Physics Colloquium

Please note that self-served coffee and cookies will be in the seminar lounge at 3:00 pm. Abstract: Since first observed at RHIC roughly 20 years ago, jet quenching has been used as an important probe of quark-gluon plasmas (meaning quark-gluon plasmas/liquids/glasmas/whatever) made in heavy-ion collisions. Now sPHENIX will carry on that RHIC tradition with new, more detailed investigations. In this talk, I will take a step back and discuss a selected set of fundamental, qualitative issues concerning the theory of how very high-energy quarks and gluons shower and lose energy inside a quark-gluon plasma. I will start with a qualitative review of a 1950s theoretical result (the Landau-Pomeranchuk-Migdal effect) that medium-induced QED shower development is significantly slowed at extremely high energy because of coherence effects in how high-energy particles interact with the medium. Those calculations were later generalized to QCD in the 1990s. But there are also more subtle interference effects, which made it unclear whether we have any real theoretical control at all in calculating high-energy in-medium QCD showers. I hope to convince you that certain theoretical "spherical cow" thought experiments give interesting and important insights into this problem.