Genesis of electroweak symmetry breaking - 1

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Genesis of electroweak symmetry breaking - 1. Tom Kibble Imperial College 13 Sep 2012. Outline Part 1. Story of idea of spontaneous symmetry breaking in gauge theories and electroweak unification from my viewpoint at Imperial College. - PowerPoint PPT Presentation

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  • Genesis of electroweaksymmetry breaking - 1Tom KibbleImperial College13 Sep 2012

    Electroweak symmetry breaking Sep 2012

  • Outline Part 1Story of idea of spontaneous symmetry breaking in gauge theories and electroweak unification from my viewpoint at Imperial College Physics after WW2: QED, renormalization theory Models of strong interactions: gauge theory, symmetry breaking Abdus Salam The idea of weak-electromagnetic unification Obstacles to unification the Goldstone theorem Part 2: Overcoming the obstacles the Higgs mechanism

    Electroweak symmetry breaking Sep 2012

  • Physics after WW2 During the war, physicists had been working on atomic weapons, radar, operational research, etc. After 1945, they went back to fundamental physics, leading to very rapid developments, initially in the simplest quantum field theory, Quantum Electrodynamics (QED) the theory of interacting electrons and photons. Perturbation theory gave excellent results to lowest order in the fine structure constant But higher order corrections were infinite.

    Electroweak symmetry breaking Sep 2012

  • Renormalization Solution was found in 1947, independently by Richard Feynman, Julian Schwinger and (in 1943) by Sin-Itiro Tomonaga all the infinities could be collected into infinite renormalizationconstants, relating m and e to m0 and e0. For this work, they sharedthe Nobel Prize for Physics in 1965. In 1948 Freeman Dyson showed that all three approaches were equivalent, and gave a proof that renormalization worked to all orders. Innovative experiments on the Lamb shift and the magnetic moment of the electron confirmed the results were correct to unprecedented accuracy. There was a gap in Dysons proof, however ...

    Electroweak symmetry breaking Sep 2012

  • Abdus Salam at 14 Abdus Salam was born in 1926, the son of a minor civil servant living in Jhang near Lahore in what is now Pakistan. At 14, he won a scholarship to Government College, Lahore with the highest marks ever recorded, making the front page of the local paper. He published his first paper at 17 an improved solution to an algebraic problem solved by Srinivas Ramanujan. At 20 he won a scholarship to Cambridge.

    Electroweak symmetry breaking Sep 2012

  • Salam in Cambridge Salam went to Cambridge in 1946 an outstanding undergraduate. Kemmer had no specific project for him, but suggested an older student of Kemmers, Paul Matthews, might have ideas for a project. Matthews suggested he try to fill an outstanding gap in Dysons proof that renormalization works to all orders in perturbation theory; it did not directly deal with the case of overlapping divergences, e.g. in Matthews returned from a brief holiday to find Salam had completely solved the problem! This work gained him an instant international reputation, with an invitation to Princeton. He was excited by recent developments in theoretical physics, and asked to be taken on as a PhD student by Nicholas Kemmer.

    Electroweak symmetry breaking Sep 2012

  • Strong interaction models The success of QED inspired physicists to look for similar theories of strong and weak interactions. or even better, a unified theory of all of them. Initially, strong interactions attracted most interest. The best guess as field theory of strong interactions was Hideki Yukawas meson theory: pions as force carriers

    Salam and Matthews (at Princeton) showed that too could be renormalized. But there was a big problem no one could make any calculations for a model with g ~ 1.

    Electroweak symmetry breaking Sep 2012

  • Field theory vs S-matrix theory Problem with a field theory of strong interactions: perturbation theory calculations are impossible with a coupling constant ~1. During 1950s, many people concluded that field theory had had its day the new rage was S-matrix theory, based on analytic properties of scattering amplitudes, especially Regge poles. Many people thought there were no elementary hadrons all were bound states of each other the self-consistent bootstrap. But in a few places, the flag of field theory was kept flying Imperial College, Harvard, ... .

    Electroweak symmetry breaking Sep 2012

  • Imperial College ca. 1960Imperial College theoretical physics group was founded in 1956 by Abdus Salam in 1959 he became the youngest FRS at age 33 I arrived in 1959 3 permanent faculty: Abdus Salam Paul Matthews John C Taylor I joined faculty in 1961 Numerous visitors: Murray Gell-Mann, Stanley Mandelstam, Steven Weinberg, Kenneth Johnson, Art Rosenfeld, ...

    Electroweak symmetry breaking Sep 2012

  • Gauge theories First example of a gauge theory beyond QED was the Yang-Mills theory (1954), gauged SU(2) intended as a theory of strong interactions, with SU(2) representing isospin same theory also proposed by Salams student Ronald Shaw, but unpublished except as a Cambridge University PhD thesis. Although this ultimately proved not to be the correct theory of strong interactions, it was the model for all subsequent gauge theories.

    Electroweak symmetry breaking Sep 2012

  • Gauge theories at Imperial Salam was convinced from an early stage that a unified theory of all interactions should be a gauge theory. There was a lot of interest in gauge theories at Imperial College my own first involvement in 1961 was to show how gravity could be viewed as a gauge theory of the Poincar group but not a renormalizable one.

    Electroweak symmetry breaking Sep 2012

  • The particle zoo Experimental particle physics grew fast cosmic ray observation with cloud chambers bubble chamber experiments with particle accelerators Discovered a huge number of new particles could they all be elementary? Search for symmetries particles arranged in multiplets, related by symmetries SU(2) isospin (Heisenberg, Kemmer) SU(3) eightfold way (Murray Gell-Mann, Yuval Neeman) ... Now understood in terms of quarks: SU(2) symmetry of (u,d), SU(3) of (u,d,s)

    Electroweak symmetry breaking Sep 2012

  • Broken symmetries These were approximate symmetries, therefore broken in some way spontaneously? Spontaneous breaking of gauge symmetry, giving mass to the plasmon, was known (not fully understood) in superconductivity. Nambu (1960) suggested a similar mechanism could give masses to elementary particles. Nambu and Jona-Lasinio (1961) proposed a specific modelphase symmetry is exact chiral symmetry is spontaneously broken

    Electroweak symmetry breaking Sep 2012

  • Unification A very important step was the discovery that the weak four-fermion interactions involved V and A rather than S, T or P. This meant that the weak interactions could be seen as due to the exchange of spin-1 W bosons. This made them seem very similar to electromagnetic interactions mediated by photons. So the question arose: could there be a unified theory of weak and electromagnetic interactions? VA theory proposed by Marshak & Sudarshan (1957) and by Feynman & Gell-Mann (1958) Because of the difficulty of calculating with a theory with large coupling constant, interest began to shift towards the weak interactions.

    Electroweak symmetry breaking Sep 2012

  • Similarity and DissimilarityElectromagneticinteraction Weakinteractionexchange ofspin-1 exchange ofspin-1 Wlong rangeshort range largeparity conservingparity violatingBut So: Can there be a symmetry relating and W? If so it must be broken

    Electroweak symmetry breaking Sep 2012

  • Early Unified Models Salam and Ward (1964), unaware of Glashows work, proposed a similar model, also based on SU(2) x U(1) though neither model used the correct representation of leptons. 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 gauge bosons are naturally massless, and in all these models symmetry breaking, giving the W bosons masses, had to be inserted by hand. Glashow (1961) proposed a model with symmetry group SU(2) x U(1) and a fourth gauge boson Z0, showing that the parity problem could be solved by a mixing between the two neutral gauge bosons.

    Electroweak symmetry breaking Sep 2012

  • Massive vector bosons Gauge theories naturally predicted massless vector bosons. If masses were added by an explicit symmetry-breaking term, then the vector-meson propagator would not bebut rather Thus we have a much worse divergence, and the theory is clearly not renormalizable. So the question started to be asked: could the symmetry breaking that gives rise to vector boson masses be spontaneous symmetry breaking?

    Electroweak symmetry breaking Sep 2012

  • Spontaneous Symmetry Breaking Often there is a high-temperature symmetric phase, and a critical temperature below which the symmetry is spontaneously broken crystallization of a liquid breaks rotational symmetry so does Curie-point transition in a ferromagnet gauge symmetry is broken in a superconductor Particle physics exhibited many approximate symmetries it was natural to ask whether they could be spontaneously broken Spontaneous breaking of symmetry occurs when the ground state or vacuum state does not share the symmetry of the underlying theory. It is ubiquitous in condensed matter physics Could this work in particle physics too?

    Electroweak symmetry breaking Sep 2012

  • Nambu-Goldstone bosons This happens because of the existence of degenerate vacuum states,labelled by a continuou