Since 1982, particle physicists have sojourned every so often in the fresh mountain air of Snowmass, Colorado, to dream up successive generations of particle accelerators.
Budgetary restrictions mean that this year’s Snowmass Community Summer Study — the first since 2001 — will be a bit less ambitious. The grass-roots planning exercise, which begins on 29 July, will last one week rather than three. And physicists will be trading the posh mountain resort for a humbler spot: Minneapolis, Minnesota.
The idea capturing physicists’ attention in the run-up to the meeting is correspondingly modest and thrifty: a low-energy but high-intensity accelerator called a cyclotron. The technology, first used more than 80 years ago, eventually ceded ground to the snazzier synchrotrons that today power facilities such as the Large Hadron Collider (LHC) at Europe’s particle-physics laboratory, CERN, near Geneva, Switzerland.
But, as physicists look for cheaper ways to test fundamental questions, cyclotrons could experience a renaissance.
“We need something totally out of the box,” says Janet Conrad, a particle physicist at the Massachusetts Institute of Technology in Cambridge, and co-spokesperson for the DAEδALUS collaboration, a proposal to generate beams of subatomic neutrinos using linked cyclotrons.
After the US Department of Energy (DOE) in 2011 closed the highest-energy US particle collider, the Tevatron, at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, US particle physicists focused on creating particle beams that were high in intensity, rather than energy, so that they could generate large amounts of data per second. That way, even if Fermilab would no longer compete with the LHC in terms of energy, it could repurpose the Tevatron’s luminous proton beams and establish an ‘intensity frontier’.
Fermilab quickly developed plans for a flagship intensity experiment, called the Long Baseline Neutrino Experiment (LBNE). It would send beams of neutrinos and antineutrinos along a 1,300-kilometer underground path to a detector in the Homestake mine near Lead, South Dakota. Neutrinos and antineutrinos are thought to behave the same in nearly every way, but the LBNE would look for crucial differences. It would monitor the way in which the three different types of neutrino morph, or oscillate, into one another as they travel along the beam, and then repeat the experiment, this time with a beam of antineutrinos. Any detected difference would indicate a fundamental asymmetry — helping to explain why the Universe contains so much more matter than antimatter.
In 2012, the DOE asked Fermilab to strip down these ambitious plans (see Nature 485, 16; 2012). Fermilab came back with a design that would cost about $800 million in its first phase. But many physicists are concerned that the scaled-back experiment will be less precise.
DAEδALUS is now emerging as a lower-cost alternative to test for matter–antimatter asymmetry. In this plan, near-stationary protons would be dumped in the center of a small cyclotron and accelerated by magnetic fields in spirals until they reached the cyclotron’s outer edge. These souped-up protons would then be injected into a second, larger cyclotron, 15 meters across, which would accelerate them further still. The resulting proton beam would then be fired at a carbon and copper target to generate particles called pions, some of which decay into antineutrinos.