Knocking on Heaven's Door (34 page)

We don’t know which theory will be realized in nature. We also don’t know when we will make new discoveries. The answers depend on what is out there, and we don’t yet know that or we wouldn’t have to look. But for any particular speculation about what exists, we know how to calculate how we might discover the experimental consequences and estimate when it might occur. In the next couple of chapters, we’ll look into how LHC experiments work, and in Part IV that follows, we’ll consider how what they might see.

THE CMS AND ATLAS EXPERIMENTS

In August 2007, the Spanish physicist and CERN theory group leader Luis Álvarez-Gaumé enthusiastically encouraged me to join a tour of the ATLAS experiment that the experimental physicists Peter Jenni and Fabiola Gianotti were planning for the visiting Nobel Prize winner T. D. Lee and a few others. It was impossible to resist the infectious enthusiasm of Peter and Fabiola, who at the time were spokesperson and deputy spokesperson of the experiment, and who generously shared an expertise and familiarity with all the details of the experiment that suffused all of their words.

[
FIGURE 29
]
Looking down from the platform above into the ATLAS pit, with the tubes that transported materials down in view.

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 evolution of the transformation Galileo and others instigated several centuries ago. Since the invention of the microscope at that time, successively advanced technology has allowed physicists to indirectly study increasingly remote distances. The study of small sizes has repeatedly revealed underlying structure of matter that can only be observed with very tiny probes.

Experiments at the LHC are designed to study substructure and interactions with a range a hundred thousand trillion times smaller than a centimeter. This is about a factor of ten smaller in size than anything any experiment has ever looked at before. Although previous high-energy collider experiments, such as those running at the Tevatron at Fermilab in Batavia, Illinois, were based on similar principles to these LHC detectors, the record energy and collision rate that the new detectors faced posed many novel challenges that forced their unprecedented size and complexity.

Like telescopes in space, the detectors, once built, are essentially inaccessible. They are enclosed deep underground and subject to large amounts of radiation. No one can access the detector while the machine is running. Even when it is not, reaching any particular detector element is extremely difficult and time-consuming. For this reason, the detectors were built to last at least a decade, even with no maintenance. However, long shut-down periods are planned for every two years of LHC running, during which time physicists and engineers will have access to many of the detector components.

In one important respect, however, particle experiments are very different from telescopes. Particle detectors don’t need to point in a particular direction. In some sense they look in all directions at once. Collisions happen and particles emerge. The detectors record any event that has the potential to be interesting. ATLAS and CMS are general-purpose detectors. They don’t record just one type of particle or event or focus on particular processes. These experimental apparatuses are designed to absorb the data from the broadest possible range of interactions and energies. Experimenters with enormous computational power at their disposal try to unambiguously extricate information about such particles and their decay products from the “pictures” experiments record.

More than 3,000 people from 183 scientific institutes, representing 38 countries, participate in the CMS experiment—building and operating the detector and analyzing the data. The Italian physicist Guido Tonelli—originally deputy spokesperson—now heads the collaboration.

In a break from CERN’s legacy of male physicists presiding, the impressive Italian donna Fabiola Gianotti also transitioned from deputy to spokesperson, this time for ATLAS, the other general-purpose experiment. She is well deserving of the role. She has a mild-mannered, friendly, and polite demeanor—yet her physics and organizational contributions have been tremendous. What makes me really jealous, however, is that she is also an excellent chef—maybe forgivable for an Italian with enormous attention to detail.

ATLAS too involves a gigantic collaboration. More than 3,000 scientists from 174 institutes in 38 countries participated in the ATLAS experiment (December 2009). The collaboration was initially formed in 1992 when two proposed experiments—EAGLE (Experiment for Ac-curate Gamma, Lepton, and Energy Measurements) and ASCOT (Apparatus with Super Conducting Toroids) joined together with a design combining features of both with some aspects of proposed SSC detectors. The final proposal was presented in 1994, and it was funded two years later.

The two experiments are similar in basic outline, but different in their detailed configurations and implementations, as is illustrated in some detail in Figure 32. This complementarity gives each experiment slightly different strengths so that physicists can cross-check the two experiments’ results. With the extreme challenges involved in particle physics discoveries, two experiments with common search targets will have much more credibility when they confirm the findings of each other. If they both come to the same conclusion, everyone will be much more confident.

The presence of two experiments also introduces a strong element of competition—something my experimenter colleagues frequently remind me about. The competition pushes them to get results more quickly and more thoroughly. The members of the two experiments also learn from each other. A good idea will find its way to both experiments, even if implemented somewhat differently in each. This competition and collaboration, coupled with the redundancy of having two independent searches relying on somewhat different configurations and technology, underlies the decision to have two experiments with common goals.

[
FIGURE 32
]
Cross sections of the ATLAS and CMS detectors. Note the overall sizes have been rescaled.