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 at RHIC, located at Brookhaven, or at the LHC at CERN. 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. Thus, 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 pair’s 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 determining the space-time evolution of the system from experiment. I have also developed phenomenological tools for determining the chemical evolution of the QCP from correlations driven by charge conservation. These correlations, at a quantitative level, have shown that the quark content of the matter created in heavy-ion collisions at the RHIC or at the LHC indeed have roughly the expected densities of up, down and strange quarks. Other work has included transport tools, such as hydrodynamics and Boltzman distributions, as applied to relativistic collisions, and methods for exact calculations of canonical ensembles with complicated sets of converted charges.
From 2009 through 2015, I am the principal investigator for the Models and Data Analysis Initiative (MADAI) collaboration, which was funded by the NSF through the Cyber-Enabled Discovery and Innovation initiative. MADAI involves nuclear physicists, cosmologists, astrophysicists, atmospheric scientists, statisticians and visualization experts from MSU, Duke and the University of North Carolina. The goal is to develop statistical tools for comparing large heterogenous data sets to sophisticated multi-scale models. In particular, I have worked to use data from RHIC and the LHC to extract fundamental properties of the matter created in high-energy heavy-ion collisions.
Selected PublicationsDetermining Fundamental Properties of Matter Created in Ultrarelativistic Heavy-Ion Collision, J. Novak, K. Novak, S. Pratt, C. Coleman-Smith, R. Woplert, arXiv: 1303.5769 (2013)
Identifying the Charge Carriers of the Quark-Gluon Plasma, S. Pratt, Physical Review Letters, 108. 212301 (2012)
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)