Knocking on Heaven's Door - Lisa Randall [109]
My fellow visitors and I donned our helmets and entered the LHC tunnel. Our first stop was a landing where we could stare down at the gaping pit beneath, as is shown in the photo in Figure 29. Witnessing the gargantuan cavern with its vertical tubes that would transport pieces of the detector from the place where we stood to the floor 100 meters below got me hooked. My fellow ATLAS tourists and I eagerly anticipated the experience we had in store.
After the first stop, we proceeded to the floor down below that housed the not-yet-completed ATLAS detector. The nice thing about the unfinished state was that you could see the detector’s innards, which would eventually be closed up and shielded from view—at least until the LHC turns off for an extended period of time for maintenance and repairs. So we had the opportunity to stare directly at the elaborate construction, which was impressively colorful and big—larger even than the nave of the Cathedral of Notre Dame.
But the size was not in itself the most magnificent aspect. Those of us who grew up in New York or any other big city are not necessarily overly impressed by enormous construction projects. What makes the ATLAS experiment so imposing is that this huge detector is composed of many small detection elements—some designed to measure distances with a precision at the level of microns. The irony of the LHC detectors is that you need such big experiments to accurately measure the smallest distances. When I now show an image of the detector in public lectures, I feel compelled to emphasize that ATLAS is not only big, but it is also precise. This is what makes it so amazing.
A year later, in 2008, I returned to CERN and saw the construction progress ATLAS had made. The ends of the detector that had been open the previous year were now closed up. I also took a spectacular tour of CMS, the LHC’s second general-purpose detector, along with the physicist Cinzia da Via and my collaborator, Gilad Perez, who appears in Figure 30.
[ FIGURE 30 ] My colleague, Gilad Perez, in front of part of the layered CMS muon detector/magnet return yoke.
Gilad hadn’t yet visited an LHC experiment, so I had the opportunity to relive my first experience through his excitement. We took advantage of the lax supervision to clamber around and even look down a beam pipe. (See Figure 31.) Gilad noted this could be the place where extra-dimensional particles get created and provide evidence for a theory I had proposed. But whether it will be evidence for this model or some other one, it was nice to be reminded that this beam pipe was where insight into new elements of reality would soon emerge.
Chapter 8 introduced the LHC machine that accelerates protons and collides them together. This chapter focuses on the two general-purpose LHC detectors—CMS and ATLAS—that will identify what comes out of the collisions. The remaining LHC experiments—ALICE, LHCb, TOTEM, ALFA, and LHCf—are designed for more specialized purposes, including better understanding the strong nuclear force and making precise measurements of bottom quarks. These other experiments will most likely study Standard Model elements in detail, but they are unlikely to discover the new high energy beyond the Standard Model physics that is the LHC’s primary goal. CMS and ATLAS are the chief detectors that will make the measurements that will, we hope, reveal new phenomena and matter.
[ FIGURE 31 ] Cinzia da Via (left) walking past the location where we could stare down the beam pipe and see inside (right).
This chapter contains a good amount of technical detail. Even theorists like me don’t need to know all these facts. Those of you interested only in the new physics that we might discover or the LHC concepts in general might choose to jump ahead. Still, the LHC experiments are clever and impressive. Omitting these details wouldn’t do justice to the enterprise.
GENERAL PRINCIPLES
In some sense, the ATLAS and CMS detectors are the logical