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Powering up the world’s biggest physics experiment

Posted: September 08, 2008

On Wednesday, the world’s largest particle accelerator – the Large Hadron Collider (LHC) – will be powered up to receive an injection of particle beams for the very first time as it begins the heroic quest to solve some of the biggest mysteries of our Universe. Astronomy Now will be attending this momentous scientific milestone via a live link-up from Westminster to the ceremony at CERN (the European Organisation for Nuclear Research) and we’ll have a full report later in the week.

The LHC has a circumference of 27 kilometres, stretching from Geneva airport in the lower left, to the open French countryside in the upper right. Image: CERN.

Buried one hundred metres below the French/Swiss border and boasting a circumference of 27 kilometres, the LHC is the largest and most ambitious physics experiment in history. By accelerating particles to speeds of 99.9999991 percent that of light, and watching what happens when they collide, it will probe questions surrounding the nature and existence of dark matter and antimatter, the weakness of gravity and even if extra dimensions exist.

The dual beams of the LHC consist of bunches of particles a few tens of centimetres in length containing 100 billion protons. There will be 3000 going around the collider in each direction at any one time, with each beam containing the energy equivalent to the Eurostar travelling at over 90 miles per hour. Sub-atomic smash-ups will occur between the beams 600 million times a second, and detectors situated along the accelerator’s circumference will be on the look-out for the resulting debris of subatomic particles.

The LHC in numbers  
Circumference 27 km
Depth 100 m
Number of detectors 6
Speed of protons 99.9999991% speed of light
No. trips beams make around LHC 11,245 /sec
Number of collisions 600 million/sec
Number of magnets 9,600
Temperature of magnets 1.6 K (-271.25 C)
Number of sensors 150 million
Data expected during experiments 700MB/sec (15 million GB/yr)
Annual power consumption

800,000 mega-Watt hours (19m Euros)

Cost to build $6 billion

On the LHC’s shopping list is the elusive higgs boson particle, which is widely believed to be the origin of the well-known property mass. The Standard Model of particle physics predicts two types of particles: mass-less particles like photons, and those with mass, like quarks (protons and neutrons that make up atomic nuclei) and electrons. The Standard Model also predicts the existence of the higgs boson and states that particles that possess mass do so because they interact with the resulting higgs field, as if they were travelling through treacle. This is important from a cosmological viewpoint, because the presence of dark matter can only be inferred from its gravitational effect, and this gravitational effect results from dark matter possessing mass. In essence, understanding the higgs boson could help scientists finally understand the ins and outs of dark matter, which itself is thought to comprise around 23 percent of the Universe’s energy density while ‘normal’ baryonic matter comprises four percent, and dark energy, which is thought to play a dominant role in the expansion of the Universe, makes up 73 percent. The LHC, therefore, has the potential to open up a treasure trove of information about the nature of the Universe.

But the LHC can’t directly detect a higgs boson; instead its presence can be detected by looking at what it decays into. However, what it decays into is determined by its mass, which is an unknown quantity. There are certain probabilities, however, and by comparing the signatures of what is detected by the LHC compared with what is expected from other processes, any significant statistical ‘anomaly’ could point towards the higgs boson.

Another mystery that the LHC will help solve is why gravity is so weak compared to the other forces of nature such as electromagnetic and nuclear forces. Gravity is well-known to play a dominant role in the Universe but it has no apparent influence at the particle level. Indeed, the 'graviton' particle hasn't actually been discovered. Suggestions as to why gravity appears so weak have resulted in the consideration of more dimensions than the three – width, depth and height – that we currently know about. Could gravity be leaking into our three dimensional Universe from another dimension? The LHC might just be capable of probing these dimensions, which could be curled up smaller than sub-atomic particles.

The detectors
There are six points along the LHC that will gather data from the particle collisions:
ATLAS (A Toroidal LHC Apparatus) is the largest detector, measuring 46 x 25 x 25 metres and will search for the higgs boson, dark matter particles and extra dimensions.
CMS (Compact Muon Solenoid) will support ATLAS detections, essentially an extra pair of ‘eyes’ that will help eliminate systematic errors.

ALICE (A Large Ion Collider Experiment, above) will look at the collisions of lead ions to recreate a form of matter that existed just after the big bang, and should shed light on how everyday matter arose.
LHCb (Large Hadron Collider beauty) will try and find out why we live in a Universe dominated by matter, and not equal amounts of matter and anti-matter. Beauty refers to the ‘beauty’ or ‘bottom’ (third-generation) quark that would be a typical decay product of the proposed higgs boson.
TOTEM (TOTal Elastic and diffractive cross section Measurement) will measure the size of protons and keep track of LHC’s luminosity, or how precisely the LHC produces collisions.
LHCf (Large Hadron Collider forward) simulates cosmic rays in a controlled environment in order to help scientists to devise ways to study naturally occurring cosmic ray collisions.
Which detector will make the first revolutionary discovery in physics? Keep up to date with LHC affairs on

Another aim of the LHC is to try and find the missing link between two of the most successful theories of physics – General Relativity and quantum theory, in which one cannot currently be explained with the other, thus presenting a major hurdle to finding the holy grail ‘theory of everything’ that has been the goal of scientists for decades. The LHC could find out what the bridge between the theories is, and confirm or refute the idea that string theory is part of the equation. String theory states that all different particles are different vibrations in higher-dimensional string structures. One of the things that string theory predicts is the existence of micro black holes, something that could be identified in the LHC in terms of the particles that they decay into. Such a result would provide the first real validation for string theory. But despite scare mongering by the popular media that the Earth will ultimately be consumed in one of these ‘man-made’ black holes, such an event is impossible since they would decay almost immediately, lacking the energy to grow, let alone be sustained.

Moreover, the sub-atomic collisions that the LHC will bare witness to are just a patch on what happens naturally in nature. In its 4.6 billion year existence, the Earth has been subject to a phenomenal number of cosmic ray collisions of far higher energies than could ever be achieved in the LHC, with no ill effects. The only real danger from the LHC is that the machine could damage itself should the magnets that keep the beams inside the collider fail. The result would be that the particle beams would destroy the magnets. To gradually test the full capabilities of the LHC, therefore, the beams won’t be switched on with full intensity at first, but will gradually be powered up over several months before getting down to serious business.

Astronomy Now will be attending the LHC powering up ceremony via a live link-up to CERN. We’ll have a full report later in the week. In the meantime, you can read more about the LHC in the July issue of Astronomy Now magazine.