Understanding almost everything
15 October 2007
Physicists at Bristol University have been part of an international collaboration involved in designing and building the Large Hadron Collider (LHC), at CERN, a hundred metres beneath the Swiss-French border.
The University has a long history of particle physics dating back to before its first physics building was opened in 1927 by Sir Ernest Rutherford, then President of the Royal Society. Indeed, the carvings over what used to be the main entrance still reflect what our benefactor, Henry Herbert Wills, considered was the ‘value to humanity’ that had been made by physics over the previous decades – the discovery of radioactivity, X-rays, the electron, isotopes and various other elementary particles. Famous names associated with the department include Paul Dirac, who predicted the existence of antimatter – more of which later – for which he won a Nobel Prize in 1933. Cecil Powell, Hans Albrecht Bethe and Nevill Mott also won Nobel prizes for their contributions to physics, and were also at Bristol. Today, another Nobel prize beckons to the team that finds the ‘Higgs boson particle’ (fondly known as ‘the Higgs’), for that, it is hoped, will explain why things have mass.
We don't know why matter exists and we can't explain why things have mass
Peter Higgs, after whom the Higgs is named, predicted that space is filled with a field, rather like a gravitational field, that permeates the entire Universe. The Higgs field plays a fundamental role in that it gives mass to every elementary particle, including the Higgs boson itself. That it exists is indicated by the fact that without it the Standard Model of particle physics – the theory that describes the funda-mental interactions between all the fundamental particles – breaks down because without the Higgs boson we cannot explain the large difference in mass between different fundamental particles that make up ordinary matter, and other particles, such as photons (particles of light), that have no mass at all. Without the Higgs in the model, everything would be as insubstantial as light. So finding the Higgs is the Holy Grail of the LHC experiment, and scientists there are confident that it will be found – assuming it exists. If they don’t find it, then it doesn’t exist and we will have to completely rethink our understanding of the basic principles of particle physics – no small task, given how long it has already taken us to get this far.
So how will they look for it? Within the LHC tunnel two beams of protons (which belong to a family of particles known as hadrons) travelling in opposite directions will be accelerated to near the speed of light. When going at full speed they will travel around the 27-kilometre tunnel more than 11,000 times a second. At four locations along the tunnel the protons will be forced to collide with each other at the rate of 40 million times per second. When this happens new particles will be formed in the collision, spraying out in all directions. It is in these collisions that the Higgs, and other particles we may not have seen before, will form and exist for considerably less than a nanosecond before they die away. While this may seem an incredibly short life, in particle physics a nanosecond is a long time.
Particles travel around the 27-kilometre tunnel more than 11,000 times a second
Detectors have been built at the four collision points in order to ‘see’ the new particles as they form. Two of these detectors – ATLAS and CMS – are what are called ‘general purpose’ detectors, while the other two – LHCb and ALICE – have been designed to detect specific effects. ATLAS and CMS are both expected to see the Higgs, but in different ways – each being needed to verify the findings of the other. Thus there is fierce competition between the two groups working on the different detectors, each determined to see the Higgs first. Seventeen Bristol physicists are among the 2,300 scientists from 36 different countries who work on CMS. In particular, Bristol physicists have helped design and build part of the detector known as the Electromagnetic Calorimeter, which measures the energy of the particles produced in collisions and will be the most important component in looking for the Higgs. These detectors are giants – the CMS is 22 metres long and 16 metres high, and the cavern required to house it is about six storeys high. It weighs 12,500 tonnes in total and because of its size, pieces are constructed on the surface – some of these alone weigh 1,500 tonnes – and lowered into place in the cavern using enormous cranes. Remarkably, for something so huge, there were only ten centimetres to spare on either side when the largest piece was lowered.
So how will they know when they have found the Higgs – or anything else, come to that? The different layers of the detectors measure different properties of the particles produced, and tracking devices reveal the paths of electrically charged particles as they fly away from the collision. The new particles are typically unstable and will rapidly ‘decay’ into a cascade of lighter, more stable and better understood particles which leave behind characteristic signatures in the different layers of the CMS, allowing them to be identified. The presence, or otherwise, of any new particles can then be inferred from these signatures. But so much data is generated in these collisions – every second it would fill all the books held by the whole of the British Library – that it is only possible to keep data from one in every million collisions. Even this requires huge computing power and enormous storage facilities. One of the most impressive parts of CERN is the computer centre, where thousands and thousands of PCs – just like the one on your desk – are lined up in banks piled high on top of each other, in a room that seems to go on forever.
At less than the cost of a pint of beer per person per year, it seems an absolute bargain
To enable scientists to access the data produced by the LHC from anywhere in the world, the LHC Grid is being developed that will link computers around the world via the internet, which was, by the way, itself invented at CERN to help people working there share results. Data from the LHC experiments will be distributed around the globe for processing and analysis, so a high-performance computing facility is being installed on the top of Bristol’s physics building which, at peak performance, will be able carry out over 13 trillion calculations per second. Thus scientists sitting in Bristol could be the first to find the Higgs – you don’t have to be at CERN to see evidence of it.
The other experiment that eight Bristol physicists are involved with is the LHCb (b for beauty), which seeks to find out why more matter than antimatter exists in the Universe, even though equal amounts were created at the time of the Big Bang. Antimatter is not the stuff of science fiction – it really does exist and will be created by the LHC so that it can be studied. Antimatter is the mirror image of matter; thus when the two come into contact they annihilate each other, which means that the Universe as we know it should not exist. But it does, suggesting that matter and antimatter behave differently in a very subtle way. In order to recreate the moment immediately after the Big Bang, when the Universe was only a hundredth of a billionth of a second old, the LHC will accelerate particles to the highest energy levels ever achieved in a laboratory. In those collisions, particles called beauty and anti-beauty quarks will be produced in pairs, just as they were the moment the Universe formed. The LHC will create about a thousand billion pairs of beauty and anti-beauty quarks per year in the hope of detecting the asymmetry that explains why it is that nature prefers matter to antimatter. It is possible that in the process this ground-breaking research will reveal a new kind of physics, not previously known about.
So what’s all this going to cost? Not very much, actually. The total cost of building the LHC over the 13-year construction period is about £2.7 billion, and the UK’s contribution to that is £511 million. The funding is provided by the Science and Technology Facilities Council. This compares very favourably with the £757 million for Wembley Stadium and £4.3 billion for Heathrow’s Terminal 5, and is dwarfed by the £9.4 billion (and still rising) cost of the London Olympic Games. What’s more, who knows what spin-offs there might be? Particle physics has already been instrumental in various medical breakthroughs. For example, the University has just been awarded a large grant by the Medical Research Council to develop radioactive tracers that will track noradrenaline, a chemical in the brain known to be associated with depression (see page 16). The noradrenaline tracer will be tracked using PET scanners, the development of which owes much to previous research done at CERN. PET scanners have recently provided significant advances in understanding how the brain works, which in turn contributes to our understanding of how to control disease. With this new collider, we might discover other technologies so far undreamt of.
Admittedly it’s the taxpayer – you and me – who foots the bill for the LHC. But at less than the cost of a pint of beer per person per year it seems an absolute bargain to me, paticularly if it’s going to help us understand, well, almost everything.