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New meaning to the word “compact” April 24, 2007

Posted by dorigo in news, personal, physics, science.

CMS stands for “Compact Muon Solenoid”. It is one of the two giant detectors being assembled as we speak inside two of the six caverns crossed by the Large Hadron Collider, the 17-miles accelerator lying four hundred feet below the pleasant ground of France and Switzerland, near Geneva.

Compact is a word one nowadays hears a lot in advertisements of electronic gizmos, cars, and the like. It is a concept which arises the pleasant feeling of something self-contained, easy to store, non-cumbersome, elegant. You want it compact, be it a cell-phone, a digicam, or a city car. Compact is fashionable, compact is practical, compact is cool.

Yet compact, for CMS, means something slightly different. I went down the pit of the Cessy site at P5 this evening, and I saw the beast with my own eyes for the first time, after years spent learning about the subtleties of its design on scientific papers and technical design reports. I tell you, CMS is not compact the way you know compactness.

No, it is not easy to store -you’d need a 20 by 50 meters pocket for that- and it is more cumbersome than almost anything else I know. But it is certainly elegant, and in some peculiar way it is also self-contained.

I was deeply impressed by touching with my hands the silvery central superconducting magnet, which while enormous, it is dwarfed by the surrounding muon detectors. Indeed, what is allegedly compact is the solenoid, if compared to the rest of the detector. The helium-cooled structure houses 20,000 ampere currents, and produces a field of 4 tesla in the tracking volume.

What impressed me most, however, was to read between the lines of what stood in front of me the incredible engineering achievement of putting together this giant mass of steel and electronics, the handling of 20 kilo-ton pieces to be lowered with millimetric accuracy into the pit. There was beauty in the construction work. Marco Zanetti, a colleague who brought me downstairs, mentioned that the cool colors I saw were actually meant to be a code: red is magnet steel, green is infrastructure where it is possible to walk, yellow tags parts needed for moving the structure, and so on. The complex really made a terrific view. 

During my tour I could take a few pictures. Feeling no shame, I resembled a Japanese visitor in Piazza San Marco while I aimed my pocket digital camera at the giant pieces already assembled in the cavern, or waiting to be lowered down the pit in the assembly building above ground. And while I did, I was thinking at what I would reply if asked why not to use the official CERN pictures of the detector. No, I did not take part in building this one. Yes, I am awed by it. No, I feel my image of scientist is not harmed if I take some pictures. I have kids too, you know…Science: It’s just my job five days a week [D.Bowie, “Rocket Man”].

So here are a few pictures, taken by yours truly this evening.


Here I am, in the LHC tunnel, next to the beam line. I do not particularly fancy wearing a helmet, but you have to compromise fashion for safety sometimes.

And here is one part of the LHC triplets of low-beta quadrupoles. This guy did not suffer damage during the test of last month, but it will still need servicing to improve the lateral supports and the resistence to longitudinal stresses. Its twin, sitting on the opposite side of CMS, is the one that got damaged.

This is the pit seen from the cavern. It looks like a small hole, but the whole CMS detector has to come down that hole, piece by piece- and the smallest pieces are 17 meters wide.

Here is a view of the cavern, with part of the detector already installed. Presently, the wedges of the central electromagnetic calorimeter are being inserted, using the red structure resembling a toy for guinea-pigs. The red crib rotates, allowing insertion of the costly wedges one by one. The wedges contain the famous lead tungstate crystals that allow a precise measurements of high energy photon showers, giving CMS the capability of detecting the super-rare decay of Higgs bosons to photon pairs, among other things. In the picture, you also see the silver cylinder of the central solenoid.

And finally, here you get a nice view of one of the sections of muon drift tubes, sandwiched by the red magnetized iron. This section is still above ground, and it will be lowered in the pit later this year.



1. Andrea Giammanco - April 24, 2007

off topic (but not completely): you will find this article quite interesting, i believe…

2. riqie arneberg - April 25, 2007

off topic perhaps. but thank you Andrea for directing me to a discussion accessible to a lay person.

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