I got here to CERN a few days ago, and things are quieter than recently. It's the start of vacation season and so the cafeteria is not so full and parking is plentiful. Lots of summer students are hanging around in the evening, and some of the meetings are sparsely attended. In the CMS experiment, the folks who have labored long and hard to build the world's biggest and most complex silicon-strip detector have earned a bit of vacation. As of about August 1, this billion-dollar baby will begin buttoning up for its journey underground into the heart of the experiment. So I found myself alone in the room with this thing, in the Tracker Integration Facility, and it is hard not to hold it in awe. "It" comprises thousands of flat detectors, each a thin rectangle of silicon crystal with microscopic aluminum strips embedded in it. These strips sense the passage of charged particles like pions, muons, electrons, and so forth, passing the tiny charge they collect to custom-designed readout chips which send the data in digital form to an army of processors which gather all the information from a single 25 nanosecond "beam bunch crossing" into a nice tight wad for later processing. If you unrolled and laid out all the silicon detectors in the CMS Tracker you could tile a tennis court. It's mind bogglingly complex, the product of hundreds of people workng for a dozen years. But there it is, and you know what? It works. Its been put through its paces on the surface and it's just about show time downstairs at LHC Point 5. I call it a billion dollar baby but it's hard to say what it really cost. That number takes into account at least some of the labor by engineers, technicians, physicists, and students all over the world, but probably not all of it. With the testing of the tracker at an end and the transport not yet begun, I am here with my CMS pixel colleagues to take advantage of our last (and first, actually) chance at seeing if our detector will fit neatly inside the tracker. We are working on the innermost detector in the CMS experiment, the pixels. Once you get so close to the collision that charged particles are millimeters apart, you can no longer use long strips to detect them but have to go to arrays of pixel sensors. Our detector may be a lot smaller than the tracker, but it has even more individual readout channels, around 45 million in total. Each pixel is 0.1 by 0.15 mm (100 by 150 microns) and is read out by a custom chip developed at PSI in Zurich, and bump bonded to the silicon sensors. (Enough jargon yet?) The data from pixels with charge above a preset threshold are sent out on a serial line, converted to an optical signal and digitized upon receipt in high speed electronics in an adjacent cavern underground, then sent up the data acquisition stream. Anyway, when all this mumbo-jumbo is done we have a big set of three-dimensional space points along the trajectory of the same particles that have also passed through the tracker. But the pixel points allow us to see what happened very close to the primary proton-proton collision vertex. Combining the pixel hits along a track we can project back to the vertex with a resolution of about 10 microns. Given that the pixels are much larger than that this is quite a trick: we rely on the fact that particles split their signal among adjacent pixels, and we can use a sort of averaging trick to get much more precise than a single pixel width. My task here, though is much more practical. We want to make sure that the pixel detectors we are building will fit inside the tracker, the two halves of the detector meshing neatly at the end of their grooves inside the tracker. Up until this point it's been an engineering project, but now we have real hardware and we need to be sure that no parts will interfere mechanically with each other. Stay tuned, and in a few days I will post some photos of that and tell you how it worked. Gulp!
