Plasma wakefield accelerators want the green light to scale up (Image: Berkeley Lab)

It is now officially a Higgs boson, but squeezing any more information out of the particle discovered last year looks tough. At least using the Large Hadron Collider at CERN. With major hurdles to building a larger atom smasher, future colliders may be tabletop affairs that use exotic matter, rather than brute force, to hit the high energies needed.

In the LHC, two beams of protons zip around a 27-kilometre circular track that accelerates them to close to the speed of light. They collide to produce Higgs bosons, which quickly decay into other particles that the LHC detects. But the smash-ups also shatter the protons into component particles that confuse things. As a result, the LHC is unlikely to tease out properties of the Higgs boson that would help extend the standard model of particle physics, which is known to be wanting.

Beams of fundamental particles, such as electrons, wouldn’t shatter, and so should provide more sensitive results. However, these don’t do well in circular machines, and linear accelerators must be exceptionally long to hit the required energies. The International Linear Collider, a proposed LHC successor, would be 30 to 50 kilometres long, posing financial and technical challenges.


So teams at the SLAC National Accelerator Laboratory in Stanford, California, are exploring a technique called plasma wakefield acceleration, which could achieve these energies in something thousands of times smaller.

The idea is to fire low-energy electrons, which are relatively cheap to produce, at a gas, ionising it to make plasma, creating a wake in the path of the electrons. A second bunch of electrons then rides the wake like a surfer, picking up energy and accelerating. About a metre of plasma is needed – which is why the devices are nicknamed tabletop accelerators. Stringing several together would let groups of electrons ride successive waves to higher energies. Eventually, an accelerator just 500 metres long might make Higgs bosons. So far, SLAC has created a plasma wave and measured the energy boost in its wake: it plans to try surfing a second bunch of electrons on it later this year.

Meanwhile, the Berkeley Lab Laser Accelerator in California is trying to create plasma wakefields using laser pulses instead of low-energy electrons. Firing a laser into plasma scatters free electrons in the superhot gas. Some regroup behind the laser pulse, where they can ride its wake. Strung together over just a few hundred metres, these modules could reach collisions packing 1 trillion electronvolts of energy, the same order of magnitude as the LHC.

Tabletop accelerators are exciting, says Steve Holmes of the Fermi National Accelerator Laboratory in Batavia, Illinois, but they are decades away from producing fundamental physics results.