27 July 2019 | New Scientist | 43the LHC, but it would allow much more precise
studies of the Higgs and other particles by
smashing together not protons, but electrons
and positrons. As these are simpler particles,
they would generate very clean collisions.
CERN, the particle physics laboratory
near Geneva, Switzerland, that hosts the LHC,
has a similar plan to the ILC, albeit up to
50 kilometres long, in the form of the Compact
Linear Collider (CLIC). But even that is dwarfed
by the lab’s proposed Future Circular Collider
(FCC), a machine that would have a whopping
100-kilometre circumference to smash
together either electrons and positrons,
or protons and protons, at energies up to
100,000 GeV. This would cost up to $25 billion.
Meanwhile, China is mulling over a Circular
Electron Positron Collider (CEPC) with an
equivalent 100-kilometre circumference,
but a more economical $6 billion price tag.
These gargantuan machines seem essential
to the future of particle physics. But for some
researchers, a cheaper, smaller alternative is
worth pursuing. As a senior scientist at CERN
during the time of the celebrated Higgs
discovery, Assman could easily have gone on
to work on ever bigger colliders. But scarcely
had the champagne corks landed at CERN than
he had chosen a new path: plasmas. “I thought
the main challenges with conventional
accelerators had been overcome,” he says of
his decision to move to DESY in 2012. “I thought
it would be cooler to innovate new technology,
and make it smaller, instead of bigger.”
After solids, liquids and gases, plasmas are
sometimes considered the fourth state of
matter: an ethereal mix of electrons and the
positively charged atomic nuclei, or ions, from
which they were stripped. As the electrons
and ions move around, tiny electric fields
are created and destroyed, making plasma
the perfect medium for carrying charged
particles. Unlike the hollow space separating
a conventional accelerator’s metal tubes, for
example, plasma is at no risk of suddenly
becoming conductive and neutralising an
electric current: it is conductive already.
Appropriately for a technology that has
been likened to particles going surfing, plasma
acceleration originated in California, a half
hour’s drive from the beach. In 1979, John
Dawson and Toshiki Tajima at the University
of California, Los Angeles, (UCLA) published a
theoretical paper that would form the basis for
all subsequent work on plasma accelerators.
The pair’s idea was to fire a laser into a gas ofdesired energy. Within reason. Even
circular accelerators have to be very
big, otherwise particles simply shed
their energy as radiation – or else get
flung out of the ring as they skid around tight
corners. Hence the LHC. Only a circular collider
27 kilometres in circumference could smash
opposing beams of protons with enough
energy – up to 13,000 gigaelectronvolts (GeVs) –
to produce the famous Higgs boson. Although
impressive for confirming a decades-old
prediction, the finding of the Higgs in 2012
has been the LHC’s only elementary particle
discovery to date, and has left particle
physicists wondering what, if anything, to
do next. “Many of us thought 27 kilometres
was the limit,” says Ralph Assman, a leading
scientist at the German accelerator lab DESY.
Very large hadron colliders
Not everyone, though. One long-mooted
successor to the LHC is the International
Linear Collider (ILC), a $7.5 billion machine
up to 30 kilometres long. Japan was set to host
it until its government failed to commit in
earnest this year. If the plan still goes ahead,
the ILC wouldn’t technically be as powerful as >
“ Plasma
accelerators
could soon
give big
beasts like
the LHC a
run for their
money”