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Posts Tagged ‘data’

First ATLAS Pixel Tracks!

Sunday, September 14th, 2008

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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pictures of muons

Friday, July 25th, 2008

Below are some nice pictures the ATLAS detector took of muons created in the atmosphere that passed through Meyrin, Switzerland on July 21, 2008 (or technically “Event Displays from M8 Week”). The lines are the paths the muons took through the ATLAS detector, according to the reconstruction software. You can see the detector elements that actually recorded something are lit up along those paths. The reconstruction software “connects the dots”.

In each image are several views from different perspectives, or projections. The top projection in each image is the view down the beam axis, sort of what will be the “proton’s eye view” during collisions. The one below that is the view from alongside the detector.

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CMS in Cyprus

Wednesday, July 9th, 2008

Hah!

If I’m quick I’ll get this in just after Peter’s blog. While ATLAS went to Berne, CMS went to Limmasol (Lemesos) Cyprus for the collaboration meeting. Now, it is rather a critical time, so the wisdom of going fairly far away is dubious, but getting the upper management and riffraff out of the hair of those really doing the work for a while is maybe not such a terrible idea. I’m told it was quiet and people could get work done at CERN. Anyway, I had a good time-Limmasol is a resort town, so it wasn’t particularly serene (think Miami Beach for Brits and rich Russians) but I did get into the countryside a little which was quite nice. Also, my buddy Chris proposed an alternative excursion to the one planned for the whole collaboration, which was very fun, since I only get to do this particular activity once every blue moon. He’s on the right, I’m on the left

In other news, we have tracks in our tracker! Triggered by Cosmics via the muon chambers, with more than 95% of it turned on! So far performance looks really excellent, I’ll provide more details soon

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Event Viewing

Thursday, May 22nd, 2008

Being able to visualize events in the detector is critical to understanding whether everything is functioning properly. But creating a program to display events in practice is incredibly difficult. I have the utmost respect for people who attempt it.

Obviously the big hurdle to event viewing is trying to display a three-dimensional detector on a two-dimensional screen. ATLAS has two solutions to this. One is Atlantis, the tried-and-true event viewer. The philosophy of Atlantis is to try and present the ATLAS detector in every two-dimensional slice possible. Such as this picture here.

Atlantis Event Viewer

From top left going clockwise, you see the full detector as if you were looking down the beam pipe, then the same cross section zoomed in on the calorimeters, then again the same cross section showing the inner detector, then a ‘bird’s eye’ view looking down on the beam pipe, and lastly a side profile of the detector (where the beam pipe is now the horizontal plane).

Atlantis as a tool is very useful but as for style… hmmm, not so much. It does have that retro look and while retro in fashion is considered acceptable, retro in computing is generally not.

Our second option is Visual Point 1 or VP1. VP1 takes the opposite approach. Going totally 3-dimensional, allowing the users to to place himself/herself at any point in the detector. In this picture, the view point is outside the calorimeter.

Atlas VP1 Viewer

The detector is just a shadow, barely seen in the picture and only the hits are shown (in yellow here). While VP1 definitely has that more modern feel, the jury is still out for me. It kind of reminds me of Tron. And it is too touchy. You accidentally hold the mouse button down too long and you are transported to some strange view point. And then you have no idea where you are, or what you are looking at.

It is a thankless job that is for sure!

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Cosmics vs Beam

Friday, March 14th, 2008

In response to my last posting about the ‘6th Milestone week’, the following questions were posed by Jacques.

Once you are satisfied with the results of this test, or any subsequent test that might be decided using cosmic rays, will you “just” have to wait for the LHC to start beaming, at which time you can immediately start gathering and exploiting the data from proton collisions, or does another heavy test campaign begin then? For how long?

Outside the difference in the data frequency/volume(is this not a 1 to 1 million ratio?), are there limitations in the current ATLAS tests due to the nature of cosmic rays? Or will the most volatile particles created by the collision decay so quickly that you can only track the results of such decays for which cosmic rays are a good subrogate to calibrate the various detectors and check they deliver consistent “tracks” whenever a given particle crosses them?
How about the calibration of the Level One Trigger in this context?

For starters, ‘satisfied with the results of this test’ is a constantly changing criterion. A year ago, satisfaction was getting just two sub-systems to run together. Today satisfaction is running all sub-systems with a level-one trigger rate of 10kHz. Next month, satisfaction will be running at 100kHz (which is the level-one rate we want to have during beam running). At the start of each ‘M-week’, we have a whole list of problems that we experienced in the previous ‘M-week’. At the end of the week, we have an entirely new list of problems which are generally more complicated and therefore more difficult to solve. So I suppose forward progress is defined as finding harder and harder problems.

We will never be in a position where we ‘just wait’ for the beam. Nor when the beam is running will we just be waiting for the data to roll in. It is a cultural trait of high energy particle physics to push the system. If we are stable with a level-one trigger rate of 100kHz, someone will suggest an idea to push that rate to 120kHz. There is a saying, ‘If it was easy, it would have been discovered already’. Thus in order to make the big discovery, we have to be willing to take risks and push the detector to its design limits and if possible beyond. And pushing the limits is all the fun!

In the period before the beam, cosmic rays aren’t a great way of testing ATLAS’ limitations. But it is all we have. Using the muon trigger chambers, the cosmic ray rate is about 100Hz. The beam will be 40 MHz which is roughly a factor of a million greater. We try to push the rate during cosmic running by using a high rate ‘random’ trigger but there are no physics events associated with these triggers. Additionally most cosmic events tend to be a single muon slicing through the detector. Whereas with the beam running there will be hundreds to thousands of particles in the detector.

Cosmic muons are very useful to study tracking in the inner detector and muon chambers. For the calorimeters, we can use them as a preliminary cross-check of our energy calibration. They are also helpful to establish the relative timing between sub-systems (which needs to be known on the nanosecond level). While cosmic muons are helpful for calibrating the part of the level-one trigger that looks at muons, it doesn’t help us much with calorimeter-based level-one trigger. The reason is that the calorimeters measure energies that are typically associated with “jets” of many particles, not a single track like those from cosmic muons.

The bottom line is cosmics are all good and fun. But if given the choice of cosmics vs beam. Give us beam!

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