Knocking on Heaven's Door - Lisa Randall [111]
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.
I am often asked when the LHC will run my experiments and search for the particular models that my collaborators and I have proposed. The answer is right away—but they are looking for everyone else’s proposals too. Theorists help by introducing new search targets and new strategies for finding stuff. Our research aims to identify ways to find whatever new physical elements or forces are present at higher energies, so that physicists will be able to find, measure, and interpret the results and thereby gain new insights into underlying reality—whatever it might be. Only after data is recorded do the thousands of experimenters, who are split up into analysis teams, study whether the information fits or rules out my models or any others that are potentially interesting.
Theorists and experimenters then examine the data that gets recorded to see whether they conform to any particular type of hypothesis. Even though many particles last only a fraction of a second and even though we don’t witness them directly, experimental physicists use the digital data that compose these “pictures” to establish which particles form the core of matter and how they interact. Given the complexity of the detectors and data, experimenters will have a lot of information to contend with. The rest of this chapter gives a sense of what, exactly, that information will be.
THE ATLAS AND CMS DETECTORS
So far we have followed LHC protons from their removal from hydro-gen atoms to their acceleration to high energy in the 27 km ring. Two completely parallel beams will never intersect, and neither will the two beams of protons traveling in opposite directions within them. So at several locations along the ring, dipole magnets divert them from their path while quadrupole magnets focus them so that the protons in the two beams meet and interact within a region less than 30 microns across. The points at the center of each detector where proton-proton collisions occur are known as the interaction points.
Experiments are set up concentrically around each of these interaction points to absorb and record the many particles that are emitted by the frequent proton collisions. (See Figure 33 for a graphic of the CMS detector.) The detectors are cylindrically shaped because even though the proton beams travel in opposite directions at the same speed, the collisions tend to contain a lot of forward motion in both directions. In fact, because individual protons are much smaller than the beam size, most of the protons don’t collide at all but continue straight down the beam pipe with only mild deflection. Only the rare event where individual protons collide head-on are of interest.
[ FIGURE 33 ] Computer image of CMS broken up to reveal individual detector components. (Graphic courtesy of CERN and CMS)
That means that although most particles continue to travel along the beam direction, the potentially interesting events contain a spray of particles that travel significantly transversely to the beam. The cylindrical detectors are designed to detect as much of these interaction products as possible, taking into account the large spread of particles along the beam direction. The CMS detector is located around one proton collision