(Inside Science) -- The next generation of particle physics just got a whole lot closer. Scientists at the Muon Ionization Cooling Experiment (MICE) have developed a revolutionary new process that, for the first time, makes a muon particle collider within reach. Such a collider could allow physicists to access energies higher than ever before, opening a door to a new frontier in fundamental physics research.
The scientists describe the process in a new paper published today in the journal Nature.
Protons and electrons: Tried and tested
Modern particle colliders either collide protons or electrons with themselves, or electrons with their antimatter counterparts -- positrons. The purpose of both is the same: smash particles together with a whole lot of energy and see what physics comes out. It took the highest-energy accelerator ever built, the Large Hadron Collider (LHC), to discover the elusive Higgs boson.
But neither protons nor electrons are ideal candidates for maximizing the energy of these collisions.
“The problem with protons is that they're actually collections of even smaller particles that are glued together,” said Chris Rogers, a particle physicist at Rutherford Appleton Laboratory in the U.K. and a member of the MICE collaboration. “So when we collide these protons, what we’re really doing is smashing together bags of particles, and each of those smaller particles only holds a fraction of the total energy of the proton.”
A collider could use fundamental particles to be more efficient, since they can’t be broken down into smaller parts. Electrons fit the profile, but there is a different problem: When electrons are accelerated in a circle, they radiate energy in a phenomenon called synchrotron radiation.
“What we need is a fundamental particle, which doesn't have the synchrotron radiation,” said Rogers. “That's what we have in muons.”
Muons are similar to electrons, in that they have the same negative electric charge -- but with about 200 times more mass. A muon collider would create exciting possibilities for the field of particle physics. Because muon collisions are more energy-efficient, a muon collider could reach higher energies than the LHC at a fraction of the size, and potentially with a much lower price tag.
“Using muons is very important because all the energy in the collision is used to create the new particles,” said Nadia Pastrone, the research director at the Italian National Institute for Nuclear Physics and the chair of the Muon Collider Working Group at CERN. “We could discover a new state of matter, or even better, precisely measure how the Higgs boson couples to other particles: one of the most important measurements in the field. This is a way to discover new physics.”
Muons: New but tricky
The problem is that muons are trickier to handle than other particles. In accelerators, muon beams are produced from an energetic proton beam hitting a target. The high-energy collision produces particles called pions, which almost immediately decay into muons. This indirect, chaotic process doesn’t make for a very narrow muon beam -- more like a diffuse beam the diameter of a watermelon. To get any meaningful physics out of the experiment, scientists needed to figure out how to get the beam’s diameter down to roughly the size of a quarter.
To make things even harder, unlike immortal protons and electrons, muons decay into other particles in just 2.2 microseconds -- and one would have to narrow, or “cool,” a beam of muons from the diameter of a watermelon to that of a quarter, then line it up to perform the experiment, all before these muons crumble into pieces.
The MICE experiment is the first demonstration of a unique, lightning-quick cooling process called ionization cooling. The diffuse muon beam is sent through a series of absorbers made of liquid hydrogen, and metal cavities filled with an electromagnetic field.
In the absorbers, the muons bounce around hydrogen atoms and knock off the atoms’ electrons, a process that takes away some of the muon’s chaotic momentum. Then the muons go through the metal cavity, where the electromagnetic field accelerates them in the forward direction.
After a long series of absorbers and cavities, the muons have coalesced into a narrow beam.
The accelerator built by MICE was about 10 meters long and 3 meters high -- about the size of a school bus. It’s no competition for the kilometers-long LHC, but it overcomes one of the biggest obstacles to the development of muon colliders.
“Muon ionization cooling was a linchpin in demonstrating the technical feasibility of muon colliders,” said Vladimir Shiltsev, a former leader of the Muon Collider Task Force at Fermi National Accelerator Laboratory in Illinois. “Given the results from the MICE experiment, people have started very seriously thinking about muon colliders as game changers. There are not many great ideas which can change the landscape of particle physics; muon colliders are one of them.”