Making Mini Big Bangs at the Large Hadron Collider
Interview with David Evans, Professor of High Energy Physics, following his Inaugural Lecture
Much has been made of the discovery of the Higgs boson – the elementary particle pivotal to the Standard Model of physics – and rightly so. Yet the so-called ‘God particle’, hypothesised for so long and finally confirmed in 2012, is responsible for only one per cent of the mass of the visible universe. The other 99 per cent is due to the Strong Force.
While the media clamoured for news of the Higgs – discovered by scientists, including some from Birmingham, working on the ATLAS collaboration at the Large Hadron Collider (LHC) at CERN – a less talked-about experiment, ALICE, was busily collecting data from mini ‘big bangs’ created within the LHC. The results could be every bit as exciting and just as vital in transforming our understanding of how the Universe works.
Along with gravity, electromagnetism and the Weak Force, the Strong Force – also known as the Nuclear Force – is one of the four fundamental forces in Nature.
Birmingham’s Professor David Evans leads the UK team at ALICE (acronym for A Large Ion Collider Experiment), one of the largest-ever experiments devoted to research in the physics of matter at an infinitely small scale.
Involving more than 1,200 scientists, it has been running for three years and is expected to continue for at least another 15.
‘Last year, at any one time, ALICE was using 36,000 computers worldwide, 24 hours a day, to process the data we’d collected,’ explains David. ‘This is why it has to be a worldwide affair.’
Although still at the ‘tip of the iceberg’ stage, the experiment has already revealed that the primordial soup from which the Universe was created was, in fact, a perfect liquid. ‘So already we’ve got the name wrong by calling it the Quark Gluon Plasma!’
The focus of ALICE is to put lead nuclei into the LHC, accelerate them to nearly the speed of light and then collide them together. This creates, for a brief instant, a sub-atomic fireball, which recreates the kind of conditions that would have existed one millionth of a second after the Big Bang. Researchers have created millions of these fireballs over the past three years.
By doing this, we create the highest temperatures and densities ever created in an experiment. The temperatures are of the order of 300,000 times hotter than the core of the Sun – you’re talking something like six trillion degrees. The density of these little fireballs is about 50 times denser than a neutron star. This really is extreme matter.
At such extremes, matter as we know it melts. So what ALICE has produced is the primordial soup – Quark Gluon Plasma – that would have existed right at the beginning of time and space.
‘Although this thing is 300,000 times hotter than the sun, it behaves like a perfect liquid – the most perfect liquid that’s ever been produced in an experiment,’ says David. ‘So it looks like the universe was born out of a perfect liquid. But there’s a long way to go.
‘What we are doing now is studying this exotic state of matter. ‘We’re doing that for two reasons: Firstly, it tells us about the evolution of the very early universe and, secondly, by studying this we can learn more about the Strong Force, which is the strongest force known to scientists.’
The Strong Force is responsible for holding together the nuclei of atoms, but also for confining the fundamental particles called quarks inside atomic nuclei called protons and neutrons.
‘You never get free quarks; they are always imprisoned inside particles like protons and neutrons. The other interesting thing is that by confining these, energy is stored up and it is this that generates 99 per cent of the mass of protons and neutrons. So 99 per cent of the mass of the visible universe is due to the Strong Force – and only one per cent due to the Higgs.
So what we want to know is how the Strong Force imprisons quarks and how it generates 99 per cent of the mass of matter.
What, ultimately, might this tell us about how the Universe works?
‘Like all pure research, discovering how things work is never straightforward, but the whole of the modern world we live in – the electronic world – stems from pure research and an understanding of electromagnetism. So the results of understanding the Strong Force could be endless and the long-term spin-offs fantastic.
‘To do this cutting-edge science, we are pushing technology to the limit and we have to keep doing that. The new technologies developed at CERN have already had amazing spin-offs elsewhere. Everyone knows the worldwide web was discovered here, but what’s less well known is that touch-screen technology was also invented at CERN.
‘At the moment we are still analysing data, but soon as the LHC starts running again in January (following a two-year shutdown to repair and upgrade it so it can produce double the energy) we’ll start taking data again.’
David has been at the University for nearly 20 years. After gaining a degree in Physics from Oxford, he chose Birmingham for his PhD, in High Energy Physics.
He liked Birmingham the moment he stepped on to campus. ‘Not only was the campus lovely, but there was a great atmosphere within the High Energy Physics Group. The first collision of nuclei had just taken place at CERN, which Birmingham was involved with, so the idea of getting involved right at the start of these experiments was very exciting.’
David was then awarded a prestigious CERN Fellowship, and moved to Geneva for two years. ‘They put me in charge of the experiment I was working on, so I was the international spokesperson.’
Since then he’s been based at the University, but spends time at CERN, near Geneva, where he is on the ALICE management board. ‘So it puts Birmingham right at the heart of decision-making at ALICE.’
As with any pure, cutting-edge science on a scale never before attempted (the LHC is the largest and most powerful particle accelerator ever built), there is a real possibility of discovering ‘new’, even more exotic physics.
It’s like going into a dark room and shining a torch around, not knowing what you’ll find, which is what makes it so exciting.