5,925 research outputs found
Thermal field theory derivation of the source term induced by a fast parton from the quark energy-momentum tensor
I derive the distribution of energy and momentum transmitted from a fast
parton to a medium of thermalized quarks, or the source term, in perturbative
thermal field theory directly from the quark energy-momentum tensor. The fast
parton is coupled to the medium by adding an interaction term to the
Lagrangian. The thermal expectation value of the energy-momentum tensor source
term is then evaluated using standard Feynman rules at finite temperature. It
is found that local excitations, which are important for exciting an observable
Mach cone structure, fall sharply as a function of the energy of the fast
parton. This may have implications for the trigger dependence of
measurements of azimuthal dihadron particle correlations in heavy-ion
collisions. In particular, a conical emission pattern would be less likely to
be observed for increasing trigger . I show that the results presented in
this paper can be generalized to more realistic modeling of fast parton
propagation, such as through a time dependent interaction term, in future
studies.Comment: Version as accepted by Physical Review D. New version has several
clarifications and added references. 5 pages, 3 figure
Exploring Causal Influences
Recent data mining techniques exploit patterns of statistical independence in multivariate data to make conjectures about cause/effect relationships. These relationships can be used to construct causal graphs, which are sometimes represented by weighted node-link diagrams, with nodes representing variables and combinations of weighted links and/or nodes showing the strength of causal relationships. We present an interactive visualization for causal graphs (ICGs), inspired in part by the Influence Explorer. The key principles of this visualization are as follows: Variables are represented with vertical bars attached to nodes in a graph. Direct manipulation of variables is achieved by sliding a variable value up and down, which reveals causality by producing instantaneous change in causally and/or probabilistically linked variables. This direct manipulation technique gives users the impression they are causally influencing the variables linked to the one they are manipulating. In this context, we demonstrate the subtle distinction between seeing and setting of variable values, and in an extended example, show how this visualization can help a user understand the relationships in a large variable set, and with some intuitions about the domain and a few basic concepts, quickly detect bugs in causal models constructed from these data mining techniques
Fundamental Vibrational Transitions of HCl Detected in CRL 2136
We would like to understand the chemistry of dense clouds and their hot cores
more quantitatively by obtaining more complete knowledge of the chemical
species present in them. We have obtained high-resolution infrared absorption
spectroscopy at 3-4 um toward the bright infrared source CRL 2136. The
fundamental vibration-rotation band of HCl has been detected within a dense
cloud for the first time. The HCl is probably located in the warm compact
circumstellar envelope or disk of CRL 2136. The fractional abundance of HCl is
(4.9-8.7)e-8, indicating that approximately 20 % of the elemental chlorine is
in gaseous HCl. The kinetic temperature of the absorbing gas is 250 K, half the
value determined from infrared spectroscopy of 13CO and water. The percentage
of chlorine in HCl is approximately that expected for gas at this temperature.
The reason for the difference in temperatures between the various molecular
species is unknown.Comment: 6 pages, 3 figures, A&A in pres
High-resolution absorption spectroscopy of the OH 2Pi 3/2 ground state line
The chemical composition of the interstellar medium is determined by gas
phase chemistry, assisted by grain surface reactions, and by shock chemistry.
The aim of this study is to measure the abundance of the hydroxyl radical (OH)
in diffuse spiral arm clouds as a contribution to our understanding of the
underlying network of chemical reactions. Owing to their high critical density,
the ground states of light hydrides provide a tool to directly estimate column
densities by means of absorption spectroscopy against bright background
sources. We observed onboard the SOFIA observatory the 2Pi3/2, J = 5/2 3/2 2.5
THz line of ground-state OH in the diffuse clouds of the Carina-Sagittarius
spiral arm. OH column densities in the spiral arm clouds along the sightlines
to W49N, W51 and G34.26+0.15 were found to be of the order of 10^14 cm^-2,
which corresponds to a fractional abundance of 10^-7 to 10^-8, which is
comparable to that of H_2O. The absorption spectra of both species have similar
velocity components, and the ratio of the derived H_2O to OH column densities
ranges from 0.3 to 1.0. In W49N we also detected the corresponding line of
^18OH
Dispersive entrainment into gravity currents in porous media
he effects of dispersion acting on gravity currents propagating through porous media are considered theoretically and experimentally. We exploit the large aspect ratio of these currents to formulate a depth-averaged model of the evolution of the mass and buoyancy. Dispersion, acting predominantly at the interface between the current and the ambient, is velocity dependent and acts to entrain fluid into the gravity current, in direct analogy to turbulent mixing. Here, we show that when the gravity current is fed by a constant buoyancy and mass flux the buoyancy of the current is self-similar and recovers the classical solution for gravity currents in porous media. In contrast, the profile and the depth-averaged concentration of the current evolve in a non-self-similar manner. The total volume of the current increases with time as due to this dispersive entrainment. We test our theoretical predictions using a suite of laboratory experiments in which the evolution of the concentration within the current was mapped using a dye-attenuation technique. These experimental results show good agreement with the early-time limits of our theoretical model, and in particular accurately predict the evolution of the depth-averaged concentration profile. These results suggest that mixing within porous media may be modelled using an effective dispersive entrainment, the magnitude of which may be set by the underlying structure of the porous medium
Intermittency in two-dimensional Ekman-Navier-Stokes turbulence
We study the statistics of the vorticity field in two-dimensional
Navier-Stokes turbulence with a linear Ekman friction. We show that the
small-scale vorticity fluctuations are intermittent, as conjectured by Nam et
al. [Phys. Rev. Lett. vol.84 (2000) 5134]. The small-scale statistics of
vorticity fluctuations coincides with the one of a passive scalar with finite
lifetime transported by the velocity field itself.Comment: 4 pages, 7 figure
SWAS and Arecibo observations of H2O and OH in a diffuse cloud along the line-of-sight to W51
Observations of W51 with the Submillimeter Wave Astronomy Satellite (SWAS)
have yielded the first detection of water vapor in a diffuse molecular cloud.
The water vapor lies in a foreground cloud that gives rise to an absorption
feature at an LSR velocity of 6 km/s. The inferred H2O column density is
2.5E+13 cm-2. Observations with the Arecibo radio telescope of hydroxyl
molecules at ten positions in W51 imply an OH column density of 8E+13 cm-2 in
the same diffuse cloud. The observed H2O/OH ratio of ~ 0.3 is significantly
larger than an upper limit derived previously from ultraviolet observations of
the similar diffuse molecular cloud lying in front of HD 154368. The observed
variation in H2O/OH likely points to the presence in one or both of these
clouds of a warm (T > 400) gas component in which neutral-neutral reactions are
important sources of OH and/or H2O.Comment: 15 pages (AASTeX) including 4 (eps) figures. To appear in the
Astrophysical Journa
Comparing different freeze-out scenarios in azimuthal hadron correlations induced by fast partons
I review the linearized hydrodynamical treatment of a fast parton traversing
a perturbative quark-gluon plasma. Using numerical solutions for the medium's
response to the fast parton, I obtain the medium's distribution function which
is then used in a Cooper-Frye freeze-out prescription to obtain an azimuthal
particle spectrum. Two different freeze-out scenarios are considered which
yield significantly different results. I conclude that any meaningful
comparison of azimuthal hadron correlation functions to RHIC data requires
implementing a realistic freeze-out scenario in an expanding medium.Comment: Contribution to the Proceedings for 2008 Hot Quarks in Estes Park,
CO, as accepted for publication in EPJ-
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