Theoretical Evaluations of the Fission Cross Section of the 77 eV Isomer of 235-U

We have developed models of the fission barrier (barrier heights and transition state spectra) that reproduce reasonably well the measured fission cross section of $^{235}$U from neutron energy of 1 keV to 2 MeV. From these models we have calculated the fission cross section of the 77 eV isomer of $^{235}$U over the same energy range. We find that the ratio of the isomer cross section to that of the ground state lies between about 0.45 and 0.55 at low neutron energies. The cross sections become approximately equal above 1 MeV. The ratio of the neutron capture cross section to the fission cross section for the isomer is predicted to be about a factor of 3 larger for the isomer than for the ground state of $^{235}$U at keV neutron energies. We have also calculated the cross section for the population of the isomer by inelastic neutron scattering form the $^{235}$U ground state. We find that the isomer is strongly populated, and for $E_n = 1 MeV$ the $(n,n'\gamma)$ cross section leading to the population of the isomer is of the order of 0.5 barn. Thus, neutron reaction network calculations involving the uranium isotopes in a high neutron fluence are likely to be affected by the 77 eV isomer of $^{235}$U. With these same models the fission cross sections of $^{233}$U and $^{237}$U can be reproduced approximately using only minor adjustments to the barrier heights. With the significant lowering of the outer barrier that is expected for the outer barrier the general behavior of the fission cross section of $^{239}$Pu can also be reproduced.Comment: 17 pages including 8 figure

Stirring Strongly Coupled Plasma

We determine the energy it takes to move a test quark along a circle of radius L with angular frequency w through the strongly coupled plasma of N=4 supersymmetric Yang-Mills (SYM) theory. We find that for most values of L and w the energy deposited by stirring the plasma in this way is governed either by the drag force acting on a test quark moving through the plasma in a straight line with speed v=Lw or by the energy radiated by a quark in circular motion in the absence of any plasma, whichever is larger. There is a continuous crossover from the drag-dominated regime to the radiation-dominated regime. In the crossover regime we find evidence for significant destructive interference between energy loss due to drag and that due to radiation as if in vacuum. The rotating quark thus serves as a model system in which the relative strength of, and interplay between, two different mechanisms of parton energy loss is accessible via a controlled classical gravity calculation. We close by speculating on the implications of our results for a quark that is moving through the plasma in a straight line while decelerating, although in this case the classical calculation breaks down at the same value of the deceleration at which the radiation-dominated regime sets in.Comment: 27 pages LaTex, 5 figure

Improved Holographic QCD

We provide a review to holographic models based on Einstein-dilaton gravity with a potential in 5 dimensions. Such theories, for a judicious choice of potential are very close to the physics of large-N YM theory both at zero and finite temperature. The zero temperature glueball spectra as well as their finite temperature thermodynamic functions compare well with lattice data. The model can be used to calculate transport coefficients, like bulk viscosity, the drag force and jet quenching parameters, relevant for the physics of the Quark-Gluon Plasma.Comment: LatEX, 65 pages, 28 figures, 9 Tables. Based on lectures given at several Schools. To appear in the proceedinds of the 5th Aegean School (Milos, Greece

Transport Properties of the Quark-Gluon Plasma -- A Lattice QCD Perspective

Transport properties of a thermal medium determine how its conserved charge densities (for instance the electric charge, energy or momentum) evolve as a function of time and eventually relax back to their equilibrium values. Here the transport properties of the quark-gluon plasma are reviewed from a theoretical perspective. The latter play a key role in the description of heavy-ion collisions, and are an important ingredient in constraining particle production processes in the early universe. We place particular emphasis on lattice QCD calculations of conserved current correlators. These Euclidean correlators are related by an integral transform to spectral functions, whose small-frequency form determines the transport properties via Kubo formulae. The universal hydrodynamic predictions for the small-frequency pole structure of spectral functions are summarized. The viability of a quasiparticle description implies the presence of additional characteristic features in the spectral functions. These features are in stark contrast with the functional form that is found in strongly coupled plasmas via the gauge/gravity duality. A central goal is therefore to determine which of these dynamical regimes the quark-gluon plasma is qualitatively closer to as a function of temperature. We review the analysis of lattice correlators in relation to transport properties, and tentatively estimate what computational effort is required to make decisive progress in this field.Comment: 54 pages, 37 figures, review written for EPJA and APPN; one parag. added end of section 3.4, and one at the end of section 3.2.2; some Refs. added, and some other minor change

Selective population of states in fission fragments from the S32+24Mg reaction

The symmetric and near-symmetric mass fission yields from the S32+24Mg reaction have been studied in a particle-particle- coincidence measurement. Evidence is presented for a selective population of states in Si28 fragments arising from the symmetric fission of the Ni56 compound nucleus. A statistical-model calculation of the expected strength to specific mutual excitations of the fission fragments is presented and compared to the experimental results. This calculation is found to describe the structures observed at high excitation energy in the fission Q-value spectra quite well. Analysis of the -ray spectra indicates, however, that a specific set of states in Si28, corresponding to a highly deformed prolate band, is populated more strongly than expected based on a purely spin-weighted, statistical decay of the compound nucleus. It is suggested that the population pattern of states in the fission fragments may reflect nuclear structure effects at the point of scission

Jet-quenching of the rotating heavy meson in a N{\mathcal{N}}=4 SYM plasma in presence of a constant electric field

In this paper, we consider a rotating heavy quark-antiquark ($q\bar{q}$) pair in a ${\mathcal{N}}$=4 SYM thermal plasma. We assume that $q\bar{q}$ center of mass moves at the speed $v$ and furthermore they rotate around the center of mass. We use the AdS/CFT correspondence and consider the effect of external electromagnetic field on the motion of the rotating meson. Then we calculate the jet-quenching parameter corresponding to the rotating meson in the constant electric field

A Pre-Landing Assessment of Regolith Properties at the InSight Landing Site

This article discusses relevant physical properties of the regolith at the Mars InSight landing site as understood prior to landing of the spacecraft. InSight will land in the northern lowland plains of Mars, close to the equator, where the regolith is estimated to be ≥3--5 m thick. These investigations of physical properties have relied on data collected from Mars orbital measurements, previously collected lander and rover data, results of studies of data and samples from Apollo lunar missions, laboratory measurements on regolith simulants, and theoretical studies. The investigations include changes in properties with depth and temperature. Mechanical properties investigated include density, grain-size distribution, cohesion, and angle of internal friction. Thermophysical properties include thermal inertia, surface emissivity and albedo, thermal conductivity and diffusivity, and specific heat. Regolith elastic properties not only include parameters that control seismic wave velocities in the immediate vicinity of the Insight lander but also coupling of the lander and other potential noise sources to the InSight broadband seismometer. The related properties include Poisson’s ratio, P- and S-wave velocities, Young’s modulus, and seismic attenuation. Finally, mass diffusivity was investigated to estimate gas movements in the regolith driven by atmospheric pressure changes. Physical properties presented here are all to some degree speculative. However, they form a basis for interpretation of the early data to be returned from the InSight mission.Additional co-authors: Nick Teanby and Sharon Keda

Heavy-flavor production and medium properties in high-energy nuclear collisions --What next?

Open and hidden heavy-flavor physics in high-energy nuclear collisions are entering a new and exciting stage towards reaching a clearer understanding of the new experimental results with the possibility to link them directly to the advancement in lattice Quantum Chromo-Dynamics (QCD). Recent results from experiments and theoretical developments regarding open and hidden heavy-flavor dynamics have been debated at the Lorentz Workshop Tomography of the Quark-Gluon Plasma with Heavy Quarks, which was held in October 2016 in Leiden, The Netherlands. In this contribution, we summarize identified common understandings and developed strategies for the upcoming five years, which aim at achieving a profound knowledge of the dynamical properties of the quark-gluon plasma