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February 18, 2005
The International Linear Collider
Part 1: A Precision Probe for the New Physics

Beginning a series on the role of Berkeley Lab researchers in planning for the proposed International Linear Collider

Berkeley Lab physicists are among groups from dozens of universities and agencies in Europe, North America, and Asia who are collaborating to design and eventually build the International Linear Collider (ILC), an extraordinary new machine — stretching at least 30 kilometers, its exact location yet to be decided — designed to explore the fundamental particles and forces of nature.

A Japanese design for a powerful electron-positron linear collider (Image courtesy KEK)

The Lab's Accelerator and Fusion Research Division and Physics Division are concentrating their efforts on two aspects of the proposed machine, the damping rings that will be essential to preparing and injecting bunches of electrons and their antiparticles, positrons, with sufficient energy and luminosity, and the detectors that will be needed to measure and study a range of high-energy events, some never yet observed in particle accelerators.

No matter where they call home, the scientists who are laboring to build the ILC make it a point to work together even as they push competing ideas. A possible model for the organization of the international effort is the 20-member-nation CERN, whose official name is the European Organization for Nuclear Research — but whose acronym is more commonly applied to the organization's laboratory for particle physics near Geneva, an accelerator 27 kilometers in diameter that lies in both Switzerland and France. Ever since it was conceived in the late 1940s, CERN has been a paradigm of international cooperation in big science. Indeed, the principal spur to timely construction of an International Linear Collider is CERN's next big accelerator, the Large Hadron Collider (LHC), now under construction and scheduled to come on-line in 2007.

Hadrons versus leptons

Hadrons are a class of particles made of quarks, held together by aptly named gluons; the most common hadrons are protons and neutrons. Protons are favorites for particle accelerators because they are massive, electrically charged, and therefore relatively easy to accelerate to high energies in large numbers.

But the real interactions in a powerful hadron collider are between quarks, the constituents of the hadrons, and the energy of the quark-quark collisions varies from event to event. The LHC will run protons into one another some 800 million times a second, with energies of 7 trillion electron volts (7 TeV) in each opposing beam; quark collisions will be so many and so complex that sophisticated techniques will be needed to filter the interesting events out of noisy chaos.

One result of this is that while the LHC agenda is rich and wide-ranging, says Berkeley Lab's Ian Hinchliffe, a theoretical physicist participating in the LHC's programs, "there are some important measurements the LHC won't be able to do with sufficient accuracy, and some it can't do at all. To complete the picture, we'll need a linear collider's accuracy and complementary measurements."

Each collision between hadrons (left) is actually a group of quark collisions with uncertain energies. Leptons are pointlike particles (right) that collide with energy that can be known precisely. (Images courtesy CERN, DESY)

Unlike hadrons, leptons — including electrons and positrons — are light, simple, and pointlike: the energy of each event can be known exactly. The challenge for the International Linear Collider is to accelerate electrons and positrons to sufficiently high energies — up to 0.5 TeV per beam — and still get enough collisions. Particles of like charge repel one another, so it's difficult to produce tight bunches of them if they have low mass. Once a sufficiently high rate of events per unit cross-section of the beam has been achieved, however, the ratio of important events to noisy background events will also be high. A mere 10,000 events of interest per year is the goal, few enough to easily store all the data, making it possible to study every event in minute detail.

The Future of Particle Physics

Starting in the 1960s from the suggestion that there are particles more basic than protons or neutrons, namely quarks, the remarkably successful Standard Model of particles and interactions has been elaborated for over 40 years. The LHC and the ILC are both designed to complete the map of Standard Model physics and to discover and explore the new physics that lies beyond its borders.

By the end of the 20th century all the particles predicted by the Standard Model had been found except one, the particle whose interactions with all the others determine their masses: the Higgs boson. Many searches for the Higgs at existing accelerators have placed limits on the Higgs's own mass, but the Higgs itself has remained elusive.

"Understanding the origin of mass will be a major intellectual and cultural achievement," says Marco Battaglia of the Physics Division, also a professor of physics at UC Berkeley, who is in charge of ILC physics studies and the research and development program within Berkeley Lab's Physics and Engineering Divisions. "But even beyond the discovery of the Higgs, there's much more to future physics."

For example, evidence from the Sudbury Neutrino Observatory in Canada and the Super-Kamiokande collaboration based in Japan indicates that neutrinos have mass and oscillate among "flavors" — electron neutrinos change into muon neutrinos, which change into tau neutrinos, which change back to electron neutrinos, and so on. Neutrino mass is a concept that requires extending the Standard Model.

There are other questions the Standard Model is unequipped to answer, notably why it has not been possible to give a unified explanation of gravity and the much stronger electromagnetic, weak, and strong nuclear forces. Nor can the Standard Model explain dark matter, thought to account for a quarter of the density of the universe. As for the even more mysterious dark energy that fills the universe and accounts for 70 percent of its density, one possibility may be a link to the Higgs boson. At one time accelerators were thought incapable of contributing to astrophysical questions like these. No longer. Together, the LHC and the ILC will open new realms of yet-to-be-explored possibilities.

The most popular candidates to extend the Standard Model are varieties of supersymmetry (SUSY), theories that assign every standard particle a massive sibling that follows a different set of statistics. All particles obey one of two different kinds of statistics: Fermi-Dirac statistics forbid any two fermions in a system — electrons orbiting an atom, say — from occupying the same quantum state, while bosons, which obey Bose-Einstein statistics, happily pile together into the same quantum state, like the photons in a laser beam.

It's convenient (if too simple) to think of fermions as building blocks of matter and bosons as carriers of the forces. Leptons, including electrons, are fermions. In fermions, the quantum number called spin is measured in half units (in the case of the electron, spin 1/2); the supersymmetric twins of leptons are called "sleptons" and are bosons with spin 0. Similarly, spin-1/2 quarks are twinned with spin-0 "squarks," while the spinless, massless, strong-force carriers known as gluons, which are bosons, have supersymmetric twins dubbed "gluinos" with spin 1/2.

Arbitrary as it sounds, the SUSY scheme goes a long way toward patching the holes in the Standard Model. All these extra particles ease the unification of the three strongest forces (and may possibly, with much effort, be extended to include gravity); the lightest SUSY particles may be good candidates for the dark matter; SUSY models may even be able to explain neutrino mass.

If SUSY models reflect reality, there won't be just one Higgs boson but several. The LHC will certainly detect at least one, but it will take a sufficiently energetic electron-positron collider to distinguish and measure them all.

In this simulation, a collision between an energetic electron and positron creates a Higgs boson and a Z boson. (Image Norman Graf)

Battaglia remarks that "the Higgs would be observable independent of its mode of decay. Its couplings to fermions and gauge bosons could be measured to very good accuracy. Even the coupling of the Higgs boson to itself could be determined, which would provide a crucial test of the Higgs mechanism" for imparting mass to all particles.

The cold frontier

There are many other aspects of the new physics the ILC will be able to explore with great precision. Hinchliffe and Battaglia point out that the Standard Model's long string of successes was made possible only because both hadron machines like Fermilab's Tevatron and lepton machines like SLAC's linear collider and CERN's Large Electron Positron collider were available to find all the Standard Model's pieces and fit them into the mosaic.

The same synergy will be needed for the new physics. Says Hinchliffe, "Together, the LHC and the International Linear Collider could validate the Higgs mechanism and explain dark matter. The LHC alone will leave many questions unanswered."

Since a powerful electron-positron linear collider was first widely discussed over a decade ago, the concept itself has been accelerating toward realization. Of the major competing technologies, one was based on very high frequency accelerating cavities operating at relatively normal temperatures (the "warm" technology) and the other involved much lower frequencies made possible by superconducting accelerating cavities (the "cold" technology). In Beijing last August the technology panel of the International Committee for Future Accelerators recommended concentrating on the cold approach.

Soon a Central Design Group will be established to develop a detailed proposal and budget for the machine and its detectors. Not long after that, the international community will have to bring itself to address the most sensitive question of all: where the ILC will be located.

Upcoming editions of Science@Berkeley Lab will examine Berkeley Lab research into what the International Linear Collider needs to succeed, including a positron damping ring to reach high luminosity and a detector that can capture and identify the most interesting events.

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