Europe’s Large Hadron Collider tests the bounds of physics – and budgets
Scientists look for technologies to push particles faster, better, and cheaper.
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Faster particles, accelerating costs
The collision energy in a proton collider must be substantially higher than might otherwise be the case because the energy is parceled out to varying degrees among all the quarks and gluons in the mix, not just among two protons.
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Researchers at the European Organization for Nuclear Research (CERN) are laying plans to upgrade the LHC’s power in about 10 years. Still, scientists say they are fast approaching the limits for affordable proton colliders, even when an international collaboration is sharing the cost. At some point on the climb up the collision-energy ladder, the protons’ heft won’t prevent them from losing increasing amounts of travel energy as they constantly shift directions in a circular ring, rendering them less practical. And the magnetic fields needed to keep a rein on the protons would grow so high that no known, or at least affordable, material could withstand the physical stresses the magnetic fields would set up.
This has prompted many physicists looking harder at electrons, and their heavier cousins, muons, to literally get more bang for the buck. Electrons have been getting a workout for years at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, Calif., and at the LHC’s predecessor at CERN, the Large Electron-Positron Collider.
Unlike protons, electrons are fundamental particles – they have no internal components. So every hit is clean; all of the energy of the collision goes into forming the heavier particles scientists are interested in. Scientists can accomplish the same physics with fewer collisions than a proton collider requires.
The proposed International Linear Collider (ILC), for instance, would smack electrons into an onrushing beam of positrons to create copious numbers of particles called the Higgs boson. It’s a particle thought to impart mass to other particles, and the LHC stands an excellent chance of finding it. But, researchers say, that’s like discovering the shoreline of a new continent. It’s an important advance, but takes repeat visits to truly explore.
The LHC uses a specific energy level to ensure its collisions are in the predicted range for detecting Higgs, as well as other phenomenon. The ILC initially would operate with a fraction of that energy, some 500 billion electron-volts instead the 14 trillion used by the LHC.
But electrons are not the perfect projectiles either. In effect, they chafe at flitting around in circles. They have so little mass that as they speed around an accelerator ring, they lose energy through a form of radiation known as synchrotron radiation. So accelerators that use electrons for high-energy physics experiments typically are straight-line, or linear, colliders. Here, too, the size of the accelerator has limits – largely economic. SLAC’s linear collider is two miles long. The ILC’s would initially span 22 miles. It would need to grow by another nine miles to achieve the highest collision energy physicists envision for it.
Ways to push particles even faster
To rein in the size of such machines, researchers are exploring several unconventional approaches to giving electrons and positrons a series of swift kicks.
Last year, for example, researchers using facilities at SLAC reported a significant proof-of-concept advance using an approach a Tour de France cyclist might appreciate.
The team sent two closely spaced pulses of electrons through a heated tube filled with lithium ions. The first pulse turned the gas into a plasma and set up a wake as it passed. Electrons from the second pulse that hit the wake got kicked to far larger energies. Over a length of just under three feet, the team accelerated a small fraction of electrons to energies they ordinarily would reach if they traveled the full length of SLAC’s two-mile tunnel.