Fermilab Associate Director for Accelerators Steve Holmes posed the big questions immediately about accelerator R&D for the proposed International Linear Collider. "Is this thing going to work?" Holmes said, introducing his talk on Monday to the EPP 2010 panel. "And what is it going to take to get us there?"
The R&D program includes significant US presence with all major subsystems of the machine, Holmes said, which is needed to mount a credible bid to host the ILC. The Fermilab ILC effort focuses on leadership in superconducting radiofrequency (SC RF) technology, establishing lab-based facilities for fabricating and testing as part of a US-based capability in high-gradient technology. The Fermilab goal, Holmes said, "is to provide the US with the best possible host site for the ILC."
Holmes told the panel that, driven by the physics goals, the luminosity goals for the ILC are very aggressive and offer significant risk, but the goal of the accelerator R&D program is to reduce that risk. He added that the energy of the beam is not the greatest risk to ILC.
The difficulty, Holmes explained in tracing the path of the beam, is that while luminosity is directly proportional to beam power, it is inversely proportional to the vertical size of the beam. The beam densities produced at the injection system must be preserved in the Linac, then maintained in the collisions at the Interaction Region. Holmes described a "huge effort to get the vertical beam size down."
The 500 GeV machine will need 17,000 accelerating cavities with a gradient performance goal of 35 MeV/meter. Holmes said close to 100 high-gradient, superconducting cavities had been produced in Europe and tested by the TESLA group at DESY in Germany. He said five 8-cavity modules used at TESLA had a gradient of 23 MeV/meter, while six 9-cell cavities tested at 35 MeV/meter. In addition, some single-cell cavities displayed gradients beyond 40 MeV/meter.
Holmes said the Proton Driver under consideration for Fermilab offered synergies with the ILC through its use of superconducting RF. Lab R&D on the Proton Driver could proceed for about 18 months independently of any ILC decisions, Holmes said, but a series of "branch points" would begin around that time. He concluded that "The answer is yes-the linear collider could be constructed and would if based on a successful R&D aimed at the identified risks."
The Proton Driver was a central focus in proposals for the lab's future accelerator neutrino program presented by Gary Feldman of Harvard University, a MINOS experimenter. Feldman said the proton driver would increase proton intensity by a factor of four for the neutrino beam, and could provide the basis for a possible future neutrino factory. As Fermilab Director Michael Witherell said in his talk on the national program, continued US leadership in neutrino physics would require building the Proton Driver. As Feldman explained, "the Proton Driver provides a step-by-step approach to measuring all of the neutrino's oscillation parameters. At each step, we gain the information necessary to efficiently plan the next step."
A major component of Fermilab's future, the accelerator-based neutrino program counts heavily on the proposed NOVA experiment, with an off-axis detector for the NuMI/MINOS beam. The result of the off-axis location is a narrow-band beam with less background and more neutrino flux at the oscillation maximum. NOVA would use a liquid scintillator detector 810 km from the lab, at Ash River, Minnesota-the farthest point north within the US on the NuMI/MINOS beam line. NOVA offers an order of magnitude greater sensitivity than MINOS for the muon neutrino to electron neutrino oscillation.
Synergies of particle astrophysics with particle physics have included early limits on neutrino properties, said Rocky Kolb, director of Fermilab's Particle Astrophysics Center. And future contributions could involve supersymmetry and dark matter, strings and dark energy, the Higgs and extra dimensions. The Center encompasses the Sloan Digital Sky Survey, the Pierre Auger Cosmic Ray Observatory, and the Cryogenic Dark Matter Search; new initiatives are the Dark Energy Survey and the Joint Dark Energy Mission, collaborations of DOE and NASA. Kolb said some of the challenges of mounting a particle astrophysics program include the lack of a "roadmap" procedure for planning future projects.
Particle astrophysics is "strongly coupled to what we do," said Fermilab Director-designate Pier Oddone, responding to a question. "It's a small but vital effort at the lab, and I would like to see it grow. But the growth would have to be in competition with other labs for pots of national funding. We would hope to generate proposals attractive enough to bring funding here." Oddone concluded: "The 'A' in 'FNAL' still stands for 'accelerator.'"