The Particle Zoo: The Search for the Fundamental Nature of Reality by Gavin Hesketh

Author:Gavin Hesketh [Hesketh, Gavin]
Language: eng
Format: epub
Tags: Science, Physics, Atomic & Molecular, Quantum Theory, General
ISBN: 9781784298692
Google: XqdnCwAAQBAJ
Amazon: B01ARXVSS2
Publisher: Quercus
Published: 2016-08-31T16:00:00+00:00

The CERN accelerator complex leading to the LHC.

That’s what’s going on inside the LHC, but what does it look like? The LHC sits about 100 m below ground, in a tunnel around 4 m high. It only takes up about a quarter of that tunnel, so there is plenty of room to stand up, and even ride a bike – the preferred method to get around the 27 km circumference when carrying out maintenance work. The protons travel round in a ‘beam pipe’, roughly 6 cm in diameter. There are two, one for the proton bunches travelling in each direction, and they have to be as empty as possible, otherwise protons will be lost in collisions with random air molecules as they travel around. So the pipes are held at a vacuum – emptier than outer space, in fact. The pipes are surrounded by the accelerating cavities and giant magnets, so if you were to go into the tunnel, these are what you’d see – interspersed with some of the cryogenics to cool the magnets, high-voltage equipment for the cavities and electronics to monitor everything. And all this requires a lot of power: when it is running, the LHC uses around 200 megawatts, much the same as a town of around 60,000 people.

Precision machines

At the start of this process we took a hydrogen atom and split it into protons and electrons. The LHC uses the protons, which are really a bag of quarks and gluons. It is impossible to know whether a quark or a gluon will get pulled out of the bag for each collision, or how much of the proton’s total energy it will carry – it will certainly be less than the full 6.5 TeV. Electrons are fundamental particles; they carry all of their energy into every collision. So it’s reasonable to wonder why the inefficient protons are being used instead of the electrons.

When particles are bent around corners, they lose some energy by emitting photons known as synchrotron radiation. Sticking with the analogy that our particles are surfing along in an electric field, synchrotron radiation is the water that is kicked up when the surfer makes a turn, and kicking up that water costs some energy. Lighter particles produce more synchrotron radiation than heavy particles. A lot more. The rate falls with the mass of the particle squared, squared. An electron is around 2,000 times lighter than a proton, and at some point it just becomes impossible to keep electrons going around in a circle at high speed due to the amount of energy they lose.

As an example, the LHC tunnel was previously used by the LEP accelerator, colliding electrons and their antimatter partner, positrons. LEP could reach an energy of just over 200 GeV, while the LHC, accelerating the much heavier protons around the same 27-km tunnel, reaches 70 times that energy. Some plans for a future electron–positron collider to reach higher energies therefore use a linac rather than a synchrotron: building a linac many kilometres long is still easier than dealing with synchrotron radiation.

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