Electroweak symmetry breaking Jan 2014 1 Spontaneous symmetry breaking in gauge theories Tom Kibble 20 Jan 2014

  • View
    215

  • Download
    0

Embed Size (px)

Transcript

  • Slide 1
  • Electroweak symmetry breaking Jan 2014 1 Spontaneous symmetry breaking in gauge theories Tom Kibble 20 Jan 2014
  • Slide 2
  • Electroweak symmetry breaking Jan 2014 2 Symmetry This talk is the story of how this idea of spontaneous symmetry breaking came to play a key role in one part of the standard model of particle physics as I saw it from my viewpoint at Imperial College But this doesnt always work the symmetry may be spontaneously broken Symmetry helps to solve problems in physics
  • Slide 3
  • Electroweak symmetry breaking Jan 2014 3 Imperial College Theory Group I joined the IC theoretical physics group in 1959, the same year its founder, Abdus Salam, became the youngest FRS at age 33. It was very lively, with numerous visitors: Murray Gell-Mann, Stanley Mandelstam, Steven Weinberg,... Interests in quantum field theory symmetries
  • Slide 4
  • Electroweak symmetry breaking Jan 2014 4 Quantum Field Theory In classical physics, wave fields (e.g. electric and magnetic fields) and particles are very different in quantum physics, they are the same described by quantum fields Bosons (force carriers) have spin where e.g. photon has spin we call it a spin-1 particle Fermions (constituents of matter) have spin electrons, protons, neutrons have spin (spin- particles) fermions obey the Pauli exclusion principle: two fermions cannot occupy the same quantum state, the foundation of atomic structure Interactions are described by quantum field theory
  • Slide 5
  • Electroweak symmetry breaking Jan 2014 5 Quantum Electrodynamics First quantum field theory was Quantum Electrodynamics (QED), the theory of interacting electrons and photons, developed in 1930s. Method of calculation perturbation theory involved a power series expansion in powers of the fine structure constant (e = proton charge = electron charge) Lowest-order calculations gave excellent results But higher order corrections were all infinite. Solution renormalization was found in 1947, independently by Richard Feynman, Julian Schwinger and (in 1943) by Sin-Itiro Tomonaga Led to amazingly accurate predictions of electron magnetic moment and Lamb shift (2s 1/2 2p 1/2 energy difference in H), etc.
  • Slide 6
  • Electroweak symmetry breaking Jan 2014 6 What next? We distinguish four types of interaction strong nuclearshort-rangecoupling strength ~ 1 electromagneticlong-rangestrength = ~ 10 2 weak nuclearshort-rangestrength ~ 10 10 gravitationallong-rangestrength ~ 10 40 After QEDs success, people searched for field theories of other interaction (or even better, a unified theory of all of them). Most interest in strong interactions there were candidate field theories, but no one could calculate with them because perturbation theory doesnt work if the small parameter is ~ 1. So perhaps weak interactions were more promising?
  • Slide 7
  • Electroweak symmetry breaking Jan 2014 7 Symmetries A huge zoo of particles were discovered, which could obviously be grouped into families, related by approximate symmetries Isospin. Heisenberg suggested the very similar proton (p) and neutron (n) could be seen as two states of a nucleon (N: N + = p, N 0 = n), with a symmetry that rotates one into the other, like spatial rotations that rotate the spin states of an electron an SU(2) symmetry now understood as a symmetry between two lightest quarks (u,d) Eightfold way. Strongly interacting particles (hadrons) could be grouped into octets and decuplets. Gell-Mann (1961) and Neeman (1961) showed that this could be explained by a more approximate SU(3) symmetry now understood as a symmetry of the three lightest quarks (u,d,s).
  • Slide 8
  • Electroweak symmetry breaking Jan 2014 8 Gauge theories This idea of replacing global by local symmetry the gauge principle gives a rationale for the existence of the electromagnetic field, and for the zero rest-mass of the photon. This idea could be applied elsewhere. There was a lot of interest at Imperial College. Salam was convinced from an early stage that a unified theory of all interactions should be a gauge theory. Quantum mechanics has a global phase symmetry physics doesnt change when we make the same phase rotation of the wave function everywhere mathematically, a U(1) symmetry QED is a gauge theory it has a special local U(1) symmetry this means we can make different rotations at different points of space and time, but it only works if there is an electromagnetic field
  • Slide 9
  • Electroweak symmetry breaking Jan 2014 9 Larger gauge theories Because isospin is an approximate symmetry, this symmetry must be broken in some way but adding symmetry-breaking terms destroys many of the nice properties of gauge theories. Because of the difficulty of calculating with a strong-interaction theory, interest began to shift to weak interactions, especially after it was found that they could be explained as proceeding via exchange of bosons W with spin 1 (units of ), like the photon ( . could there be a unified theory of weak and electromagnetic? First example of a gauge theory beyond QED was the Yang-Mills theory (1954), a gauge theory of isospin SU(2) symmetry. same theory also proposed by Salams student Ronald Shaw, but unpublished except as a Cambridge University PhD thesis ultimately not correct theory of strong interactions, but the foundation for all later gauge theories.
  • Slide 10
  • Electroweak symmetry breaking Jan 2014 10 Similarity and Dissimilarity Electromagnetic interaction Weak interaction exchange of spin-1. exchange of spin-1 W long range short range large parity conserving (mirror-symmetric) parity violating (non-mirror-symmetric) But So: Can there be a symmetry relating and W ? If so it must be broken
  • Slide 11
  • Electroweak symmetry breaking Jan 2014 11 Early Unified Models Salam and Ward (1964), unaware of Glashows work, proposed a similar model, also based on SU(2) x U(1). Big question: could this be a spontaneously broken symmetry? (first suggested by Yoichiro Nambu) The first suggestion of a gauge theory of weak interactions mediated by W + and W was by Schwinger (1956), who suggested there might be an underlying unified theory, incorporating also the photon. But in all these models symmetry breaking, giving the W bosons masses, had to be inserted by hand and models with spin-1 bosons with explicit masses were known to be non-renormalizable. Glashow (1961) proposed a model with symmetry group SU(2) x U(1) and a fourth gauge boson Z 0, showing that the parity problem could be solved by a mixing between the two neutral gauge bosons.
  • Slide 12
  • Electroweak symmetry breaking Jan 2014 12 Nambu-Goldstone bosons vacuum breaks symmetry: choose and set So (NG boson) cubic and quartic terms Spontaneous breaking of a continuous symmetry existence of massless spin-0 Nambu-Goldstone bosons. NG bosons correspond to spatial oscillations in e.g. Goldstone model: complex scalar field with
  • Slide 13
  • Electroweak symmetry breaking Jan 2014 13 Goldstone Theorem There were known (not well understood) counter-examples in condensed matter, e.g. superconductivity (Nambu 1960, Philip Anderson 1963). No observed massless scalars no spontaneous breaking of a continuous symmetry ! Weinberg: Nothing will come of nothing; speak again! (King Lear) That NG bosons are massless in any relativistic theory was apparently proved by Weinberg & Salam, published with Goldstone (1962) I was very interested when in 1964 Gerald Guralnik (a student of Walter Gilbert, who had been a student of Salam) arrived at Imperial College as a postdoc to find that he had been studying this problem, and already published some ideas about it. We began collaborating, with another US visitor, Carl Richard Hagen. We (and others) solved problem.
  • Slide 14
  • Electroweak symmetry breaking Jan 2014 14 Mass generation mechanism Solution was found by three groups Englert & Brout (1964), Higgs (1964), Guralnik, Hagen & TK (1964) gauge theories are not like other field theories: masslessness of NambuGoldstone bosons and gauge bosons cancels out, combining to create massive gauge bosons. All three proposed (from different viewpoints) essentially the same model for spontaneous symmetry breaking in the simplest U(1) gauge theory, i.e. a broken version of electrodynamics it involves introducing a new scalar (spin-0) field, with sombrero potential (as in Goldstone model) spontaneous symmetry breaking occurs when this field acquires a non-zero average value this gives mass to other fields it interacts with, in particular the gauge bosons.
  • Slide 15
  • Electroweak symmetry breaking Jan 2014 15 Electroweak unification By 1964 both the mechanism and Glashows (and Salam and Wards) SU(2) x U(1) model were in place, but it still took three more years to put the two together. Further work on the detailed application of the mechanism to non- abelian theories (TK, 1967). This work helped, I believe, to renew Salams interest. The three papers on the Higgs mechanism attracted very little attention at the time. The boson attracted even less interest. Unified model of weak and electromagnetic interactions of leptons proposed by Weinberg (1967) essentially the same model was presented independently by Salam in lectures at IC in autumn of 1967 and published in a Nobel symposium in 1968 he called it the electroweak theory.
  • Slide 16
  • Electroweak symmetry breaking Jan 2014 16 Later developments In 1973 the key prediction of the theory, the existence of neutral curren