ATLAS and CMS

The Compact Muon Solenoid: The CMS detector in its cavern. Protons travel through the orange pipe to the centre of the detector where the particles collide. Image © CERN.

ATLAS and CMS are the two ‘general-purpose’ detectors at the LHC. They’re looking for any new particles or unknown physics which the LHC’s record-breakingly high energies might allow us to observe for the first time.

Download detectors infosheet (PDF) »

The Higgs boson

Probably the most famous goal of ATLAS and CMS is to spot the Higgs boson—a particle predicted independently and almost simultaneously by three groups of physicists, including Peter Higgs, which allows all other particles to have mass. It’s a bizarre problem in modern physics that we can explain the properties of subatomic particles to incredible precision, but we’re unable to explain why they have mass—in other words, what makes it hard to accelerate or decelerate them.

The Higgs boson is the only particle in the ‘standard model’ of particle physics which hasn’t been observed. There are two types of particle in the model: fermions, which are ‘stuff’ (eg electrons, and the quarks which make up protons and neutrons); and bosons, which transmit forces (eg photons, which are particles of light, and transmit electromagnetic forces). The Higgs boson is one of the latter and, if it exists, would be responsible for giving particles mass. The theory says that the Universe is filled with a sticky soup of Higgs particles, and those particles which interact most strongly with the Higgs particles are bogged down by them. This is what we think of as mass—a tiny, light electron barely sees the Higgs particles, whilst a proton (which is 2,000 times heavier) is wading through a dense sea of them.

If we find it, it would be the last piece in the standard model’s mathematical house of cards and, further new physics notwithstanding, our understanding of subatomic particles would be nearly complete. If we don’t, it’s perhaps more exciting for particle physicists, because suddenly a cornucopia of new potential theories is unleashed, ready for the LHC to test.

Dark matter

Astronomers have known for some time that there’s something missing in our understanding of the Universe. We can predict with extremely high precision how objects in the cosmos should orbit one-another: in the Solar System, for example, this works extremely well, and we can predict the motion of planets and moons with incredible accuracy. However, when we try to apply these laws to whole galaxies, something goes wrong: stars near the edges of the galaxies are rotating around galactic cores much more quickly than we’d expect given the matter we can see with telescopes. This effect scales up to clusters of galaxies too. We can measure their mass using ‘graviational lensing’, where gravity bends the light from further-away objects. The curvature of the light’s path is related to the amount of mass bending it, and this allows us to deduce that there’s much more there than just the visible mass in the clusters.

What this mass is composed of we just don’t know, but it’s possible that it’s made up of heavy, weakly-interacting subatomic particles. The hope is that some of these dark matter particles will be produced in the LHC.

New particles, unknown physics

It’s easy to forget when we have some very definite ideas of what we might find at the LHC that we might see some things which we’ve never thought of before. The LHC is the highest energy particle collider which has ever been built, and might well allow us to create particles which are completely new to science. ATLAS and CMS would be the likely places that these kinds of results would be unearthed.

Sensitive measurements of the energies of particles created in a smash allow us to work out the mass of a new particle created thanks to E = mc². Then, more subtle measurements of what kind of daughter particles follow and their speed and direction of flight will hopefully allow scientists to work out exactly what they’ve made and work out what, if any, effect this has on existing theories—or entirely new ones!