Page last updated at 22:51 GMT, Friday, 20 November 2009
Guide to the Large Hadron Collider



The Large Hadron Collider (LHC) will smash two beams of particles head-on at super-fast speeds, recreating the conditions in the Universe moments after the Big Bang, writes BBC science reporter Paul Rincon.

Alice time projection chamber (Cern/A. Saba)
The Alice detector will investigate the moments after the Big Bang

Scientists hope to see new particles in the debris of these collisions, revealing fundamental new insights into the nature of the cosmos.

They will be looking for new physics beyond the Standard Model – the framework devised in the 1970s to explain how sub-atomic particles interact.

The Standard Model comprises 16 particles – 12 matter particles and four force-carrier particles. The Standard Model has worked remarkably well so far.

But it cannot explain the best known of the so-called four fundamental forces: gravity; and it describes only ordinary matter, which makes up but a small part of the total Universe.

Also, one of the most important particles in the Standard Model – the Higgs boson – has yet to be found in an experiment.

Today, the Standard Model is regarded as incomplete, a mere stepping stone to something else. So the LHC should help reinvigorate physics' biggest endeavour: a grand theory to explain all physical phenomena in Nature.

However, some physicists point out that Nature has a habit of throwing curve balls. And some of the most exciting discoveries at the LHC could be those that no-one expects.

THE HUNT FOR THE HIGGS

There is an essential ingredient missing from the Standard Model. Without it, none of the 16 particles in the scheme would have any mass.

An extra particle is required to provide all the others with mass – the Higgs boson. This idea was proposed in 1964 by physicists Peter Higgs, Francois Englert and Robert Brout.

According to their theory, particles acquire mass through their interactions with an all-pervading field, called the Higgs field, which is carried by the Higgs boson. It is the only Standard Model particle that has yet to be observed experimentally.

CMS (M. Brice/Cern)
The CMS is one of two LHC experiments looking for the Higgs

As such, the search for the Higgs has become something of a cause celebre in particle physics. Finding the Higgs is one of the main science objectives for the LHC.

The Atlas and CMS experiments are both designed to see it, if it is there. This means that scientists working on these respective experiments will be competing to see it first.

The US Tevatron particle accelerator, though less powerful than the LHC, has been engaged in the hunt for the Higgs.

IN THE DARK

All the matter that we can see in the Universe – planets, stars and galaxies – makes up a minuscule 4% of what is actually out there. The rest is dark energy (which accounts for 73% of the cosmos) and dark matter (23%).

Dark energy cannot be observed directly, but it is responsible for speeding up the expansion of the Universe – a phenomenon that can be detected in astronomical observations.

Artist's impression of dark matter distribution (Nasa/Esa/Richard Massey-Caltech)
Astronomers have mapped dark matter's distribution, but have no idea what it is

Like dark energy, dark matter can only be detected indirectly, as it does not emit or reflect enough light to be seen. But its presence can be inferred through its effects on galaxies and galaxy clusters.

Physicists know virtually nothing about the nature of either dark energy or dark matter. But they can speculate.

According to one idea, dark matter could be made up of "supersymmetric particles" - massive particles that are partners to those already known in the Standard Model.

A leading dark matter candidate is the neutralino, the lightest of these "super-partners". And some theoretical physicists have proposed a link between the Higgs mechanism and dark energy.

MIRROR, MIRROR

Each basic particle of "ordinary" matter has its own anti-particle. Matter and antimatter have the same mass, but opposite electric charge.

For example, a proton has an anti-particle called an anti-proton (a proton with a negative charge). An electron has an anti-particle called a positron (an electron with a positive charge).

In the same way that an ordinary proton and electron can come together to form a hydrogen atom, an anti-proton and a positron can form an atom of anti-hydrogen.

When a particle of ordinary matter meets its anti-particle, the two disappear in a flash, as their mass is transformed into energy.

They are said to "annihilate" one another. But equal amounts of matter and anti-matter must have been produced in the Big Bang.

So why did matter and anti-matter not completely annihilate each another after the birth of the Universe?

Today, we live in a Universe almost entirely composed of ordinary matter. Scientists will use the LHC to investigate why this is, and what happened to all the anti-matter.

DOUBLE TROUBLE

Attempts to unify gravity with the other fundamental forces have come to a startling prediction: that every known particle has a massive "shadow" partner particle.

Atlas wheel (Cern)
Atlas is one of the experiments that could find evidence for supersymmetry

All particles are classified as either fermions or bosons. A particle in one class has superpartner in the other class, "balancing the books" and doubling the number of particles in the Standard Model.

For example, the superpartner of an electron (a fermion) is called a selectron (a boson). Evidence for supersymmetry would enable the "unification" of three fundamental forces - the strong, weak, and electromagnetic – helping to explain why particles have the masses they have.

It would also give a boost to string theory – one stab at a grand "theory of everything". But string theory is not dependent on discovering evidence for supersymmetry.

OTHER DIMENSIONS

In addition to the four dimensions we already know about, string theory predicts the existence of six more.

Some physicists even think the existence of these extra dimensions could explain why gravity is so much weaker than the other fundamental forces. Perhaps, they argue, we are not feeling its full effects.

This might be explained if its force was being shared with other dimensions. If these extra dimensions do exist, the LHC could be the first accelerator to detect them experimentally.

At high energies, physicists could see evidence of particles moving between our world and these unseen realms. For example, they could see particles suddenly disappear into one of these dimensions.

Alternatively, particles originating from an extra dimension could suddenly appear in our world.

THE HOLE TRUTH

According to some physicists, the LHC can operate at high enough energies to generate mini-black holes.

However, the vast majority of particle physicists say there is no need for alarm. If any should be created, they should evaporate quickly.

How a black hole might look if it is generated in the collider (Atlas)
How a black hole might look if it is generated in the collider

A report dealing with the collider's safety acknowledged the possibility that the LHC could create these primordial black holes.

The report says: "If microscopic black holes were to be singly produced by colliding the quarks and gluons inside protons, they would also be able to decay into the same types of particles that produced them.”

The suggestion that black holes could be made in the LHC has stoked fears that one of these micro-black holes could swell in size, swallowing up the Earth.

But physicists stress that any such phenomena would be short-lived and thus would pose no threat to our planet.

The size of everything (BBC)



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