As physicists dig deeper into the Energy Frontier, they find new particles through their decays into known ones.

Layers of Discovery

Jun 22, 2012

(Reprinted from Fermilab Today.)

Three thousand years ago, the inhabitants of ancient Crete used a writing system called Linear B. Archaeologists in the mid-20th century managed to decipher it because they had a working knowledge of Mycenaean, an early form of Greek. Linear A, an earlier and still-unknown language, may someday be understood through its relationship with Linear B.

Discoveries often make other discoveries possible, even in particle physics. Most particles created in proton collisions are not produced in the first instant, but through successive chains of decay. For instance, top quarks are far too short-lived to make it from the collision point to the first layer of the CMS detectors, even at speeds approaching the speed of light. A top quark immediately decays into a bottom quark and a W boson. The bottom quark lives long enough to travel a fraction of an inch, but the W boson instantly decays into two more quarks. The quarks form dozens of mesons and baryons, some of which are unstable while others live for 10 nanoseconds or more (a long time). Only these last survivors are directly observed.

Each step in this decay chain was once considered new physics. The discovery of each stage made the next one possible: As with Linear B, scientists could reconstruct the top quark because they knew the language of bottom quarks and W bosons.

CMS scientists are currently engaged in a new expedition to search for massive particles that decay into pairs of high-energy top quarks. It is particularly challenging because the many decay products of each top quark overlap one another in the detector. These scientists had to invent sophisticated algorithms for disentangling the fragments of each W boson and bottom quark— so that they could reconstruct energetic top quarks, so that they might reconstruct a new particle, Z-prime, decaying into them.

Each step brings us closer to understanding the origins and fundamental structure of the universe, but it relies on the accumulated wisdom of generations of physicists.