglH -y6n4> a + ifa 1> + - ig r 0. (13.124) Now consider the effect of adding an explicit chiral symmetry breaking term of the form to the linear Lagrangian. Then the axial vector current will no longer be conserved. The new term only affects the equation for 4> ay which becomes U< t> + m2(j)a - 2V'< j> a + gnij> ip -c , and this new equation, when used in the calculation of Eq....

## Ba0 f dxxal

To complete the reduction to standard form (the first step), note that the combined denominator D always has the form where k is the internal loop momentum and Q is a vector function of the external r omenta and the Feynman parameters. Thus the square of the denominator can always be completed by shifting k k' Q, which gives This shift must also be carried out in the numerator N, which assumes the general form N No + k' N + k' r + MXW + + - , (C.8) where the Ni are tensors which do not depend...

## Ps

The derivation of these results for the Dirac theory is identical to that for the KG theory review the arguments which led from Eq. (4.73) to Eq. (4.78) . Each of these two expressions for differs from its KG counterpart which is (4.76b) for the first and (4.78) for the second only in the sign of the negative energy term. And here, as in the KG theory, the propagation of the negative energy states backward in time is interpreted as the propagation of the corresponding antiparticle states...

## [7Bf 7 [VBi 6j47

*For a discussion of the d functions, see Rose (1957). *For a discussion of the d functions, see Rose (1957). will use this result in the next chapter when we construct the most general solution to the Dirac wave equation. To complete the discussion of the homogeneous Lorentz group, we need only to find the representation of the time reversal transformation S T). This will be postponed until Chapter 8, where time reversal will be discussed in some detail. In this section we discuss the...

## Jo J 27t

B (2 1 )5(2- f,2 e)r(e 1) Fig. 16.8 The three diagrams which contribute to the fourth order self-energy, with their diverging subdiagrams enclosed in a shaded box. Diagrams (A) and (B) have nested divergences, while (C) has overlapping divergences. the subtraction by a factor of two as needed, giving The troublesome e 1 log(-q2 j,2) term has been canceled, the infinite part is constant, and the renormalization program can be carried through. Review of the Fourth Order Calculation We conclude...

## Info

Which, in the limit e 0, gives k 30, where (30, defined in Eq. (17.25), determines how the coupling constant runs. If k is positive, the theory is asymptotically free. The key to the demonstration is therefore the calculation of Zg. From Eq. (16.121), there are three equivalent forms for Zg where we ignore the possibility of using Z4 (why ). The diagrams which contribute to the six renormalization constants which enter (17.32) are shown in Fig. 17.5. We will choose the last combination in...

## SwJdvf0dx

Note that the elog (Y fi2) term cancels, insuring that the singular term, while of order 1 e2, is nevertheless a constant, and hence a legitimate counterterm. To complete the proof (which will not be done here) the argument must be extended to graphs with more than one loop. Finally, we consider the self-energy graphs and the problem of overlapping divergences. A general method for subtracting the divergences from any Feynman graph, including those with overlapping divergences, was developed by...

## F d4k J 2TT4

The electron self-energy, denoted by (p), is related to My. by Fig. 11.4 Feynman diagram for the electron self-energy. Note that the loop momentum k cannot be constrained by momentum conservation. Fig. 11.4 Feynman diagram for the electron self-energy. Note that the loop momentum k cannot be constrained by momentum conservation. Fig. 11.5 First three of an infinite class of Feynman diagrams which defines the dressed electron propagator. Fig. 11.5 First three of an infinite class of Feynman...

## Pzy nTkmrL [Ldz

X + a* a* e_ fc+fcm z+i u'n+< *,m't a* am a an e*(fc-fc )z_*( n_ 'm)t L-fl a- e_2iWnt + fc- anflU e2 -4 - 2fcnat a . However, the first two terms sum to zero, because they are odd when n is changed to -n (recall kn k-n but w_n). Hence which expresses the total momentum as a vector sum of the momentum of each phonon (kn) times the number of phonons with that momentum (a a,,). The full vector character of the momentum operator is only partially illustrated by this one-dimensional example, where...

## J 777711771 772 J dfxdf2 dridr2 1 771771 f2r2 jiiir2i2

- J < 771*71 J drji 771 j 772772 j < 772772 -1. (14.143) Now, suppose we generalize the quadratic form tjt to fjAr , which can be written Then, the integral (14.143) generalizes to J dfjidfj2dT idri2 e J df idfj2drndr 2 fjxAl3 rj-j) fjkAki ru) J df idfj2dr idT 2 m tjj fjk r)t Ai j Ake - det A , or, if an i is inserted in the exponent, where n is the dimension of the matrix A. The result, which can be generalized to arbitrarily large matrices, will be very useful in the following sections....

## Decay Rates

Fhe differential decay rate AW a(T) will be defined to be the probability that a state a will decay into state 3 in the time interval T 2, -T 2 divided by the total time T (hence a rate). Formally Under most experimental circumstances, when a decaying system is isolated from the apparatus, the measurement is made over a time interval long compared to the internal time scale of the system, and we may therefore take the limit as T oo. This limiting rate is denoted AWpa, Later, we will see that...

## Meuep Ael K

'For a clear discussion of the definition and properties of helicity states see the classic paper by Jacob and Wick JW 59 , Note that the ratio of this decay channel to the muon decay channel is Hence the decay of the n into p + i> M is 10,000 times more probable, even though the phase space is much smaller. For a discussion of phase space, refer back to Eq. (9.21). This surprising result is an example of a general feature of vector (and axial vector) interactions. Generalizing the discussion...

## V0ksvykselk et2 h

Recall the matrix form of the Dirac spin sums, Fig. 10.10 Feynman diagrams for Compton scattering. With these new additions to Rules 2-4, we have most of the basic rules needed for elementary calculations. We are still missing the rules associated with renor-malization (so far, all renormalization constants have been set to unity), closed loops, and some of the rules associated with isospin. We have also not discussed the electrodynamics of spin 0 and 1 bosons. We will cover...

## Introduction To Renormalization

We now turn to the general question of how to calculate the higher order terms which arise in the perturbation expansion for the S-matrix. We have chosen to introduce this study by examining all the terms which arise in second order QED. The discussion therefore serves two purposes the specific examples we study are of practical importance, and they also are rich enough to illustrate most of the issues which will arise in a general study. A survey of all Feynman diagrams generated by second...

## Interaction Of Radiation With Matter

In this chapter, the Lagrangian obtained in the last chapter is used to show how atomic decay is explained by field theory. Then the famous Lamb shift is calculated and discussed. The Lamb shift is the splitting between atomic levels with the same total angular momentum but different orbital angular momentum and cannot be explained without the use of field theory. The largest such splitting is between the 2Sx 2 and 2Pi 2 levels and is a noticeable feature of the hydrogen atom spectrum. Finally,...

## Fc2

Is the state of mjt2 photons with momentum k2 and polarization q2. (b) Discuss the physical significance of your result. 2.4 The orbital part of the angular momentum operator fllL was defined in Eq. (2.60). Prove that it contains no terms of the form ah a , , or 0-n,a0-n',a'< and also show that where ljt) is the state of one photon with momentum k and polarization a. What is the significance of this result Relativistic Quantum Mechanics and Field Theory FRANZ GROSS Copyright 2004 WILEY-VCH...

## Interacting Field Theories

With this chapter we begin a systematic study of interacting field theories in which all particles are described by relativistic quantum fields. This means that all particles are handled in a similar way and that the annihilation or creation of particles or particle pairs can be treated in a way consistent with the description of their scattering. The particles are isolated from their surroundings and interact only with each other, so that momentum (as well as energy) is conserved, and the...

## Problems

4.1 Solve the manifestly covariant form of the Klein-Gordon equation for the ground state of the hydrogen atom. Specifically, assume and show that the ground state wave function can be written Find e, 3, and E. Then examine the nonrelativistic limit by projecting out the 4> + and 4> - components defined in Eq. (4.36). Interpret your results and compare with the Schrodinger theory. 4.2 Calculate the fine structure splitting of the energy levels for a pion bound in an atom with charge Ze....

## [efeixeifMx

Where ' refers to the derivative of with respect to its argument x. Finally, for future reference note that For the study of the structure of hadrons, we are interested in positive energy solutions (antiquarks will be described by their positive energy charge conjugate states) for which m is both less than and greater than the bound state energy E. There will be two kinds of solutions, depending on the parity (or the sign of k) of the state. If A > 0, i k, and for solutions in the vicinity of...

## J xj Ac J d4x 0 Ac

In order to obtain a definite solution for the EM fields, the arbitrariness associated with the gauge freedom (2.15) must be removed so that the fields can be uniquely specified everywhere. This process is referred to as gauge fixing and involves imposing some constraints on the fields which will fix the gauge function Ac and remove the gauge freedom. Two popular choices for the constraint, or choice of gauge, are the Lorentz and Coulomb gauges, defined by the constraints There are advantages...

## Of The Electromagnetic Field

We now use the techniques developed in Chapter 1 to quantize the electromagnetic (EM) field. This system is one of the most important in physics but is also one of the most complicated. The EM field appears to be two coupled three-vector fields, but through Maxwell's equations and gauge invariance, it can be reduced to a single four-vector field with only two independent components. The elimination of these redundant components, which are connected with the gauge invariance of the system, poses...

## R1 r5a rla

D (1-0 - Jo de(l-0 (11-126) This value was first calculated by Schwinger in 1948 Sc 48 . The current agreement between theory and experiment represents an impressive confirmation of the correctness of QED.* The magnetic moment is often expressed in terms of the gyromagnetic ratio g related to the magnetic moment by where, as we saw in Chapter 5, the value predicted by the Dirac equation is g 2. It turns out that the departure from this value, usually expressed in terms of the For a recent...

## Feynman Rules

In this Appendix we collect together all of the rules for the calculation of relativistic cross sections and decay rates. The rules fall into two parts. There are rules for the calculation of the cross section and decay rates from the relativistic scattering matrix, M, and then there are rules for calculating M in a given theory. The former are quite general, but the latter, referred to as the Feynman rules, depend on the specific theory. The rules for calculation of relativistic decay rates...

## C

Fig. 12.27 The complex u plane showing motion of uq, the end point of the contour C'2, as Mg increases. The contour Cj begins at the fixed point (mi + i)2 and m be deformed in order to avoid the moving singularity. This displays the imaginary part as a dispersion integral with singularities along the real axis from a to uq. The upper limit, uq, will turn out to be the same 0 which appears in (12.105), but the lower limit (which, in this application, is actually three numbers describing two...

## A755A detA755132

tp x S- A t5S A ip x detA p5 i , 5.133 which is the correct transformation law for a pseudoscalar if A e 1 Normalization of Dirac wave functions. Note that the normalization integral, which involves ip rp, can be expressed in terms of the following density Iiis makes it clear that it is the fourth component of a four-current, which is conserved. We already wrote this conservation law in covariant form in Eq. 5.14 in terms of the Dirac adjoint it is The appearance of the factor y E m in the...