Higgs Hunters Talk

FAQs / Classification help (Read me first!)

  • jokergirl by jokergirl moderator

    I thought I'd start to add a few FAQs in case it helps anyone who is confused (like me!).

    Classification FAQs

    Q: What are we classifying?

    • A: We are classifying lines that seem to sprout from a common point that is NOT the center. These are called "Off-center vertexes" (OCVs). Ideally, the lines should not extend over that common point, but practically, we see that the computer often makes it look like they do (so-called "backwards" lines). Use your own judgement in this as much as you can, and use the discussion button to get a look at the zoomed-in and side view to verify your guesses when you are starting out. You'll get the hang of it after a few examples!

    Q: Do I mark single white lines as OCVs?

    • A: No, just ignore them.

    Q: What are those colourful circles or dots I see sometimes?

    • A: They are artifacts from the computer. Ignore them.

    Q: Are all line crossings OCVs?

    • A: No. Remember that we are looking at a 2d representation of a 3d picture (+time). Sometimes the paths of some particles just seem to cross in one of the views. Use your own judgement and don't worry, if it's unclear in one view the data for the other view will make up for it!

    Q: What if I can't find any OCVs in a picture?

    • A: That's OK. Not all pictures have OCVs. Don't mark anything (except if you see something weird), just click "OK" and move on to the next picture.

    Q: What qualifies as "something weird"?

    • A: @andy.haas: "Lots of tracks in one tight bunch, or coming out at weird angles but not forming a vertex, or a big amount of tracks on one side but the other side, or lots of energy in the calorimeter (the red/green areas beyond the tracker), or things in the muon system (the outermost detector)... what ever looks weird to you!"

    Interface FAQs

    Q: What do I see in the Normal view/Zoom view?

    • A: You are looking along the axis where the particles travel. This means you are looking at the collision head-on, and the detector is shown cut through the point of collision. The rings on the outer edge are the muon detector (blue), the calorimeters (red and green) and grey detector fields as we approach the center.

    Q: What do I see in the Slice view?

    • A: You are looking at the particle beam at 90 degrees. Basically, you are seeing the detector "sliced" open lengthwise. Because the particles that are being collided come in a stretched-out packet, the re might be more than one collision point - consider the vertical center of the image, the "beam", as the center in this view. Other than that there is not much to see, except for more grey detector fields symmetrically around the center.

    Q: Where is the center in the slice view?

    • A: The center in slice view is in the middle, vertically, of the image. Yes, the entire middle line. The line where the colourful dots are.

    Q: Can we get zoom/extra views?

    • A: No. There is a reason for this that the scientists have thought about. Basically, to make sure all classifications are based on the one view that they present to us, to level the playing field.

    Q: Why are some lines a different colour?

    • A: The only colours of consequence are green (the muons coming from the center) and the red dashed line, which is not a track at all. Other than that, the colours don't mean anything specific, but are markers for that the computer thinks the lines come from the same vertex. (Read more about the colours and dots on the detectors below!)

    Q: What are the colourful areas on the edge of the circle?

    • A: They represent different parts of the Atlas detector. The orange and green rings are calorimeters, and the blue ring on the outside is the muon spectrometer. You can see a diagram here: http://www.atlas.ch/detector.html
      Rectangular areas in those rings are energy measurements.

    Q: Will there be more help on "toughies"?

    • A: Yes, we are planning to add a few more examples here on the forum, at least.

    Q: Why are some images marked as "simulated data"?

    • A: Some of the images are simulations to match our answers to known data. This helps tune in how to interpret the data that comes out of Higgs Hunters, and helps us train on nice, easy-to-spot vertices. You should see less of those as the project goes on.

    More advanced info below!

    More about the interface (Colours, lines and rectangles oh my!)

    A more detailed description of the ATLAS detector interface. Please bear in mind that I am not a physicist but am puzzling all of this together by statements that the scientists have left on the forum and the documentation on various webpages. This is also not meant to be a description of how the ATLAS detector works (read the excellent blog posts here and here on that matter instead), but mostly deals with a better explanation of what we are seeing in the pictures.

    (Scientists, please correct me and I'll amend if I got something wrong?)

    • The tracking detectors can detect and display the tracks of charged particles. These are the lines we see in the images. Neutral particles such as photons and neutrons cannot be traced, but they still show up as energy in the calorimeters. So do particles that were detected but removed by the computer (more on that in a second).
    • The collision point is in the center of the image. You can see at least two green tracks (muons) come from that central collision. There will usually also be a dashed red line going from the center. The dashed line represents the direction of imbalance of momentum when all the detected particles are combined, and is not a particle track in itself.
    • The lines are curved because the particles are charged particles inside a strong magnetic field. The higher the particle momentum, the less the curvature.
    • The computer is trying to remove any other lines that originate from the center, so anything else you see should be tracks that the computer considers not to come from the original collision. (They might still look like they do, see "backwards tracks" and other artifacts)
    • Apart from green and the red dashed line, the colours of the lines have no special meaning. The computer will try to colour tracks coming from the same vertex in the same colour, but it gets it wrong more often than not, so use your own judgement.
    • The calorimeters can detect energy both from charged and neutral particles. This shows up as yellow blocks in the red and green rings. White trapezoid fields mark areas where especially much energy has been detected ("jets"). Red dots on the muon detector also mean energy detected there.
    • There are sometimes also red, blue, yellow and green rectangular markings on the outside of the ring, starting at the muon detector. The red ones are always at the end of muon lines, so they are obviously the energy measured from our muons. Blue ones often appear opposite a muon and indicate a jet that has likely come from the decay of a bottom quark. Yellow and green ones mean photons and electrons, respectively (?).

    Some interesting things seen so far

    More interesting threads

    Some selected quotes:


    The main focus of this search is finding two or more tracks starting
    at the same point which is displaced from the proton-proton collision
    point. This collision point is where the two green tracks (muons)
    meet. All these tracks (white or coloured) are reconstructed from
    signals in the tracking detectors which only detect charged particles.
    The tracks that point back very precisely to the collision point
    (except for the two muons) are not displayed to make it easier to look
    for the ocvs. The calorimeters (red and green regions) detect and
    absorb both charged and neutral particles and the yellow blocks in
    these regions represent energy deposited by both charged and neutral


    Good one! The blue lines in the outside indicate "jets" which are
    tagged as having likely come from the decay of a bottom quark. These
    quarks are fairly rare compared to the more common up/down or even
    strange/charm quarks, since they are much heavier (~5 GeV, one 25th of
    the Higgs mass). We identify them by looking for .... displaced
    vertices! The bottom quark goes ~3mm before it decays. They are thus a
    background to the kinds of ocv's you folks are looking for. The "jets"
    are collimated sets of hadrons (strongly interacting particles) and
    photons which hits the calorimeters. Their energies are measured
    there, and is plotted with these funny gray curved trapezoids.


    Yes, muons are drawn in green. (Sometimes there are other tracks that
    are also green, sorry. But the muons go farther, into the outermost
    muon detector, and have thick red lines at the end of them.) You may
    notice that there are at least 2 muons in almost every event you see.
    We've selected the events to have a Z boson decay to a pair of muons.
    This greatly enhances the chance that there's a Higgs boson decay in
    the same event! It's great that you're so interested in more about our
    detectors and particle physics - I agree it's exciting stuff. We can
    surely help you all find more info on ATLAS and the LHC online!


  • Marcuandy1981 by Marcuandy1981

    Thanks for this 😃


  • DZM by DZM admin

    This is awesome! I was just thinking last night that we needed something like this. Thank you!!!



    excellent FAQ....thank you. challenging project so far, more FAQ info the better imo 😃


  • Mark_..._a_physicist_of_sorts by Mark_..._a_physicist_of_sorts

    Is there anyway to get back to an image we chose not to talk about? Guess who just clicked the wrong option after classifying a ?muonjet? - nnnnnngggggh!!? 😦


  • jokergirl by jokergirl moderator

    Whoops, I left this thread unlocked by accident after my last edit. Mark, could you ask this question again in its own thread in the tech support forum, please? I'd like to keep this thread free of clutter.