The Johns Hopkins Gazette: March 19, 2001
March 19, 2001
VOL. 30, NO. 26


Physicists Prep Proposal for Particle Collider

Scientists hoping U.S. will commit to Electron-Positron Linear Collider

By Michael Purdy

Johns Hopkins Gazette Online Edition

High-energy physicists from across the United States will gather at Johns Hopkins from March 19 to 21 to consider and refine a proposal for a major new U.S. particle collider.

Researchers coming to the meeting are advocates of a proposal known as the Electron-Positron Linear Collider. Many believe such a collider could help confirm a theory known as supersymmetry, resolving many of the unanswered questions in their present understanding of elementary particles and the forces that act on them.

American scientists also feel an American commitment to build the proposed collider could help maintain the United States' leadership role in particle physics, a status many think the United States began to relinquish when it pulled the plug on the Superconducting Supercollider in the early 1990s.

"That decision was a real blow to the high-energy physics community, and ended up forcing many of us to go for extended periods of time to collider facilities in Europe," said Morris Swartz, professor of physics in the Krieger School of Arts and Sciences and the organizer of the meeting.

The gathering at Hopkins, anticipated to draw about 100 researchers, will be the third meeting of advocates of the linear collider in the past three years. At the meetings, researchers have been fine-tuning design proposals, developing a better sense of what such a collider might reveal and working to avoid some of the traps the Superconducting Supercollider fell into a decade ago.

They are building the case for a linear collider in anticipation of a larger meeting of American and international physicists this summer in Snowmass, Colo. At that meeting, sponsored by the American Physical Society, advocates of a variety of collider proposals will present their best arguments for their proposals to the larger community of physicists.

Results from that discussion will be presented to a new High Energy Physics Advisory Subpanel jointly sponsored by the Department of Energy and the National Science Foundation.

"We're going through a cyclic period of reassessment of our goals and long-term plans in high-energy physics," said Jonathan Bagger, a Hopkins professor of physics and mathematics and co-chair of the DOE/NSF subpanel. "For the past few years, the community's been very involved in projects at Stanford and at Fermilab in Illinois, and in making sure the Large Hadron Collider or LHC, a new particle collider at the CERN research facility in Europe, goes through.

"An upgraded Fermilab Tevatron goes online this month, the facility at Stanford is online, and the LHC seems assured, so now the community's looking ahead to see what else is needed," Bagger said.

Bagger noted that the Fermilab Tevatron is the highest energy particle collider in the world for now, but it will be surpassed in 2005 when the LHC starts producing data.

"The U.S. won't be at the energy frontier then, and we have to ask ourselves, What does that mean? Do we want to host another facility at the energy frontier, or is our country content with sending our physicists abroad to use these facilities elsewhere?" Bagger said.

The High Energy Physics Advisory Subpanel will present to funding agencies advice and suggestions on those questions and other issues, "painting a vision of particle physics over the next 20 years," according to Bagger.

Researchers who favor the linear collider proposal are hoping to earn the approval of the subpanel and the physics community at large by working to keep the costs of their proposal down, staying away from problems that sank the Superconducting Supercollider, and showing that the linear collider can complement the abilities of the LHC.

"For example, we're probably going to propose that the site of the new collider be assigned, not selected through a competition like the Superconducting Supercollider, and to save on costs we will suggest that it be constructed as part of an existing national lab rather than as a facility of its own," Swartz said.

The design of the linear collider also could reduce costs. Colliders are typically built in a ring-shaped structure, with particles orbiting around the ring at high speeds. Scientists can divert them from their orbits to create collisions at several places in the ring.

"These types of colliders have produced a very rich range of data that make up about 90 percent of the 'bible' of particle physics," Swartz said. "But to increase the energies available to you in such colliders, you have to increase the storage ring size quadratically. In other words, getting five times the energy means you have to make the ring 25 times larger. And cost scales more or less the same way."

The fundamental problem, Swartz explained, is that electrons radiate away energy when they travel in a curve.

"To avoid this, the linear electron-positron collider would use state-of-the-art technology and techniques to shoot the particles in straight lines," Swartz said. "In this type of particle collider, the cost for increased energy only goes up linearly, or five times the energy for about five times the cost."

Increasing energy is the key for high-energy physicists because breaking matter down into its most fundamental constituents means slamming particles together harder and harder.

By doing this, physicists have been able to observe and characterize three principal families of fundamental particles that convey force and make up matter: leptons, fermions and bosons. This "standard model" works very well at conventional energies, Swartz noted, but starts to break down at higher energies.

Physicists see a potential way out of the problems: A theory called supersymmetry may solve the difficulties with "partner" particles that correspond to types of particles already confirmed to exist. Scientists are hopeful that the LHC will provide a first glimpse of such particles, but they're going to have to look carefully at the enormous amounts of data that will be produced by LHC.

An electron-positron collider, Swartz said, could complement the abilities of the LHC by revealing more detailed information on the many subtle properties of supersymmetric particles. He said similar proposals for new linear colliders are receiving serious consideration in Europe and Asia.

"It doesn't have to be an either/or question as we consider the future of particle physics," Bagger noted. "Particle physics is highly collaborative, and we can't say what the future will be like without talking to many people. But facilities like these do take a long time to build, so if we want to have something up and running not too long after the LHC comes online, we have to start identifying our priorities."