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Should a photon-photon collider precede the ILC?

Should the International Linear Collider (ILC) be the next big project for high energy physics? Or should a smaller, less expensive collider be the next step? Former director of Japan's KEK laboratory and former International Committee on Future Accelerators chairperson Hirotaka Sugawara proposes that the HEP community build a photon-photon collider prior to building the ILC. When asked "Why the rush?" he replied, "Why should we wait?"

Two major factors could answer Sugawara's question. First, whether the Large Hadron Collider (LHC) at CERN successfully detects the Higgs boson in a low-energy range and, second, whether or not international support for the ILC falls short of the six billion dollar price tag.

The concept of a photon collider has been explored by physicists for about a few decades now, with possible implementations considered for a variety of accelerator facilities, but most recently for the ILC. A photon collider was being thought of as an upgrade of rather than a precursor to the ILC, capable of exploring a range of physics in complementary ways to either electron-positron or proton-proton collisions, the two avenues pursued by the ILC and LHC. Sugawara wants to reverse that order and build a photon-photon collider first.

A photon-photon collider might tempt physicists eager to study the Higgs boson. If the Higgs has a mass in the low-energy range as most theories predict, then a photon-photon collider could generate Higgs bosons directly, requiring only 160 GeV of energy. A photon-photon collider has often been referred to as a Higgs factory due to its potential to produce copious quantities of Higgs particles and study them in detail.

The lower energy requirement doesn't make the physics any easier, but it would be a less expensive alternative. Matter colliders like the LHC and the ILC can only produce a Higgs in conjunction with a Z boson, which requires an extra 100 GeV in energy. That extra energy shows up on the electric bill and can mean a difference of millions of dollars in operating costs.

Sugawara emphasizes that cost is the primary motivating factor for building a photon-photon collider first. The ILC can't materialize unless organizers can get governments and funding agencies to support it. With high-energy physics suffering budget cuts world wide, and the LHC still trying to get on its feet, it's a tough time to pitch such an idea. Sugawara predicts that the cost of a photon-photon collider would be well under half that of the ILC, making it more appealing to potential funders.

"If there is a financial threshold over which a government will not fund the ILC, and if that threshold is half the cost, then a photon-photon collider might make sense," says Tor Raubenheimer, head of the Accelerator Physics Group at SLAC. But if governments are willing to fully fund the ILC, Raubenheimer says it is unlikely that the community will hold back from building it. Sugawara supports building one either way.

Then again, Sugawara argues that a photon-photon collider could also advance accelerator technology before the ILC is built. "From the time that it is decided to build a particle accelerator to the time the physics actually starts can easily be a decade," Raubenheimer says. Right now the ILC would operate at 500 GeV, but by the time it is fully constructed scientists may well want it to operate at 1 or 2 TeV. Such an upgrade might not be fiscally or technologically possible on a fully constructed machine. As Sugawara puts it, "We would build the machine to study the machine." In other words he believes the potential collider would advance accelerator technology and understanding before a major investment is made in the ILC.

Of course, it may not come down to people at all. Results from the LHC might change everything. If the Higgs is not in the low-energy range, a photon-photon collider wouldn't be able to find it. However, Sugawara still believes the machine could be beneficial, and that plans should be made in the next 3-5 years to build one. With planning overlap he says the ILC could still come online in less than 15 years.

"My view is the minority," Sugawara says. "I think we should just build it. People think from the beginning it should have some big discovery. My view is different. I think we should build it to see what direction we should go in."

Raubenhimer says, "If the LHC comes back with a low mass Higgs and low mass supersymmetry--some really exciting results--I think that will motivate the [ILC] committee to get together and make the ILC happen." If the LHC doesn't provide such thrilling results, it may push back the ILC timeline.

Sugawara, who now lives and works in Washington, DC, says he needs a global consensus in the physics community to make a photon-photon collider happen. Slowly but steadily, through presentations all over Europe and the United States, he is trying to build that consensus. He brought his presentation to the ILC Steering Committee, in charge of promoting the construction of the ILC through international collaboration, on October 31.


Read the technical details (PDF) of one photon-photon collider design in a document prepared when the TESLA project was a contending design for the International Linear Collider.