Knocking on Heaven's Door - Lisa Randall [110]
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