Next-Generation Accelerators Could Hold Key to Dark Matter, Energy

At the dawn of the 21^st century, physicists have attained a thorough knowledge of the particles and forces that characterize ordinary matter. Meanwhile, astrophysical and cosmological observations have revealed that our picture of the universe is incomplete: 95% of the cosmos is made not of ordinary matter, but of dark matter and dark energy.

In order to answer these fundamental questions, astrophysical observations of the relics of the Big Bang must be compared with data from physics experiments, and for that, we need to plan a new linear collider to operate concurrently with the Large Hadron Collider (LHC) currently under construction at CERN, according to speakers at a special session on the next generation of particle accelerators at the APS April meeting in Denver, Colorado.

"Cosmology and particle physics are joined at the hip," said Michael Turner (University of Chicago), currently on leave as assistant director for mathematical and physical sciences at NSF. He emphasized that the two fields share many of the same pressing questions. He believes that particle accelerators complement telescopes, the primary means of investigation in cosmology and astrophysics. The latter essentially use the universe itself as a laboratory and let Nature set up the experiments, since the enormous dynamical range of cosmological conditions can't be created on Earth. But accelerators provide controlled, repeatable conditions.

Turner outlined some of the knowledge gained from experiments conducted at accelerator facilities. Most notably, in the 1970s, quarks were found to be the basic building blocks of Nature. However, "There are still many questions that remain to be answered, and those answers will all require particle accelerators," he said.

Those questions include determining the exact nature of dark matter, hopefully by directly producing that particular brand of particle—an achievement that many scientists rank on a par with Copernicus' recognition in the 16^th century that Earth was not the center of the solar system. String theory predicts seven undiscovered dimensions of space that may give rise to much of the apparent complexity of particle physics. Discovering those extra dimensions would change our understanding of the birth and evolution of the universe. And string theory could even reshape our concept of gravity. There is also the question of dark energy, most notably figuring out "why nothing weighs so little," said Turner.

According to Gerald Dugan of Cornell University, the particle physics community has reached a consensus that the next linear collider should be an electron-positron linear collider, with an initial center of mass energy of 500 GeV. This can be later upgraded to 1000 GeV, and ideally operated concurrently with the LHC. The International Linear Collider Steering Committee (ILCSC) was established to make a recommendation on whether the rf accelerating system should make use of superconductivity or should operate at room temperature. Each of these has its own benefits and drawbacks. A decision will be made by the end of 2004.

ILSCS will then establish a conceptual design, followed by a detailed engineering design. The new machine will cost significantly more than the LHC—most of that cost was for the magnets, with little need to build a surrounding infrastructure, since the CERN facility already existed. The current estimated cost is about $6 billion, and roughly 10% of that will need to be spent upfront just to determine the feasibility of the project. The more advanced design studies will enable scientists to nail down a more exact price range.

Dugan admitted that ITER, for instance, is experiencing a political site selection problem. The future linear collider will face similar challenges, and hopefully the scientific community can learn from the ITER experience. For one thing, the focus is on making it a truly international effort from the very beginning. In fact, international collaboration is almost a necessity because of the high cost of building the new facility. "These machines are very big and very expensive, and one nation alone cannot bear the cost," said Turner. "The next accelerator will be financed by the world, and the case for it must be extremely well made in order to justify the expenditure."

The first step is convincing other physicists. Unity is essential. And ultimately, chemists, biologists, Congress and the general public will also have to be won over. The particle physics community can only accomplish this by effectively communicating to all of those groups the importance of the scientific questions to be answered by the new collider, Turner said.