Christopher E. Coleman-Smith

Constraining the initial state granularity with bulk observables in Au+Au collisions at \sqrt{s_{\rm NN}}=200 GeV

In this paper we conduct a systematic study of the granularity of the initial state of hot and dense QCD matter produced in ultra-relativistic heavy-ion collisions and its influence on bulk observables like particle yields, mT spectra and elliptic flow. For our investigation we use a hybrid transport model, based on (3+1)D hydrodynamics and a microscopic Boltzmann transport approach. The initial conditions are generated by a non-equilibrium hadronic transport approach and the size of their fluctuations can be adjusted by defining a Gaussian smoothing parameter σ. The dependence of the hydrodynamic evolution on the choices of σ and tstart is explored by means of a Gaussian emulator. To generate particle yields and elliptic flow that are compatible with experimental data the initial state parameters are constrained to be σ = 1 fm and tstart = 0.5 fm. In addition, the influence of changes in the equation of state is studied and the results of our event-by-event calculations are compared to a calculation with averaged initial conditions. We conclude that even though the initial state parameters can be constrained by yields and elliptic flow, the granularity needs to be constrained by other correlation and fluctuation observables.

Quantifying properties of hot and dense QCD matter through systematic model-to-data comparison

We systematically compare an event-by-event heavy-ion collision model to data from the CERN Large Hadron Collider. Using a general Bayesian method, we probe multiple model parameters including fundamental quark-gluon plasma properties such as the specific shear viscosity η/s, calibrate the model to optimally reproduce experimental data, and extract quantitative constraints for all parameters simultaneously. Furthermore, the method is universal and easily extensible to other data and collision models.

Results of a systematic study of dijet suppression measured at the BNL Relativistic Heavy Ion Collider

We present a systematic study of the dijet suppression at RHIC using the VNI/BMS parton cascade. We examine the modification of the dijet asymmetry Aj and the within-cone transverse energy distribution (jet-shape) along with partonic fragmentation distributions z and jt in terms of: q̂; the path length of leading and sub-leading jets; cuts on the jet energy distributions; jet cone angle and the jet-medium interaction mechanism. We find that Aj is most sensitive to q̂ and relatively insensitive to the nature of the jet-medium interaction mechanism. The jet profile is dominated by q̂ and the nature of the interaction mechanism. The partonic fragmentation distributions clearly show the jet modification and differentiate between elastic and radiative+elastic modes.

Classification of initial state granularity via 2D Fourier expansion

A new method for quantifying fluctuations in the initial state of heavy ion collisions is presented. The initial state energy distribution is decomposed with a set of orthogonal basis functions which include both angular and radial variation. The resulting two-dimensional Fourier coefficients provide additional information about the nature of the initial state fluctuations compared to a purely angular decomposition. We apply this method to ensembles of initial states generated by both Glauber and color glass condensate Monte-Carlo codes. In addition initial state configurations with varying amounts of fluctuations generated by a dynamic transport approach are analyzed to test the sensitivity of the procedure. The results allow for a full characterization of the initial state structures that is useful to discriminate the different initial state models currently in use. Communicated by Steffen Bass

Implementing the LPM effect in a parton cascade model

Abstract Parton Cascade Models (PCM [K. Geiger, B. Muller, Nucl. Phys. B369 (1992) 600–654; S. A. Bass, B. Muller, D. K. Srivastava, Phys. Lett. B551 (2003) 277–283; Z. Xu and C. Greiner, Phys. Rev. C 76, 024911 (2007); D. Molnar and M. Gyulassy, Phys. Rev. C 62, 054907 (2000)]), which describe the full time-evolution of a system of quarks and gluons using pQCD interactions are ideally suited for the description of jet production, including the emission, evolution and energy-loss of the full parton shower in a hot and dense QCD medium. The Landau-Pomeranchuk-Migdal (LPM) effect [L. D. Landau, I. J. Pomeranchuk, Dolk. Akad. Nauk. SSSR 92 (92); A. B. Migdal, Phys. Rev. 103 (6) (1956) 1811–1820], the quantum interference of parton wave functions due to repeated scatterings against the background medium, is likely the dominant in-medium effect affecting jet suppression. We have implemented a probabilistic implementation of the LPM effect [K. Zapp, J. Stachel, U. A. Wiedemann, Phys. Rev. Lett. 103 (2009) 152302] within the PCM which can be validated against previously derived analytical calculations by Baier et al (BDMPS-Z) [R. Baier, Y. L. Dokshitzer, A. H. Mueller, S. Peigne, D. Schiff, Nucl. Phys. B478 (1996) 577–597; R. Baier, Y. L. Dokshitzer, S. Peigne, D. Schiff, Phys. Lett. B345 (1995) 277–286; R. Baier, Y. L. Dokshitzer, A. H. Mueller, S. Peigne, D. Schiff, Nucl. Phys. B483 (1997) 291–320; B. Zakharov, JETP Lett. 63 (1996) 952–957; B. Zakharov, JETP Lett. 65 (1997) 615–620]. Presented at the 6th International Conference on Physics and Astrophysics of Quark Gluon Plasma (ICPAQGP 2010).

A “Helium atom” of space: Dynamical instability of the isochoric pentahedron

We present an analysis of the dynamics of the equifacial pentahedron on the Kapovich-Millson phase space under a volume preserving Hamiltonian. The classical dynamics of polyhedra under such a Hamiltonian may arise from the classical limit of the node volume operators in loop quantum gravity. The pentahedron is the simplest nontrivial polyhedron for which the dynamics may be chaotic. We consider the distribution of polyhedral configurations throughout the space and find indications that the borders between certain configurations act as separatrices. We examine the local stability of trajectories within this phase space and find that locally unstable regions dominate although extended stable regions are present. Canonical and microcanonical estimates of the Kolmogorov-Sinai entropy suggest that the pentahedron is a strongly chaotic system. The presence of chaos is further suggested by calculations of intermediate time Lyapunov exponents which saturate to non zero values.

Jet modification in a brick of QGP matter

We have implemented the LPM effect into a microscopic transport model with partonic degrees of freedom by following the algorithm of Zapp & Wiedemann. The Landau-Pomeranchuk-Migdal (LPM) effect is a quantum interference process that modifies the emission of radiation in the presence of a dense medium. In QCD this results in a quadratic length dependence for radiative energy loss. This is an important effect for the modification of jets by their passage through the QGP. We verify the leading parton energy loss in the model against the leading order Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPS-Z) result. We apply our model to the recent observations of the modification of di-jets at the LHC.We have implemented the LPM effect into a microscopic transport model with partonic degrees of freedom by following the algorithm of Zapp & Wiedemann. The Landau-Pomeranchuk-Migdal (LPM) effect is a quantum interference process that modifies the emission of radiation in the presence of a dense medium. In QCD this results in a quadratic length dependence for radiative energy loss. This is an important effect for the modification of jets by their passage through the QGP. We verify the leading parton energy loss in the model against the leading order Baier-Dokshitzer-Mueller-Peigne-SchiffZakharov (BDMPS-Z) result. We apply our model to the recent observations of the modification of di-jets at the LHC.