Overview: Discovering the Quantum Universe
Mysteries of the Terascale
Science Magazine's 125 Questions. Question #1: "What is the Universe made of?"
Related Questions:
- Are there undiscovered principles of nature: new symmetries, new physical laws?
- How can we solve the mystery of dark energy?
- Are there extra dimensions of space?
- What happened to the antimatter?
Later in this decade, experiments at the Large Hadron Collider at CERN will break through to the Terascale, a region of energy at the limit of today's particle accelerators where physicists believe they will find answers to questions at the heart of modern particle physics.
The LHC will expose the Terascale to direct experimental investigation. Present-day experiments suggest that it harbors an entirely new form of matter, the Higgs boson, that gives particles their mass. Beyond that, physicists believe that the Terascale may also hold evidence for such exotic phenomena as dark matter, extra dimensions of space, and an entire new roster of elementary superparticles.
The first target is the Higgs. Over the past few decades, theoretical breakthroughs and precision experiments have led to the construction of the standard model of particle physics, which predicts that an omnipresent energy field permeates the cosmos, touching everything in it. Like an invisible quantum liquid, it fills the vacuum of space, slowing motion and giving mass to matter. Without this Higgs field, all matter would crumble; atoms would fly apart at the speed of light.
So far, no one has ever seen the Higgs field. To detect it, particle accelerators must first create Higgs particles and then measure their properties. The LHC is designed with enough energy to create Higgs particles and launch the process of discovery.
To determine how the Higgs really works, though, experimenters must precisely measure the properties of Higgs particles without invoking theoretical assumptions. Such precise and model-independent experiments are a hallmark of linear collider physics, not available in the complex experimental environment of the LHC. A linear collider could determine if the Higgs discovered at the LHC is the one-and-only Higgs. Does it have precisely the right properties to give mass to the elementary particles? Or does it contain admixtures of other new particles, heralding further discoveries? A linear collider would be able to make clean and precise determinations of critical Higgs properties at the percent level of accuracy.
A Higgs discovery, however, will raise a perplexing new question: According to our present understanding, the Higgs particle itself should have a mass a trillion times beyond the Terascale. Although the Higgs gives mass to Terascale particles, its own mass should be much, much greater. Why does the Higgs have a mass at the Terascale?
For years, theorists have tried to explain this mystery, devising multiple possibilities including supersymmetry, extra dimensions and new particle interactions. Which, if any, of the theories is correct? Sorting that out is a task for the LHC and a linear collider. The LHC will have enough energy to survey the Terascale landscape. Then a linear collider could zoom in to distinguish one theory from another.
Theories of supersymmetry and extra dimensions, for example, predict new particles that are close relatives of the Higgs. Some of these particles would be difficult to detect or identify at the LHC, and difficult to distinguish from the Higgs itself. Linear collider experiments would have unique capabilities to allow physicists to identify these particles and pinpoint how they are related to ordinary matter.
The Terascale may hold the answers to still more of particle physics' most basic questions. The dominance of matter over antimatter in the universe remains a mystery, but part of the answer could lie in undiscovered interactions that treat matter and antimatter slightly differently — that is, in undiscovered sources of the matter-antimatter asymmetry physicists call CP violation. At the LHC, it will be difficult to extract CP information about Terascale physics. Experiments at a linear collider, however, could detect and measure new sources of matter-antimatter asymmetry.
Mapping the Terascale will take physicists far into new scientific territory, as complex theoretical frameworks come face to face with experimental data. From this clash of theory with data will arise a profoundly changed picture of the quantum universe. |
