T-2, Nuclear and Particle Physics, Astrophysics and Cosmology

Initial Conditions of Conserved Charges in Heavy-Ion Collisions

Matt Sievert

Ultra-relativistic collisions of heavy nuclei provide a unique experimental window into the early universe and into extreme sectors of the QCD phase diagram by producing an exotic state of matter known as the quark-gluon plasma (QGP). The essential characteristic of the QGP, which is formed at temperatures sufficiently high to melt hadrons into a plasma of delocalized quarks and gluons, is a nearly vanishing fluid viscosity which makes the QGP among the most “perfect liquids” in nature. This nearly-ideal fluid flow, however, makes the final distributions of detected hadrons especially sensitive to the initial geometry of the fireball shortly after the collision. As such, tremendous effort has been expended to distinguish the features of the data arising from the final-state QGP evolution from the features reflecting the fluctuating initial state itself. At top RHIC and LHC energies, the initial energy density of a heavy-ion collision is composed almost entirely of gluons, with the hydrodynamic evolution reflecting energy-momentum conservation over the lifetime of the QGP fireball. However, quarks, which constitute a minority of the overall energy density, also carry other conserved charges such as baryon number and electric charge which are sensitive to entirely different transport properties of the QGP. In this talk, I will present a new model for reconstructing the initial distribution of quarks and antiquarks in a heavy-ion collision by sampling the (g à q qbar) splitting function over the initial energy density. In this way, we provide a new numerical tool which can be used to supplement any model for the initial energy density with the associated conserved charges. As a result, we find a strong flavor dependence of the initial geometries of different quarks, as characterized by their initial eccentricities. Importantly, we find that the strange quark geometry differs significantly from the geometry of the bulk energy density in an event, reflecting the geometry of the hot spots rather than the geometry of the bulk. This new tool for the initial conditions, when coupled to a charge-conserving viscous hydrodynamics code, will open the door to studying a wealth of new charge- and flavor-dependent correlations and transport parameters of the QGP.


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