Next-generation accelerator to go 'cold'

This event image was taken at the Stanford Linear Accelerator Center (SLAC; Menlo Park, CA) using the 307-megapixel CCD in the SLAC Large Detector (SLD) vertex detector.

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This event image was taken at the Stanford Linear Accelerator Center (SLAC; Menlo Park, CA) using the 307-megapixel CCD in the SLAC Large Detector (SLD) vertex detector.1 The view was along the beam line of the SLAC Linear Collider (SLC), and shows a Z-particle decay into a gluon jet (red) and two jets containing a beauty and an anti-beauty quark (blue, yellow). The heavy-quark jets show detached vertices ­relative to the Z decay vertex. Experimental experience and research results obtained since the SLD was installed in the SLC in 1991 will play a major role in the development of a next-generation linear collider (LC) projected for completion in 2014, even though an international commission decided in August to use a competing tech­nology in the next-generation LC.

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The two-mile-long linear accelerator in the SLC produces 50-GeV beams of positrons and electrons; two curving magnetic arcs transport the separate positron and electron beams from the end of the linear accelerator to a single collision point inside the SLD, which stands six stories high and detects polarized Z particles produced in the collisions, with the ultimate aim of measuring crucial parameters related to particle physics.

The success of the SLC, completed in 1989, has helped to validate the concept of a next-generation LC, in which beams from two head-to-head linear accelerators would achieve even higher energy electron-positron collisions. The success of the SLD, installed in the SLC in 1991 and eventually growing to more than 300 million channels, has helped to validate the concept of pixellated (CCD) silicon detectors arranged concentrically around the inner wall of the vertex drift chamber of the accelerator to detect charged particle emissions.

Improvements that will be required in detector technology for a 20-mile-long next-generation LC producing 0.5-TeV beams initially (and eventually climbing to 1.5 TeV) was a frequent topic of discussion at the 5th International Symposium on Development and Application of Semiconductor Tracking Detectors held in Hiroshima, Japan, in June. While the fairly long readout time of CCDs due to the need to shift charge across rows and columns of pixels for sequential readout was adequate for the repetition rates of the SLC, next-generation LC CCD detectors would have to significantly boost readout speeds and radiation hardness over currently available technology.

Alternative approaches include hybrid pixel detectors based on reduced-area high-resistivity silicon strips that allow separate optimization of detector and readout chip technologies; CMOS-based monolithic active-pixel sensors with a potential to greatly improve impact resolution as well as charge-collection efficiency; and a depleted field-effect transistor, originally developed for x-ray applications, that combines detector and amplification properties.1, 2

The recommendation last August by a panel of 12 physicists to build the next-generation LC using superconducting (“cold”) technology, proposed by the Deutsches Elektronen-Synchrotron (Hamburg, Germany), rather than conventional room-temperature (“warm”) technology proposed by SLAC and its partner KEK (Ibaraki, Japan), is also intended to facilitate technology development by focusing international resources on one solution. But the decision came as a bit of a blow to researchers at SLAC

“We are certainly disappointed that our warm technology was not selected,” said Jonathan Dorfan, director of SLAC and chair of the International Committee for Future Accelerators. “However, the high-energy-physics worldwide community has taken a huge and necessary step forward by making this selection and has crossed a critical threshold in the realization of the dream that SLAC helped initiate-building a frontier energy linear collider.”3


1. H. Sardrozinski, presentation at IEEE2000 NSS-MIC.
2. H. Park, presentation at 5th Intl. Symposium on Development and Application of Semiconductor Tracking Detectors, Hiroshima, Japan (June 14-17, 2004).
3. M. Schwartz, Stanford Report (Sept. 1, 2004).

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