Top quark cross section

Although cross section has little to do with a literal slice of the top quark, it can tell us about the quark’s fundamental properties.

Last week’s Physics in a Nutshell described the strange way that particle physicists use the term “cross section” to mean a reaction rate. For instance, the proton-proton to top-antitop cross section is related to the probability that two protons will interact and produce a top quark and an antitop quark. This idea of cross section has historical roots in the fact that the collision probability of a stream of spherical balls is proportional to the cross sectional area of those balls. Today, the term is used because it specifies the collision probability in a way that is independent of the number of particles in the stream, so that results from one accelerator can be compared with results from another.

But why measure collision probabilities at all? Back when particles were thought to be hard spheres, it was an indirect way of measuring their size. In our current understanding, each particle is a quantum cloud of probable positions, and even if two particle clouds pass through each other, they may or may not interact. The probability of interaction is only partly determined by position—it is also affected by a fundamental property known as coupling, the strength of connection between the colliding particles and the result of the collision. When protons collide, the probability of making a top-antitop pair depends on the coupling of gluons inside the proton with the top quarks. By measuring this cross section, scientists learn how strongly gluons and top quarks are related, one link in the web that connects all fundamental particles.

In a recent paper, CMS physicists presented a new measurement of proton-proton to top-antitop cross section, using 60 times more data than the previous measurement. The extra precision that the large data set provides not only tests our understanding of the gluon-top coupling, it also checks assumptions about the density of gluons in the proton and quantifies a background for other searches. As the heaviest particle known, the top quark couples more tightly than any other to the Higgs boson and anything related to the origin of mass—the more we know about it, the better.

—Jim Pivarski

The physicists pictured above carefully measured the production cross section of top-antitop pairs in proton collisions.
The physicists pictured above made significant contributions to CMS missing energy reconstruction, ranging from data quality monitoring tools to novel approaches of reducing dependence on overlapping events (pileup).