Transcript Higgs boson
Genesis of electroweak symmetry breaking – 2 Tom Kibble Imperial College 14 Sep 2012 Electroweak symmetry breaking Sep 2012 1 Outline How did we escape the impasse created by the Goldstone theorem? • Obstacles to unification — Goldstone theorem • The Higgs mechanism • Electroweak unification • Experimental confirmation • Discovery of the Higgs boson • Is it really the Higgs boson? • What next? Electroweak symmetry breaking Sep 2012 2 Nambu-Goldstone bosons • Spontaneous breaking of a continuous symmetry existence of massless spin-0 Nambu-Goldstone bosons. • e.g. Goldstone model L * V V 21 ( * 21 2 )2 — vacuum breaks symmetry: i 0 0 e — choose 0 2 1 ( 1 i 2 ) and set 2 2 2 1 V 1 cubic and quartic terms So 2 m12 2 , m22 0 (Goldstone boson) • This was believed inevitable in a relativistic theory Electroweak symmetry breaking Sep 2012 3 Are massless bosons inevitable? • No massless spin-0 bosons had been seen, though they should be easy to see • If they are an inevitable consequence of spontaneous symmetry breaking, that rules it out • So among would-be unifiers there was a lot of discussion about whether there was any way around the Goldstone theorem • There were known counter-examples in condensed matter physics, in particular in superconductivity, but it was generally believed that massless bosons are inevitable in a relativistic theory • When Steven Weinberg spent a sabbatical at Imperial in 1962, he and Salam developed a proof of the Goldstone theorem for relativistic theories Electroweak symmetry breaking Sep 2012 4 Goldstone Theorem • Proof (Goldstone, Salam & Weinberg 1962): assume 1. symmetry corresponds to conserved current: j 0 (0) i d 3 x [(0), j 0 (0,x)] 2. there is some field whose vev is not invariant: 0 0 0, thus breaking the symmetry Let f (k) i d 4 x e ikx 0 [(0), j (x)] 0 1 k f (k) 0 k0f 0 (k0 ,0) 0 f 0 (k0 ,0) (k0 ), 0 • No observed massless scalars continuous symmetry ! 1,2 dk0 f 0 (k0 ,0) 0 massless particle in intermediate state no spontaneous breaking of a Electroweak symmetry breaking Sep 2012 5 Counter-examples • There were known counter-examples in condensed matter, e.g. superconductivity (Philip Anderson 1963). • Where long-range forces are involved, j 0 does not imply k0f 0 (k0 ,0) 0 k0f (k0 ,0) d x (0e 0 4 ik0 x0 ) 0 [(0), j 0 (x)] 0 d 4 x 0 [(0),0 j 0 (x)] 0 d 4 x 0 [(0),k j k (x)] 0 surface integral at infinity • But this could not happen in a relativistic theory because commutators vanish at spacelike separation ! Electroweak symmetry breaking Sep 2012 6 Impasse • In a relativistic theory, there seemed to be no escape — spontaneous symmetry breaking implied the existence of zero-mass spinless bosons — since no such bosons had been seen, spontaneous symmetry breaking was ruled out — other models with explicit symmetry breaking were clearly divergent, giving infinite results • 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 found the solution — though we were not the first to do so. Electroweak symmetry breaking Sep 2012 7 Higgs mechanism • The argument fails in the case of a gauge theory, e.g. in Coulombgauge QED, commutators do not vanish at spacelike separation. — Englert & Brout (1964), Higgs (1964), Guralnik, Hagen & TK (1964) • Higgs model (gauged Goldstone model): D ieA again set 1 2 L D * D 41 F F V F A A ( 1 i 2 ) V 21 ( * 21 2 )2 1 B A 2 e F B B L 21 11 41 F F 21 212 21 e22B B cubic terms ... Thus the massless gauge and Goldstone bosons have combined to give a massive gauge boson. But: there is more to it. Electroweak symmetry breaking Sep 2012 8 Gauge modes F j e22B L , B 0 1 B A 2 0 are also satisfied for any 2 so long as e (gauge invariance of original model) • Field equations • To tie down not only B but also Aand gauge condition: ,2 we need to impose a • With B 0 the Coulomb gauge condition 2 0 (or constant) • However the Lorentz gauge condition 2 satisfy 0 k Ak 0 requires A 0 only requires that 2 — in this manifestly covariant gauge, the Goldstone theorem does apply, but the Goldstone boson is a pure gauge mode. Electroweak symmetry breaking Sep 2012 9 How is the Goldstone theorem avoided? • This is the particular question asked by Guralnik, Hagen & TK • There is another way of expressing the proof of the Goldstone theorem — for a relativistic theory, j 0 would seem to imply dQ 0, Q d 3 x j 0 (x) dt i 0 (0),Q 0 0 • But if Q is time-independent, the only intermediate states that can contribute are zero-energy states which can only appear if there are massless particles. • The broken symmetry condition is • So what is wrong? — Q does not exist Electroweak symmetry breaking Sep 2012 10 No total charge operator • When the symmetry is spontaneously broken, the integral Q d 3 x j 0 (x) does not define a conserved charge operator. • For example, in the simple Higgs model, j e(1D2 2D1) e 2 e22 A L e22B L • So Q e22 d 3 x B0 (x) L which is clearly not time-independent, and in fact does not exist as a self-adjoint operator. Electroweak symmetry breaking Sep 2012 11 Unitary inequivalence • The non-existence of Q is related to the fact that spontaneous symmetry breaking implies a degenerate vacuum. We chose a state 0 in which 0 0 2 • But we could equally well have chosen any other phase: a state for which e i 2 iQ 0 • If Q existed, eiQ would be unitary, and we would expect e • But in fact 0 0 and indeed 0 (x1)L (xn ) 0 for 0 • 0 and belong to orthogonal Hilbert spaces, carrying unitarily inequivalent representations of the canonical commutation relations • This is a defining property of spontaneous symmetry breaking Electroweak symmetry breaking Sep 2012 12 Electroweak unification • The three papers on the Higgs mechanism attracted very little attention at the time. • By 1964 both the mechanism and Glashow’s (and Salam and Ward’s) SU(2) x U(1) model were in place, but it still took three more years to put the two together. • I did further work on the detailed application of the mechanism to non-abelian theories (TK, 1967). This work helped, I believe, to renew Salam’s 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. Electroweak symmetry breaking Sep 2012 13 Electroweak model • The electroweak model of Weinberg and Salam was basically Glashow’s SU(2) x U(1) model together with a doublet of Higgs fields interacting with leptons. — gauge fields: W ,Z 0 , — Higgs fields: complex doublet, four real fields — three give masses to W ,Z 0, fourth is physical Higgs — leptons form left-handed doublet e ,eL — and a right-handed singlet eR Electroweak symmetry breaking Sep 2012 14 Later developments • Both Salam and Weinberg speculated that their theory was renormalizable. This was proved by Gerard ’t Hooft in 1971 — a tour de force using methods developed by his supervisor, Tini Veltman, especially the computer algebra programme Schoonship. • In 1973 the key prediction of the theory, the existence of neutral current interactions — those mediated by Z0 — was confirmed at CERN. • This led to the Nobel Prize for Glashow, Salam & Weinberg in 1979 — but Ward was left out (because of the ‘rule of three’?). • In 1983 the W and Z particles were discovered at CERN. • ’t Hooft and Veltman gained their Nobel Prizes in 1999. Electroweak symmetry breaking Sep 2012 15 The Higgs boson • In 1964, the Higgs boson had been a very minor and uninteresting feature of the mechanism — the key point was the Higgs mechanism for giving the gauge bosons masses and escaping the Goldstone theorem. • But after 1983 it started to assume a key importance as the only missing piece of the standard-model jigsaw. The standard model worked so well that the boson (or something else doing the same job) more or less had to be present. • Finding the Higgs was one of the main objectives of the LHC. Electroweak symmetry breaking Sep 2012 16 Discovery of the Higgs boson • Two great collaborations, Atlas and CMS have over a 20-year period designed built and operated marvellous detectors. CMS under construction Possible Higgs event • Result: almost certain discovery of the Higgs (of some kind) Electroweak symmetry breaking Sep 2012 17 Giving mass to particles • Originally, the Higgs field had one principal purpose — to give masses to the W and Z bosons. • But then it became clear that it would also give masses to the other particles it interacts with, e.g. the electron. An interaction term with with coupling constant g implies a mass m : g . • This means the Higgs interacts most strongly with the most massive particles, a prediction that has been tested at LHC. • It is often loosely said that ‘the Higgs gives mass to all other particles’. That is not entirely true. It does give mass to almost all the known elementary particles, except the neutrinos. • But protons and neutrons acquire most of their mass from the gluons that bind quarks together, by a quite different mechanism — and we have no idea where the tiny neutrino masses come from. Electroweak symmetry breaking Sep 2012 18 Is it really the Higgs? • The evidence for a particle at around 125 GeV is now very strong, but is it the standard-model Higgs? • There is still a lot of work to do in the next few months — we know it is a boson of even spin, but is it definitely 0, not 2? — we know it decays into the expected channels, but are the branching ratios those predicted by the standard model? — there are some hints of possible discrepancies • I think it is almost surely some sort of Higgs, but there are other possibilities beyond the standard-model Higgs — e.g. in a supersymmetric extension of the standard model, there are several Higgs particles, of which this would be the lightest. That would be a very exciting possibility, which might explain dark matter. Electroweak symmetry breaking Sep 2012 19 Is this the end of particle physics? • No! There are many outstanding questions still to be answered. • The standard model is wonderfully successful, but it is a mess — it has something like 20 arbitrary parameters whose values we cannot predict, e.g. ratios of particle masses, or why 3 generations — it is not a unified model, being based on the symmetry group SU(3) x SU(2) x U(1), with three independent coupling strengths • There are suggestions that all three interactions become truly unified at an energy scale of about 1015 GeV — one of the reasons for favouring supersymmetry is that this grand unification idea works much better in a supersymmetric extension of the standard model. • The standard model does not include gravity — for that perhaps we need string theory or M-theory or •••. Electroweak symmetry breaking Sep 2012 20 The End • I wish to conclude by acknowledging my huge debt to my mentor and inspiration, Abdus Salam • He was a brilliant physicist, an inspiring leader, a skilled diplomat, and a warm and generous man. • It was a very sad loss when he died prematurely in 1996. Electroweak symmetry breaking Sep 2012 21 Electroweak symmetry breaking Sep 2012 22