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Section 1: Introduction

The physical universe challenges us over a wide span of distances, ranging over more than 35 orders of magnitude, from subatomic scales (< 10-14 meters) to the dimensions of galaxies (1021 meters) and beyond. In recent years, scientists working at both ends of the scale—particle physicists probing the basic building blocks of matter and cosmologists studying the structure of the universe on the largest observable scales—have started to converge on a common picture of how the universe expanded from a hot, dense "particle soup" shortly after the Big Bang to form galaxies, stars, and planets. Impressive as this "cosmic convergence" is, important questions still remain: Is there a Higgs particle responsible for giving particles mass? What is the nature of the dark matter that dominates the mass in galaxies, including our own Milky Way? And, why is a mysterious force dubbed dark energy causing the expansion of the universe to speed up? To address these questions, physicists have planned a variety of experiments that use accelerators, telescopes, and detectors deep underground. They hope to find some of the answers in the next decade.

Fundamental particles of the Standard Model.

Figure 1: Fundamental particles of the Standard Model.

Source: © Wikimedia Commons, License: CC 3.0 Unported. Author: MissMJ, 27 June 2006. More info

Particle physicists have already made significant progress in understanding the subatomic end of the scale. They have enshrined their discoveries in the Standard Model of particle physics. This theory is so apparently perfect that no crack has yet appeared despite experimentalists' best efforts to devise ever-more precise tests. Yet, at the same time, it is so fatally flawed as to convince theorists that behind the Standard Model must lie a better theory that encompasses and expands upon it.

The evidence for dark matter and dark energy, although they remain completely mysterious, is perhaps the most significant hints that the Standard Model is incomplete. We shall learn about that evidence and the theoretical problems it causes in Units 10 and 11. But even before these cosmological clues surfaced, observations of the behavior of particles called neutrinos and theoretical problems in extending the Standard Model to much higher energies had suggested that something was missing. Literally thousands of theoretical papers in the literature propose everything from string theory to extra dimensions and from supersymmetry to multiple universes as remedies for the Standard Model's known flaws. The Large Hadron Collider (LHC) at the CERN laboratory in Geneva, Switzerland—the highest-energy particle accelerator ever built—will put the Standard Model to its most rigorous tests ever and tell us which, if any, of the many theories beyond the Standard Model bear any resemblance to reality. This unit details the discoveries of successive subatomic particles and will analyze what each contributed to the Standard Model.