Visions: The Coming Revolutions in Particle Physics

Wonderful opportunities await particle physics over the next decade, with the coming of the Large Hadron Collider to explore the 1-TeV scale (extending efforts at LEP and the Tevatron to unravel the nature of electroweak symmetry breaking) and many initiatives to develop our understanding of the problem of identity and the dimensionality of spacetime. PACS: 12.15.-y 13.85.-t 12.60.-i 14.80.Bn FERMILAB–CONF-02/058–T 1 Galileo’s Three Revolutions In this talk, I want to evoke some of the revolutionary developments I believe will come over the next two decades in particle physics. To set the context, and because we find ourselves in Italy, I’d like to begin by recalling three revolutions we identify with Galileo and his time. Eppur si muove. In the public mind and in popular literature, Galileo is remembered chiefly for his part in completing the Copernican revolution, establishing that humans do not occupy a privileged location in the universe. It’s a great achievement and a good story, given texture by Galileo’s complex persona and by the richness of his relationship with the inquistorial Church [1]. Cimenti. The Copernican revolution is a scientific movement accomplished. We scientists revere Galileo no less for his contribution to the scientific method. For it was during Galileo’s time that humans found the courage to reject Authority. They learned instead to read nature by doing experiments, subjecting their hypotheses to unremitting trials by ordeal that Galileo called cimenti. The notion that experiment, not eloquence, is the arbiter of what is true revolutionized mankind’s relationship with nature. The minute particular. An essential element of civilization is human curiosity about the world and a thirst to comprehend nature. Until five centuries ago, the questions our ancestors wondered about were broad and the explanations they advanced were sweeping but vague. Asking great questions—seeking to explain everything about the world all at once—led to extremely limited answers. Science as we know it took shape in Galileo’s time when humans learned that asking limited questions could lead them to universal insights. In contemporary ∗Fermilab is operated by Universities Research Association Inc. under Contract No. DEAC02-76CH03000 with the United States Department of Energy. Visions: The Coming Revolutions in Particle Physics 2 American discourse at least, the received wisdom holds that all the great questions have been answered and that today’s scientists—gifted though they might be—are dealing with ever narrower research topics. This canard betrays an ignorance of how science has been done ever since it became worthy of the name. You can help explain to the world what science is. To be most effective, you must make the connection between your own minute particular (the search for the Higgs boson, the width of the W , the mass of the top, or whatever) and the universal understanding we are trying to build. Are all the great scientific revolutions in the distant past? Not at all: We are here to discuss the revolution-in-progress that we expect experiments with the Large Hadron Collider to complete. This is the radically new and simple conception of matter brought about by the development of gauge theories and the recognition that quarks and leptons are the basic constituents of matter— at the current limits of our resolution. The gauge-theory synthesis is part of a larger change that we are living through—no, making—in the way humans think about their world. The recognition that the human scale is not privileged, that we need to leave our familiar surroundings the better to understand them, has been building since the birth of quantum mechanics. As it emerges whole, fully formed, in our unified theories and renormalization group equations, the notion seems to me both profound and irresistible. I find it fully appropriate to compare this change in perception with the shifts in viewpoint we owe to Copernicus and Einstein. 2 Our Picture of Matter We base our understanding of physical phenomena on the identification of a few constituents that seem elementary at the current limits of resolution of about 10−18 m, and a few fundamental forces. The constituents are the pointlike quarks {(u, d)L, (c, s)L, (t, b)L} and leptons {(νe, e)L, (νμ, μ)L, (ντ , τ )L}, with strong, weak, and electromagnetic interactions specified by SU(3)c ⊗ SU(2)L ⊗ U(1)Y gauge symmetries. The electroweak theory is founded on the weak-isospin symmetry embodied in the quark and lepton doublets and weak-hypercharge phase symmetry, plus the idealization that neutrinos are massless. In its simplest form, with the electroweak gauge symmetry broken by the Higgs mechanism, the SU(2)L ⊗ U(1)Y theory has scored many qualitative successes: the prediction of neutral-current interactions, the necessity of charm, the prediction of the existence and properties of the weak bosons W± and Z. Over the past ten years, in great measure due to the beautiful experiments carried out at the Z factories at CERN and SLAC, precision measurements have tested the electroweak theory as a quantum field theory, at the one-per-mille level [5, 6, 7]. Last year, our colleagues working at LEP made a heroic push to discover the Higgs boson [8]. The search will intensify again in a few years at the Tevatron and the Large Hadron Collider. The quark model of hadron structure and the parton model of hard-scattering processes have such pervasive influence on the way we conceptualize particle 1A recent article in FermiNews [2] makes the challenge uncomfortably plain. 2For surveys of the electroweak theory, with references, see Ref. [3, 4].