Posterior distributions for physical parameters describing relativistic heavy-ion collisions, such as the viscosity of the quark-gluon plasma, are extracted through a comparison of hydrodynamicbased transport models to experimental results from 100A GeV + 100A GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). By simultaneously varying six parameters and by evaluating several classes of observables, we are able to explore the complex intertwined dependencies of observables on model parameters. The methods proved a full multi-dimensional posterior distribution for the model output, including a range of acceptable values for each parameter, and reveal correlations between them. The breadth of observables and the number of parameters considered here go beyond previous studies in this field. The statistical tools, which are based upon Gaussian Process emulators, are tested in detail and should be extendable to larger data sets and a higher number of parameters.

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, m T 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 t start 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 t start = 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.

We systematically compare an event-by-event heavy-ion collision model to data from the 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. The method is universal and easily extensible to other data and collision models.

We use the semi-analytic model ChemTreeN, coupled to cosmological N-body simulations, to explore how different galaxy formation histories can affect observational properties of Milky Way-like galaxies' stellar haloes and their satellite populations. Gaussian processes are used to generate model emulators that allow one to statistically estimate a desired set of model outputs at any location of a p-dimensional input parameter space. This enables one to explore the full input parameter space orders of magnitude faster than could be done otherwise. Using mock observational data sets generated by ChemTreeN itself, we show that it is possible to successfully recover the input parameter vectors used to generate the mock observables if the merger history of the host halo is known. However, our results indicate that for a given observational data set the determination of "best fit" parameters is highly susceptible to the particular merger history of the host. Very different halo merger histories can reproduce the same observational dataset, if the "best fit" parameters are allowed to vary from history to history. Thus, attempts to characterize the formation history of the Milky Way using these kind of techniques must be performed statistically, analyzing large samples of high resolution N-body simulations.

We determine the limit of the lowest achievable photoemitted electron temperature, and therefore the maximum achievable electron brightness, from unstructured photoemitting materials producing dense relativistic or nonrelativistic photoelectron beams. The limit is given by electron heating that occurs just after emission into vacuum, and is due to poorly screened Coulomb interactions equivalent to disorder induced heating seen in ultracold neutral plasmas. We first show that traditional analytic methods of Coulomb collisions fail for the calculation of this strongly coupled heating. Instead, we employ an N -body tree algorithm to compute the universal scaling of the disorder induced heating in fully contained bunches, and show it to agree well with a simple model utilizing the tabulated correlated energy of one component plasmas. We also present simulations for beams undergoing Coulomb explosion at the photoemitter, and demonstrate that both the temperature growth and subsequent cooling must be characterized by correlated effects, as well as correlation-frozen dynamics. In either case, the induced temperature is found to be of several meV

A new method to quantify 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.

The quantum efficiency QE of Cu (111) is measured for different impinging light angles with photon energies just above the work function. We observe that the vectorial photoelectric effect, an enhancement of the quantum efficiency due to illumination with light with an electric vector perpendicular to the sample surface, is stronger in the more surface sensitive regime. This can be explained by a contribution to photoemission due to the variation of the electromagnetic potential at the surface. The contributions of bulk and surface electrons can then be determined.Photoemission from metals has been studied for more than a century both from the experimental and the theoretical point of view [1,2]. Many aspects of this phenomenon are well understood and explained, but others, such as quantitative theoretical prediction of relative peak intensities [3] and total photoemission yield [4], need further development. Not only is this of theoretical interest, it has practical application in the design of photocathodes for Free Electron Lasers (FELs) and ultrafast electron diffraction.In this letter we show experimental measurements of the total photoemission quantum efficiency's QE dependence on the incidence angle θ of the impinging light. Cu(111) was chosen as a sample due to its robust nature and its well known and experimentally verified band structure [5][6][7]: this allows us, through tuning the incident photon energy hν, to control the relative proportions of surface and bulk electrons emitted. As expected [8,9], the three step model [10], which predicts a QE proportional to the absorbed part (1 − R(θ)) of the incident photon energy, needs to be corrected to account for the more effective emission from the electric-field component perpendicular to the sample's surface. Since the intensity of this behavior, known as the vectorial photoelectric effect, increases with the surface sensitivity of the emission process, it is directly related to the well-known surface photoelectric effect [11][12][13], due to the variation at the sample surface of the perpendicular component A ⊥ of the light electromagnetic potential.The quantum efficiency was measured as a function of the incidence angle θ of the impinging photons in the range −63 • < θ < 57 • with 5 • steps (θ = 0 • indicates normal incidence) for two different values of the photon energy hν 1 = 5.44 eV and hν 2 = 5.74 eV; data are compared to results of Ref. 9, obtained with hν 3 = 6.28 eV.Considering the Cu(111) projected band structure shown in Fig. 1, with work function φ = 4.94 eV, surface state and bulk band gap bottom binding energies E SS = 5.35 eV and E BG = 5.8 eV respectively at k = 0, an estimation of the probed initial states can be made for (111) band structure [5][6][7]. States probed by photons of energy hν1 = 5.44 eV, hν2 = 5.74 eV and hν3 = 6.28 eV are highlighted in blue, states probed by hν2 and hν3 are highlighted in violet, states probed by hν3 only are highlighted in black. hν1, exciting all the surface state (SS) and only a few bulk states, is ...

Background: There has been much recent investigation into dijets at the LHC, predictions for dijets at RHIC scales are lacking.Purpose: We present a systematic study of the dijet suppression at RHIC using the VNI/BMS parton cascade. Methods: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:q; the path length of leading and sub-leading jets; cuts on the jet energy distributions; jet cone angle and the jet-medium interaction mechanism.Results: We find that Aj is most sensitive toq and relatively insensitive to the nature of the jet-medium interaction mechanism. The jet profile is dominated byq and the nature of the interaction mechanism. The partonic fragmentation distributions clearly show the jet modification and differentiate between elastic and radiative+elastic modes.Conclusions: Dijets at RHIC scales are strongly modified by a QGP medium, measurements of their suppression will provide a vital additional data point for understanding the modification of hard probes.