CMS, one of the large general purpose detectors being built for the Large Hadron Collider, is performing detailed tests of one of the fundamental forces in the universe: gravity. The detector has been built in a large hanger-type building above ground. However, the accelerator is 100 meters underground, and if CMS wants to record collisions, it has to go underground too. So CMS is being carefully, oh so carefully, lowered in sections to its collider hall by a giant gantry crane. If you think of the detector as a giant cylindrical can, the sections being lowered are transverse slices of the can and look like large thick disks. Luckily, the ability to separate the detector into these disks, and then reassemble them, was built into the detector’s design.
A particularly large chunk of the detector â€” its heaviest piece â€” was lowered this week. CMS stands for Compact Muon Solenoid and it’s the solenoid itself (preassembled with the central portion of the detector) that made the journey underground Wednesday. The solenoid is a large magnet, generating a 4 Telsa magnetic field (100,000 times stronger than the Earth’s magnetic field) with a total stored energy of 2.66 GigaJoules (equivalent to half a tonne of TNT), and is responsible for our ability to observe tracks and measure the energy of charged particles. It’s an essential and expensive component of the detector.
This test of gravity was a challenging engineering feat. The solenoid weighs 1950 metric tons â€” as much as 5 jumbo jets â€” and is 16 meters tall, 17 meters wide, and 13 meters long. It had a 20 centimeter clearance with the walls of the shaft leading underground. The gantry crane supported the detector by 4 massive cables, each with 55 strands, and operated by a hydraulic jacking system with sophisticated monitoring and control systems. The process took about 10 hours, which is a long time to hold one’s breath!
Luckily, we have a fairly thorough understanding of Newtonian gravity (unlike quantum gravity) and this lowering experiment was able to confirm our calculations. In other words, the solenoid now safely rests 100 meters underground! All in all, 15 slices of the detector must be lowered, with the solenoid being piece number 8. The last slice will make its descent this Summer, just in time to complete the assembly and record the first collisions this Fall.