By Eric Hand

Cosmic-ray experiments would suffer, unless budgets increased, under a list of priorities for high-energy physics given to the US Department of Energy (DOE) by independent advisers last week.

The DOE should instead fund one massive dark-matter detector, one major dark-energy experiment and a high-energy gamma-ray detector, according to the report, which was presented on 23 October in Washington DC.

For current budget trends, the report did not endorse Auger North, a US$127-million array of 4,400 cosmic-ray detectors in southeastern Colorado. This is due partly to the success of its southern counterpart, the Pierre Auger Observatory in Argentina, which is close to tracing the origin of high-energy cosmic rays to massive black holes in galactic centres, says Steve Ritz, chair of the 15-person committee that wrote the report. "You no longer need to invoke new physics to explain [high-energy cosmic rays]," says Ritz, an astrophysicist at the University of California, Santa Cruz.

The report is the first in which US high-energy physicists have set priorities for particle astrophysics. The committee considered four budget scenarios, ranging from the relatively grim DOE budget for the 2008 fiscal year in years going forwards, to budgets that would more than double over the next decade. The DOE takes the advice "very seriously", says Dennis Kovar, the agency's associate director of science for high-energy physics.

Even in the tightest scenario, dark-matter experiments fared well -- both because they are relatively cheap and because there are hints that discovery of these undetected particles, which make up nearly a quarter of the Universe's mass-energy, could be imminent. If budget trends continue, however, the report says that only one next-generation detector can be chosen from among competing groups, which propose ten-tonne vats of liquid argon or xenon, or cryogenic silicon detectors set deep underground.

Dark energy -- the mysterious vacuum force that makes up most of the rest of the Universe's mass-energy -- remains a priority, although the experiments are expensive. Unless budgets increase, the report suggests, the DOE will have to choose between supporting a space-based project, the Joint Dark Energy Mission, or a major ground-based telescope: either the 8.4-metre Large Synoptic Survey Telescope (LSST) or BigBOSS, a 4-metre telescope.

"We don't have the luxury of a lot of redundancy," says Patricia Burchat, a physicist at Stanford University in California and a member of the LSST project. "We can't afford it." Ritz says that so far, the agency hasn't looked at the trade-offs between ground- and space-based approaches to dark energy.

For high-energy gamma-ray astrophysics, the report supports the Advanced Gamma-ray Imaging System, an array of about 50 telescopes spread over a square kilometre -- but only if the consortium merges with a similar European proposal, the Cherenkov Telescope Array