Researchers at the U.S. Department of Energy's (DOE) SLAC National Accelerator Laboratory in California and the University of California, Los Angeles, have found a way to smash electrons and their antimatter opposites, positrons, together at higher energies than possible before. The breakthrough could be used to create future particle accelerators that can find a cavalcade of exotic particles. The research is published in the journal Nature.
In the Large Hadron Collider at CERN, protons are smashed together at nearly the speed of light around a large circular ring. The collisions lead to the formation of exotic particles that can be studied, such as the Higgs boson and, it’s hoped in the near-future, dark matter.
But, while this method of particle acceleration is effective, it is limited because protons are quite big, relatively speaking. The number of exotic particles produced is relatively small, meaning researchers must perform collisions many, many times to have a hope of seeing something of particular interest.
Thus, researchers have considered electron-positron collisions as a more attractive proposition. While such accelerators have been built before, which must be linear, it is difficult to accelerate these smaller subatomic particles to high energies. This is because a linear accelerator tens of kilometers long would be needed; currently the biggest, run by SLAC, is only 3.2 kilometers (two miles) long.
So scientists needed a way to produce high energies in short linear accelerators. The solution for electrons was already known; higher energies can be achieved by sending a bunch through an ionized gas, or plasma. A second bunch can then "surf" the wake that is produced (like a boat in water) to reach higher speeds and energies. But so-called plasma wakefield acceleration does not work for positrons. A bunch trailing another will lose its shape and even slow down, rather than speed up.
In this latest research, though, it was found that using just a single bunch of positrons produced the desired effect. Its interaction with the plasma created a wake at the front that accelerated the positrons at the back of the bunch, while the bunch was also able to retain its shape. The effect occurred after the bunch had traveled 10 centimeters (four inches) through the plasma. It could make high-energy electron-positron collisions more feasible.
Electrons "surfing" are shown left, and positrons on the right. Bunches are inside the dotted lines. W. An/UCLA.
"These collisions are simpler and easier to study," said SLAC's Michael Peskin, a theoretical physicist not involved in the study, in a statement. "Also, new, exotic particles would be produced at roughly the same rate as known particles; at the LHC they are a billion times more rare."
Unfortunately, there are no plans to build a plasma-based accelerator at the moment. However, the researchers said that existing linear accelerators could be upgraded to have a short plasma section, allowing high-energy electron-positron collisions to be achieved.
"It's conceivable to boost the performance of linear accelerators by adding a very short plasma accelerator at the end," said lead author Sébastien Corde of SLAC in the statement. "This would multiply the accelerator's energy without making the entire structure significantly longer."
Simply smashing. Sorry.