Building the next- gen atom smasher
Even bigger colliders will be need to unravel the mysteries of particle physics. CATHAL O’CONNELL looks at what’s on the drawing board.
THE LARGE HADRON COLLIDER is a wonder of the modern world, our version of the great pyramid at Giza, but built by 10,000 scientists and engineers.
It’s a giant ring of superconducting magnets, 27 kilometres around, running 100 metres beneath the surface of France and Switzerland. Physicists use it to rev protons up to 99.999999% the speed of light before smashing them together to reveal the building blocks of the universe.
Simply building the LHC was a monumental achievement – it has the largest refrigeration system on the planet and its magnets are coiled with enough superconducting wire to stretch to the Sun and back five times.
And it works. In 2012, physicists used it to discover the Higgs boson, the last unverified part of the standard model of particle physics, with a special place in the family of fundamental particles because it gives the others their mass. Its identification has been hailed as one of the great scientific breakthroughs in human history.
But the biggest machine ever built is not big enough. The LHC’S operator, the Geneva-based European Organisation for Nuclear Research (CERN), still sees a long working life ahead of the collider, with about another 15 years of groundbreaking research to come. Yet physicists know that some of the biggest challenges in particle physics lie just outside its reach.
There are two “big questions” the LHC’S successor will try to answer, says Geoff Taylor, a particle physicist at the University of Melbourne who leads Australia’s research contribution to the LHC: “What are the details of the Higgs? And what is the dark matter particle – or particles?”
Finding the Higgs boson was, in the words of distinguished British physicist Peter Knight, “the physics version of the discovery of DNA”; and, like the discovery DNA or any other great breakthroughs in science, it provoked more questions than it provided answers.
At just 130 times the mass of the proton, the Higgs was a lot lighter than many physicists expected. Why? We’ve so far only found one Higgs particle, yet some theories predict at least four others – do they exist? Does the Higgs mechanism, which gives mass to most subatomic particles,
work just as the standard model says it should? What exactly can the Higgs decay into? The LHC can’t answer these questions because it can’t produce Higgs bosons in great enough numbers or in conditions clean enough to observe clearly.
In order to do that, we need a new collider, a so-called “Higgs factory” calibrated to produce the boson en masse.
The new field of Higgs physics could tie up many of the loose ends in the standard model of particle physics, but the ultimate prize would be to go beyond the standard model itself, and thereby solve one of the great mysteries in all physics.
From the seemingly off-kilter dance of galaxies, we know the universe is made of more than what meets the eye – that about 80% of the material in the universe. is “dark matter”.
Many physicists think dark matter must be made of a new kind of particle, or perhaps whole families of new particles. It was hoped the LHC might produce them in collisions – perhaps in the form of so-called “superpartners” of the common electrons and quarks we’re made of.
As British theoretical physicist Peter Higgs has said, the boson that bears his name is “not the most interesting thing that the LHC is looking for”.
Alas, despite some titillating signals, no more new particles have yet been found. To uncover these particles, if they exist, will need much more energetic collisions than the LHC can ever muster.
Already, the wheels are in motion to set up the LHC’S successors. Three gargantuan projects are on the cards: one in Japan, one in China and one in Europe. Each would, in turn, become the largest experiment ever devised.
PROTON COLLIDERS LIKE THE LHC ARE THE SLEDGEHAMMERS OF PARTICLE PHYSICS – USED TO BREAK NEW GROUND.
INTERNATIONAL LINEAR COLLIDER, JAPAN — The first, or at least easiest, cab off the rank is the International Linear Collider (ILC) destined for Japan, says Taylor.
Imagine a gun barrel 15.5 kilometres long. Imagine a second barrel, pointing down that of the first. Now imagine firing two bullets at each other at almost the speed of light and photographing the collision. That’s the ILC in a nutshell.
The ILC is an electron-positron collider, so each “bullet” is actually a bunch of 20 billion electrons or positrons.
On the face of it, the ILC’S collision energy of 500 gigaelectronvolts seems puny compared to the LHCS 13 teraelectronvolts. But it’s a totally different animal.
Proton colliders like the LHC are the sledgehammers of particle physics – used to