Charm quark’s light reveals riddles in theory

Events containing a charm quark and a photon allow physicists to shed light on the behavior of the strong force.

When DZero physicists scrutinize events in their data that contain a charm quark and a photon, they can shed light on the behavior of the strong force. In the Standard Model, it is difficult to accurately predict quark production because of the complex strong force interactions involved. Instead, theorists build models of the strong force that must be constrained by experimental data. On the other hand, the interaction of photons, or particles of light, with quarks is well understood. So when a photon is involved in an event with quarks, it lights the way to test different models of the strong force, as was done in a recent analysis from DZero.

The first challenge in this analysis was to identify events that contained a photon and a charm quark, instead of some other flavor of quark. A quark will turn into a spray of particles, called a jet, after it is produced. While lighter quarks turn into a jet immediately, heavier charm quarks and bottom quarks form short-lived composite particles that may travel a few millimeters before decaying. This displaced decay within a jet allows it to be identified as containing either a charm quark or bottom quark. Properties of the displaced decay allowed analyzers to determine the ratio of charm quarks to bottom quarks found in the data.

The analyzers compared the events containing a photon and a charm quark to various theoretical models in two ways. The total rate of production for these events was measured for different ranges of the photon’s energy. While some predictions described either the low or the high photon energy range well, no single model described the full range properly. Analyzers also compared the data to theoretical models of the ratio of photon and charm quark production to photon and bottom quark production. They again found that no single prediction accurately described the data, but suggested specific adjustments that led to good agreement. This information will provide important constraints that will improve future models of the strong force interaction.

Mike Cooke

These physicists made major contributions to this analysis.
Comparing the Standard Model prediction of a process to the data is often performed by simulating individual events of that process many times, perhaps millions or more. This team coordinates the production of a shared library of simulated events for use in DZero analyses.