Seeing Protons at the LHC

Last week at the Moriond conference Jorg Wenninger gave an interesting presentation on the status of the LHC. One of his slides showed the spot of light that is emitted by the proton beam.

Spot of light produced inside the LHC, image courtesy of Jorg Wenninger.

Spot of light produced inside the LHC.

When a charged particle, like a proton, is accelerated it emits some photons. Depending on the strength of the acceleration the photons can having different energies corresponding to radio waves, visible light, or even X-rays. At the LHC the protons go around a 27 kilometer (16.8 mile) ring at nearly the speed of light. The protons are kept on track by a series of 1232 bending magnets (dipoles). When you go around a corner in a fast car you can feel you body being flung outward, this is because the car is being accelerated inward. (Otherwise the car would keep going in a straight line!)  Each time a proton goes around the LHC ring it looses one billionth of its energy. With the planned operating energy in 2010 and 2011, a proton while going through each dipole will loose about one trillionth of its energy, which would be about 7 electron-Volts (the energy a single electron picks up going through a 7 Volt battery). A video camera positioned at the end of the dipole magnet can pick up this light, which can be used to monitor the size and position of the beam.

This is really the first time a beam of protons has been directly seen using light in the lab. The technical term for this light is synchrotron radiation. In previous machines the synchrotron radiation from protons has been too feeble to see. The amount of synchrotron radiation goes inversely with the mass of the particle squared; this is the reason that the highest energy machines use protons rather than electrons. Electrons are 2000 times lighter than protons, so they would have 4 million times as much synchrotron radiation. If electrons were being used instead of protons, the electricity bill would at least 4 million times larger just to keep the electrons up to speed!

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2 Responses to “Seeing Protons at the LHC”

  1. W W Mackay says:

    “This is really the first time a beam of protons has been directly seen using light in the lab.” This is an incorrect claim. Synchrotron radiation from protons was seen decades
    ago at the SPS.
    See “Proton beam profile measurements with synchrotron light” by R. Bossart et al. in
    Nuclear Instruments and Methods, Volume 184, 349 (1981).

  2. terning says:

    Thanks for the correction!