9281488
On Computing Upper Limits to Source Intensities
A common problem in astrophysics is determining how bright a source could be and still not be detected in an observation. Despite the simplicity with which the problem can be stated, the solution involves complicated statistical issues that require careful analysis. In contrast to the more familiar confidence bound, this concept has never been formally analyzed, leading to a great variety of often ad hoc solutions. Here we formulate and describe the problem in a self-consistent manner. Detection significance is usually defined by the acceptable proportion of false positives (background fluctuations that are claimed as detections, or Type I error), and we invoke the complementary concept of false negatives (real sources that go undetected, or Type II error), based on the statistical power of a test, to compute an upper limit to the detectable source intensity. To determine the minimum intensity that a source must have for it to be detected, we first define a detection threshold and then compute the probabilities of detecting sources of various intensities at the given threshold. The intensity that corresponds to the specified Type II error probability defines that minimum intensity and is identified as the upper limit. Thus, an upper limit is a characteristic of the detection procedure rather than the strength of any particular source. It should not be confused with confidence intervals or other estimates of source intensity. This is particularly important given the large number of catalogs that are being generated from increasingly sensitive surveys. We discuss, with examples, the differences between these upper limits and confidence bounds. Both measures are useful quantities that should be reported in order to extract the most science from catalogs, though they answer different statistical questions: an upper bound describes an inference range on the source intensity, while an upper limit calibrates the detection process. We provide a recipe for computing upper limits that applies to all detection algorithms.
117207881
3D Simulations for a Micron‐Scale, Dielectric‐Based Acceleration Experiment
An experimental program to demonstrate a dielectric, slab‐symmetric accelerator structure has been underway for the past two years. These resonant devices are driven by a side‐coupled 800‐nm laser and can be configured to maintain the field profile necessary for synchronous acceleration and focusing of relativistic or nonrelativistic particles. We present 3D simulations of various versions of the structure geometry, including a metal‐walled structure relevant to ongoing cold tests on resonant properties, and an all‐dielectric structure to be constructed for a proof‐of‐principle acceleration experiment.
18073215
DEVELOPING PETA-SCALABLE ALGORITHMS FOR BEAM DYNAMIC SIMULATIONS
Peta-scalable software packages for beam dynamic simulations are being developed and used at Argonne national laboratory. The standard Particle-In-Cell (PIC) method and direct Vlasov solvers in up to 4 dimensions have been developed and benchmarked with respect to each other. Both of them have successfully run on 32 thousands processors on BG/P at Argonne Leadership Computing Facility. Peta-scale computing facility at ANL provides suitable environment for simulating beams in large scale. We have simulated charged beams through traditional PIC method as well as direct solving of the Vlasov equation in higher dimensions. Several scalable Poisson solvers have been developed and incorporated with these methods. High-order numerical methods have been adopted for solving the Poisson and Vlasov equations. Preliminary results on direct Vlasov solvers have been obtained in up to 4 dimensions. Domain decomposition method has been used for the parallelization in these software packages, and good scaling has been achieved. These packages have been successfully applied in end-to-end simulation of linear accelerators and large scale accelerator design optimizations. There are still lots of places can be improved for the PIC method and lots of challenges exist in the direct Vlasov solvers, therefore, more efforts are needed in both algorithms and applications. INTRODUCTION Plasma and charged beams are of great importance in modern science and technology. Their researches become more and more rely on the simulations. Simulating plasma and charged beams has three different methods: microscopic model, kinetic model and fluid model. In the microscopic model, each charged particle is described by 6 variables (x, y, z, z y x v v v , , ). Therefore, for N particles, there are 6N variables in total. This requires solving the Vlasov equation in 6N dimensions, which exceeds the capability of current supercomputers for large N. On the other end is the fluid model which is the simplest model, because it treats the plasma as a conducting fluid with electromagnetic forces exerted on it. This leads to solving the Magneto-hydrodynamics (MHD) equations in 3D (x, y and z). MHD solves for the average quantities, such as density and charge, which makes it difficult to describe the fine structure in the plasma. Between these two models is the kinetic model, which solves for the charge density function by solving the Boltzmann or Vlasov equations in 6 dimensions (x, y, z, z y x v v v , , ). The Vlasov equation describes the evolution of a system of particles under the effects of self-consistent electromagnetic fields. The model been used in current beam dynamics simulations is the kinetic model. There are two different ways to solve it. The dominant one is the so called Particle-In-Cell (PIC) method, which utilizes the motion of the particles along the characteristics of the Vlasov equation using a Lagrange-Euler approach [1, 2]. The PIC method has the advantages of fast speed and easy implementation, but similar to MHD, it is hard to capture the fine structures in the plasma. Furthermore, there is noise associated with the finite number of particles in the simulations. With petascale computing, one-to-one simulation can be realized for 9 10 particles. But for more intense beams, PIC method still uses macro particles. The other way to solve the kinetic model is to solve the Vlasov equation directly. But the challenge is the high dimensions. For example, in order to simulate beam in 3D, Vlasov equation has to be solved in 6D. This clearly needs peta-scale computing, and the peta-scalable algorithms are critical to the success. During the past 5 years, we have developed several software packages to meet these demands. This paper presents our efforts on developing peta-scalable algorithms to simulate charged beams in linear accelerators. PARALLEL POISSON SOLVERS In both approaches, Poisson’s equation has to be solved in 2D or 3D to account for the space charge effect. In designing peta-scalable algorithms, it is usually the most challenging part. Therefore, we present our work on this first. Several different methods have been adopted and are given in following: Fourier Method This is the most standard method for solving the Poisson’s equation in box region with Cartesian coordinate system. The potential has been expanded in Fourier series in all three directions. Periodic and Dirichlet zero boundary conditions have been applied in all three directions. ___________________________________________ * This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. jin_xu@anl.gov ∑ ∑ ∑ −
119023494
Shear viscosity of nuclear matter
In this talk I report my recent study on the shear viscosity of neutron-rich nuclear matter from a relaxation time approach. An isospinand momentum-dependent interaction is used in the study. Effects of density, temperature, and isospin asymmetry of nuclear matter on its shear viscosity have been discussed. Similar to the symmetry energy, the symmetry shear viscosity is defined and its density and temperature dependence are studied.
118333521
Isospin-dependent pion in-medium effects on the charged-pion ratio in heavy ion collisions
Using results from the chiral perturbation theory for the s-wave interaction and the {Delta}-resonance model for the p-wave interaction of pions with nucleons, we evaluated the spectral functions of pions in asymmetric nuclear matter with unequal proton and neutron densities. We find that in hot dense neutron-rich matter the strength of the spectral function of positively charged pions at low energies is somewhat larger than that of negatively charged pions. In a thermal model, this isospin-dependent effect slightly reduces the ratio of negatively charged to positively charged pions that are produced in heavy ion collisions induced by radioactive beams. The relevance of our results to the determination of the nuclear symmetry energy from the measured ratio of negatively to positively charged pions produced in heavy ion collisions is discussed.
118385674
A systemic study of 14 southern infrared dark clouds with N2H+, HNC, HCO+ and HCN lines
We have studied 14 southern infrared dark clouds (IRDCs) using the data taken from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey and the GLIMPSE and MIPSGAL mid-infrared survey of the inner Galaxy. The physical and chemical characteristics of the 14 IRDCs are investigated using N2H+(1-0), HNC(1-0), HCO+(1-0), and HCN(1-0) molecular lines. We find that the 14 IRDCs are in different evolutionary stages from the "starless" to the sources with an UCHII region. Three IRDCs are detected to have the star forming activities. The integrated intensity ratios I(HCO+)/I(HCN), I(N2H+)/I(HCN), and I(HNC)/I(HCN) are all about 1.5, which is different from the previous measurements, suggesting that the integrated intensity ratios may be affected by the cloud environments. The integrated intensities of HNC, HCO+ and HCN show a tight correlation for the 14 IRDCs, implying a close link to the chemical evolution of these three species in the IRDCs. The derived excitation temperature for each IRDC is less than 25 K. We estimate the abundances of the four molecules from 10^(-11) to 10^(-9), and the average abundance ratios N(HNC)/N(HCN) = 1.4+ -0.50, N(HNC)/N(HCO+) = 1.74 + - 0.22, N(HCN)/N(HCO+)= 1.21 + - 0.41.
53135280
Azimuthal jet flavor tomography with CUJET2.0 of nuclear collisions at RHIC and LHC
A bstractA perturbative QCD based jet tomographic Monte Carlo model, CUJET2.0, is presented to predict jet quenching observables in relativistic heavy ion collisions at RHIC/BNL and LHC/CERN energies. This model generalizes the DGLV theory of flavor dependent radiative energy loss by including multi-scale running strong coupling effects. It generalizes CUJET1.0 by computing jet path integrations though more realistic 2 + 1D transverse and longitudinally expanding viscous hydrodynamical fields contrained by fits to low pT flow data. The CUJET2.0 output depends on three control parameters, (αmax, fE, fM ), corresponding to an assumed upper bound on the vacuum running coupling in the infrared and two chromo-electric and magnetic QGP screening mass scales (fEμ(T ), fMμ(T )) where μ(T ) is the 1-loop Debye mass. We compare numerical results as a function of αmax for pure and deformed HTL dynamically enhanced scattering cases corresponding to (fE = 1, 2, fM = 0) to data of the nuclear modification factor, RAAf(pT, ϕ; s,b$$ \sqrt{s}, b $$) for jet fragment flavors f = π, D, B, e at s$$ \sqrt{s} $$ = 0.2 − 2.76 ATeV c.m. energies per nucleon pair and with impact parameter b = 2.4, 7.5 fm. A χ2 analysis is presented and shows that RAAπ data from RHIC and LHC are consistent with CUJET2.0 at the χ2/d.o.f < 2 level for αmax = 0.23 − 0.30. The corresponding q^$$ \widehat{q} $$(Ejet, T )/T3 effective jet transport coefficient field of this model is computed to facilitate comparison to other jet tomographic models in the literature. The predicted elliptic asymmetry, v2(pT ; s$$ \sqrt{s} $$, b) is, however, found to significantly underestimated relative to RHIC and LHC data. We find the χv22$$ {\chi}_{v_2}^2 $$ analysis shows that v2 is very sensitive to allowing even as little as 10% variations of the path averaged αmax along in and out of reaction plane paths.
118687423
Generic constraints on the relativistic mean-field and Skyrme-Hartree-Fock models from the pure neutron matter equation of state
We study the nuclear symmetry energy S(ρ) and related quantities of nuclear physics and nuclear astrophysics predicted generically by relativistic mean-field (RMF) and Skyrme-Hartree-Fock (SHF) models. We establish a simple prescription for preparing equivalent RMF and SHF parametrizations starting from a minimal set of empirical constraints on symmetric nuclear matter, nuclear binding energy, and charge radii, enforcing equivalence of their Lorenz effective masses, and then using the pure neutron matter (PNM) equation of state obtained from ab initio calculations to optimize the pure isovector parameters in the RMF and SHF models. We find that the resulting RMF and SHF parametrizations give broadly consistent predictions of the symmetry energy J and its slope parameter L at saturation density within a tight range of 2a nd 6 MeV, respectively, but that clear model dependence shows up in the predictions of higher-order symmetry energy parameters, leading to important differences in (a) the slope of the correlation between J and L from the confidence ellipse, (b) the isospin-dependent part of the incompressibility of nuclear matter Kτ , (c) the symmetry energy at suprasaturation densities, and (d) the predicted neutron star radii. The model dependence can lead to about 1–2 km difference in predictions of the neutron star radius given identical predicted values of J and L and symmetric nuclear matter (SNM) saturation properties. Allowing the full freedom in the effective masses in both models leads to constraints of 30 J 31. 5M eV, 35 L 60 MeV, and −330 Kτ −216 MeV for the RMF model as a whole and 30 J 33 MeV, 28 L 65 MeV, and −420 Kτ −325 MeV for the SHF model as a whole. Notably, given PNM constraints, these results place RMF and SHF models as a whole at odds with some constraints on Kτ inferred from giant monopole resonance and neutron skin experimental results.