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Would a transversal beam of electrons increase the energy collision in the LHC?

  1. Jun 25, 2012 #1
    Hi All,

    Protons repel each other and in order to make them collide in the LHC huge amounts of energy have to be put in place, my question is; if we launch a beam of electrons transversal to the beam of protons and right in the collision point of the protons, would not this increase the energy at which the protons collide since the electrons would minimize the protons repulsion?

    I imaging that to be able to squeeze a beam of electrons there we would also need lots of energy but I wonder if it would pay off.

    Thank you.
    Last edited: Jun 25, 2012
  2. jcsd
  3. Jun 25, 2012 #2
    Well, estimate the potential energy associated with the proton repulsion at the collision point and compare it to the their kinetic energy. Is it relevant? not really.

    The reason allot of energy has to be put in the protons is that we wish to collide at them at very high energies, in order to produce particles with more mass. But they would collide at much lower energies.
  4. Jun 25, 2012 #3


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    This is rather strange. The amount of space-charge for the proton beam is actually quite small since there's an extremely small number of protons to start with (significantly less than 1 nC per bunch). So the perceived problem that you are talking about here doesn't matter much.

    Furthermore, there's a reason one uses magnets such as a solenoid or quadrupoles. One can FOCUS these beams to a very small point at various locations along the beamline. So getting them to get to a small cross-section isn't an issue either.

    NOTE that at the LHC currently, the problem isn't too few collisions, but it is getting to be too many! Read the issue on a possible LHC data pile-up!

  5. Jun 25, 2012 #4


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    e2/r = (e2/ħc)(ħc/r) = (1/137)(197.5 MeV-f/1 f) = 1 or 2 MeV

    Actually the goal is to make the protons collide any way at all, dumping their tremendous kinetic energy into an interaction, and if they want to do this via a photon that's fine. The probability of a collision at 8 TeV being elastic is negligible.
  6. Jun 25, 2012 #5
    Oh, I see, so the repulsion among protons is negligible in the collision compared to their kinetic energy, that would explain it. Thank you all!
  7. Jun 25, 2012 #6


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    It is about 20nC (1.3*10^11 protons) per bunch.
    Space charge effects are not negligible and the beam/beam interaction close to the collision points has to be taken into account.

    In addition to Bill_K's calculation: This 1-2 MeV is related to the last femtometers of the proton/proton approach, while the collisions take place anywhere within ~10cm length.

    An electron beam parallel to the beam can be used to cool a proton beam (and therefore reducing its width and increasing the collision rate), this is called electron cooling.
    However, the LHC protons are so high-energetic that the required electron accelerator would be quite long and expensive. The acceleration itself is used to focus the protons, and this is enough to get a high collision rate for several hours.
  8. Jun 25, 2012 #7
    Oh, wow! so it even has a name; electron cooling. I guess the question wasn't that bad after all! :tongue: Though the beam is just parallel, never transversal, right? OK.

    Thank you mfb!
    Last edited: Jun 25, 2012
  9. Jun 26, 2012 #8


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    It is parallel to give the beams some time to exchange thermal energy. The protons have a design path and energy, but in addition they have some random motion around these design values, which is similar to a proton gas with some "temperature". If you add "cool" electrons to the beam, the protons can transfer a part of their thermal energy to the electrons.
  10. Jun 27, 2012 #9
    Oh I see! Well that makes sense. Thanks again mfb!
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