Executive Summary

Lighting Out for the Terascale

“I reckon I got to light out for the territory ahead of the rest…”
-- Mark Twain, Huckleberry Finn

Particle physicists are about to light out for a vast new scientific terra incognita. When they do, later in this decade, they will encounter a territory of discovery that many of them have theorized and dreamed about all their lives. This unexplored country is the Terascale, named for the Teravolts of particle accelerator energy that will open it up for scientific discovery. The next generation of particle accelerators are physicists' tickets to the Terascale and the mysteries that it harbors about the nature of the physical laws that govern the universe. Once they've seen the Terascale, physicists believe, the universe will never look the same.

Although physicists have yet to explore the Terascale, they have ideas of what they may find. The past 30 years of experiment and theory have produced many clues and predictions of its features and contours — a detailed travel guide to a country that no one has yet visited. Experiments at the Large Hadron Collider at CERN in Europe will soon show what relation the theorists' guidebook bears to Terascale reality. Real data from these experiments will rewrite the theorists' Guide to the Quantum Universe.

About certain features of the Terascale, most predictions agree. Most physicists expect to find the Higgs boson — or, if not the Higgs, whatever it is that does Higgs's job of giving mass to the particles of matter. Experiment and theory so far all seem to say that SOMETHING like the Higgs exists at the energy of the Terascale to keep the universe and everything in it from flying apart at the speed of light. The LHC experiments will very likely discover it. When they do, the discovery will be a triumph of technology and human understanding. Less certain, but also distinctly likely, are discoveries of dark matter, extra dimensions of space, “superpartners” for all the familiar particles of matter, parallel universes — and completely unexpected phenomena.

Like the discovery of an uncharted continent, the exploration of the Terascale at the LHC will transform forever the geography of the universe. But there will be limits to the LHC's view. A true grasp of Terascale physics will require a source of comprehensive and nuanced information of a different kind. Along with the LHC, physicists propose a second particle accelerator for Terascale discoveries, one that would use different particles — electrons instead of the LHC's protons — and different technology. With LHC discoveries pointing the way, this linear collider would provide the missing perspective on the Terascale and write the guidebook's absent chapters.

If, for example, the LHC experiments were to spot a Higgs particle, or something that looks like a Higgs, a linear collider would move in for a close-up. Is it in fact the Higgs? Is it all alone, or does it have relatives? How does it interact with the particles around it?

The LHC experiments may well identify candidates for dark matter, the unseen mystery substance that outweighs visible matter in the universe five to one. A dark matter sighting by the LHC would be an extraordinary discovery; and again, a linear collider could discover the information physicists need: Is it really dark matter? Does it have all the characteristics that dark matter must have? Does it make up all of dark matter, or only a fraction? If LHC experiments see evidence for supersymmetry, extra dimensions or parallel universes, a linear collider would have the ability to discover their true nature.

With the perspective from the LHC experiments, a linear collider could range across the physics of the Terascale. It would also offer another unique capability. Using coordinates from LHC discoveries, it could detect the quantum effects of phenomena occurring at energies far beyond the Terascale, acting as a telescope from the Terascale to the energy regions Einstein dreamed of, where all of nature's different forces may become one single force.

A linear collider's design would allow it to function as an all-terrain explorer of the Terascale, adaptable to investigate in depth what the LHC discovers. The more information the LHC uncovers about the Terascale, the more discoveries a linear collider would make.

The definitive map of the Terascale must await the results of experiments at these next-generation accelerators — the LHC a soon-to-be reality, the linear collider now at the stage of a proposal. Discovering the Quantum Universe gives a best estimate of the questions the experiments will answer about this new scientific territory, following three themes: Mysteries of the Terascale; Light on Dark Matter; and Einstein's Telescope.