Knocking on Heaven's Door - Lisa Randall [108]
For any given model of physics beyond the Standard Model, any predicted small discrepancies—where the inner workings of an as-yet-unseen theory would make a visible difference—would be a big clue as to the underlying nature of reality. The absence of such discrepancies so far tells us the level of precision or how high an energy we need to find something new—even without knowing the precise nature of potential new phenomena.
The real lesson of effective theories, introduced in the opening chapter, is that we only fully understand what we are studying and its limitations at the point where we see them fail. Effective theories that incorporate existing constraints not only categorize our ideas at a given scale, but they also provide systematic methods for determining how big new effects can be at any specific energy.
Measurements concerning the electromagnetic and weak forces agree with Standard Model predictions at the level of 0.1 percent. Particle collision rates, masses, decay rates, and other properties agree with their predicted values at this level of precision and accuracy. The Standard Model therefore leaves room for new discoveries, and new physical theories can yield deviations, but they must be small enough to have eluded detection up to now. The effects of any new phenomena or underlying theory must have been too small to have been seen already—either because the interactions themselves are small or because the effects are associated with particles too heavy to be produced at the energies already probed. Existing measurements tell us how high an energy we require to directly find new particles or new forces, which can’t cause bigger deviations to measurements than current uncertainties allow. They also tell us how rare such new events have to be. By increasing measurement precision sufficiently, or doing an experiment under different physical conditions, experimenters search for deviations from a model that has so far described all experimental particle physics results.
Current experiments are based on the understanding that new ideas build upon a successful effective theory that applies at lower energies. Their goal is to unveil new matter or interactions, keeping in mind that physics builds knowledge scale by scale. By studying phenomena at the LHC’s higher energies, we hope to find and fully understand the theory that underlies what we have seen so far. Even before we measure new phenomena, LHC data will give us valuable and stringent constraints on what phenomena or theories beyond the Standard Model can exist. And—if our theoretical considerations are correct—new phenomena should eventually emerge at the higher energies the LHC now studies. Such discoveries would force us to extend or absorb the Standard Model into a more complete formulation. The more comprehensive model would apply with greater accuracy over a larger range of scales.
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.
CHAPTER THIRTEEN
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