For nearly three decades, the United States has hosted the world's most powerful particle collider – a critical tool scientists use to probe the nature of matter and the origins of the universe.

This week, the director of the Fermi National Accelerator Laboratory, which operates the machine, announced that the lab would pull the plug on the device, known as the Tevatron, at the end of the current fiscal year.

The announcement marks the second high-profile US science and technology program to undergo significant transition in 2011. Word of the Tevatron's retirement comes as NASA's shuttle program works its way through its final two scheduled flights.

In each case, fiscal challenges have prompted presidential administrations to seek ways the US can remain an influential player – but with sustainable budgets. And the rising cost of ambitions in both spheres have dictated a higher degree of international participation on future projects than has been the case historically.

To some in the field, the loss of the Tevatron – with no next-generation US replacement – represents evidence of erosion eating away at America's scientific leadership. Others see it as a transition that still allows for cutting-edge physics.

The US still hosts a powerful collider at the Brookhaven National Laboratory. But it's only capable of about 10 percent of the Tevatron's collision energy, and it's designed to answer a different set of research questions.

Either way, the announcement that the Tevatron's Nobel-Prize-winning program will end has been anticipated for years, acknowledges Stuart Henderson, the lab's associate director for accelerators. But it was disappointing, he says.

In Europe, the European Organization for Nuclear Research, known by its French acronym CERN, brought its Large Hadron Collider on line in late 2009. The LHC is designed to smack protons together at energy levels seven times higher than those achieved at the Tevatron. The startup came a year late, after an initial attempt in 2008 uncovered electrical problems that required complicated repairs.

Scientists anticipate discovering new particles and evidence of new physics in the sub-atomic debris those LHC collisions will generate.

Throughout construction of the LHC in Europe, there was an understanding on this side of the Atlantic "that there would be an end to colliding beams here at Fermilab," Dr. Henderson says. In many respects, he says, the program at Fermilab has gone on longer than many originally envisioned.

But last summer, researchers at Fermilab announced that a much-sought elementary particle that the LHC also has on its Most Wanted list – a particle known as the Higgs boson, and sometimes called the "God particle" – might be within reach of collision energies the Tevatron was achieving at Fermilab. The Higgs boson is a hypothesized particle associated with a quantum field that imparts mass.

That announcement generated a good deal of excitement at the lab, Henderson says.

But to take advantage of the new information, the lab would have to keep the Tevatron running for up to three more years. In the end, the US Department of Energy was unable to find the money without threatening the health of other research programs the high-energy physics community had set as priorities.

In a letter to the chairman of the DOE's High Energy Physics Advisory Panel (HEPAP), which had looked at options for extending the Tevatron's run, William Brinkman, DOE's director of science, explained that "the current budgetary climate is very challenging and additional funding had not been identified."

Rather than pushing to build US colliders that keep the country at the forefront of the energy frontier, the advisory panel has recommended pushing what it calls the intensity frontier – especially in probing the properties of particles known as neutrinos.

The energy frontier leads to discoveries of particles hypothesized to have existed in the smallest fraction of a second after the big bang, which formed the observable universe. "It requires a lot of energy to reach them," Henderson explains.

At the intensity frontier, physicists are looking for very rare interactions and processes. "Hidden in those processes are extremely sensitive measures of our present understanding of particle physics," he says.

The stars of the intensity frontier, neutrinos, are particles which have minuscule mass and rarely interact with matter. Yet physicists say these sub-atomic no-see-ums may play important roles in the ongoing evolution of the universe and also could point to new physics beyond the standard model.

"We want to be the world leader in intensity-frontier physics," Henderson says. Indeed, Fermilab is already moving in that direction, with a strong endorsement from a HEPAP subgroup that focuses on particle physics.

Getting there could be a challenge, because the US, hoping for a future machine, the International Linear Collider, to supplant the Tevatron, in effect deferred to Japan as the leader on the high-intensity neutrino frontier, says Lawrence Sulak, a physicist at Boston University who currently is at CERN on a sabbatical. [Editor's note: The original version of this paragraph was less specific about the goal of American physicists.]

For the past decade, physicists in Japan have led an international collaboration on the T2K project. A proton accelerator near Tokai generates a beam of neutrinos, which is aimed at the Super Kamiokande neutrino detector in the Japanese Alps near Toyama.

In November, the underground detector at Super Kamiokande recorded its first neutrinos from the beam.

It would take a decade for the US to catch up, Dr. Sulak says, and he worries that the effort could get quashed "before we get there."

As if to underscore his concern, the Deep Underground Science and Engineering Laboratory, planned for a one-time gold mine near Lead, S.D., faces what proponents say they hope is a temporary budget problem.

Planners envision the lab as home to detectors that will receive the high-intensity beam of neutrinos that post-Tevatron Fermilab will generate. So far, some $300 million has been committed to the project.

But just before the Christmas holiday, a federal science panel refused to approve an additional $29 million to, among other things, ensure the mine's safety as the lab's design progresses, and to maintain infrastructure for experiments already installed in the mine.

In a statement released last month, the DUSEL team expressed its hope that the issues triggering a thumbs-down on the $29 million "can be addressed to the satisfaction of all parties to this important initiative."