| Calls to protect devils, kangaroos Animal rights activists are using National Threatened Species Day today to call for an end to permits to cull Tasmanian forester kangaroos. abc.net.au |
High-energy tomography: Can you tell what it is yet? | Jon Butterworth Detailed software models of particle physics detectors are an essential part of designing and using them. GEANT, the huge open-source software project developed for them, is also used for space and medical applicationsAfter a brief last-minute panic, I gave my talk on Atlas at the UK high-energy physics forum, which finished today in Abingdon. One of the pictures I showed is below.Each dot in there is a vertex - a point of origin of two particle tracks measured in the Atlas detector. These particles don't come directly from the proton-proton collisions in the LHC. They are "secondaries" created when a fast moving particle crashes into some of the material in the detector. The more material in a particular region, the more chance of a crash and so the more vertices. The density of dots basically allows us to map out the structure of our detector.Note the scale on the picture - this is structure 40 millimetres or so from the LHC beam and about 30 millimetres wide. The main repeated shapes show the silicon detector modules in the Atlas pixel detector. The ears on the side are cooling pipes, and you can also make out cables and the carbon-fibre shell on the right.I also showed this, rather similar, picture:This is a computer simulation of the same thing. It is not perfect, but it gives you an idea of the remarkably detailed software model we have of our detector. Not just of where it is, but the shapes, and how dense they are and what they are made of, so we can simulate the way particles from the LHC collisions pass through the detector and are in the end measured.Amongst other things, this is another essential ingredient in the calibration of the detectors which I wrote about here, since every time a particle crashes into a bit of material, it loses energy, and we need to understand those losses.Building models like this is done for practically all particle physics experiments. There's an open-source software project called GEANT, initiated at Cern but now with many collaborators, which provides a toolkit for putting together materials and geometries and simulating how various particles interact with them.This is of course a useful thing to do in many situations and GEANT is now pretty widely used in space and medical applications as well as physics. Lewis Dartnell (who is a researcher in the UCL Institute for Origins) used it to see how far underground bugs on Mars would have to hide to escape cosmic radiation.The pictures come from this paper which we (Atlas) sent to ICHEP. Fabiola Gianotti, our spokesperson, showed it in her talk. (By the way, in particle-physics-land "spokesperson" means "head of the collaboration", not the PR department.)In other meeting highlights, Jeff Forshaw of "Why Does E=mc2" fame was arguing about what a particle is really, when you get right down to it. At 3am in the bar. I missed this, since I went to bed at 1am, so possibly I still don't know. But it's something to do with all those dots.Jon Butterworthguardian.co.uk © Guardian News & Media Limited 2010 | Use of this content is subject to our Terms & Conditions | More Feeds guardian.co.uk |
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