ALICE

Michael Weber: “Studying the quark-gluon plasma via low-mass di-electrons”

How did the early universe look like?

According to the theory of Quantum Chromo Dynamics (QCD), matter at sufficient high temperatures and/or densities undergoes a phase transition to a plasma of deconfined quarks and gluons, known as QGP (quark-gluon plasma).

How to study this early-universe “soup”, that features specific properties due to the presence of free colour charges, so different than the ordinary nuclear matter we see today?

Unfortunately, we cannot access this era directly by astronomical observations, but we are able to reproduce similar conditions as in the early universe by colliding heavy ions, such as gold and lead, at ultra-relativistic energies. To investigate the properties of the QGP we exploit signatures measurable in the final state particles reconstructed in large size detectors as ALICE (A Large Heavy-Ion Experiment) at the CERN-LHC (Large Hadron Collider).

What is the temperature of the system produced in ultra-relativistic heavy-ion collisions? What is its space-time evolution and its life time? Why do protons and neutrons weigh 100 times more than the quarks they are made of?

The New Frontier Group at the Stefan-Meyer-Institut collaborates in the ALICE experiment with the measurement of low mass di-leptons. Leptons, e.g. electrons and positrons, are produced throughout the whole collision history but do not interact with the colour charge of the QGP bringing first-hand information of the different stages of the medium to the final state. The mass of particles that decay in the QGP into electron-positron pairs is therefore directly accessible and can give insight into the generation of hadron masses in general. The QGP emits thermal radiation, which also can be measured with electron-positron pairs. A final goal is to map the temperature throughout the full evolution of the produced hot and dense system.