Quantum Field Theory (Ph.D.)

Quantum Field Theory (Ph.D.) (Spring '23, Spring '24)


Text and reference books: mainly the Quantum Field Theory textbooks by by M. Peskin, D. Schroeder [P] and M. Srednicki [S] and "QFT and the Standard Model" by M. Schwartz [Sw] (going back and forth). Some excellent reference books overall and for various subtopics include the books by S. Weinberg, by A. Zee, by L. Ryder, by B. Hatfield, and by di Francesco et al [diF] (on conformal field theory), and others. Outline of contents:

Lattice models to QFT (chain of oscillators with nearest neighbour interactions to scalar fields) [my notes, also [diF sec.2.1.1] [P sec.2.3]]; generalities on relativity + quantum mechanics to QFT [partly S ch.1, P ch.2]; relativistic scalar field, canonical quantization [S ch.3] [P ch.2]; propagators and causality [P sec.2.4]; spin statistics for scalar bosons [S ch.4];

Lorentz group Lie algebra and its representations [Sw 10.1-10.3] [S] [P]; fermions and spinor representations, Dirac equation [Sw 10.1-10.3] [P] [S]; spinors and Lorentz transformations [Sw 10.3].

LSZ reduction, scalar field theory [S ch.5] [P ch.4]; free field correlation functions via Feynman path integrals [S ch.8] [P ch.9]; path integrals for interacting phi^3 scalar field in perturbation theory [S ch.9]; scattering amplitudes, Feynman rules [S ch.10] [P 9.2-9.3]; tree amplitudes, Mandelstam variables, scattering and differential cross-sections (briefly) [S ch.11] [P 5.4]; loop amplitudes and divergences;

phi^4 scalar field theory and perturbative 1-loop renormalization, calculationally (Feynman parameters, dimensional regularization, Euclidean continuations, renormalized perturbation theory at 1-loop, fixing mass/coupling counterterms [P, 6.3, 7.5, 10.2]; field strength renormalization and a brief glimpse at the Kallen-Lehmann spectral representation [P 7.1].

Other regularization schemes [P 6.3, 7.1] [Sw 15.4, 16.1, App.B]: recall the 1-loop momentum integral by dim.reg.; then use Pauli Villars and evaluate via Euclideanizing; then differentiate under integral sign and evaluate, then integrate to obtain the log-regulator matching with Pauli-Villars.

Example [Sw]: phi^3 theory and the 1-loop integral --> real-valued vs imaginary part. Unitarity of the S-matrix and the optical theorem [P 7.3] [Sw 24.1]. Working this out explicitly for 1-loop contribution to propagator and verifying imaginary part arising from intermediate states going on-shell etc. Unitarity for tree-level amplitudes.

The Veneziano amplitude (open strings) and analyticity properties (or what is Not a quantum field theory!) [from Green,Schwarz,Witten, "Superstring Theory" vol.1, ch.1, beginning].

very brief glimpse at MSbar (minimal subtraction) [P 11.4]; broad recap of renormalization schemes; an overview of Wilsonian renormalization (Euclidean continuations and statistical physics, critical phenomena; physical UV cutoffs; integrating out thin momentum shells, low energy effective actions). [P ch.12]

Integrating out thin momentum shells in phi4 theory explicitly at 1-loop, and RG flows of couplings; running of the coupling towards lower energies, the beta-function; Wilson-Fisher fixed point. [P 12.1]

Renormalization conditions at running scales; the Callan-Symanzik equation and beta-functions; explicit evaluation for phi4 theory [P 12.2]; generalities on RG flow equations and beta-functions; a very cursory glimpse of QCD, asymptotic freedom and strong coupling IR. [P 12.2-12.5]

* midsem, endsem, assignments (or reading project)

Suggested follow-up reading:
U(1) gauge theory (Maxwell electromagnetism), free field quantization (radiation gauge, Lorentz gauge, negative norm states etc);
QED and elementary scattering processes [P ch.4-7, Sw ch.8-9, 13];
Nonabelian gauge theories and the Standard Model [P part III, Sw part IV].

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