FERMILAB–PUB–15/058–T Electroweak Symmetry Breaking in Historical Perspective Chris Quigg∗ Theoretical Physics Department Fermi National Accelerator Laboratory P.O. Box 500, Batavia, Illinois 60510 USA The discovery of the Higgs boson is a major milestone in our progress toward understanding the natural world. A particular aim of this article is to show how diverse ideas came together in the conception of electroweak symmetry breaking that led up to the discovery. I will also survey what we know that we did not know before, what properties of the Higgs boson remain to be established, and what new questions we may now hope to address. I. INTRODUCTION The first goal of this article is to sketch how a broad range of concepts, drawn mainly from weak-interaction A lively continuing conversation between experiment phenomenology, gauge field theories, and condensed- and theory has brought us to a radically simple con- matter physics, came together in the electroweak the- ception of the material world. Fundamental particles ory. The presentation complements the construction of the electroweak theory given in my pre-discovery article, called quarks and leptons are the stuff of direct expe- 26 rience, and two new laws of nature govern their in- “Unanswered Questions in the Electroweak Theory”. teractions. Pursuing clues from experiment, theorists Presentations similar in spirit may be found in Refs. have constructed the electroweak theory1–3 and quantum 27,28. Next, I will briefly summarize what we now know chromodynamics,4–7 refined them within the framework about the Higgs boson, what the discovery has taught of local gauge symmetries, and elaborated their conse- us, and why the discovery is important to our conception quences. In the electroweak theory, electromagnetism of nature. Finally, I will address what remains to find and the weak interactions—so different in range and ap- out about the 125-GeV Higgs boson and what new ques- parent strength—are ascribed to a common gauge sym- tions are opened by its existence. For example, we need metry. We say that the electroweak gauge symmetry is to discover what accounts for the masses of the electron broken, by dynamics or circumstances, to the gauge sym- and the other leptons and quarks, without which there metry of electromagnetism. would be no atoms, no chemistry, no liquids or solids— no stable structures. In the standard electroweak theory, The electroweak theory and quantum chromodynam- both tasks are the work of the Higgs boson. Moreover, standard model ics (QCD) join to form the of particle we have reason to believe that the electroweak theory is physics. Augmented to incorporate neutrino masses and imperfect, and that new symmetries or new dynamical lepton mixing, the standard model describes a vast ar- principles are required to make it fully robust. Through- ray of experimental information. The gauge theories of out the narrative, I emphasize concepts over technical the strong, weak, and electromagnetic interactions have details. been validated by experiment to an extraordinary degree as relativistic quantum field theories. Recent textbook treatments of QCD and the electroweak theory may be found, for example in Refs. 8–11. II. EXPERIMENTAL ROOTS OF THE ELECTROWEAK THEORY Until recently, the triumph of this new picture has been incomplete, notably because we had not identified the arXiv:1503.01756v3 [hep-ph] 11 May 2015 agent that differentiates electromagnetism from the weak This section is devoted to a compressed evocation of interaction. The 2012 discovery of the Higgs boson by the how the phenomenology of the (charged-current) weak ATLAS12 and CMS13 Collaborations working at CERN’s interactions developed, in order to establish what a suc- Large Hadron Collider capped a four-decades-long quest cessful theory would need to explain. A superb source for for that agent. [Further details of the discoveries are re- the experimental observations that led to the creation of ported in Refs. 14–17.] The observations indicate that the standard model is the book by Cahn & Goldhaber,29 the electroweak symmetry is spontaneously broken, or which discusses and reproduces many classic papers. hidden: the vacuum state does not exhibit the full sym- Becquerel’s discovery30 of radioactivity in 1896 is one metry on which the theory is founded. Crucial insights of the wellsprings of modern physics. In a short time, into spontaneously broken gauge theories were developed physicists learned to distinguish several sorts of radioac- a half-century ago by Englert & Brout,18 Higgs,19,20 and tivity, classified by Rutherford31 according to the char- Guralnik, Hagen, & Kibble.21 All the experimental infor- acter of the energetic projectile emitted in the sponta- mation we have22–25 tells us that the unstable 125-GeV neous disintegration. Natural and artificial radioactivity particle discovered in the LHC experiments behaves like includes nuclear β decay, observed as an elementary scalar consistent with the properties an- A A ticipated for the standard-model Higgs boson. Z (Z + 1) + β− , (1) → 2 where β− is Rutherford’s name for what was soon iden- Detecting a particle that interacts as feebly as the neu- tified as the electron and AZ stands for the nucleus with trino requires a massive target and a copious source of charge Z and mass number A (in modern language, Z neutrinos. In 1953, Clyde Cowan and Fred Reines39 used protons and A Z neutrons). Examples are tritium β the intense flux of antineutrinos from a fission reactor and 3 3− 3 decay, H1 He2 + β−, neutron β decay, n p + β−, a heavy target (10.7 ft of liquid scintillator) containing → 214 214 → 28 and β decay of Lead-214, Pb Bi + β−. about 10 protons to detect the inverse neutron-β-decay 82 → 83 For two-body decays, as indicated by the detected reaction ν¯+p e++n. Initial runs at the Hanford Engi- → products, the Principle of Conservation of Energy & Mo- neering Works were suggestive but inconclusive. Moving mentum says that the β particle should have a definite their apparatus to the stronger fission neutrino source at energy. What was observed, as experiments matured, the Savannah River nuclear plant, Cowan and Reines and was very different: in 1914, James Chadwick32 (later to their team made the definitive observation of inverse β discover the neutron) showed conclusively that in the de- decay in 1956.40 cay of Radium B and C (214Pb and 214Bi), the β energy Through the 1950s, a series of experimental puzzles follows a continuous spectrum. led to the suggestion that the weak interactions did not 41 The β-decay energy crisis tormented physicists for respect reflection symmetry, or parity. In 1956, C. S. years. On December 4, 1930, Wolfgang Pauli addressed Wu and collaborators detected a correlation between the 60 an open letter33 to a meeting on radioactivity in Tübin- spin vector J~ of a polarized Co nucleus and the direc- 42 gen. In his letter, Pauli advanced the outlandish idea tion pˆe of the outgoing β particle. Now, parity inver- of a new, very penetrating, neutral particle of vanish- sion leaves spin, an axial vector, unchanged ( : J~ J~) P → ingly small mass. Because Pauli’s new particle interacted while reversing the electron direction ( :p ˆe pˆe), P → − very feebly with matter, it would escape undetected from so the correlation J~ pˆe should be an “unobservable” null any known apparatus, taking with it some energy, which quantity if parity is· a good symmetry. The observed would seemingly be lost. The balance of energy and mo- correlation is parity violating. Detailed analysis of the mentum would be restored by the particle we now know 60Co result and others that came out in quick succession as the electron’s antineutrino. Accordingly, the proper established that the charged-current weak interactions scheme for beta decay is are left-handed. By the same argument, the parity op- eration links a left-handed neutrino with a right-handed A A Z (Z+1)+ β− +¯ν . (2) neutrino. Therefore a theory that contains only νL would → be manifestly parity-violating. What Pauli called his “desperate remedy” was, in its way, Could the neutrino indeed be left-handed? M. Gold- very conservative, for it preserved the principle of energy haber and collaborators inferred the electron neutrino’s and momentum conservation and with it the notion that helicity43 from the longitudinal polarization of the recoil the laws of physics are invariant under translations in nucleus in the electron-capture reaction space and time. 152 m 152 e− + Eu (J = 0) Sm∗(J =1)+ νe After Chadwick’s discovery of the neutron in 1932 in → (3) highly penetrating radiation emitted by beryllium irra- | γ +152 Sm . diated by α particles,34 Fermi named Pauli’s hypothet- → ical particle the neutrino, to distinguish it from Chad- A compendious knowledge of the properties of nuclear wick’s strongly interacting neutron, and constructed his levels, together with meticulous technique, enabled this four-fermion theory (what we would today call a low- classic experiment. energy effective theory) of β decay, which was the first Following the observation of maximal parity violation step toward the modern theory of the charged-current in the late 1950s, a serviceable effective Lagrangian for weak interaction.35 In retrospect, nuclear β decay was the weak interactions of electrons and neutrinos could be the first hint for flavor, the existence of particle families written as the product of charged leptonic currents, containing distinct species. That hint was made mani- fest by the discovery of the neutron, nearly degenerate GF µ V A = − νγ¯ µ(1 γ5)e eγ¯ (1 γ5)ν + h.c., (4) in mass with the proton, which suggested that neutron L − √2 − − and proton might be two states of a nucleon, with the n - p mass difference attributed to electromagnetic ef- where Fermi’s coupling constant is GF =1.1663787(6) 5 2 × fects.