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Seth Zenz | Imperial College London | UK

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First ATLAS Pixel Tracks!

I’m on my 18th hour on training shift since Saturday morning, getting in as much time in the control room as I can, and it’s been a very exciting time. One of my colleagues has just discovered that, last night, we recorded the first cosmic ray tracks in the ATLAS pixel detector!

First ATLAS Pixel Detector Track!

This is very exciting news for us; we’re working right up to the wire to make sure our pixel detector is able to run stably along with the rest of the detector. Collisions are coming soon soon soon!

Update (Sept 15): In response to two excellent questions in the comments, I wrote in a little more detail what you’re looking at in the picture. I figure the explanations might as well go in the entry:

1. What’s the perspective? Where’s the LHC?

You’re looking at the inner part of the ATLAS detector, which is wrapped around one of the collision points of the LHC. The large image in the upper left is a cross-section of the detector; the white dot in the very center is where the LHC beam pipe is. The image along the bottom shows the same tracks from the side; the LHC beam pipe isn’t shown, but it would run horizontally (along the Y’ = 0 cm line).

2. What do the dot colors mean? What’s the line?

All the dots are the actual points at which we have a signal from our detector. The red dots represent the signal that we think was left by a charged particle when it passed through, and the red line is the path we think that particle took (i.e. the “track”). The green dots are also signals in the detector, but we think they’re random firings in our electronics, because we can’t make any tracks out of them.

It may look like a lot of electronic noise, because there are more hits from random firings than from the track. But remember that there were only one or two tracks to be found, whereas we have over eighty million pixels in our detector. Thus the fraction of noisy pixels was actually quite small, and certainly didn’t interfere with finding the track. We also have a list of especially noisy pixels that we can “mask” (i.e. ignore), which will bring down the noise by quite a lot but which we haven’t begun to use yet.

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18 Responses to “First ATLAS Pixel Tracks!”

  1. Well Done! Congratulations to all ATLAS pixel folks, I know from experience how difficult it is to get pixel tracks out (actually I completely forgot to blog about that, I think that says something about how busy the experts are).

    Glad to see nice cosmics. I also like the event display, honestly it’s much better than the CMS event display.

  2. Seth Zenz says:

    Hi Freya, thanks! This is actually one of three or four different ATLAS event display programs — I find this one simple but very functional.

    I’m sorr–I mean, delighted–to hear that CMS got its pixel tracks ahead of ours. ;)

  3. Harry Trump says:

    Does this image represent looking down on the LHC from above, or more of a cross section of the tube?

  4. pravin says:

    You all are creating history, CERN will be remebered after thousands of years also…….

  5. Seth Zenz says:

    You’re looking at the inner part of the ATLAS detector, which is wrapped around one of the collision points of the LHC. The large image in the upper left is a cross-section of the detector; the white dot in the very center is where the LHC beam pipe is. The image along the bottom shows the same tracks from the side; the LHC beam pipe isn’t shown, but it would run horizontally (along the Y’ = 0 cm line).

  6. Bilja says:

    I am assuming that the red dots and lines represent cosmic ray tracks, but what about the green dots? What do they represent?

  7. Seth Zenz says:

    Great question, Bilja. I’ve been meaning to write an entry about how tracking works, and obviously it would have helped here!

    All the dots are the actual points at which we have a signal from our detector. The red dots represent the signal that we think was left by a charged particle when it passed through, and the red line is the path we think that particle took (i.e. the “track”). The green dots are also signals in the detector, but we think they’re random firings in our electronics, because we can’t make any tracks out of them.

    It may look like a lot of electronic noise, because there are more hits from random firings than from the track. But remember that there were only one or two tracks to be found, whereas we have over eighty million pixels in our detector. Thus the fraction of noisy pixels was actually quite small, and certainly didn’t interfere with finding the track. We also have a list of especially noisy pixels that we can “mask” (i.e. ignore), which will bring down the noise by quite a lot but which we haven’t begun to use yet.

  8. Gordon says:

    So, that means the track is “found” computationally…

  9. Seth Zenz says:

    Hi Gordon. Yes, that’s right, they’re found by computer. This is quite necessary, because our automated trigger system needs tracks to decide whether to save an event at all! Of 40,000,000 possible collisions per second, about 100,000 are selected for our “Level 2″ trigger, which includes a quick track-finding algorithm. In the end, only 100 events will be saved, so there will be many, many tracks that are only “seen” by the computer and never by humans at all.

  10. Bilja says:

    Thanks for the explanations, Seth. That helps a lot and makes the images so much more interesting. :)

  11. Jaz says:

    Is nice to be hearing about what’s going on there. Is enjoyable, interesting and insigtful!!
    Thankyou
    Jaz

  12. Coin says:

    In the topmost “cross-section”, is there a difference between the black and dark gray pixels within the circle?

  13. Gordon says:

    Thanks Seth…

    I assume that when the collisions begins, the tracks that are computed are spiral (i.e. the particles being observed will be under the influence of a magnetic field) rather than linear like the cosmic ray, would that be right?

    Cheers mate.

  14. Ceri says:

    Great explanation about the track fitting and hushing the noisy pixels!

  15. Tachi says:

    Thanks for the detailed explanation, I love physics nd want to understand this as much as I can :)

  16. Seth Zenz says:

    Hi Coin. I don’t see any black pixels. Black is just the background. And I suspect they grey pixels are “bad” in some way, but I have to admit I don’t know what the issue is. Also, note that the small grey dots, outside the second white circle, are the SCT — the detector outside the pixel detector, which I don’t know much about at all. Only the hits inside that circle are from the pixels.

    Hi Gordon. The tracks curve when our magnetic field is on, and of course you’re right that we need it on for collisions. The curvature of the tracks will tell us the particle momentum, which is the primary reason for building tracking systems in the first place. Note that very energetic tracks will still look nearly straight — they’ll be curved, but not very much.

    Thanks for the questions and encouragement, everyone!

  17. Gordon says:

    Thanks Seth and happy higgs hunting.

    Cheers,
    G

  18. Tim J says:

    It’s fascinating getting some insight into what you’re doing. I’ve waited for this ever since I first heard about superstring theory 20 years ago. I just hope our questions are leaving you time to drink some coffee and do some science ;-)

    By the way I think this sort of public outreach via blogs etc. is really important, so I wish you well with that too.

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