Knocking on Heaven's Door - Lisa Randall [77]
Even after these funding and budget problems were resolved, LHC development still wasn’t entirely smooth sailing. Lyn in his talk described how a series of unforeseen events periodically slowed down construction.
Certainly no one involved in excavating the cavern for the CMS (Compact Muon Solenoid) experiment could have foreseen digging into a fourth-century Gallo-Roman villa. The property boundaries were parallel to the farm field boundaries that exist to this day. Excavation was halted while archaeologists studied buried treasure, including some coins from Ostia, Lyon, and London (Ostium, Lugdunum, and Londinium at the time the villa was occupied). Apparently the Romans were better at establishing a common currency than modern Europe, where the euro still hasn’t displaced the British pound and the Swiss franc as a means of exchange—particularly annoying for British physicists arriving at CERN who don’t have the currency required to pay for a taxi.
Compared to CMS’s travails, the 2001 excavation of the ATLAS cavern proceeded relatively uneventfully. Digging the cavern involved removing 300,000 metric tons of rock. The only problem they faced is that once the material was removed, the cavern floor began to rise slightly—at the rate of about a millimeter each year. This might not sound like much, but the movement could in principle interfere with the precise alignment of the detector pieces. So the engineers needed to install sensitive metrology instruments. They are so effective that they not only detect ATLAS movements, but are sufficiently sensitive to have registered the 2004 tsunami and the Sumatra earthquake that triggered it, as well as others that came later.
The procedure for building the ATLAS experiment deep underground was rather impressive. The roof was cast on the surface and suspended by cables while the walls were built up from below until the vault could sit on them. In 2003, the completed excavation was inaugurated with a celebration, notable for the presence of an alpine horn echoing inside, which in Lyn’s description was a source of great amusement. Installation and assembly of the experimental apparatus subsequently followed with the components lowered one by one until ultimately the ATLAS experiment was assembled with this “ship in a bottle method” in the excavated cavern belowground.
CMS preparations, on the other hand, continued to face rough seas. It once again got into trouble during excavation since it turned out that the CMS site was infelicitously placed not only over a rare archaeological site but also over an underground river. With the heavy rains that year, the engineers and physicists discovered to their surprise that the 70-meter-long cylinder they inserted into the ground to transport materials down had sunk 30 centimeters. To deal with this unfortunate hindrance, the excavators created walls of ice along the cylinder walls to freeze the ground and stabilize the region. Supporting structures to stabilize the fragile rock around the cavern also had to be installed, including screws up to 40 meters in length. Not surprisingly, the CMS excavation took longer than foreseen.
The only saving grace was that because of CMS’s relatively compact size, experimenters and engineers had already been considering constructing and assembling it on the surface. Constructing and installing components is a lot easier aboveground, and everything is faster since there is more room to work in parallel. This aboveground construction had the added critical benefit that the cavern problems wouldn’t further delay construction.
However, as you might imagine, it was a rather daunting prospect to lower this enormous apparatus—which is something I had a chance to think about when I first visited CMS in 2007. Indeed, lowering the experiment was no easy task. The largest piece began its 100-meter descent into the CMS pit, carried by a special crane, at the dauntingly