Thermal photon production in Gubser inviscid relativistic fluid dynamics

The Gubser solution to inviscid relativistic fluid dynamics is used to examine the role of transverse expansion on the energy spectrum of photons radiated by quark-gluon plasma. Transverse flow is shown to be a modest effect on the energy spectrum of photons as a whole, despite its large effect on rare high-energy photons produced at low temperatures. An exact expression is derived for the volume of the plasma as a function of its temperature. A simple formula is obtained for the energy spectrum of high-energy thermal photons, which is used to relate the inverse slope $T_{\textrm{eff}}$ of the photon spectrum at energy $E$ to the maximum temperature of the plasma $T_0$, finding $T_{\textrm{eff}} \approx T_0/(1+\frac{5}{2} \frac{T_0}{E})$.Comment: 6 pages, 5 figures; v2: added exact formula for Gubser volume, fixed definition of effective temperature, updated text accordingl

Applications of emulation and Bayesian methods in heavy-ion physics

Heavy-ion collisions provide a window into the properties of many-body systems of deconfined quarks and gluons. Understanding the collective properties of quarks and gluons is possible by comparing models of heavy-ion collisions to measurements of the distribution of particles produced at the end of the collisions. These model-to-data comparisons are extremely challenging, however, because of the complexity of the models, the large amount of experimental data, and their uncertainties. Bayesian inference provides a rigorous statistical framework to constrain the properties of nuclear matter by systematically comparing models and measurements. This review covers model emulation and Bayesian methods as applied to model-to-data comparisons in heavy-ion collisions. Replacing the model outputs (observables) with Gaussian process emulators is key to the Bayesian approach currently used in the field, and both current uses of emulators and related recent developments are reviewed. The general principles of Bayesian inference are then discussed along with other Bayesian methods, followed by a systematic comparison of seven recent Bayesian analyses that studied quark-gluon plasma properties, such as the shear and bulk viscosities. The latter comparison is used to illustrate sources of differences in analyses, and what it can teach us for future studies.Comment: 52 page

Initial conditions for hydrodynamics from kinetic theory equilibration

We use effective kinetic theory to study the pre-equilibrium dynamics in heavy-ion collisions. We describe the evolution of linearized energy perturbations on top of out-of-equilibrium background to the energy-momentum tensor at a time when hydrodynamics becomes applicable. We apply this description to IP-Glasma initial conditions and find an overall smooth transition to hydrodynamics. In a phenomenologically favorable range of $\eta/s$ values, early time dynamics can be accurately described in terms of a few functions of a scaled time variable $\tau T/(\eta/s)$. Our framework can be readily applied to other initial state models to provide the pre-equilibrium dynamics of the energy momentum tensor.Comment: 5 pages, 5 figures, Proceedings for the 26th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2017), February 5-11 2017, Chicago, Illinois, US

Les ateliers d'écriture à l'école : étude d'une pratique d'accompagnement à l'apprentissage de l'écriture

La question centrale de ce mémoire est d’étudier les ateliers d’écriture en classe, au travers du filtre du rapport à l’écriture. En introduction, le constat de départ est une difficulté des élèves dans la maîtrise de leur écrit. Dans un premier temps, nous nous penchons sur l’écriture, son apprentissage chez l’enfant, puis nous regardons du côté de l’École, pour voir ce qu’elle demande, si cela a évolué. En deuxième partie, c’est l’atelier d’écriture qui est analysé : qu’est-ce que c’est, son historique. Vient la partie expérimentation : ce qui a été pratiqué dans une classe comme ateliers d’écriture, l’enquête auprès des élèves sur leur rapport à l’écriture. Nous analysons là les résultats de cette enquête, ce que nous pouvons comprendre du rapport à l’écrit des enfants, comment les ateliers d’écriture tirent parti, s’accommodent, contribuent au rapport à l’écriture des élèves. En conclusion, nous revenons sur le constat de départ pour réfléchir sur une pratique des ateliers d’écriture comme pratique d’accompagnement à l’écriture efficace et étayée par les avancées en didactique de l’écriture. Nous constatons également notre avancée en compréhension des difficultés d’apprentissage de l’écriture grâce à cette recherche. En annexes, nous avons restitué les programmes officiels, notre démarche complète de pratique des ateliers d’écriture en classe, les entretiens avec les élèves, des exemples de textes produits en atelier

Computational budget optimization for Bayesian parameter estimation in heavy ion collisions

Bayesian parameter estimation provides a systematic approach to compare heavy ion collision models with measurements, leading to constraints on the properties of nuclear matter with proper accounting of experimental and theoretical uncertainties. Aside from statistical and systematic model uncertainties, interpolation uncertainties can also play a role in Bayesian inference, if the model's predictions can only be calculated at a limited set of model parameters. This uncertainty originates from using an emulator to interpolate the model's prediction across a continuous space of parameters. In this work, we study the trade-offs between the emulator (interpolation) and statistical uncertainties. We perform the analysis using spatial eccentricities from the T$_\mathrm{R}$ENTo model of initial conditions for nuclear collisions. Given a fixed computational budget, we study the optimal compromise between the number of parameter samples and the number of collisions simulated per parameter sample. For the observables and parameters used in the present study, we find that the best constraints are achieved when the number of parameter samples is slightly smaller than the number of collisions simulated per parameter sample.Comment: 11 pages, 7 figure