The discovery of the atomic structure of matter in terms of particles - negatively-charged electrons orbiting a nucleus of positive protons and neutral neutrons - has underpinned much of human progress over the past 100 years. Electronics, new materials and modern medicine, for example, are ultimately all the result of pushing back the frontiers in our understanding of the fundamental particles that make up matter and the forces by which they interact.

Physicists now have a very good description of Nature's basic building blocks called the Standard Model. It comprises 12 types of particle - six quarks, of which two kinds make up the familiar proton and neutron, and six lighter particles called leptons such as the electron. Also included are four fundamental forces - electromagnetism, the strong and weak nuclear forces and gravity - each mediated by characteristic 'force' particles. Every particle also has an antimatter partner with identical but opposite properties such as charge.

Definitive evidence for the Standard Model has come from colliding beams of particles at very high energies in large accelerators. For example, the W and Z particles which mediate the weak force were discovered in 1983 using a circular particle collider called the Proton-Antiproton Collider at CERN, the European Laboratory for Particle Physics in Geneva. The Large Electron Positron Collider (LEP) subsequently built at CERN, and also the Stanford Linear Collider and the Tevatron Collider at Fermilab in the US, then probed the properties of these particles and their interactions in exquisite detail.

Higgs and other mysteries
These experiments showed that the Standard Model works well. However, it is far from complete and there are many unanswered questions. For instance, why do the particles have the masses they have and where does the mass come from? The favoured idea is that a particle called the Higgs boson (named after Peter Higgs of Edinburgh University) gives the other particles a certain mass depending on how strongly it interacts with them. Another conundrum is what has happened to all the antimatter in the Universe. Matter and antimatter must have been created in equal amounts just after the Big Bang some 12 to 15 billion years ago. The cause might be that some particles behave slightly differently from their antiparticles. Finally, physicists would like to formulate an ultimate theory, going beyond the Standard Model, that truly unifies all the matter and force particles and explains how they formed after the Big Bang. A strong component of these unified theories is that all the known particles may have yet another type of partner called a supersymmetric particle.

Physicists are already searching for signs of the Higgs boson or supersymmetric particles in existing colliders. However, to investigate properly these and other new phenomena requires a collider that will achieve much higher energies - 70 times that of LEP - replicating the searingly hot conditions in the Universe one million-millionth of a second after its birth. This machine, the Large Hadron Collider (LHC), is now being built by international teams, with major participation from the UK. Constructed in the same underground tunnel as LEP, it will collide intense beams of protons at 7 teraelectronvolts (million million electronvolts). The LHC will be ready by 2005.

New technology
The advanced technology that has to be developed for such a project is amazing. Five thousand powerful and highly sophisticated superconducting magnets will steer and focus two counter-rotating bunches of particles through a 16-mile long, high-vacuum circular tube until they smash into one another at a rate of 40 million times a second. Near each of the four crossing points of the beams will be a giant detector, the size of a mansion, to capture and measure new particles produced in the collisions.

ALICE detector
cross section

Two detectors, ATLAS and the Compact Muon Solenoid (CMS), will aim to observe the Higgs and supersymmetric particles, perhaps uncovering totally new phenomena (as often happens in research). They are extraordinarily complex pieces of engineering, robust enough to work efficiently in the expected extreme conditions of high radiation. The electronics and computer systems attached to the detectors will have to deal with an incredible amount of data. Each of the one billion particle collisions a second will produce hundreds of particles to be sorted through. Incredible skill is needed to pinpoint the handful of events that will reveal key results.

ATLAS detector

The two other detectors will study further fundamental phenomena. LHCb will be looking for evidence of the asymmetry in particle behaviour that explains the missing antimatter in the Universe. It will search for particles called B-mesons produced in the proton collisions. Another experiment, ALICE, will detect a different set of events: the LHC will be run so as to collide beams of heavy ions (such as lead) instead of protons. The idea is to create the very hot 'soup' of quarks and gluons (which mediate the strong force) that existed one millionth of a second after the Big Bang before conditions cooled enough for them to become locked into composite particles like protons and neutrons.

CMS detector

We don't know exactly what we'll find when the LHC starts up but it is certain to yield new insights into the structure of matter. As well as giving us a more profound understanding of creation and the nature of reality, the discovery of unexpected phenomena may open the door to technologies as yet undreamt of - perhaps cheap forms of energy, new methods of communication or even novel space propulsion. This is truly the science of the third millennium.