Light from Cascading Partons in Relativistic Heavy-Ion Collisions

We calculate the production of high energy photons from Compton and annihilation processes as well as fragmentation off quarks in the parton cascade model. The multiple scattering of partons is seen to lead to a substantial production of high energy photons, which rises further when parton multiplication due to final state radiation is included. The photon yield is found to be proportional to the number of collisions among the cascading partons.Comment: revised version: 4 pages, 4 figures, uses REVTEX

Shear Viscosity in a Perturbative Quark-Gluon-Plasma

Among the key features of hot and dense QCD matter produced in ultra-relativistic heavy-ion collisions at RHIC is its very low shear viscosity, indicative of the properties of a near-ideal fluid, and a large opacity demonstrated by jet energy loss measurements. In this work, we utilize a microscopic transport model based on the Boltzmann equation with quark and gluon degrees of freedom and cross sections calculated from perturbative Quantum Chromodynamics to simulate an ideal Quark-Gluon-Plasma in full thermal and chemical equilibrium. We then use the Kubo formalism to calculate the shear viscosity to entropy density ratio of the medium as a function of temperature and system composition. One of our key results is that the shear viscosity over entropy-density ratio $\eta/s$ becomes invariant to the chemical composition of the system when plotted as a function of energy-density instead of temperature.Comment: 11 pages, 8 figures: version #2 contains some revisions and added references to clarify relationship to previously published wor

Laboratory Tests of Gravitational Physics Using a Cryogenic Torsion Pendulum

Progress and plans are reported for a program of gravitational physics experiments using cryogenic torsion pendula undergoing large amplitude torsional oscillation. The program includes a UC Irvine project to measure the gravitational constant G and joint UC Irvine - U. Washington projects to test the gravitational inverse square law at a range of about 10 cm and to test the weak equivalence principle.Comment: 17 pages, 11 figures, contribution to the 10th Marcel Grossman Conference Proceedings (Rio de Janeiro, July 20 - 26, 2003) - changed wording in first paragraph of section

Space-time evolution of bulk QCD matter

We introduce a combined fully three-dimensional macroscopic/microscopic transport approach employing relativistic 3D-hydrodynamics for the early, dense, deconfined stage of the reaction and a microscopic non-equilibrium model for the later hadronic stage where the equilibrium assumptions are not valid anymore. Within this approach we study the dynamics of hot, bulk QCD matter, which is being created in ultra-relativistic heavy ion collisions at RHIC. Our approach is capable of self-consistently calculating the freezeout of the hadronic system, while accounting for the collective flow on the hadronization hypersurface generated by the QGP expansion. In particular, we perform a detailed analysis of the reaction dynamics, hadronic freezeout, and transverse flow.Comment: 24 pages, 27 figure

Gravitino Dark Matter and Cosmological Constraints

The gravitino is a promising candidate for cold dark matter. We study cosmological constraints on scenarios in which the gravitino is the lightest supersymmetric particle and a charged slepton the next-to-lightest supersymmetric particle (NLSP). We obtain new results for the hadronic nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into the gravitino, the associated lepton, and a quark-antiquark pair. The bounds from the observed dark matter density are refined by taking into account gravitinos from both late NLSP decays and thermal scattering in the early Universe. We examine the present free-streaming velocity of gravitino dark matter and the limits from observations and simulations of cosmic structures. Assuming that the NLSP sleptons freeze out with a thermal abundance before their decay, we derive new bounds on the slepton and gravitino masses. The implications of the constraints for cosmology and collider phenomenology are discussed and the potential insights from future experiments are outlined. We propose a set of benchmark scenarios with gravitino dark matter and long-lived charged NLSP sleptons and describe prospects for the Large Hadron Collider and the International Linear Collider.Comment: 51 pages, 20 figures, revised version matches published version (results unchanged, JHEP style used, figures replaced with new high-quality figures, typos corrected, references added

Phoebe 2.0 – Triple and multiple systems

Some close binary formation theories require the presence of a third body so that the binary orbit can shrink over time. Tidal friction and Kozai cycles transfer energy from the binary to its companion, resulting in a close inner binary and a wide third body orbit. Spectroscopy and imaging studies have found 40% of binaries with periods less than 10 days, and 96% with periods less than 3 days, have a wide tertiary companion. With recent advancements in large photometric surveys, we are now beginning to detect many of these triple systems by observing tertiary eclipses or through the effect they have on the eclipse timing variations (ETVs) of the inner-binary. In the sample of 2600 Kepler EBs, we have detected the possible presence of a third body in ∼20%, including several circumbinary planets. Some multiple systems are quite dynamical and feature disappearing and reappearing eclipses, apsidal motion, and large disruptions to the inner-binary. phoebe is a freely available binary modeling code which can dynamically model all of these systems, allowing us to better test formation theories and probe the physics of eclipsing binaries

A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air

We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140–150 nmol mol-1, which is higher than today's values. CO mole fractions rose by 10–15 nmol mol-1 from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a ˜ 30 nmol mol-1 decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe

Super-Earths: A New Class of Planetary Bodies

Super-Earths, a class of planetary bodies with masses ranging from a few Earth-masses to slightly smaller than Uranus, have recently found a special place in the exoplanetary science. Being slightly larger than a typical terrestrial planet, super-Earths may have physical and dynamical characteristics similar to those of Earth whereas unlike terrestrial planets, they are relatively easier to detect. Because of their sizes, super-Earths can maintain moderate atmospheres and possibly dynamic interiors with plate tectonics. They also seem to be more common around low-mass stars where the habitable zone is in closer distances. This article presents a review of the current state of research on super-Earths, and discusses the models of the formation, dynamical evolution, and possible habitability of these objects. Given the recent advances in detection techniques, the detectability of super-Earths is also discussed, and a review of the prospects of their detection in the habitable zones of low-mass stars is presented.Comment: A (non-technical) review of the literature on the current state ofresearch on super-Earths. The topics include observation, formation, dynamical evolution, habitability, composition, interior dynamics, magnetic field, atmosphere, and propsect of detection. The article has 44 pages, 27 figures, and 203 references. It has been accepted for publication in the journal Contemporary Physics (2011

Strangeness Production at RHIC in the Perturbative Regim

We investigate strange quark production in Au-Au collisions at RHIC in the framework of the Parton Cascade Model(PCM). The yields of (anti-) strange quarks for three production scenarios -- primary-primary scattering, full scattering, and full production -- are compared to a proton-proton baseline. Enhancement of strange quark yields in central Au-Au collisions compared to scaled p-p collisions increases with the number of secondary interactions. The centrality dependence of strangeness production for the three production scenarios is studied as well. For all production mechanisms, the strangeness yield increases with $(N_{\rm part})^{4/3}$. The perturbative QCD regime described by the PCM is able to account for up to 50% of the observed strangeness at RHIC.Comment: 10 pages, 4 figures, IOP forma

The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB

We have investigated the dynamical interaction of low- and-intermediate mass stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first paper, we examine the structures generated by the stellar winds during the Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind history predicted by stellar evolution. The influence of the external ISM is also taken into account. We find that the wind variations associated with the thermal pulses lead to the formation of transient shells with an average lifetime of 20,000 yr, and consequently do not remain recorded in the density or velocity structure of the gas. The formation of shells that survive at the end of the AGB occurs via two main processes: shocks between the shells formed by two consecutive enhancements of the mass-loss or via continuous accumulation of the material ejected by the star in the interaction region with the ISM. Our models show that the mass of the circumstellar envelope increases appreciably due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar model). We also point out the importance of the ISM on the deceleration and compression of the external shells. According to our simulations, large regions (up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars are expected. These large regions of gas are formed from the mass-loss experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical Journa