To decay, in particle physics lingo, is to morph from one particle into another. “In decay” might well describe the state of experimental particle physics in the United States, if the country doesn’t make a strong push in coming years to host the world’s next big particle smasher. That, anyway, is the view of the multidisciplinary Committee on Elementary Particle Physics in the 21st Century, in a recent report issued by the U.S. National Research Council (NRC).

Particle accelerators whip up protons, lighter particles, nuclei, or ions to near light speed, then let them crash into each other or into a target to release their enormous energy as a sunburst of short-lived, highly unstable particles. The bigger the machine and the more energetic the collisions, the more exotic the particles dislodged.

Since 1983 the United States has had the accelerator with the highest beam energy, the Tevatron, at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. That torch passes to Europe next year when CERN’s Large Hadron Collider (LHC), near Geneva, switches on, beginning perhaps a 15-year reign as top collider. If all goes well, the LHC will discover the long-sought Higgs boson, the presumed source of matter’s mass.

Even more injurious to the U.S. physicist’s pride than seeing the Europeans put the capstone onto the spectacularly successful Standard Model of particle physics is the prospect that by 2010 the country will have no major accelerator-based experiments. The two leading machines, the Tevatron and the PEP-II, at the Stanford Linear Accelerator Center, and the Cornell Electron Storage Ring (CESR) are all set to expire in the next few years. The sole survivors may be the Relativistic Heavy Ion Collider (RHIC), at Brookhaven National Laboratory, in Upton, N.Y.; the Continuous Electron Beam Accelerator, in Virginia; and a pair of neutrino experiments at Fermilab, MINOS and MiniBooNE. Fermilab’s BTeV, an experiment that would have extended the Tevatron’s life, got the axe last year, as did a proposed particle physics experiment at Brookhaven called RSVP.

“It’s a critical time,” says Pier Oddone, director of Fermilab.

U.S. particle physics never recovered from the cancellation in 1993 of the Superconducting Super Collider (SSC), a proton–antiproton collider that was to have been even bigger than the LHC. Since then, the field’s center of gravity has started to shift. Japan began studying antimatter with its KEKB machine and plans to continue a strong neutrino physics program at its Tsukuba J-PARC facility, now under construction. Europe will soon enter the neutrino game with OPERA, involving CERN and Italy’s Gran Sasso lab.

Where the mantle of particle-smashingest country will go after the LHC completes its work depends on who hosts construction of the multibillion-dollar International Linear Collider (ILC), an electron–positron machine meant to be the next really big thing but still in preliminary design. The NRC report urges the government to spend up to US $500 million on linear collider R&D to lure the project to the United States.

The results obtained from the biggest colliders are increasingly arcane and ever harder to explain to the general public and their elected representatives. But the results aren’t trivial. A decade ago, Fermilab’s Tevatron identified the last of the quarks, the constituents of neutrons and protons. Earlier this year, Brookhaven scientists, simulating big bang conditions in the RHIC, reported that the so-called quark–gluon plasma they were investigating actually behaves much more like a liquid. Yet Brookhaven, suffering funding cuts from the federal government, was able to continue those experiments only when a wealthy local donor coughed up $13 million to support them.

PHOTOS: CLOCKWISE FROM TOP: REIDER HAHN/FERMILAB; CERN; KEK