A giant particle accelerator will help physicists better understand the workings of nature

The most complex scientific instrument ever constructed will start up in 2007. The particle accelerator being built at CERN, the European particle-physics laboratory near Geneva, will dominate international fundamental physics for the next 15 years.

Over the centuries scientists have refined their understanding of exactly what makes up the universe. Since the 1960s the existence of different particles, each now thought to be fundamental, has been postulated and observed. So far physicists have seen 16 of them. The problem is that none of these particles confers mass on the others—yet we know from nature that something must do this.

The likeliest explanation is that there is a 17th fundamental particle that has not yet been seen. According to this theory, an invisible field fills the whole of space. When different particles interact with this field, they gain mass, much as anything moving through treacle gets slowed down by it. The larger the interaction, the more mass the particle would have.

As the universe is pervaded by such fields that convey other characteristics, the proposal is plausible. It was first made in the 1960s by Peter Higgs of the University of Edinburgh in Scotland. The Higgs field cannot be observed directly, yet, as with other fields, there should be a particle associated with it that can be detected. In this case, it is a particle called the “Higgs boson”. Unfortunately no one has ever seen one.

That may soon change. Deep underground at CERN lies what will be the world’s most powerful particle accelerator. By the end of 2007 two tubes within the 27km-long (17-mile) circular tunnel will have the air pumped out of them to create the most perfect vacuum on the planet, comparable with that of outer space. The tubes will then be filled with pulses of fast-moving protons, a familiar (albeit non-fundamental) particle. The protons will be accelerated to speeds marginally below that of light itself. Two beams, each containing billions of protons, will circulate in opposite directions in the two separate tubes. But four experiments will run in cavernous halls on the ring, where the beams will collide with such energy that the impact will resemble the universe a split second after the Big Bang.

The four experiments are known by their acronyms. The biggest two are called ATLAS and CMS. Each employs roughly 2,000 scientists and engineers from at least 150 institutions in 35 countries (the best American particle physicists will work at CERN, as will the top Russian, Chinese and Indian scientists). In terms of volume, ATLAS is the bigger: its equipment is the size of a five-storey house and occupies eight times as much space as CMS. But CMS, which weighs about the same as 40 large aircraft, is twice as heavy as ATLAS. These are general-purpose experiments, designed to find the Higgs particle or, failing that, to tell physicists what it is that they see in the extreme conditions created by the impacts.

Such collisions produce a plethora of new and exotic particles that exist fleetingly before they themselves decay into other items of interest that soon settle back down into everyday matter. Both ATLAS and CMS almost completely cover the point at which the protons smash into one another, maximising their chances of detecting interesting events. Each performs the same set of measurements on the particles emerging from the impact, capturing their paths, energies and identities. But each experiment does this differently.

Scientists working on each obviously want to be the first to see incontrovertible evidence of the existence of the Higgs particle. Nobel prizes are awarded for such things. But the healthy competition between the two experiments is tempered by the need for collaboration. Both experiments share the same accelerator technology and the extraordinarily fast data analysis needed to distinguish between collisions that are interesting and ones that are not. Moreover, any glimpse of new physics would need to be confirmed by the other experiment.

The other two experiments each have a specific goal. In the Big Bang matter and antimatter were created in equal amounts but somehow the antimatter disappeared. An experiment called LHCB will examine why.

Then there’s ALICE, “a large ion collider experiment”. At times the protons will be removed from the ring and lead ions will be injected instead. Many of those entities thought to be fundamental have never been seen alone, only in combination with other fundamental particles. Although the lead collisions will be far messier than those between protons, the result may be to see isolated basic building blocks of nature for the first time.

As well as the hunt for the Higgs particle, the new machine will look for evidence that will help scientists distinguish between theories of what lies just beyond their present understanding of the world. These include “supersymmetry”—the idea that every matter particle has a partner force particle—and “dark matter”, thought to be more abundant than the stuff scientists can now see. Such discoveries will be for 2008 and beyond.