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Dark matter, electron-positron

  1. Feb 19, 2013 #1
    Hi I have a question about a popular science article from BBC, you can ofcourse read the whole article but the part that left me wondering was his which I quoted right here.

    """Prof Ting said that in its first 18 months of operation, AMS had witnessed 25 billion particle events. Of these, nearly eight billion were fast-moving electrons and their anti-matter counterparts, positrons"""

    So how come the detector detect electrons and positrons if positron is the electron's antiparticle , how come the positron hasn't already annihilated with the electron after they were produced , how did they came so long way being separate to enter the detector?
    And when they enter or before they enter how come the detector distinguish them as they would probably annihilate each other in the moment of detection?
    Ok share your thoughts.
  2. jcsd
  3. Feb 19, 2013 #2

    Simon Bridge

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    because it was built to do so.
    When the two particles are created they are travelling away from each other, so they cannot annihilate.
    They have a high speed.
    They are detected in different places. The details are in how the detector works - how it detects stuff.

    i.e. an electron hitting an metal plate would give it a net negative charge - a positron hitting one would annihilate with one of it's electrons producing a net positive charge. The photon produced could also be detected via a photomultiplier.
  4. Feb 19, 2013 #3
    Ok I was maybe a bit unclear I understand that the detector ir built so to capture these particles and all that , the questions wasn't so much about how does the detector detect those particles rather how those particles survive all the way to make it to the detector without annihilating each other.

    Once in the detector the powerful magnet sets the electron apart from the positron with it's field lines but I wonder how did they make it till the detector all this way without annihilating at the very birth of their journey? Maybe a little deeper explanation would be much appreciated.
  5. Feb 19, 2013 #4

    Simon Bridge

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    I said: When the two particles are created they are travelling away from each other, so they cannot annihilate.

    A particle anti-particle pair created at rest with respect to each other would never reach the detector and you won't detect them. The probability that the two particles will annihilate, even when quite close together, can still be quite small. Look up "positronium".
  6. Feb 22, 2013 #5
    The only thing I can add to what Simon said is that when they are created, e+ and e- have high momenta. In a typical collision of a particle with a fixed target, the e-e+ produced will have their longitudinal momenta alligned to conserve momentum, yet their transverse momenta will be in the opposite direction. The magnitude of the transverse momenta is such, that particles finally "choose" to move away from each other.
    If, say, they were produced at rest, they would finally produce bound state which would annihilate into 2 (or 3) gammas..
    Hope this helps.
  7. Feb 23, 2013 #6


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    That would be too hard to detect.

    High-energetic electrons and positrons look very similar in particle detectors - they fly through the material, and produce electron/hole pairs in silicon detectors, Cherenkov photons in Cherenkov detectors, transition radiation in transition radiation detectors and so on. Almost all particle detectors have a magnetic field, and this allows to distinguish positrons and electrons: They have a different charge, so they are deflected in different directions.

    The annihilation "probability" (cross-section) is small for high-energetic positrons. They can travel through the universe easily - as long as they don't hit any significant massive object and slow down there.
  8. Feb 23, 2013 #7

    Simon Bridge

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    Well yes of course.
    The comment is addressing a different point. Well ... I hoped it would.
    Deflection in a magnetic field is the more usual way to check.
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