Physics Colloquium - Krishna Rajagopal

Zoom Recording ID: 81343043189 UUID: pCDBsyZTSYmAQS9Ux0wALA== Meeting Time: 2026-02-11 09:40:51pmGMT

Title:  “Hot and Cold Quark Soup Under Pressure”

Abstract: Ultrarelativistic heavy ion collisions reproduce tiny droplets of the trillions-of-degrees-hot liquid with pressures 10 million trillion trillion times Earth’s atmospheric pressure. This stuff that filled the microseconds-old universe is conventionally called quark-gluon plasma (QGP) but is better thought of as hot quark soup.  Over the past twenty years, data obtained via recreating this primordial fluid in “Little Bangs” have shown that it is the most liquid liquid in the universe, making it the first complex matter to form as well as the source of all protons and neutrons. After a look at how we have learned this, I will focus on the road ahead, in particular on new probes being developed to answer questions like: How can we use jets to see the inner workings of QGP and understand how a strongly coupled liquid emerges from quarks and gluons? And, how does the droplet of QGP ripple after it has been probed by a passing jet? Cold quark soup, at pressures almost as high, can be found at the centers of the heaviest neutron stars — where the inward pressure of the surrounding star prevents it from exploding. Cold quark soup is expected to be the quark analog of a superconductor, a prediction that may be within reach of coming astrophysical observations of neutron stars and their mergers.  And, in a sense most surprising of all, recent work points to the possibility that ordinary protons remember the pressure and entropy of the hot quark soup from which they were born during the Big Bang.