A recipe. Build two pipes, each about 6 centimetres wide and 27 kilometres long. Bend them both into a circle and cool to 1.9 kelvin, about 300 °C below room temperature. Fill with protons – about a hundred billion of them to start with – all travelling as close to the speed of light as possible. Add a magnetic field a hundred thousand times more powerful than the Earth’s to steer the particles through the pipes. And make sure the protons in each pipe move in opposite directions.
Now here’s the exciting bit. Align the two pipes so that the particles collide. About twenty protons should smash into each other, creating showers of other particles. Take a good look at each one. If you spot a particle you don’t recognise, shout.
Finally, repeat forty million times. Each second.
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A joke? A pipe dream perhaps? Not according to scientists at CERN, the world’s largest particle physics laboratory, which spans the French-Swiss border near Geneva. This machine, which physicists have been discussing for ten years, will smash together particles such as protons and ions of the heavy metal lead, collectively known as hadrons. It should start operating in 2002 and run for twenty years. If all goes according to plan, physicists will be able to recreate the fiery conditions that existed in the first moments of the Universe (see ‘What will the LHC find?’).
The finest minds in the world are developing the technology needed to make the LHC work. The project falls into two parts – building the accelerator to smash protons and ions together, and building the detectors to measure the particles created in these collisions.
The LHC will be constructed 100 metres underground and will share the 27-kilometre tunnel now occupied by the Large Electron Positron (LEP) collider, CERN’s largest existing accelerator. The LHC will be a far more complex project than the earlier LEP. For a start, it will smash counter-rotating beams of protons which bend in opposite directions under the same magnetic field. Physicists could have used two completely separate pipes and two sets of magnets – the solution adopted for the recently abandoned Superconducting Supercollider in the US. But CERN engineers chose a more complex design using two pipes housed in one set of magnets cleverly computed to bend each stream in the same direction. Not only does this design save space, allowing the machine to fit in the same tunnel as the LEP, but it also saves 25 per cent of the cost of two separate pipes.
Finding a way to bend the beams is only part of the problem, according to Lyn Evans, the Welshman in charge of building the LHC. More difficult is devising a way to control them. The Americans chose the twin pipe design to allow them to regulate each beam independently. ‘But with the two-in-one structure, adjusting one beam will always affect the other beam,’ says Evans. ‘We have overcome this by inserting correcting magnets at regular intervals around the LHC that adjust the beams separately.’
Each proton beam will travel around the LHC at energies of up to 7000 gigaelectronvolts (GeV), explains Evans, compared with the 90 GeV of the positrons and electrons in the LEP. As a result, the LHC needs the much stronger fields created by superconducting magnets to bend the beams. ‘One of the beauties of superconducting wire is that it uses very little power,’ says Evans. The LHC will consume no more electricity than the LEP.
BIG CHILL
But there is one drawback. Superconducting wire works only at temperatures close to absolute zero, so each LHC magnet has to sit in a bath of superfluid helium at 1.9 K (-271.1 °C). Superfluid helium is an extremely effective heat conductor which, even in very small volumes, can remove large amounts of heat with very little movement. A mere trickle running through a pipe at the top of the apparatus is enough to remove any excess heat.
Ordinary liquid helium at 4.5 K has been produced since the beginning of the century, says Philippe Lebrun, who is in charge of developing the cryogenic system for the LHC. The problem is producing superfluid helium on the scale needed at CERN. Eight cryogenic plants situated every 3.3 kilometres around the circuit will pump 700 000 litres of liquid helium through 40 000 leak-proof junctions to cool 31 000 tonnes of LHC machinery. When the machine is switched off each December for its annual three-month overhaul, the helium must be stored: a problem that requires some lateral thinking. One possible solution would be to invite a helium supplier to build a massive storage facility and distribution centre at CERN in return for a regular supply of helium.
Producing ordinary liquid helium at 4.5 K is relatively easy and CERN already has the largest liquefaction facility in the world. ¿ìè¶ÌÊÓƵs will lower the temperature a further 2.6 degrees by reducing the pressure from a single atmosphere (roughly 1 bar) to 16-thousandths of an atmosphere (16 millibars). As the pressure drops, the liquid boils and the vapour carries away heat, causing the liquid to cool further – in the same way that an aerosol can cools as its contents are sprayed. But the challenge facing engineers is designing and building the equipment to recycle this vapour en masse to make the system efficient. A machine known as a cold compressor sucks away the vapour to prevent an increase in pressure that would halt the cooling process. Once removed, the machine compresses the gas to return it to atmospheric pressure and pipes it back into the cooling system.
Almost 24 kilometres of the 27-kilometre ring will operate at 1.9 K. Only straight sections where experiments are performed will escape the cold. Once inside the machine most of the liquid will stay there and must be insulated from heat from the outside world. ¿ìè¶ÌÊÓƵs will minimise the thermal contact with the surroundings by designing weight-bearing supports for the magnets which are good thermal insulators. Lebrun says metal is too good a conductor for this purpose and the most likely materials will be glass fibre or carbon fibre that will shoulder around 10 000 kilograms in conditions of extremely high vacuum and low temperature.
The design of the magnets is also challenging engineers. As the current rises in the copper-clad niobium-titanium wires, the magnetic field reaches an intense 9 tesla. (A strong horseshoe magnet has a field of about half a tesla.) At this strength, forces equivalent to the weight of several hundred tonnes build up inside the magnet – enough to blow the structure apart. And if any of the superconducting wires budge during operation, the field collapses and the magnet becomes useless. To prevent movement, each magnet is tightly clamped in nonmagnetic collars and surrounded by iron yokes.
The collapse of the magnetic field is known as quenching and, although rare, the magnets must be specially designed to cope with its effects. Quenching can also occur when a magnet heats up; if part of the beam smashes into it, for example. The magnet loses its superconduc-tivity and the sudden increase in resistance causes a large, rapid rise in temperature. Engineers expect an increase of 200 K in a matter of seconds creating huge thermal stresses as the metals expand. Within milliseconds of detecting the quench, a computer triggers built-in electric heaters to raise the temperature of the magnet evenly to spread the forces throughout the structure.
TAKING THE TUBE
Each of the 1300 tubes housing the dipole magnets and the superfluid helium will be 13 metres long, over a metre wide and weigh 40 tonnes. But in the centre of the magnets, the pipes carrying the protons will be a mere 56 millimetres across. Before the beam can run, scientists will have to create an extremely high vacuum inside these pipes to minimise the chances of protons colliding with molecules of air. Roughly three hundred billion molecules of air will be pumped out of every litre of the pipe lowering the pressure to ten million times less than atmospheric pressure. Once the vacuum has been achieved it must be maintained while the machine is running. When the beam of protons is bent it emits ultraviolet light known as synchrotron radiation. The light bombards the pipe causing molecules of gas absorbed within the metal to be released. This process, known as outgassing, destroys the vacuum and ruins the beam.
Engineers at CERN hope to overcome this problem by lining each pipe with a sieve-like inner tube to absorb the radiation. Although the inner tube still outgasses, it absorbs the ultraviolet radiation, preventing it from reaching the outer pipe. Molecules of gas will pass through holes in its surface and hit the cooler surface behind it. At a temperature of 1.9 K, the outer tube acts like flypaper and the gas molecules stick to its surface because the cold robs them of their kinetic energy. The radiation will keep the 50 kilometres of inner tube several degrees warmer than its surroundings and the tubing must be carefully insulated from the outer pipe since any heat leak could cause the magnets to quench.
Getting the beam running is one thing, stopping it is quite another. At full power, the circulating protons will have the energy of several kilograms of high explosive – enough to rip an expensive hole in the vacuum tube, damage the superconducting magnets and cause helium to leak into the pipe. According to Evans, within a ten-thousandth of a second of anything going wrong or when an experiment is over, special magnets will kick the beams into two special graphite targets. The beam dumps are situated at a tangent to the ring and are designed to heat up to over 1000 degreeC in a fraction of a second.
A very different set of problems face scientists designing the three LHC particle detectors. These experiments will have to record the paths of particles to within fractions of a milli-metre, process enormous amounts of information in microseconds and operate for long periods while being bombarded by intense radiation. Two of the detectors, ATLAS and CMS, will look at collisions between protons, verifying each other’s results, while a third, ALICE, will study heavy ion collisions. They will be enormous. Far larger than any LEP detector. The CMS detector, for example, will be the size of a small office block, 14 metres high, 20 metres long and weigh 12 000 tonnes. Engineers hope to shoehorn them into the underground, cathedral-like spaces now partially filled by the LEP detectors.
READY WRAPPED
Part of the reason for their size is that LHC detectors will be far more complex than those at the LEP because of the sheer number of particles they must measure. The detectors are made of several layers wrapped around the point of collision like an onion. Each layer measures a different property of the particles. For example, at a distance of about 1.5 metres from the centre an electromagnetic calorimeter measures the position and direction of photons and electrons (almost a third of all the particles produced) as they stream through the detector. Each layer is subdivided into many small detector elements rather like the grains in photographic film. The larger the number of elements, the greater the resolution of the detector. The electromagnetic calorimeter will contain 200 000 elements, and ATLAS as a whole will house around 10 million elements to measure the vast number of particles produced.
Researchers cannot hope to look at every collision individually. Instead, a computer must be programmed to find the interesting events – about one in every hundred million – and several hundred million events occur each second. Nick Ellis, who is coordinating research on collecting and selecting the data to be analysed, says that the information accumulates so quickly that only a small number of events will be examined in detail. Most of the data will be thrown away. But determining which data to discard and which to keep is an enormous task that computers will perform in three stages.
As each event occurs, the information from the ten million detector elements will be stored in simple memory devices such as capacitor arrays and random access memories. The memories are tiny – a chip the size of a fingernail can accommodate several thousand capacitors – and will be situated in the apparatus near the detectors that feed them. The data are in the form of simple voltages that represent the energy deposited by a particle in a small part of the detector, for example.
While the information is temporarily stored inside the detector, a custom-built processor about 50 metres from the experiment uses a small amount of the data to build up a low-resolution snapshot of each event. The processor compares this picture against a predetermined checklist of important features of collisions that must be saved. The checklist involves about 30 questions such as: is the energy of a particle greater than a certain number. If it is, the data pass on to another question. Each step takes around 25 billionths of a second to complete. As the data moves on, information from another event enters the processor. As points on the checklist are ticked off, the processor reduces and summarises the information from each event. The final step occurs within three millionths of a second of the collision and produces a simple yes or no result. Yes, and the information about the event is sent to the second level for further analysis. No, and it is lost forever, overwritten by information from another collision.
The result of the race to empty the buffers before they overflow is that about a hundred thousand events each second will be pumped into the second level. The data passes out of the detector to large racks of electronic memory banks about 100 metres away which allow a free circulation of air to cool them down. Physicists at CERN want to process the second level information using powerful desktop-type computers that they hope will be available cheaply in several years’ time. A ‘farm’ of a thousand of these machines will reconstruct high-resolution snapshots of small areas of the detector where interesting particles have been spotted and decide which events to save by again comparing the results with a more detailed checklist. Although the second level handles fewer events, it looks at each in more detail, taking perhaps several thousandths of a second to complete its task.
The third and final level will fully reconstruct approximately a thousand events each second. Some information will be bread-and-butter measurements; vital for calibrating the detectors but of no real interest to physicists and the third level will summarise these. But it must keep the raw data for important events and must spot them reliably. Here decision making becomes crucial and scientists have yet to finalise how it will be done. But researchers must make the third level as flexible as possible to cope with the new discoveries that physicists predict the LHC will uncover.
That’s the theory. In practice, as with the rest of the plans for the LHC, there are many problems still to solve. For example: the electronics inside the detector generate large amounts of heat which has to be dispersed. Gas or liquid flowing through pipes could cool the apparatus. But scientists on the ATLAS project are considering the novel option of sprinkling the sensors with water droplets which will absorb heat as they evaporate, much in the same way as sweat cools the human body. Preserving information inside the detectors will be tough. A single square centimetre of an LHC detector, a mere 30 centimetres from the collision, will be exposed to about ten million charged particles each second and this intense radiation can wreak havoc with electronic circuitry.
According to Peter Jenni, joint spokesperson for the ATLAS project, radiation damages the structure of silicon chip detectors and memories reducing their performance. Gallium arsenide chips, which are more expensive, may be used in parts of the detector that receive very high doses of radiation because gallium arsenide has a stronger lattice than silicon and is therefore less likely to be damaged.
TIME CHALLENGE
Given the financial green light, Chris Llewellyn-Smith, director-general of CERN, expects the LHC to be running by 2002 and producing its first results by 2003. A challenging timetable, according to Ugo Amaldi who led the team that built the DELPHI experiment at the LEP in six years during the 1980s. ‘It is difficult to see how one could build detectors ten times as complex in only eight years,’ he admits, ‘but it would be possible given enough money.’ He adds that to meet the timetable, the detectors could start producing results before they are completed. But Jenni, who is responsible for sticking to the timetable, is more optimistic. ‘There are a huge number of problems that must be solved,’ he concedes, ‘but CERN’s track record in sticking to timetables is good. I see no reason why we cannot meet the deadline.’
Although work is well under way and prototypes have been built, the 19 European governments expected to pay for the LHC have yet to give their final authorisation. Llewellyn-Smith is responsible for selling the idea and is confident of success: ‘The LHC is now scientifically mandatory. And I think the politicians will believe this argument.’
Llewellyn-Smith is promoting the LHC to European governments as a technology boost for industry. The physicists designing the LHC detectors plan to contract most of the work to companies that will be best able to exploit the technology in future applications. But this has its own problems. ‘Industry requires extremely precise designs,’ says Amaldi, ‘and there can be no late changes to the specification because of the extra expense this entails.’ Most of the LEP detectors, he says, were built in university laboratories which allowed far more flexibility in the design stages.
Designing and building the LHC is obviously an enormous challenge. But the real reason why scientists work on the LHC is simply the quest for knowledge – the desire to understand the Universe on a fundamental level. With quiet understatement, Peter Jenni explains: ‘The results we expect to get out of the LHC will answer basic questions about particle physics and even of the Universe. That’s what makes it worthwhile.’ But these are no ordinary questions. With the answers will come scientific fame and fortune. If it is successful, one thing is for sure: the LHC will be a Nobel prize-winning recipe. What will the LHC find?
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What will the LHC find?
Fifteen billion years ago, the Universe was created in the big bang. Then, at the beginning of time, matter and energy existed in the fundamental forms which physicists need to recreate in order to study the laws that governed the early Universe. ¿ìè¶ÌÊÓƵs believe these laws underlie all the laws of physics we see today. The LHC is the tool that physicists will use to study them.
Researchers think they know what to look for. One of their goals is to find the Higgs particle which they believe endows all other particles with mass. Mass is a form of energy and the Higgs particle is thought to be heavy. Theorists put an upper limit on its mass at about 1000 gigaelectronvolts (GeV) making it a thousand times as heavy as a proton. Current experiments at the LEP have looked for the Higgs at energies up to 90 GeV and will eventually look at energies up to 180 GeV. The LHC will search for the Higgs up to and beyond the 1000 GeV mark.
If the Higgs exists, many physicists expect to find an army of other particles called sparticles. Supersymmetry, the theory that predicts their existence, also provides an elegant framework linking gravity with the other fundamental forces of nature. Sparticles may be plentiful in deep space and could make up some of the mass that astronomers know is there because of its gravity but cannot see. This so-called dark matter influences the shape of huge structures such as galaxies and may form up to 99 per cent of the Universe.
The LHC may also help answer why the Universe is made of matter rather than antimatter. The clue to this problem is a particle called the B meson which the LHC will create in large quantities. Physicists hope that precise measurements of the way in which B mesons decay will reveal why there was a slight excess of matter over antimatter in the first fraction of a second after the big bang. This would explain why, after annihilation, the Universe was left exclusively with matter.
With collisions of greater and greater energy, physicists can mimic conditions closer and closer to the big bang. One way of increasing the collision energy is to use heavier battering rams. Using the LHC, physicists will collide ions of the heavy metal lead at energies of up to 1 250 000 GeV. Lead ions are more than two hundred times heavier than protons, and the resulting impacts should create bubbles of hot dense matter that existed for only 10-35 seconds after creation. This mixture of strong force particles called gluons and quarks is known as a quark-gluon plasma and is thought to exist at temperatures above a million billion degrees.
In the longer term, physicists hope to collide protons with electrons using the LHC together with the existing LEP accelerator. Protons are made of three quarks that are bound together by the strong force. The electron, however, is unaffected by the strong force, and is an ideal tool for examining the interactions between two of the other fundamental forces of nature – the electromagnetic force and the weak force. The electroweak interaction is responsible for the fact that every neutrino in the Universe is left-handed, spinning to the left around its direction of travel, while every antineutrino spins to the right. Experiments with the LHC and the LEP could explain why.