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Particle detection during collisions

  1. Sep 29, 2010 #1
    I want to know the ways newly created particles are detected during collisions such as in LHC or Fermilab, i.e either they check the EM force, electric charge being observed or other things.....
  2. jcsd
  3. Sep 29, 2010 #2


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    There are different methods for different particles.

    For example, muon detection in the ATLAS experiment is mostly done with chambers consisting of tubes filled with a gas mixture. If a muon traverses the tube it ionizes some of the gas, which sets off an electrical signal on a high voltage wire running down the center of the tube. The signal is processed at the readout side of the chamber and then sent elsewhere for storage and subsequent analysis.

    Of course, a tube only gives you coordinates on two axes (along the beam pathand radially from the beam path). For the third coordinate you rely on triggering mechanisms, which also select which events get recorded.

    I suggest you look through LHC's public outreach pages. They're pretty good.

  4. Sep 29, 2010 #3


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    There are actually only two principles on which people detect elementary particles: by their ionizing effect on matter in one way or another ; or by exciting molecules, atoms, crystals. The particles themselves, their E-fields or whatever are, to my knowledge, almost never directly observed, only their effect on matter. The point is that one single energetic particle can do a lot of ionization or excitation.

    When matter gets ionized, charges are set free, and then there can be electrical means to try to see these charges ; when systems get excited, they will de-excite and send out light for instance. Sensitive instruments try to observe these tiny electrical or light signals.

    From the characteristics of these signals, one can sometimes also deduce certain parameters of the particle that came by, like its mass.
  5. Sep 29, 2010 #4
    But particles like neutrino or neutron are neutral, how do we detect these particles which have no + or - Ion?
  6. Sep 29, 2010 #5


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    Well, they have to undergo an interaction which sets into motion a charged particle.
    For neutrons, if the neutrons are energetic, then they can collide elastically with a nucleus and set the nucleus in motion (as an ion so there is your moving charged particle). If the neutrons aren't energetic, then you have to use a material which undergoes a nuclear reaction with the neutron, and has end products which are fast charged particles (for instance, U-235 can undergo fission with a slow neutron: the fission products are the charged particles then).
  7. Sep 29, 2010 #6
    for information on how to detect newly created particles at accelerator experiments.

    Many newly created particles have very short lifetimes, and even with time dilation, they do not reach the detectors before decaying. In this case, the concept of invariant mass [ (mc2)2 = E2 - (pc)2 ] four-momenta is used. By analyzing all the detected decay particles energies and momenta, it is possible to identify the original newly created particle's rest mass and momentum. See


    Correcting for missing energy and momentum imbalance from escaping neutrinos and detector inefficiencies is done by careful analysis of all the collision tracks.

    Bob S
  8. Oct 2, 2010 #7
    OK...just one more thing...how does a particle like Muon which is also electromagnetically zero (like neutron) gets detected.....
  9. Oct 2, 2010 #8
    The muon and anti-muon are both charged, ± 1 electron charge, and have a mass about 206 times the electron mass. There is no uncharged muon. Both have a decay lifetime of about 2.2 microseconds. Both are easily deflected by the large magnets in the LHC detectors, and leave charged particle tracks in the particle detectors. See


    Bob S
  10. Oct 2, 2010 #9
    He might mean a muon neutrino.
  11. Oct 3, 2010 #10
    Sorry....I mean to say Pion....how do we detect it?
  12. Oct 4, 2010 #11
    how do we detect pion?
  13. Oct 4, 2010 #12
    There are 3 pions, two charged and one neutral.

    The charged pion lifetime is about 26 nanoseconds, so with even a minimal time dilation, the charged pions will reach the charged particle detectors at LHC before decaying to a muon and a muon neutrino.

    The neutral pion decays to two gammas in ~8 x 10-17 seconds (plus a 1% branching to a positron and electron), so the detectors in LHC will detect the two decay gammas as electromagnetic cascades. The pi-zero mass, energy, and direction are reconstructed using the invariant mass theorem mentioned in an earlier post. Of course, the LHC detector is simultaneously (within a nanosecond) detecting 100's of other tracks, many of which are probably also pi-zero decays.

    Bob S
  14. May 31, 2011 #13
    Hi, sorry to bump this one, but I have been trying to find some information on how the data received from detectors look throughout the 'analysis phases'. I'm no wiz at maths, so my question is this, I guess:

    Are there any layman papers/explanations/exemplifications of a (successful, historical) particle detection process? Starting with some images from any kind of detector, and then explaining what the following analysis was that resulted in the 'eureka! -we have a particle!' ?

    I have tried to google this a while now, but couldn't find anything quite down at my level... =)

    Thanks a lot!

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