arXiv subjects

Scott Pratt

Publications and source records attributed to Scott Pratt.

At least 55 records · Page 3Linked to original sources

arXiv2011

FRIB Theory Users Group Report: Joint ATLAS-HRIBF-NSCL-FRIB SuperUsers Meeting 18-20 August 2011

The FRIB (Facility for Rare Isotope Beams) Theory Users Group participated in the Joint ATLAS-HRIBF-NSCL-FRIB SuperUsers Meeting, hosted by Michigan State University August 18-20, 2011. Prior to the meeting a survey of the FRIB Theory Users Group was conducted to assess the health of the low-energy nuclear theory community and to identify perceived areas of need, in anticipation of the new demands on theory that will accompany FRIB. Meeting discussions focused on survey results and on possible responses. These discussions are summarized here.

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arXiv2010

Effects of Momentum Conservation and Flow on Angular Correlations at RHIC

Correlations of azimuthal angles observed at RHIC have gained great attention due to the prospect of identifying fluctuations of parity-odd regions in the field sector of QCD. Whereas the observable of interest related to parity fluctuations involves subtracting opposite-sign from same-sign correlations, the STAR collaboration reported the same-sign and opposite-sign correlations separately. It is shown here how momentum conservation combined with collective elliptic flow contributes significantly to this class of correlations, though not to the difference between the opposite- and same-sign observables. The effects are modeled with a crude simulation of a pion gas. Though the simulation reproduces the scale of the correlation, the centrality dependence is found to be sufficiently different in character to suggest additional considerations beyond those present in the pion gas simulation presented here.

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arXiv2010

Coupling Relativistic Viscous Hydrodynamics to Boltzmann Descriptions

Models of relativistic heavy ion collisions typically involve both a hydrodynamic module to describe the high density liquid-like phase and a Boltzmann module to simulate the low density break-up phase which is gas-like. Coupling the prescriptions is more complicated for viscous prescriptions if one wants to maintain continuity of the entire stress-energy tensor and currents. Derivations for the viscosity for a gas are reviewed, which then lead to expressions for changes in the phase space occupation based on simple relaxation-time pictures of viscosity. These expressions are shown to consistently reproduce the non-equilibrium components of the stress-energy tensor. An algorithm for generating a Monte Carlo sampling of particles with which to initiate the Boltzmann calculations is also presented.

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arXiv2010

Charge conservation in RHIC and contributiuons to local parity violation observables

Relativistic heavy ion collisions provide laboratory environments from which one can study the creation of a novel state of matter, the quark gluon plasma. The existence of such a state is postulated to alter the mechanism and evolution of charge production, which then becomes manifest in charge correlations. We study the separation of balancing charges at kinetic freeze-out by analyzing recent result on balancing charge correlations for Au+Au collisions at $\sqrt{s_{NN}}=200 {GeV}$. We find that balancing charges are emitted from significantly smaller regions in central collisions compared to peripheral collisions. The results indicate that charge diffusion is small and that the centrality dependence points to a change of the production mechanism. In addition we calculate the contributions from charge-balance correlations to STAR's local parity violation observable. We find that local charge conservation, when combined with elliptic flow, explains much of STAR's measurement.

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arXiv2010

Explaining Angular Correlations Observed at RHIC with Flow and Local Charge Conservation

The observation of fluctuations of parity-odd angular observables at RHIC has been interpreted as a signal of a local parity violation. We show how the observed signal can be explained by local charge conservation at freeze-out combined with elliptic flow. Calculations from a blast wave model, which overlays thermal emission onto a collective flow profile, are shown to account for the experimentally observed signal.

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arXiv2010

Alternative Contributions to the Angular Correlations Observed at RHIC Associated with Parity Fluctuations

Recent measurements at RHIC of angular correlations of same-sign vs. opposite sign pairs have been interpreted as evidence for large-scale fluctuations of parity-odd fields. In this paper, we provide alternative explanations of the same phenomena based on correlations from charge and momentum conservation overlaid with elliptic flow. These effects are shown to produce correlations with similar magnitudes as those measured. Other correlations are also considered, but estimates of their size suggest they are inconsequential.

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arXiv2009

Universal Flow in the First fm/c at RHIC

We will show how both elliptic and radial flow generated during the first fm/c at RHIC can be expressed in a form which is quasi-independent of the state of matter and depends only on the initial energy density profile. Descriptions based on partons or classical fields, thermalized or highly anisotropic, all lead to the same collective velocity given a few easily satisfied conditions. This significantly narrows the uncertainty for initializing hydrodynamic prescriptions.

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arXiv2009

The Long Slow Death of the HBT Puzzle, Proceedings for QM 2009

At the onset of the RHIC era femtoscopic source sizes inferred from two-particle correlations at RHIC defied description with hydrodynamic models. This failure, which became known as the HBT puzzle, now appears to be solved. The source of the discrepancy appears to be a conspiracy of several factors, each of which contributed to making the evolution of RHIC collisions more explosive. These included pre-equilibrium flow, using a stiffer equation of state and adding viscosity.

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arXiv2009

Femtoscopically Probing the Freeze-out Configuration in Heavy Ion Collisions

Two-particle femtoscopy reveals the space-time substructure of the freeze-out configuration from heavy ion collisions. Detailed fingerprints of bulk collectivity are evident in space-momentum correlations, which have been systematically measured as a function of particle type, three-momentum, and collision conditions. A clear scenario, dominated by hydrodynamic-type flow emerges. Reproducing the strength and features of the femtoscopic signals in models involves important physical quantities like the Equation of State, as well as less fundamental technical details. An interesting approximate "factorization" in the measured systematics suggests that the overall physical freeze-out scale is set by final state chemistry, but the kinematic substructure is largely universal. Referring to previous results from hadron and lepton collisions, we point to the importance of determining whether these "universal" trends persist from the largest to the smallest systems. We review theoretical expectations for heavy ion femtoscopy at the LHC, and point to directions needing further theory and experimental work at RHIC and the LHC.

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arXiv2009

Femtoscopy in Relativistic Heavy Ion Collisions and its Relation to Bulk Properties of QCD Matter

Using a viscous hydrodynamic model coupled to a hadronic cascade code, numerous features of the dynamics and equilibrium properties are explored for their impact on femtoscopic measurements. The equation of state, viscous parameters and initial conditions are investigated. We find that femtoscopy is affected by numerous model features at the 10% level, and that by including features and adjusting unknown parameters, one can explain experimental source size measurements to better than 10%.

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arXiv2009

Universal Flow in the Early Stages at RHIC

Pre-thermal flow plays an important role in the final state evolution of heavy ion collisions at RHIC. We show that flow has universal features for a wide range of models. This significantly reduces the uncertainty in initializing hydrodynamic models at RHIC.

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arXiv2008

The Long Slow Death of the HBT Puzzle

Femtoscopic measurements at RHIC have been hailed as a source of insight into the bulk properties of QCD matter. However, hydrodynamic models, which have been successful in reproducing other observables have failed to satisfactorily explain femtoscopic two-particle correlation measurements. This failure has been labeled the "HBT puzzle". In this talk, I present correlations using a hybrid model composed of a viscous hydrodynamic module and a hadronic cascade. After incorporating early acceleration, viscosity and a stiffer equation of state, the extracted source sizes come much closer to experiment.

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arXiv2008

Resolving the HBT Puzzle in Relativistic Heavy Ion Collision

Two particle correlation data from the Relativistic Heavy Ion Collider have provided detailed femtoscopic information describing the space-time structure of the emission of pions. This data had avoided description with hydrodynamic-based approaches, in contrast to the success of hydrodynamics in reproducing other classes of observables. This failure has inspired the term ``HBT puzzle'', where HBT refers to femtoscopic studies which were originally based on Hanbury-Brown Twiss interferometry. Here, the puzzle is shown to originate not from a single shortcoming of hydrodynamic models, but the combination of several effects: mainly including pre-thermalized acceleration, using a stiffer equation of state, and adding viscous corrections.

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arXiv2008

Universal Flow in the First Stage of Relativistic Heavy Ion Collisions

In the first moments of a relativistic heavy ion collision explosive collective flow begins to grow before the matter has yet equilibrated. Here it is found that as long as the stress-energy tensor is traceless, early flow is independent of whether the matter is composed of fields or particles, equilibrated or not, or whether the stress-energy tensor is isotropic. This eliminates much of the uncertainty in modeling early stages of a collision.

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arXiv2008

Viscosity at RHIC: Theory and Practice

Hydrodynamic behavior and the associated discussions of viscosity at RHIC has inspired a r enaissance in modeling viscous hydrodynamics. An explanation of Israel-Stewart hydrodynamics is presented here, with an emphasis on the tangible benefits compared to Navier Stokes.

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arXiv2007

Formulating Viscous Hydrodynamics for Large Velocity Gradients

Viscous corrections to relativistic hydrodynamics, which are usually formulated for small velocity g radients, have recently been extended from Navier-Stokes formulations to a class of treatments based on Israel-Stewart equations. Israel-Stewart treatments, which treat the spatial components of the s tress-energy tensor tau_ij as dynamical objects, introduce new parameters, such as the relaxati on times describing non-equilibrium behavior of the elements tau_ij. By considering linear resp onse theory and entropy constraints, we show how the additional parameters are related to fluctuatio ns of tau_ij. Furthermore, the Israel-Stewart parameters are analyzed for their ability to prov ide stable and physical solutions for sound waves. Finally, it is shown how these parameters, which are naturally described by correlation functions in real time, might be constrained by lattice calcu lations, which are based on path-integral formulations in imaginary time.

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arXiv2007

Tsunamis, Viscosity and the HBT Puzzle

The equation of state and bulk and shear viscosities are shown to be able to affect the transverse dynamics of a central heavy ion collision. The net entropy, along with the femtoscopic radii are shown to be affected at the 10-20% level by both shear and bulk viscosity. The degree to which these effects help build a tsunami-like pulse is also discussed.

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arXiv2007

Extending the Reach of Hydrodynamics

Recent and ongoing improvements to hydrodynamic treatments at RHIC are extending the physics reach of hydrodynamics, and improving the phenomenology. Here, the links between technological improvements and the extension of physics are emphasized.

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