Selected Publications: Resolving the HBT Puzzle in Relativistic
Heavy Ion Collisions, S. Pratt, Physical
Review Letters 102, 232301 (2009)
Sonic booms at 1012 Kelvin,
S. Pratt, Viewpoint, Physics 1, 29 (2008)
Origins of bulk viscosity in relativistic
heavy ion collisions, S. Pratt and K. Paech,
Phys.Rev. C 74, 014901 (2006)
Clocking hadronization in relativistic heavy
ion collisions with balance functions, S.
Bass, P. Danielewicz and S. Pratt, Phys. Rev.
Lett. 85, 2689 (2000)
Canonical and Microcanonical Calculations
for Fermi Systems, S. Pratt, Phys. Rev. Lett.
84, 4255 (2000)
My research centers on the theoretical description and interpretation of relativistic heavy ion collisions. In these experiments, heavy nuclei such as gold or lead, are collided head on at ultrarelativistic energies. The resulting collisions can create mesoscopic regions where temperatures exceed 1012 Kelvin. At these temperatures, densities become so high that hadrons overlap which makes it impossible to identify individual hadrons and one attains a new state of matter, the strongly interacting quark gluon plasma. The QCD structure of the vacuum, which through its coupling to neutrons and protons is responsible for much of the mass of the universe, also melts at these temperatures. Unfortunately, the collision volumes are so small (sizes of a few time 10-15 m) and the expansions are so rapid (expands and disassembles in less that 10-21 s) that direct observation of the novel state of matter is impossible. Instead, one must infer all properties of the matter from the measured momenta of the outgoing particles. Thus, progress is predicated on careful and detailed modeling of the entire collision.
Modeling heavy ion collisions invokes tools and methods from numerous disciplines: quantum transport theory, relativisitic hydrodynamics, non-perturbative statistical mechanics, and traditional nuclear physics -- to name a few. I have been particularly involved in the development of femtoscopic techniques built on the phenomenology of two-particle correlations. After their last randomizing collision, a pair of particles will interact according to the well-understood quantum two-body interaction. This results in a measurable correlation which can be extracted as a function of the pairs center of mass momentum and relative momentum. Since the correlation is sensitive to how far apart the particles are emitted in time and space, it can be used to quantitatively infer crucial properties of the space-time nature of the collision. These techniques have developed into a field of their own, and have proved invaluable for testing theoretical models, as they represent the best chance to directly test the spatial and temporal development of the models, which is crucial if one is to determine fundamental bulk properties of the matter, such as the pressure or viscosity.
Femtoscopic source radii (in femtometers) characterize the size of the outgoing phase space cloud for particles of given momentum, kt. Experimentally determined sizes from two-particle correlations taken by the STAR collaboration (stars) are compared to model predictions (circles).
In the Fall of 2009, we began a major new initiative funded by the NSF Cyber-Enabled Discovery Initiative. We have established the Models and Data Analysis Initiative (MADAI) with the goal of creating an infrastructure from which sophisticated models and simulations can be compared to large-scale heterogenous data sets. One goal of MADAI will be to bring data from the Relativistic Heavy Ion Collider, RHIC, (a billion-dollar facility at Brookhaven National Laboratory) and make statistical comparison with models such as the ones I have been working on over the years. Similar challenges in astrophysics, meteorology and biology will also be addressed. The collaboration includes researchers from Geography, Mathematics, Physics and Astronomy at MSU, and from Physics, Statistics and Visualization at Duke and the University of North Carolina. I serve as Principle Investigator for the project as well as contribute to the modeling and analysis for RHIC experiments.