It is probably the greatest, biggest and most expensive experiment on earth. It is the most powerful physics experiment ever built. It lies underground on the French-Swiss border, and is nearly 27 km in circumference – 26 659 m, to be exact – or, if you prefer, some 17 miles. It belongs to the European Organisation for Nuclear Research (better known by its French acronym, Cern). It is the $9-billion-odd Large Hadron Collider (LHC).
Its job is to try to answer some of the most fundamental questions in science. Where does mass come from? What is dark matter? Does the universe contain more than the four dimensions everyone is familiar with (length, breadth, height and time) and, if it does, how many, and where are they exactly? What was the universe like just after the Big Bang? And so on.
It will do so by accelerating protons, confined and directed (in a vacuum in the ‘beam pipe’ inside the LHC) by magnetic fields produced by some 9 300 magnets, to speeds just slightly below the speed of light in a vacuum, and then smashing them into one another, and observing what happens. These collisions will produce temperatures more than 100 000 times hotter than in the heart of the sun.
To achieve all this, and more, has required the building of a mighty machine that is itself an impressive monument to modern technology. The LHC’s operating temperature is 1,9 ˚K – and 0 ˚K is absolute zero, below which temperatures cannot fall – using liquid helium as the coolant.
The operating temperature of the LHC is colder than the temperature of deep outer space. Indeed, part of the LHC is the world’s largest fridge, equivalent to 150 000 domestic refrigerators. The LHC vacuum is comparable to the vacuum of outer space.
The combined strands of the superconducting cable in the LHC would, if laid end to end, encircle the equator 6,8 times – if all the filaments of these cables were separated out, and then laid end to end, they would cover the distance to the sun and back five times, with enough left over to stretch to the moon and back a few times as well.
The cavern that holds the LHC’s Atlas experiment is big enough to contain the nave of Paris’s Notre Dame cathedral. When in operation, it will consume an average of 120 MW of power.
Under the circumstances, it is not surprising that it suffered a glitch a few days after first being switched on, in September – a faulty electrical connection between two magnets caused a leak of the supercold helium, leading to damage which forced the shutdown of the LHC and required the repair and cleaning of 53 magnets, at a cost of some £20-million.
However, the LHC is programmed to be shut down every European winter anyway – electricity costs would be too high to operate it – so the delay is not as great as it would initially seem. It should be restarted this coming European summer.
Although the LHC belongs to, and is operated by, Cern, scientists from all over the world are involved in its fundamental experiments. These scientists include South Africans, who are involved in two of the LHC’s four suites of experiments, including the flagship Atlas experiment.
More precisely, a team at the University of Johannesburg (UJ) is involved with Atlas, while a team from the University of Cape Town (UCT) and the iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) is participating in an experiment designated Alice.
Atlas is an acronym for A Toroidal LHC ApparatuS, and the core of the experiment is a detec- tor in the form of a gigantic barrel, 46 m in length, with a diameter of 25 m, and a mass of 7 000 t, containing eight 25-m-long superconducting magnet coils arranged to form a cylinder around the beam pipe, which runs through the centre of the detector.
Alice stands for A Large Ion Collider Experiment, and its detector is also in the form of a barrel, smaller but heavier than that of Atlas, the Alice detector being 26 m in length and 16 m in diameter, with a mass of 10 000 t.
“Atlas is the largest LHC experi- ment – it’s got about 2 500 scientists in it, and it’s asking the most important questions for which the LHC was designed,” explains UJ associate physics professor Simon Connell, who heads the UJ team involved in the experiment. “One of these would be: Is there such a particle as the Higgs Boson? But attached to that is the nature of dark matter and the question of hidden dimensions.”
The Higgs Boson, or the Higgs particle, is currently a theoretical construct to explain how mass exists. Saying that mass is inherent in matter is simply not good enough, especially when dealing with matter at the quantum (sub-atomic) level.
“The aim of particle physics is to be able to explain the universe with a small set of particles. The smaller the number of particles, the better,” highlights UJ PhD student and Atlas researcher Claire Lee. Everything and everyone in the universe are made up of these particles, in differing combinations.
These particles rejoice in such exotic names as quarks, muons, gluons and neutrinos, and there are different versions of each of these – such as top quarks and bottom quarks.
Thus, protons and neutrons, once thought to be fundamental (nondivisable) particles, are now known to be made up of different combinations of different types of quarks, held together by gluons. Electrons, however, are still considered to be fundamental particles.
The Standard Model
“Now, we’ve got this theory, called the Standard Model. And the Standard Model explains the electromagnetic force, which includes visible light; it explains the strong nuclear force, which holds protons and neutrons together inside the nucleii of atoms, and it explains the weak nuclear force, which is responsible for nuclear decay and without which the sun would not burn,” she elucidates.
“So these are three out of the four fundamental forces we know of – the fourth is gravity. Unfortunately, the Standard Model is not compatible with gravity. But, when you look at the small distance scales and the small masses of subatomic particles, gravity isn’t really that important. So the Standard Model works exceptionally well, even without gravity.”
It is important to understand that forces and fields do not exist in and of themselves; they are all carried by particles – each force or field has its own carrier particle. Thus, light is carried by photons.
“The beauty of the Standard Model is that it gives us a whole bunch of particles and it tells us what particles make up the proton, the neutron – which themselves are the particles that make up the nucleus of an atom – and it also tells us why the nucleus is the size it is,” she says. “But what it doesn’t tell us is why the masses of the particles in the atom are very different.
The electron, for example, is something like two hundred thousand times lighter than the top quark. We know that it’s lighter, but we don’t understand why. Everything has mass, but, in the Standard Model, the easiest way we can write things down means everything is massless. That’s obviously a big problem. We need something to explain why particles have mass.”
It was in 1964 that British physicist Peter Higgs came up with the basic idea of how matter gets mass. Refined over many years with contributions from other physicists, the concept is that the universe is permeated by a field, now called the Higgs Field, carried by specific particles – Higgs Bosons.
As other particles move through the Higgs Field, the Higgs Bosons impose a kind of drag on them, which gives them their mass – this drag is their mass. Different types of particles interact with the Higgs Bosons to different degrees – the more interactions with the Higgs that a particle has, the greater its mass.
“It’s much the same as if you have a room full of people, and somebody famous walks in, and all the people want to meet this famous person, so they cluster around him, and that celebrity is slowed down as he moves through the room, because of his interactions with the people.
But an unknown person who enters the room is not slowed down so much, because people don’t cluster around him and so he has fewer interactions,” says Lee, citing the metaphor of University College, London, professor David Miller.
Even with the LHC, the Higgs cannot be detected directly. It exists, independently, for such a short time that it cannot be picked up by any detector. But it does decay into other particles, and these particles can be detected. It is these latter particles that the Atlas detector will pick up, and the scientists will add up the momenta of these decay products and, if the result comes out at a mass close to that predicted for the Higgs particle, that will strongly suggest that the Higgs Boson exists.
“The problem is that there are at least ten different ways the Higgs particle can decay. A Higgs could decay, for example, into four electrons, or into four muons, or into two photons,” she cautions. “So there are a whole series of different teams in the Higgs research group, and each is looking for different decay patterns for the Higgs. We, at UJ, are probably going to be involved in looking for Higgs decay into exotic particles – that is, things beyond the Standard Model.”
Exotic particles are also, currently, purely theoretical, but could, if they exist, help make up the mysterious dark matter – so called because it neither emits nor reflects light – which makes up 23% of the universe. (Normal matter and energy comprise less than 5% of the universe, while the very mysterious dark energy accounts for 72%.)
Although Alice is getting less media attention that Atlas, South Africa’s involvement in this LHC experiment began earlier and involved the design and construction of a key part of the detector. South Africa joined Atlas last July, but has been part of Alice since 2003.
“South African involvement was originally to design, construct, and implement the Di-Muon High Level Trigger for Alice,” reports UCT/Cern Research Unit researcher Dr Bruce Becker. “An online system was designed to decide, in real time, whether to record an event from a collision detected by Alice or not.
Hence, its designation as a trigger – it triggers data readouts, which happen at a rate of about a kiloHertz (1 000 times a second). The Trigger is a filter on all the data coming out of the experiment, and it has been installed and is located right at the front end of the electronics processing chain of the Alice experiment.”
Alice seeks to examine the first few moments of the universe, immediately after the Big Bang, when matter was superhot and superdense, and not even protons and neutrons, let alone complete atoms, existed.
“Alice seeks to condense matter, by colliding it, to get back to the densities of the very early universe,” he explains. “The aim is to recreate a strange state called deconfinement, a state in which quarks and gluons moved around freely, not bound in neutrons and protons, in what is called a quark-gluon plasma. The intent with Alice is to recreate this situation and measure its properties.”
Free quarks do not exist in nature today, so the quark- gluon plasma that will be created in the LHC will exist only very briefly. “But it will leave signals which will show it existed,” he points out. “We, in Cape Town, are particularly interested in what physicists call quarkonia, which are combinations of quarks and antiquarks, and especially the heavy ones, the beauty quarks and antibeauty quarks, and the charm and anticharm.” (The different types of quarks have whimsical names.) “These decay into pairs of muons, and we can detect muons. So we can use these to study the quark-gluon plasma.”
But how does a team in South Africa use the results of incre- dibly high energy proton colli- sions in the LHC, in Europe? The solution is the Grid.
“Grid computing is federating computer resources,” explains Becker, who is also coordinator of the South African National Computer Grid. “In Europe, national networks have been federated together to study experiments and the concept has taken off hugely there. It is used in many fields, such as earth observation, bioinfomatics, and not only particle physics.
South Africa has never had anything like this. Only recently have we created a national research network, known as SANReN. So we are now developing computer sites in this country, using this concept, and employing the relevant software.”
“The Grid is a massive, new, computing infrastructure, invol- ving both network linkages and high-performance computing clusters, linked around the world,” sums up Connell. “It provides tens of thousands of computers, working together with very fast interconnections.
And all the data from the LHC is going to just flow on to the Grid. So, from anywhere in the world, you can launch applications – imagine them as Web robots – that go and find the data you seek, and analyse it. It’s really the advances in computing that have made it possible for all countries, everywhere, to participate.
Here, at UJ, we have set up such a high-performance computing cluster, loaded on the special software, including authentication and security, so that we can access the Grid, launch analysis tasks on to all the Cern data from Atlas. This way, we will be able to do our own search for the Higgs Boson.”
In exactly the same way, the Alice research team in Cape Town will be able to access the LHC data they require, without having to travel to Europe.
There are many other mysteries and theories that the LHC will explore over the coming decades, and the instrument is bound to throw up as many questions as, if not more than, it answers.
One such theory which needs testing by LHC experiments is supersymmetry. “This is the idea that every normal particle has a supersymmetric partner, a kind of shadow particle that we don’t see,” says Connell.
Supersymmetry makes possible the concept that, just after the Big Bang, there was only one force in the universe, not the current four, and only one class of particle. “So the universe would have been in an incredibly simple and symmetric state. That is necessary if you want to imagine the universe essentially emerging out of a vacuum, in a very simple state.”
And if the results of the LHC experiments confound expectations, and do not give the hoped-for results? “The theoretical physicists will be very happy,” admits Lee. “Because that would mean we don’t actually understand the universe as well as we think we do. And there’s always more excitement in not knowing what you’re looking for, knowing that there is something else out there that you don’t understand yet.”