Mapping the Frontier

The top quark is the most massive particle in the Standard Model, and might be pointing to what lies beyond.

Regular readers of Fermilab Today may be familiar with the top quark. For 15 years after its discovery in 1995, top quarks could only be produced by the Fermilab Tevatron. This changed with last year’s start-up of the LHC, when scientists saw top quarks in the CMS detector.

The top quark is, in a sense, at the extreme edge of the Standard Model. It is by far the most massive fundamental particle known to exist. Its enormous mass may be key to understanding the mystery of why particles have any mass at all— whatever that reason is, it most strongly manifests in top quarks.

Today’s featured CMS result is a measurement of the top quark’s mass and production rate at the LHC. Unlike any other quark, top quarks decay rapidly into a W boson and a b quark. Each of these can decay many different ways, giving scientists a choice in how to search for it. In this paper, scientists look for the following pattern: top and anti-top quarks produced in pairs, which together decay into two W bosons and two b quark jets, with both W bosons decaying into a lepton (electron e or muon μ) and a neutrino. Though the neutrino escapes undetected, it makes its presence known by shifting the balance of the other particles. Everything is observed at once in the detector, making these events very complicated to study.

Precision measurements of known particles provide important clues about the physics beyond them. Before the top quark was discovered, precision measurements of the W and Z bosons predicted the top quark’s mass fairly accurately. Similarly, the W, Z, and top quark properties together imply that the as-yet undiscovered Higgs boson has a mass just beyond the current searches. Indeed, the rocky shores of this distant outpost are ideal for disembarking into the unknown.

— Jim Pivarski

The U.S. physicists shown above played an essential role in this analysis.
The above Fermilab employees are crucial to the administrative support of the US CMS project.