Sunday, December 28, 2014

Particle Physics: "Discovering the Expected"

Let me tell you the tale of two Nobel Prizes—well, almost. The first Prize I want to tell you about was awarded to Wilhelm Röntgen in 1901 for the discovery of X-rays. The details of this discovery are fascinating in their own right, but the salient point for us is that Röntgen was not looking for X-rays at all. Instead, he was studying the behavior of various types of vacuum tubes. The unexpected shimmering of a piece of his equipment that contained barium made him suspect that something unusual was happening. He was in Stockholm to collect his medal within six years.

The second Nobel Prize I want to tell you about is different in two important ways. First, it hasn’t been awarded yet, and may never be. Second, it involves what is, in some sense, the opposite of an unexpected discovery. The scientists involved knew what they were looking for: an exceedingly rare particle produced when two protons are smashed together. In fact, only once in about 10 billion collisions does this particle occur. As a result, far from taking into consideration an unexpected data source like Röntgen did, they threw away 99.995 percent of their raw data because it was too voluminous to be recorded. I’m talking about the discovery, on July 4, 2012, of the Higgs boson.

The Higgs represented one of the most important physics discoveries in decades: the final piece of the so-called Standard Model of particle physics, which describes the elementary particles and their interactions. It was also a triumph of big science. The Higgs experiment at the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research near Geneva, Switzerland, generated tens of petabytes of data, demonstrating a computational prowess unprecedented in the history of particle physics. It was positive proof that science driven by big data could extend our observational sphere in important ways. But, by requiring scientists to begin experiments with a rough idea of what they are looking for, did it also alter how we think of accidental discoveries, of the type that led not just to X-rays, but also to positrons, superconductors, and the fractional quantum Hall effect? The answer is a subtle one.
The Atlas Detector: The Atlas detector is arranged in onion-like layers of specialized sub-detectors. The innermost part, closest to where the protons collide, is called the inner tracker. It contains tens of millions of finely etched silicon traces or dots, each representing a single pixel, and can be used to measure the trajectory of charged particles to an accuracy of 10-millionths of a meter. The next layer is a calorimeter, or energy detector, which uses liquid argon to detect electrons and photons resulting from collisions. After that is a scintillator to measure the energies and directions of particles such as pions. And finally, these components are surrounded by a muon detector, as muons are not stopped by any of the previous sub-detectors. Together with the Compact Muon Solenoid detector, the rate of data generation is greater than the world’s combined Internet capacity.