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Particle accelerators such as the LHC work by accelerating electrons or protons close to the speed of light in opposing directions through an incredibly cold tunnel, until they eventually smash into each other.

I am pretty sure there is vacuum in these tunnels. Why are they cold? Afaik. vacuum contains only a few particles so they probably do not interfere with the experiments. Does heat radiation affect the experiments?

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    $\begingroup$ because they use superconducting magnets to generate highly intense and homogeneous magnetic fields, for example. $\endgroup$ – AccidentalFourierTransform May 29 '16 at 11:15
  • $\begingroup$ As an aside when experimental particle physicists say "the tunnel" they don't mean the vacuum volume of the beam line but the human accessible space in which the beam-line is constructed. You sometimes hear "the pipe" for the former. The tunnel is maintained at a reasonable temperature for humans. $\endgroup$ – dmckee May 29 '16 at 20:26
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You need to distinguish between the cooling of the superconducting magnets and the cooling of the beam line. I'm not sure it's clear which, if either, of these the article is referring to.

The superconducting magnets are cooled for the rather obvious reason that otherwise they wouldn't be superconducting. I don't think there's much more to say about this.

The beam line is cooled for a rather different reason. The vacuum inside the beam line has to be extremely good otherwise the protons would collide with gas molecules and the beam would rapidly attenuate. The walls of the beam line are cooled so that any gas molecules that strike the interior surfaces will stick to them. In effect the aim is to freeze any gas inside the beam line onto the internal surfaces. There is a nice article about this here.

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