Quantum Field Theory I - ETH Z
Quantum Field Theory ILecture NotesETH Zurich, HS14Prof. N. Beisert
c 2014 Niklas Beisert, ETH ZurichThis document as well as its parts is protected by copyright.Reproduction of any part in any form without prior writtenconsent of the author is permissible only for private,scientific and non-commercial use.
Contents0 Overview0.1 Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.2 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Classical and Quantum Mechanics1.1 Classical Mechanics . . . . . . . . .1.2 Hamiltonian Formulation . . . . . .1.3 Quantum Mechanics . . . . . . . .1.4 Quantum Mechanics and Relativity1.5 Conventions . . . . . . . . . . . . .2 Classical Free Scalar Field2.1 Spring Lattice . . . . . . .2.2 Continuum Limit . . . . .2.3 Relativistic Covariance . .2.4 Hamiltonian Field .13.43.73.93.133 Scalar Field Quantisation3.1 Quantisation . . . . . . .3.2 Fock Space . . . . . . .3.3 Complex Scalar Field . .3.4 Correlators . . . . . . .3.5 Sources . . . . . . . . . .4 Symmetries4.1 Internal Symmetries . .4.2 Spacetime Symmetries .4.3 Poincaré Symmetry . . .4.4 Poincaré Representations4.5 Discrete Symmetries . .4.1. 4.1. 4.5. 4.9. 4.11. 4.15.5.15.15.45.75.105.125.155.185.206 Free Vector Field6.1 Classical Electrodynamics . . . . . . . . . . . . . . . . . . . . . . .6.2 Gauge Fixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.16.16.35 Free Spinor Field5.1 Dirac Equation and Clifford Algebra5.2 Poincaré Symmetry . . . . . . . . . .5.3 Discrete Symmetries . . . . . . . . .5.4 Spin Statistics . . . . . . . . . . . . .5.5 Grassmann Numbers . . . . . . . . .5.6 Quantisation . . . . . . . . . . . . . .5.7 Complex and Real Fields . . . . . . .5.8 Massless Field and Chiral Symmetry3.
6.36.46.5Particle States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6Casimir Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11Massive Vector Field . . . . . . . . . . . . . . . . . . . . . . . . . . 6.157 Interactions7.1 Interacting Lagrangians . . . . . . . . . . . . . . . . . . . . . . . .7.2 Interacting Field Operators . . . . . . . . . . . . . . . . . . . . . .7.3 Perturbation Theory . . . . . . . . . . . . . . . . . . . . . . . . . .8 Correlation Functions8.1 Interacting Time-Ordered Correlators8.2 Time-Ordered Products . . . . . . .8.3 Some Examples . . . . . . . . . . . .8.4 Feynman Rules . . . . . . . . . . . .8.5 Feynman Rules for QED . . . . . . .9 Particle Scattering9.1 Scattering Basics . . . . . . . . . . .9.2 Cross Sections and Matrix Elements .9.3 Electron Scattering . . . . . . . . . .9.4 Pair Production . . . . . . . . . . . .9.5 Loop Contributions . . . . . . . . . .10 Scattering Matrix10.1 Asymptotic States . . . .10.2 S-Matrix . . . . . . . . . .10.3 Time-Ordered Correlators10.4 S-Matrix Reconstruction .10.5 Unitarity . . . . . . . . . .7.17.17.67.9.8.1. 8.1. 8.2. 8.5. 8.12. 1411 Loop Corrections11.1 Self Energy . . . . . . . . . . . . .11.2 Loop Integral . . . . . . . . . . . .11.3 Regularisation and Renormalisation11.4 Counterterms . . . . . . . . . . . .11.5 Vertex Renormalisation . . . . . . .11.111.111.411.611.911.13.4
Quantum Field Theory IChapter 0ETH Zurich, HS14Prof. N. Beisert18. 12. 20140OverviewQuantum field theory is the quantum theory of fields just like quantum mechanicsdescribes quantum particles. Here, a the term “field” refers to one of the following: A field of a classical field theory, such as electromagnetism. A wave function of a particle in quantum mechanics. This is why QFT issometimes called “second quantisation”. A smooth approximation to some property in a solid, e.g. the displacement ofatoms in a lattice. Some function of space and time describing some physics.Usually, excitations of the quantum field will be described by “particles”. In QFTthe number of these particles is not conserved, they are created and annihilatedwhen they interact. It is precisely what we observe in elementary particle physics,hence QFT has become the mathematical framework for this discipline.This lecture series gives an introduction to the basics of quantum field theory. Itdescribes how to quantise the basic types of fields, how to handle their quantumoperators and how to treat (sufficiently weak) interactions. We will focus onrelativistic models although most methods can in principle be applied tonon-relativistic condensed matter systems as well. Furthermore, we discusssymmetries, infinities and running couplings. The goal of the course is a derivationof particle scattering processes in basic QFT models.This course focuses on canonical quantisation along the lines of ordinary quantummechanics. The continuation of this lecture course, QFT II, introduces analternative quantisation framework: the path integral.1 It is applied towardsformulating the standard model of particle physics by means of non-abelian gaugetheory and spontaneous symmetry breaking.What Else is QFT? There are many points of view.After attending this course, you may claim QFT is all about another 1000 ways totreat free particles and harmonic oscillators. True, these are some of the fewsystems we can solve exactly in theoretical physics; almost everything else requiresapproximation. After all, this is a physics course, not mathematics!If you look more carefully you will find that QFT is a very rich subject, you canlearn about many aspects of physics, some of which have attained a mythologicalstatus: anti-particles, anti-matter,1The path integral is much more convenient to use than canonical quantisation discussed here.However, some important basic concepts are not as obvious as in canonical quantisation, e.g. thenotion of particles, scattering and, importantly, unitarity.5
vacuum energy,tachyons,ghosts,infinities,mathematical (in)consistency.Infinities. How to deal with infinities?There is a famous quote due to Dirac about QED (1975): “I must say that I amvery dissatisfied with the situation, because this so-called ‘good theory’ doesinvolve neglecting infinities which appear in its equations, neglecting them in anarbitrary way. This is just not sensible mathematics. Sensible mathematicsinvolves neglecting a quantity when it is small – not neglecting it just because it isinfinitely great and you do not want it!”This is almost true, but QFT is neither neglecting infinities nor in an arbitraryway.Infinities are one reason why QFT is claimed to be mathematically ill-defined oreven inconsistent. Yet QFT is a well-defined and consistent calculationalframework to predict certain particle observables to extremely high precision.Many points of view; one is that it is our own fault: QFT is somewhat idealised; itassumes infinitely extended fields (IR) with infinite spatial resolution (UV);2 thereis no wonder that the theory produces infinities. Still, it is better to stick toidealised assumptions and live with infinities than use some enormous discretesystems (actual solid state systems).There is also a physics reason why these infinities can be absorbed somehow: Ourobservable everyday physics should neither depend on how large the universeactually is (IR) nor on how smooth or coarse-grained space is (UV).We can in fact use infinities to learn about allowable particle interactions. Thisleads to curious effects: running coupling and quantum anomalies.More later, towards the end of the semester.Uniqueness. A related issue is uniqueness of the formulation. Alike QM, QFTdoes not have a unique or universal formulation.For instance, many meaningful things in QM/QFT are actually equivalence classesof objects. It is often more convenient or tempting to work with specificrepresentatives of these classes. However, one has to bear in mind that only theequivalence class is meaningful, hence there are many ways to describe the samephysical object.The usage of equivalence classes goes further, it is not just classes of objects.Often we have to consider classes of models rather than specific models. This issomething we have to accept, something that QFT forces upon us.2The UV and the IR are the two main sources for infinities.6
We will notice that QFT does what it wants, not necessarily what we want. Forexample, we cannot expect to get what we want using bare input parameters.Different formulations of the same model naively may give different results. Wemust learn to adjust the input to the desired output, then we shall find agreement.We just have to make sure that there is more output than input; otherwise QFTwould be a nice but meaningless exercise because of the absence of predictions.Another nice feature is that we can hide infinities in these ambiguities in aself-consistent way.Enough of Talk. Just some words of warning: We must give up some views onphysics you have become used to, only then you can understand something new.For example, a classical view of the world makes understanding quantummechanics harder. Nevertheless, one can derive classical physics as anapproximation of quantum physics, once one understands the latter sufficientlywell.Let us start with something concrete, we will discuss the tricky issues when theyarise.Important Concepts. Some important concepts of QFT that will guide ourway: unitarity – probabilistic framework.locality – interactions are strictly local.causality – special relativity.symmetries – exciting algebra and geometry.analyticity – complex analysis.0.1PrerequisitesPrerequisites for this course are the core courses in theoretical physics of thebachelor syllabus: Classical mechanics (brief review in first lecture)Quantum mechanics (brief review in first lecture)Electrodynamics (as a simple classical field theory)Mathematical methods in physics (HO, Fourier transforms, . . . )0.21.2.3.4.5.6.ContentsClassical and Quantum MechanicsClassical Free Scalar FieldScalar Field QuantisationSymmetriesFree Spinor FieldFree Vector res)lectures)lectures)
7.8.9.10.11.InteractionsCorrelation FunctionsParticle ScatteringScattering MatrixLoop ctures)lectures)Indicated are the approximate number of 45-minute lectures. Altogether, thecourse consists of 53 lectures including one overview lecture.0.3ReferencesThere are many text books and lecture notes on quantum field theory. Here is aselection of well-known ones: M. E. Peskin, D. V. Schroeder, “An Introduction to Quantum Field Theory”,Westview Press (1995) C. Itzykson, J.-B. Zuber, “Quantum Field Theory”, McGraw-Hill (1980) P. Ramond, “Field Theory: A Modern Primer”, Westview Press (1990) M. Srendnicki, “Quantum Field Theory”, Cambridge University Press (2007) M. Kaku, “Quantum Field Theory”, Oxford University Press (1993) online: D. Tong, “Quantum Field Theory”, lecture l online: M. Gaberdiel, “Quantenfeldtheorie”, lecture notes (in m .Peskin & Schroeder may be closest to this lecture course, but we will not follow itliterally.8
Quantum Field Theory IETH Zurich, HS14Chapter 1Prof. N. Beisert29. 09. 20141Classical and Quantum MechanicsTo familiarise ourselves with the basics of quantum field theory, let us review someelements of classical and quantum mechanics. Then we shall discuss some issues ofcombining quantum mechanics with special relativity.1.1Classical MechanicsConsider a classical non-relativistic particle in a potential. In Lagrangianmechanics is described by the position variables q i (t) and the action functionalS[q] 1 2Z t2 dt L q i (t), q̇ i (t); t .(1.1)S[q] t1A typical Lagrangian function isL( q, q ) 12 m q 2 V ( q).(1.2)with mass m and V (q) as the external potential.A classical path extremises (minimises) the action S. One therefore determines thesaddle point δS 0 by variation of the action3 Z t2 L LiiδS dt δq (t) i δ q̇ (t) i q q̇t1 Z t t2 Z t2d L L Lii (1.3)dt δq (t)d δq (t) i . q i dt q̇ i q̇t1t t1The first term is the equation of motion (Euler–Lagrange)δS Ld L i 0.iδq (t) qdt q̇ i(1.4)The second term due to partial integration is the boundary equation of motion;usually we ignore it.41In many cases, L is time-independent: L(q i , q̇ i ; t) L(q i , q̇ i ).A single time derivative q̇ i usually suffices.3Einstein summation convention: there is an implicit sum over all index values for pairs ofmatching upper/lower indices.4More precisely, we usually fix the position qi (tk ) const. (Dirichlet) or the momentum L/ q̇ i (tk ) 0 (Neumann) at the boundary.21.1
Example. Throughout this chapter we will use the harmonic oscillator and thefree particle as an example to illustrate the abstract formalism. The harmonicoscillator is described by the following Lagrangian function and correspondingequation of motionL( q, q ) 12 m q 2 21 mω 2 q 2 , m( q ω 2 q) 0.(1.5)For ω 0 this system becomes a free particle.1.2Hamiltonian FormulationThe Hamiltonian framework is the next step towards canonical quantummechanics.First, define the momentum pi conjugate to q i as5pi L q̇ i(1.6)and solve for q̇ i q̇ i (q, p; t).6 Phase space is defined as the space of the positionand momentum variables (q i , pi ).The Lagrangian function L(q, q̇; t) is replaced by the Hamiltonian functionH(q, p; t) on phase space. We define H(q i , pi ; t) as the Legendre transformation ofL H(q, p; t) pi q̇ i (q, p; t) L q, q̇(q, p; t); t .(1.7)Let us express the equations of motion through H: A variation of the Hamiltonianfunction w.r.t. all q i and pi readsδH δpi q̇ i δq i L, q i(1.8)where we substituted the definition of the momenta pi twice to cancel four furtherterms that arise. We use the Euler–Lagrange equation and momenta to simplifythe variation furtherδH δpi q̇ i δq i ṗi .(1.9)Comparing this expression to the general variationδH δq i ( H/ q i ) δpi ( H/ pi ) we obtain the Hamiltonian equations of motionq̇ i H piandṗi H. q i(1.10)Next, we introduce the Poisson bracket for two functions f, g on phase space{f, g} : 56 f g f g. i q pi pi q iThis is a choice, one might also use different factors or notations.We suppose the equation can be solved for q̇.1.2(1.11)
The Poisson bracket allows to express the time evolution for phase space functionsf (q, p; t) 7df f {H, f }.(1.12)dt tIn particular, this works well for the functions f (q, p; t) q i and f (q, p; t) pi , andyields the canonical equations of motion.Example. For the harmonic oscillator we find p m q andmmω 2 21 2 m 2 2H p · q q 2 q p ω q .222m2(1.13)The Hamiltonian equations of motion read H1 q {H, q} p , pm H mω 2 q.p {H, p } q(1.14)A convenient change of variables reads a 1(mω q i p) ,2mω a 1(mω q i p) .2mω(1.15)Using these coordinates of phase space, the Poisson brackets read{f, g} i f g f g. i ai ai a i ai(1.16)We obtain a separated first-order time evolution for a, a H ω a · a, a iω a, a iω a .(1.17)Note that the Poisson bracket with this Hamiltonian simply counts the degree of afunction in a vs. a F F{H, F } iωai i iωa i.(1.18) a a iE.g. for F (a)m (a )n one finds {H, F } iω(m n)F .1.3Quantum MechanicsLet us revisit the canonical quantisation procedure highlighting the harmonicoscillator. Quantum field theory is built on the same methods, and we shallencounter the same kinds of problems, yet in some more elaborate fashion.In canonical quantisation, classical objects are replaced by elements of linearalgebra:7The Hamiltonian H is itself a phase space function.1.3
the state (q i , pi ) becomes a vector ψi in a Hilbert space V ; a phase space function f becomes a linear operator F on V ; Poisson brackets {f, g} become commutators i 1 [F, G].8The equation of motion for a state (Schrödinger) is a wave equationi d ψ(t)i H ψ(t)i.dt(1.19)The (normalised) wave function of a state has a probabilistic interpretation: hφ ψi 2 is the probability of finding the system described by ψi in the state φi.This requires the following essential features of wave functions: hψ ψi is positive; hψ ψi can be normalised to 1 by scaling ψi; hψ ψi is conserved.Conservation requires 1d hψ ψi hψ (H H † ) ψi 0.dti (1.20)Therefore, the Hamiltonian must be hermitian (self-adjoint). We then have timeevolution with a unitary operator U (t2 , t1 ) ψ(t2 )i U (t2 , t1 ) ψ(t1 )i.(1.21)The matrix element hψ F ψi describes the expectation value of the operator F instate ψi. Curiously, it obeys the quantum analog of the classical time evolution d F1hψ F ψi hψ [H, F ] ψi.(1.22)dt ti Example. For the harmonic oscillator and free particle we need to represent thecanonical commutation relations[q̂ i , p̂j ] i {q i , pj } i δji(1.23)on phase space. We introduce a basis of position eigenstates qi. The position andmomentum operators q̂ i and p̂i then act as9q̂ i qi q i qi,p̂i qi i qi. q i(1.24)We introduce a wave function ψ(t, q) h q ψ(t)i to represent a general state ψ(t)ion phase spaceZ ψ(t)i dd q ψ(t, q) qi.8(1.25)Poisson brackets cannot always be translated literally to commutators; the idea ofquantisation is to represent them up to “simpler” terms, i.e. up to polynomials of lower degree inthe operators and of higher orders in .9As usual in quantum mechanics, the action of the operators on the states effectively invertsthe order of terms in operator products. Plain insertion of q̂ i q i and p̂i i / q i into thecommutation relations leads to the wrong sign.1.4
By construction, the position operator q̂ i acts by multiplying the wave function byq i . The momentum operator p̂i effectively acts by the derivative i / q i on thewave function.10 For acting directly on the wave function ψ(t, q) we can thus writeq̂ i ψ(t, q) ' q i ψ(t, q),p̂i ψ(t, q) ' i ψ(t, q) . q iThe Hamiltonian acting on the wave function reads 2 2mω 2 2 q .H' 2m q2The free particle is solved exactly by momentum eigenstates (Fouriertransformation)ZZdd p i 1 p · qdi 1 p · q pi d q e qi, qi e pi.(2π )d(1.26)(1.27)(1.28)The momentum eigenstate pi is an energy eigenstate with E p 2 /2m.For the harmonic oscillator we use the operators ai and a†i which act on a wavefunction as 11 (mω q i p) ' a mω q , q2mω2mω 11 †(mω q i p) ' .(1.29) a mω q q2mω2mωThey obey the commutation relations[ai , a†j ] δji .(1.30)The quantum Hamiltonian has an apparent extra vacuum energy E0 12 d ωcompared to its classical counterpart H ω a† · aH 21 ωai a†i 21 ωa†i ai ω a† · a 21 d ω ω a† · a E0 .(1.31) One can add any numerical energy E0 to the Hamiltonian. This has no effect onany commutation relations, it merely shifts the frequencies by a commonamount. E0 is largely irrelevant for physics.11 This has the same e
Quantum Field Theory I Chapter 0 ETH Zurich, HS14 Prof. N. Beisert 18.12.2014 0 Overview Quantum eld theory is the quantum theory of elds just like quantum mechanics describes quantum particles. Here, a the term \ eld" refers to one of the following: A eld of a classical eld
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This is a writeup of my Master programme course on Quantum Field Theory I (Chapters 1-6) and Quantum Field Theory II. The primary source for this course has been ‹ Peskin, Schröder: An introduction to Quantum Field Theory, ABP 1995, ‹ Itzykson, Zuber: Quantum Field Theory, Dover 1980, ‹ Kugo: Eichtheorie, Springer 1997,
This is a writeup of my Master programme course on Quantum Field Theory I (Chapters 1-6) and Quantum Field Theory II. The primary source for this course has been Peskin, Schröder: An introduction to Quantum Field Theory, ABP 1995, Itzykson, Zuber: Quantum Field Theory
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