Energy loss in a partonic transport model including bremsstrahlung processes

A detailed investigation of the energy loss of gluons that traverse a thermal gluonic medium simulated within the perturbative QCD--based transport model BAMPS (a Boltzmann approach to multiparton scatterings) is presented in the first part of this work. For simplicity the medium response is neglected in these calculations. The energy loss from purely elastic interactions is compared to the case where radiative processes are consistently included based on the matrix element by Gunion and Bertsch. From this comparison gluon multiplication processes gg -> ggg are found to be the dominant source of energy loss within the approach employed here. The consequences for the quenching of gluons with high transverse momentum in fully dynamic simulations of Au+Au collisions at the RHIC energy of sqrt(s) = 200 AGeV are discussed in the second major part of this work. The results for central collisions as discussed in a previous publication are revisited and first results on the nuclear modification factor R_AA for non-central Au+Au collisions are presented. They show a decreased quenching compared to central collisions while retaining the same shape. The investigation of the elliptic flow v2 is extended up to non-thermal transverse momenta of 10 GeV, exhibiting a maximum v2 at roughly 4 to 5 GeV and a subsequent decrease. Finally the sensitivity of the aforementioned results on the specific implementation of the effective modeling of the Landau-Pomeranchuk-Migdal (LPM) effect via a formation time based cut-off is explored.Comment: 40 pages, 20 figures, 1 tabl

Perturbative QCD Calculations of Elliptic Flow and Shear Viscosity in Au+Au Collisions at sNN=200\sqrt{s_{NN}}=200 GeV

The elliptic flow $v_2$ and the ratio of the shear viscosity over the entropy density, $\eta/s$, of gluon matter are calculated from the perturbative QCD (pQCD) based parton cascade Boltzmann approach of multiparton scatterings. For Au+Au collisions at $\sqrt{s}=200$A GeV the gluon plasma generates large $v_2$ values measured at the BNL Relativistic Heavy Ion Collider. Standard pQCD yields $\eta/s\approx 0.08-0.15$ as small as the lower bound found from the anti-de Sitter/conformal field theory conjecture.Comment: 4 pages, 6 figures, new results added in Figs 1, 2, and 3, version published in PR

Constraining the interaction between dark sectors with future HI intensity mapping observations

We study a model of interacting dark matter and dark energy, in which the two components are coupled. We calculate the predictions for the 21-cm intensity mapping power spectra, and forecast the detectability with future single-dish intensity mapping surveys (BINGO, FAST and SKA-I). Since dark energy is turned on at $z\sim 1$, which falls into the sensitivity range of these radio surveys, the HI intensity mapping technique is an efficient tool to constrain the interaction. By comparing with current constraints on dark sector interactions, we find that future radio surveys will produce tight and reliable constraints on the coupling parameters.Comment: 12 pages, 7 figure

The role of the gluonic gg<->ggg interactions in early thermalization in ultrarelativistic heavy-ion collisions

We ``quantify'' the role of elastic as well as inelastic ggggg pQCD processes in kinetic equilibration within a pQCD inspired parton cascade. The contributions of different processes to kinetic equilibration are manifested by the transport collision rates. We find that in a central Au+Au collision at RHIC energy pQCD Bremstrahlung processes are much more efficient for momentum isotropization compared to elastic scatterings. For the parameters chosen the ratio of their transport collision rates amounts to 5:1.Comment: 5 pages, 3 figures, to appear in the proceedings of Workshop for Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions (Hot Quarks 2006), Villasimius, Sardinia, Italy, 15-20 May 200

Multinomial latent logistic regression

University of Technology Sydney. Faculty of Engineering and Information Technology.We are arriving at the era of big data. The booming of data gives birth to more complicated research objectives, for which it is important to utilize the superior discriminative power brought by explicitly designed feature representations. However, training models based on these features usually requires detailed human annotations, which is being intractable due to the exponential growth of data scale. A possible solution for this problem is to employ a restricted form of training data, while regarding the others as latent variables and performing latent variable inference during the training process. This solution is termed weakly supervised learning, which usually relies on the development of latent variable models. In this dissertation, we propose a novel latent variable model - multinomial latent logistic regression (MLLR), and present a set of applications on utilizing the proposed model on weakly supervised scenarios, which, at the same time, cover multiple practical issues in real-world applications. We first derive the proposed MLLR in Chapter 3, together with theoretical analysis including the concave and convex property, optimization methods, and the comparison with existing latent variable models on structured outputs. Our key discovery is that by performing “maximization” over latent variables and “averaging” over output labels, MLLR is particularly effective when the latent variables have a large set of possible values or no well-defined graphical structure is existed, and when probabilistic analysis is preferred on the output predictions. Based on it, the following three sections will discuss the application of MLLR in a variety of tasks on weakly supervised learning. In Chapter 4, we study the application of MLLR on a novel task of architectural style classification. Due to a unique property of this task that rich inter-class relationships between the recognizing classes make it difficult to describe a building using “hard” assignments of styles, MLLR is believed to be particularly effective due to its ability to produce probabilistic analysis on output predictions in weakly supervised scenarios. Experiments are conducted on a new self-collected dataset, where several interesting discoveries on architectural styles are presented together with the traditional classification task. In Chapter 5, we study the application of MLLR on an extreme case of weakly supervised learning for fine-grained visual categorization. The core challenge here is that the inter-class variance between subordinate categories is very limited, sometimes even lower than the intra-class variance. On the other hand, due to the non-convex objective function, latent variable models including MLLR are usually very sensitive to the initialization. To conquer these problems, we propose a novel multi-task co-localization strategy to perform warm start for MLLR, which in turn takes advantage of the small inter-class variance between subordinate categories by regarding them as related tasks. Experimental results on several benchmarks demonstrate the effectiveness of the proposed method, achieving comparable results with latest methods with stronger supervision. In Chapter 6, we aim to further facilitate and scale weakly supervised learning via a novel knowledge transferring strategy, which introduces detailed domain knowledge from sophisticated methods trained on strongly supervised datasets. The proposed strategy is proved to be applicable in a much larger web scale, especially accounting for the ability of performing noise removal with the help of the transferred domain knowledge. A generalized MLLR is proposed to solve this problem using a combination of strongly and weakly supervised training data

Investigation of shear stress and shear flow within a partonic transport model

Starting from a classical picture of shear viscosity we construct a steady velocity gradient in the partonic cascade BAMPS. Using the Navier-Stokes-equation we calculate the shear viscosity coefficient. For elastic isotropic scatterings we find a very good agreement with the analytic values. For both elastic and inelastic scatterings with pQCD cross sections we find good agreement with previously published calculations