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Particle accellerators: How do they get the particles to hit eachother?

  1. Apr 24, 2012 #1
    I've never understood how they do this.

    Particles being extremely, extremely tiny, how do they manage to send them around the accelerator a million times until they reach almost the speed of light, then when they reach that speed, crash them together?

    My question is how do they get them to avoid each other until they hit almost light speed, then when they hit almost light speed, how do they get them to crash together?
  2. jcsd
  3. Apr 24, 2012 #2


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    In the LHC, the two counter-rotating beams are kept separate, and they only cross the beams in a few places. They use magnetic fields to steer and focus the beams in the locations where they want the beams to interact. There is no control over the individual particles - they just cross the two beams and rely on chance to have some of them collide. Imagine two machine guns firing at each other - most bullets will miss, but occasionally they will hit. Since there are a huge number of particles in the beams, there are enough collisions to analyze.
    Last edited: Apr 24, 2012
  4. Apr 24, 2012 #3


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    That is why you hear the term "luminosity", and the attempt to get this as high as possible. It gives an indication of how much and how often one gets collisions. The majority of particles passing through each other in each bunch do NOT collide. So the collision rate is statistical.

  5. Apr 24, 2012 #4
    LHC has two proton beams in separate accelerators colliding at crossing points inside each detector. The Fermilab Tevatron (shut down last September) had a proton beam colliding with an antiproton beam, both beams in the same accelerator. Because protons and antiprotons have the same mass but opposite charge, both beams had the same closed orbit (trajectory), but in the opposite direction. So collisions inside each detector was automatic, and some extra effort was needed to prevent the two beams from colliding elsewhere. Each beam bunch had ~ 1012 particles, and the desired interaction rate is of the order of 1 per "crossing."

    The "cross section" for pp and p-bar p collisions is given in the plots on page 12 of http://pdg.lbl.gov/2011/reviews/rpp2011-rev-cross-section-plots.pdf, so the probability of one particle in one beam hitting one particle in the other beam is roughly 4 x 10-26 cm2 divided by the beam cross sectional area at the colliding point.
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