University of Minnesota
School of Physics & Astronomy


Mysterious Charm

Andy Julin
Andy Julin
Alex Schumann

Andy Julin is a fourth year graduate student in experimental particle physics. He is part of the BESIII collaboration working under Professor Ron Poling and Dan Cronin-Hennessy. BESIII is an electron-positron detector located in Beijing, China which analyzes charm physics. Charm physics is the name given to the study of the charm or c quark, a subatomic particle that makes up other subatomic particles such as mesons and hadrons.

Julin is currently doing DD ̅ cross-section measurements from the ψ(3770) near threshold, which is the amount of energy required to produce the D meson and its anti-particle, D ̅. The collaboration is trying to learn more about how the two states on opposite sides of the threshold seem to be interfering with one another. These states are the ψ(3770) and the ψ(3686), each named after their mass value. The ψ(3770) is just above the threshold and primarily decays to a pair of D and D ̅ mesons. Part of the process for producing the DD ̅ pair has a component coming from the ψ(3686). This is mysterious since the ψ(3686) is just below the threshold and should not be able to produce DD ̅ by itself. Julin’s measurements have been studying the effect of this interference, and to see if it can explain an anomalous shape for the cross-section.

Julin must process a lot of events of electron/positron collisions. Knowing what energy to start with and identifying the tracks the particle makes through the detector can determine what other particles were produced in the collision. Though the collider is in Beijing, Julin is able to do all his work here, by file transfer, and with Monte Carlo samples generated on the computers here. He then uses these Monte Carlo samples, pre-built simulations of collision events designed for this specific detector, to do his study. "We can model the individual components that should form the shape we are looking for, combine all those together compared to the total, and get an idea of what should be there. We then assign values to certain physical parameters, and vary those amounts with the fitting program to get as close as possible to the actual data."

Julin says his research is part of the collaboration’s larger effort to understand the strong and weak forces. "There’s still a lot we don’t understand. The behavior of these two particles near the threshold, and how they relate to each other.” BESIII is fortunate that the electron positron collider is able to target specific energies very well. In contrast, the Large Hadron Collider uses protons, which are composite particles. Electrons and positrons are fundamental, and all of the energy in the collision is transferred to the new particles produced. This allows BESIII to produce particles like the ψ(3770) more directly than other collider types. “This is a unique situation. We have the opportunity to get something no one else can get."