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Supercollider imaging

  1. Sep 9, 2011 #1
    OK, here's my first foray into the physics section, and I might as well start by giving you all a good indication of my overwhelming ignorance.

    Here's my questions: The images one sees around from colliders such as the LHC, you know, the curly lines showing particles going off in different directions after atoms or ions are smashed together, how do we get them? Do ALL particles give off EM? Are there any that need to be imaged in some other way, and what way could that be? And does the energy and wavelength of the EM indicate the type of particle, or what differentiates them? How would we know when we "see" the first Higgs boson, for example?

    Thank ye kindly in advance.

    [Not a homework question, I am waaay too old for that.]
  2. jcsd
  3. Sep 9, 2011 #2


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    Charged particles ionise the medium inside the chamber, and the position of those ions is then detected. In old times a bubble formedaround such ion and was then photographed, in modern gas chambers the position of the ion is reconstructed from charges inducing on a combination of sense wires (complicated as RTG tomography, but the base idea is as simple, as with old-fashioned bubble chamber)

    Only charged particles leave "continuous" tracks (continuous - multiple points per mm).
    As the chamber is in magnetic field - the tracks are bended, so we may know the particles charge (+ or -) and momenta. There is a bigger problem with neutral particles - they are not seen directly. But gamma photons from time to time kick some electrons with Compton's scattering, and those electrons make small spirals (they have so low energy, that are not sligtly bended, but spirals with a few mm radius). Gammas may also produce pairs of particle/antiparticle, which start from the same point, their sum momentum points towards centre of the event. Neutral hadrons (mostly pi^0) are seen either as they form a shower of multiple particles if they collide with some nucleus, or they decay, and products of that decay may be identified and sum of momenta points towards neutral particle origin. Neutrinos are not detected.

    Exotic particles (like Higgs...) are never seen directly. They live too short. But the signature of them are products of their decay, fulfilling some constraints on total mass/energy.
    Last edited: Sep 9, 2011
  4. Sep 9, 2011 #3
    So, it's a combination of direct EM measurements for some particles, and indirect via induced ion movements, reconstructed as an image, that we are "seeing?"
  5. Sep 9, 2011 #4


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    Pretty much.
  6. Sep 10, 2011 #5


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    Actually even EM measurements are not "direct".
    Even in a bubble chamber it was not "direct" - charged particle ionised medium, later on those ions were seeds to form bubbles of vapour, bubbles got photographed, and what we'd seen was those photographs.
    In modern gas drift chambers particle ionises gas, those ions drift towards electrodes, multiply in high fields neer the electrode wires, then electric pulses are measured, and their intensity and timings are used for computer based reconstruction of the positions, where original ions got created.

    Measurement of neutral particles is just one step more indirect: they must either decay or scatter electrons, or collide with nucleons, to produce charged particles, which in turn ionise gas.

    But, the tradition is strong: on most of images you may see in popular articles (SciAm), only tracks of charged pictures are shown - so those pictures look similarily to old cloud/bubble chamber photographs.

    PS. Gas tracking detectors are not the only used. They are common, but some experiments (e.g. CMS at LHC) utilize silicon (semiconductor) tracking detectors, and in some combination of different techniques is used.

    But general idea is the same for all: particle ionises the medium and those ions/free electrons are then amplified, detected and positioned electronically.
  7. Sep 10, 2011 #6
  8. Sep 10, 2011 #7
    Funny, 25 years ago as an undergraduate I worked for a physics department graduate student and her advisor on identifying particles in streamer chamber events - she had three views from what were then exotic CCD cameras, and I used and help improve a simple track finding algorithm to work out 3D particle paths from the three views, after manually clicking points on each separate track in each view on a VAX mainframe terminal. Mostly protons as I recall, these were like 100 MeV heavy ion collisions, but occasionally you'd see electrons, pions, alphas, and heavier nuclear chunks. I wonder how the technology of analyzing these collisions has improved over the past 25 years.
  9. Sep 10, 2011 #8


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    My direct knowledge is also a bit outdated - I left Univ 15 years ago, but since electronic readout replaced scanning photographs, all track identification, track matching and identification of common particles is done automatically. Some part of that identification is even done online, such that simple signatures may be used to trigger for readout of the whole detector.
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