The Large Hadron Collider

Engineering the void

A vacuum is filled by virtual particles ready to pop into existence if additional energy is supplied. Rather than going to waste, they can have plenty of interesting uses. 

The Large Hadron Collider summons superlatives

Basically a machine for accelerating protons to high speed and then letting them glide around a 27km ring-shaped course over and over again, LHC is the largest scientific instrument ever built. It is the best time machine in existence as it recreates a tiny portion of the very early universe; when two protons are violently smashed together in LHC's tunnel far beneath the bucolic countryside outside Geneva, the ensuing fireball resembles a small speck of the primordial matter that filled space less than a nanosecond after the big bang. 

The LHC is the world's most powerful microscope, as the wreckage spewing from the proton-proton collision zone - streams of daughter particles - can be analysed to provide clues about the inner structure of matter down to less than a billion-billionth of a metre. 

Consider the origin of all those secondary particles constituting the collision debris. They weren't there a moment before the collision, so where did they come from? After the collision they are streaming away by the hundreds or thousands, pulled out of nothingness, pulled out of the vacuum by the mighty exertion of the LHC's double beams of protons, each racing around the ring tunnel in opposite directions and aimed to maximally jostle each other.

Vacuum might be devoid of ordinary matter but it is not empty. It is filled perpetually by virtual particles, particles in stand-by mode ready to pop into existence if additional energy is supplied, and that's precisely what LHC's beams do. Courtesy of the energy conversion process enshrined in Albert Einstein's E=mc2 equation, the beams' energy of motion (and at an unprecedented energy of seven trillion electron volts per proton there is plenty of motion) can, at the head-on collision point, promote many of those virtual denizens of the vacuum into particles in the real world, where they can be detected.

Actually, one must think of two vacuum realms. There is the physics vacuum, the staging ground for producing telltale daughter particles amid beam-beam collisions. We'll come back to that later. For now, I'd like to concentrate on the engineering vacuum, which is what I call the portion of the LHC apparatus evacuated of all air molecules.

This vacuum system, the largest in the world, consists of three parts. First, the pipes carrying the beams around the accelerator must be emptied in order to reduce extraneous interactions between lingering air and the high-energy protons. The vacuum achieved is about 10-10 torr (or about 10-8 Pa). The second vacuum surrounds the cryostat apparatus used to cool LHC's thousands of superconducting magnets. These magnets, which deflect the protons into their proper trajectories around the ring, operate without any electrical resistance if they are sufficiently cold.

This is accomplished by the world's largest cooling system. About 37,000 tonnes of equipment is maintained at a temperature of only 2K. Vacuum, besides its other interesting properties, is an excellent thermal insulator. The same principle that keeps coffee hot in a Dewar flask also helps to keep the chilled portions of LHC cold. The third vacuum system helps insulate the pipes that carry the main coolant, liquid helium, to the magnets. All together, more than 10,000m3 of space are held at high vacuum.

Why use superconducting magnets in the first place, necessitating such a massive refrigeration? The magnets, which suffer no electrical losses, save a lot on energy costs. Also, in the superconducting mode, they can carry higher currents (which produce higher magnetic fields) into the same amount of coil.

Ironic juxtapositions

So here, beneath the CERN laboratory in Geneva, we are offered some ironic juxtapositions. Some of the coldest temperatures anywhere (much of the space between galaxies is at an equivalent temperature of 3K or more) prevail only centimetres away from a place (the collision point) experiencing some of the warmest temperatures - trillions of degrees for some LHC collisions. Furthermore, the LHC vacuum, representing the aforesaid nothingness, attends upon those particle smash-ups, which represent some of the densest agglomerations of matter known to science. Once the pipes are emptied of air molecules (creating the engineering vacuum), scientists try to fill the physics vacuum with collision-produced daughter particles.

To begin with, LHC will operate with proton beams. Later, and on a part-time basis, beams of heavy ions will also be used. Lead atoms, with all their electrons stripped off, will be sped up and then clashed in place of protons. The goal here, as it is with the Relativistic Heavy Ion Collider (RHIC) machine at the Brookhaven National Laboratory in the US, is to study the breakup of large nuclei into their constituent protons and neutrons, and the melting of those protons and neutrons into their constituents, quarks and gluons, into a nuclear liquid referred to as quark-gluon plasma.

And here is where the vacuum can pose a problem, an obstacle, to the efficient study of heavy-ion collisions at CERN. This problem was not unexpected because theorists had been worrying about it for some time. But it is only now, with LHC about to come into play with its immense energy, and because of the availability of RHIC as a laboratory for testing future conditions at LHC, that the problem could be dealt with. The problem concerns a category of collision, or near collision, in which two ions (each hurtling along in its own opposing beam) pass by each other with enough leeway so that they largely proceed unaffected in their courses. Their grazing interaction, however, is so close that some of their combined energy of motion converts virtual particles lurking in the vacuum into real particles. Not to create a major splash of secondary particles signifying a serious collision. No, in this class of peripheral interactions only two new particles are created, an electron-antielectron pair. The vacuum is hardly disturbed in the process.

Quenching magnets

And yet something interesting occasionally happens with those particles. One of them, the anti-electron (also called a positron) goes off to oblivion while the electron, still in the vicinity, hitches a ride with one of the heavy ions (which, after all, possess a huge positive charge, 82 in the case of lead ions). This ion, bearing now a slightly lower electric charge, will behave slightly differently from its fellow ions as they race through the ring of powerful magnets.

Going a certain distance, the modified ion will leave its neighbouring ions and smash into the pipe carrying the beams, thus heating up the pipe and surrounding magnet coils. This loss of ions, over time, not only saps the beam of its particles, but the renegade ion is a danger to the equipment, either because it could deposit unwanted radiation or because it would (in sufficient numbers) warm up the magnets, causing them to 'quench', that is, to lose their superconducting state. 

In this way, what at one time was a novel physics phenomenon - the production of pairs of particles from the vacuum - has become something of a nuisance. A recurring theme in science is the idea that today's much-sought process can become tomorrow's tiresome background, a thing to be factored out or avoided. The unwelcome appearance of pair-production loss of beam particles isn't much of a factor in the proton-proton interactions, but can represent a larger imposition when using heavy ions.

For that reason, researchers have sought to observe this effect at the nearest equivalent to the LHC heavy-ion set-up, namely the RHIC machine, where collisions have been proceeding for some years already. And in a recent experiment the investigators, a collaboration of RHIC and LHC based scientists, looked at the matter. Roderick Bruce, John M Jowett and Simone Gilardoni of CERN, Angelika Drees, Wolfram Fischer and Steve Tepikian of Brookhaven, and Spencer R Klein of the Lawrence Berkeley, National Lab found what they were looking for, a tiny splash of energy amounting to about .004W, or about what a firefly puts out. The RHIC beam for these tests consisted of copper ions, each with a charge of +29 and carrying 6.3 TeV of energy (about 100 GeV per nucleon). These results were published in the 5 October 2007 issue of the journal Physical Review Letters.

According to Jowett, this troublesome class of events, referred to as bound-free-pair production (or BFPP, the bound referring to the electron and the free to the positron), will be much more formidable at LHC than at RHIC. First of all, the pair production scales as the atomic number of the nucleus (or the charge of the nucleus, denoted by the letter Z) raised to the seventh power. The LHC heavy-ion collisions will use beams composed of lead ions. The more highly charged nucleus and the larger energies (574 TeV per lead nucleus) mean the BFPP process should be some 100,000 times more prominent at LHC than in the test at RHIC. This would amount to about 25W, the equivalent of a reading lamp. That doesn't sound like much but, when deposited in the ultra-cold (1.9K) magnets of the LHC, it could bring them to the brink of quenching out of their superconducting state, interrupting the operation of the huge machine.

Vacuous challenges

The LHC personnel can probably live with this amount of warming. Jowett, who first worked on LHC design matters in 1984, says that the latest theoretical estimates of the heavy-ion peripheral events at LHC and the actual measurement of the pair-production events at RHIC (at the lower energy used there) suggests that the loss of beam intensity at LHC will be manageable. The pair-production mechanism, while important, is not the only process that saps the beam of ions. Some ions are lost when they are scattered by residual gas molecules in the beam pipe (no vacuum is perfect). Other ions are lost when they shuck individual neutrons; such an ion will possess the same charge as before but will have a different mass, and this disrupts its orbital properties. Still other ions are lost by interacting with nearby ions in the same beam. All of these mechanisms added together will deplete the beam over the course of a few hours. After this, the beam will have to be injected from scratch again.

Even with the vacuum now better understood, there are still many challenges ahead in the coming years for LHC. Getting the proton beams up and running will be the main job for 2008. Then comes the directed study of colliding protons, and, later still, heavy-ion collisions. And even when the scientists and engineers are producing all the collisions they could wish for, there is still the massive task of processing all the data. Proton-proton collisions will occur much more frequently than for the heavy ions, but with the ions (and don't forget that each lead ion comprises 208 protons and neutrons) the collisions are much messier, producing many more daughter particles to be tracked and accounted for. 

Heavy ion collisions at LHC are expected to occur at a rate of about 10,000 per second; and each event will log up to 100 megabytes of data, for a rate of about a terabyte every second. Perhaps in some future experiment, computer scientists can find some way of storing all this information not in any material substrate such as a DVD or a sector of a magnetic wafer, but in some digitally-accessible version of the vacuum. Instead of letting that plenitude of virtual particles go to waste, they could be used to fill out the ultimate computational medium. 

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