Back at CERN! Good to be here, more students coming in, everybody getting pretty excited. Recent progress in the cavern is the installation of the beam pipe on one side. I guess this is pretty hard to imagine, so I thought a picture might help:
So, on the very bottom of the picture you see these copper looking flanges (they’re not copper, I believe they are Aluminum, but it is just a trick of the lighting). That is the very tip of the first Endcap plug. Underneath those flanges will be the Electromagnetic Endcap, we hope! (there’s been some delay, so it is now “critical path”) Extending from that is the beampipe (eventually the plug will slide over the beampipe) which then penetrates the barrel part of the detector. It might help to have a quick glance at the cartoon before trying to understand the following: from the outside radius moving inward (it is cylindrically symmetric, so this the easiest way to describe it) what you can see is
- Muon Chambers/Magnetic Field return – the “M” in CMS stands for Muons! This the is the red layers of steel interspersed with aluminum (silver) muon chambers- there are four cylindrical layers of chambers. The red steel in between is where the magnetic field goes so that it can form a continuous loop (magnetic fields have to go in a closed loop, and they much prefer to go through steel than air). They measure muons via the ionization trail they leave behind, and we know they are muons because anything else coming from the interaction region in the center either won’t penetrate this far to the outer detectors or won’t leave a trail at all (those would be neutrinos…) More…
- Next step in radially: Solenoidal Magnet – the “S” stands for Solenoid. This is the grey collar that looks like it has a whole bunch of metal bands going from the outside to the inside, and every few bands you can see some green cables. The bands are actually cable trays, carrying the cable connections to the inner detectors. The Solenoid itself is 13 m long and has an inner diameter of about 6 m and since it is superconducting cavities inside we can get to about 4 T, making it the most powerful magnet in the world in terms of energy stored. More…
- Inside of that (we say “inside the Vacuum Tank”, which is what holds the solenoid) there’s a space where the endcap fits in (sort of like a cork) and then the Hadron Calorimeter, which is has a beveled end (i.e. as it goes in at an angle, you can only see the bottom half, where the aluminum is shinier because of the angle-it caught more of the flash, I guess) – it too is covered with aluminum cable trays connecting the inside to the outside, but there are two places just after 3:00 and before 9:00 where there are no trays and you can (barely) see the brass which is the real thing. The HCAL is made out of interleaved layers of 4 mm think scintillator and 50 mm thick copper, about 15 layers in all, in fact a similar idea to the muon chambers but on a much finer scale. Hadrons (pions, protons, etc) create a shower of particles which create light in the scintillator which is colllected and read out. The more light, the more particles, the more energy of the initial particle. More…
- Continuing inward you get to the Electromagnetic Calorimeter, which is made up of Lead Tungstate crystals but again, all you (barely) see now is the containers for these crystals, which are only about 23 cm long. This is the ring of “tan trapezoids” in the picture, again you can see a difference at 3:00 and 9:00 where the plug and barrel will meet. The ECAL is made to measure photon and electron energy – because these particles are light, they interact differently than all the others, so using a particular material selects these guys out – they undergo “showering” much like what the hadrons do in the hadron calorimeter, but via and electromagnetic interaction rather than a strong interaction, and thus the difference. More…
- Inside that, the dark blob with bits of green around it (those cables, finally reaching their destination) is the CMS Tracker. This is the detector I work on! There’s actually two parts, the Silicon Strip detector and the Pixel Detector, both of which operate in essentially the same way and similar to the muon chambers – any charged particle moving through will create some ionization (though in semiconductors these are “electron – hole” pairs, not “electron – ion” pairs) which can be measured, and by tracing the ionization path you know where that particle went. Better Tracker pictures might help give you the idea. Actually, the pixel detector is not yet installed, although they had a successful dry run earlier this year, but that goes in after the beampipe is prepared. All told, these guys have something like 70 million channels to read out at 40 MHz – think you can buy a 70 Mpix camera that does 40,000,000 frames/sec? More…
So that kinda gives a glimpse of the detector and how it surrounds the beampipe where the interactions will occur (only at a few places, one of which should be the dead center of our detector). There is of course much more information, and it is still hard to get a feel for the size of things without actually seeing it (from edge to edge is 15 m, and along the length it is 13 m) but if you don’t come to Switzerland soon, this will have to do. By the way, for completeness, the “C” is for “Compact”. Now, you may not think that something the size of a house is particularly compact, but if you think we’re big, have a look at ATLAS which is like twice the size, but half the weight – we have much higher density (all that steel!) so we are “Compact”.
Also, there are these really cool virtual tours of ATLAS and CMS from Peter McCready but they’re a bit old now. There’s several of each detector from different vantage points, I encourage you to poke around on your own.
