June 11, 2024

PhD Jan Hammelmann

Fluctuations in the hadronic phase of heavy-ion collisions

Jan Hammelmann completed his doctorate in the group of FIAS Senior Fellow Hannah Elfner.

One of the fundamental forces of nature is the strong force, or Quantum Chromodynamics (QCD), which describes the interactions between elementary particles called quarks and gluons. At the temperatures we exhibit in our daily life, quarks and gluons form bound states known as hadrons, such as protons and neutrons. However, at extremely high temperatures, hadrons melt, allowing quarks and gluons to move freely in a state called Quark-Gluon Plasma (QGP), which likely existed just after the Big Bang.

A key question in QCD research is the understanding of the transition between hadrons and the QGP. Similar to how water can exist as a liquid, gas or solid depending on temperature and pressure, QCD matter has its own phase diagram. Researchers proposed a critical point at the end of a first-order phase transition in the QCD phase-diagram, which is actively searched for through heavy-ion collisions. These collisions create conditions of high density and temperature at which the QGP might form.

One way to search for the critical point is by studying fluctuations of conserved charges, observed as higher moments of the distribution of particle numbers measured event-by-event. During his PhD, Jan Hammelmann studied these fluctuations in the hadronic phase of heavy-ion collisions using a transport model called SMASH, developed in the group of Hannah Elfner.

One of his projects focused on determining the effects of global charge conservation and hadronic interactions, such as resonance formation/decay and charge annihilation, on the higher moments. In another project, he studied the evolution of critical fluctuations in the hadronic medium by initializing SMASH with particle distribution functions. These were derived from the principle of maximum information entropy and fitted to a model that includes a critical point. In a third project, Hammelmann examined how specific hadronic interactions affect the shear viscosity and diffusion coefficients of conserved charges in QCD. These transport coefficients are vital for under- standing the dynamics of heavy-ion collisions. Comparing the results from the transport model with other findings provides valuable insights into the hadron chemistry and interactions.

Hammelmann decided not to pursue a career in academia and is currently looking for a suitable job.

PhD defence of Jan Hammelmann (right).