LHC at 3.5TeV

The LHC passed a new record in the early hours of yesterday morning, both of the beam surpassed 3.5TeV at 5:23am on the 19th March. They still have a way to go to achieve the full capability of the machine, which is designed to probe matter in order to discover the forces which govern our universe, in particular, to look for the origin of mass - and see if the proposed Higgs Boson exists. A TeV is a Tera-Electron-Volt, 1012 eV or 1 000 000 000 000 eV. An eV is the amount of energy given to an electron when it is accelerated through 1Volt. 3.5TeV is 5.6x10-7 Joules, or 0.56 μJ. For comparison, it takes about 1J to lift an apple through 1metre. By this standard, 3.5TeV doesn't sound a lot, but when lifting an apple by 1m, you're lifting approximately 1028 atoms, giving each atom roughly 10-28 Joules, so the LHC is providing each particle in the beam with about 1021 times more energy, that's 1 000 000 000 000 000 000 000 times more.


The question some may have is this: What is a Hadron, and why collide them? Particle physicists build colliders as they allow objects to approach very closely, and simulate the conditions of the early universe, this reveals the structure of matter at a fundamental level.

At CERN, for many years, the large accelerator was LEP, the electron-positron collider. Positrons are like positive electrons, they are the antimatter version of the electron (and had themselves to be manufactured in a particle accelerator). Colliding these particles. With particle accelerators, energy is the key - and as electrons are light, for a given energy the speeds are very much higher. This leads to increased problems with synchrotron radiation and the like, which provides an upper limit on the energy obtainable for a given size of machine.

To get to higher energies, more massive particles are needed, and the LHC uses protons.

Matter comes in two fundamental varieties, there are leptons (which don't feel the strong force, but do feel the weak force). Electrons fall into this class, along with the three neutrinos, muons and taus.

The other major class is the hadrons. These are not fundamental particles, but consist of quarks (of which there are six). The most common hadrons are protons and neutrons, which, along with electrons, are the building blocks for everyday matter. Neutrons cannot be accelerated as they have no charge, so the LHC collides protons.

The collisions are much more complex than for LEP. One reason is that a proton is a compound object, each proton contains three quarks, two 'up' quarks and one 'down'.

LHC cross-section For LEP, the electrons and positrons had the same charge, and so could be accelerated in the same beam tube - travelling in opposite directions. At the LHC, two beam tubes are needed, and the tubes are made to cross at the collision sites. In the photo, the beam tubes are the two tubes on the bottom, and the shape of the magnetic dipole can be seen. The top tube carries coolant.

CERN is the most outstanding piece of 'blue sky' research on the planet, and a hugely impressive piece of engineering.

I've visited the site twice, once when the LEP was still installed, and again last year, the day before the LHC was switched on again following the incident where the coolant system failed.

The collider uses very high currents to produce large magnetic fields, and this requires superconducting magnets. This in turn requires very low temperatures (though there are other reasons for low temperatures too), and liquid helium keeps everything nice and chilly. Unfortunately, there was a fault which caused a slight heating, this meant that superconductivity was lost, and there was then rapid heading as 2000kA flowed through material with resistance. The liquid helium turned to gaseous helium, and tried to expand a few hundred-fold. It couldn't, so the pressure rapidly rose, and the casing popped, filling the tunnel with liquid Helium. This was a catastrophic failure, and so violent that it pushed the machine out of alignment. This is called a 'quench'.

A new quench prevention system was fitted, and the machine is being started up again. The cool temperatures cause each section of the LHC to shrink considerably, this itself poses mechanical problems. The picture below shows a cutaway of the beam tube. At the join between sections, one piece of the beam tube slides into another, and to maintain the vacuum in the beam tube, the whole assembly is contained within metallic bellows. It's a beautiful piece of engineering, very elegant.

Beam tube expansion

For those who can't see the adventure in this, who say 'what's that good for', I pity you. Blue sky research, true research, cannot proceed on the basis of utility. If we did that, nothing new would be discovered except by pure dumb luck (which does play a part in scientific advance). Virtually everything in the modern world started with blue sky research. I could try to answer that utilitarian question by citing the spin offs (including the technology you're using to read this) - but to do so would be to legitimise the question. The LHC is a beautiful thing, and I wish it every success.

Live Beam info can be seen online, and as well as an explanation.