Born and raised in South Africa, Claire has held a love for astronomy and physics since she was young. She is interested inparticular in the relation between the two fields of particle physics and cosmology. She is currently working towards a PhD on the search for the Higgs boson with the ATLAS experiment at the Large Hadron Collider.

Astronomers and particle physicists used to work independently from each other, but now more than ever there is significant overlap between the fields. The Large Hadron Collider, the world's largest and highest-energy particle accelerator, aims to use the science of the very small to answer questions of the grandest scales imaginable. Particle physicist Claire Lee explains how this is possible.

From Quarks to the Cosmos: exploring the Universe through particle physics

Since the dawn of time, people have wondered about the Universe: how it began, why it looks the way it does, when and how it will end. Astronomers have contemplated the formation of stars and galaxies, why they group together into clusters and superclusters, why these clusters are all moving away from each other at an increasing rate, despite gravity trying to pull them back together... the list goes on.

A murky Universe

To find the answers to these questions, astronomers turn their telescopes to the sky, scanning through wavelengths from high energy gamma rays to the lowest energy radio sources, searching through space for the next clue in our quest to solve the myriad of puzzles about space.

To astronomers, the Universe is murky for the first 380 000 years after the Big Bang; we can't see what happened using telescopes. But we can using particle physics. Back then the Universe was so hot it was just a primordial soup of the basic particles that make up everything we see in the Universe today. Using particle accelerators to probe inside protons, we can see way back in time, all the way back to a tiny fraction of a second after the Big Bang.

The Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, is the world's largest particle accelerator — a 27 km ring of over 9000 magnets that crosses the border of France and Switzerland. The superconducting magnets are cooled with liquid helium to 1.9 K, making the LHC the coldest place in the Universe.

Inside the accelerator, two counter-rotating beams of protons travel very close to the speed of light and collide head-on in the centres of four large detectors, 600 million times per second. The detectors: ALICE, ATLAS, CMS and LHCb, are each designed differently in order to learn something new about the Universe.

A massive problem

In particle physics we have the Standard Model, which describes all the elementary particles that make up the Universe we see. The Standard Model has a major problem, however: the simplest version of the theory requires the masses of all the particles to be zero. Now this is obviously not the case, the Universe has given us a whole zoo of particles with a wide range of masses. We can't say we really understand how the Universe works until we know how everything in it got its mass.

Fortunately in 1964 some physicists (most notable among them Peter Higgs after whom the mechanism was named) came up with a theory for giving the particles the masses they have. According to this theory, as the Universe cooled during the first fraction of a second after the Big Bang a field suddenly materialised. We call this a phase change, much the same way liquid water changes to ice as it cools. Before this phase change all the different elementary particles had zero mass and moved around at the speed of light. But once the Higgs field had "frozen out", the particles started to get dragged to slower velocities, some to a greater degree than others. The more a particular particle was slowed, the more of its total energy was concentrated into mass.

In particle physics a field is made up of a whole lot of particles — bosons — that carry the field's effects. The Higgs field is thus made up of many Higgs bosons, popping in and out of existence all the time. It's the way that each particle interacts with these Higgs bosons that determines how much it is slowed down, and hence, how much mass it has. If the Higgs boson really does exist, then there is a good chance that it will be created at the LHC and detected in the ATLAS or CMS detectors.

What's the (dark) matter?

The Standard Model of particle physics describes all the particles in the Universe that we can see. But there's something else out there that we can't see: the aptly-named Dark Matter.

Dark Matter is pretty easy to find if you know where to look, because it's almost everywhere. Or perhaps I should say it's easy to see its effects: gravitational lensing, for example, can detect huge halos of dark matter by measuring how much light is bent as it passes a cluster of galaxies. From General Relativity, we can calculate the mass of the cluster by how much it bends the path of the light. But this ends up being far more than the mass we would get if we just added up all the stuff we can see.

The problem with Dark Matter is that we can't see it — the only way we interact with it is through the gravitational force — and we really have no idea what it is. But there's about five times more of it than all the normal matter, and it's responsible for the large scale structure of the Universe: the way the galaxies group themselves into clusters, the clusters into superclusters, and so on.

Thankfully, particle physics has a possible solution: supersymmetry. Supersymmetry extends the Standard Model of particle physics by matching each particle of one kind with a new partner particle of another kind. These new particles are much heavier than their partners which is why we don't see them with our detectors, even if they do exist. The lightest one of these particles — called the neutralino — is the best candidate we have for all that elusive Dark Matter out there, and it's possible that the LHC will have enough energy to create neutralinos in its collisions.

Where did all the antimatter go?

Another thing that puzzles physicists is why we have anything in the Universe at all. At the Big Bang, equal amounts of matter and antimatter were created. But when one particle of matter meets one particle of antimatter, they annihilate in a burst of energy, leaving no particles behind. Somehow, though, there must have been a slight excess of matter, so that when all the antimatter had annihilated, the Universe was left with all the matter we see today. The aim of the LHCb experiment is to find out just why there was this matter-antimatter asymmetry that ultimately gave us the Universe.

Probing the Bang

Finally, proton-proton collisions aren't the only ones that will happen at the LHC. The ALICE detector is specifically designed to look at collisions between lead ions. These collisions will recreate the conditions of extremely high densities and temperatures that existed a fraction of a second after the Big Bang. With the LHC, physicists will be able to take a look at what the Universe was like just when it began, learning more about the most fundamental constituents of matter and the first hundred microseconds of the evolution of our Universe.

Breaking boundaries

Even with the amazing discoveries we have had over the centuries, we are still a far way from a complete understanding of how the Universe works. Traditionally, astronomers would use their telescopes to study the sky and physicists would create particle collisions inside their detectors, with very little overlap between the two fields. But now we are entering a new era where the boundaries between astronomy and particle physics are no longer clearly defined, where we can draw conclusions about the very large by investigating the very small. We still have a number of questions about our Universe, and now particle physics may be able to give us some answers.

This feature article was written as part of the Cosmic Diary Cornerstone project for the International Year of Astronomy 2009. To find out more, check out www.cosmicdiary.org and www.astronomy2009.org.