5,645 research outputs found
Sonic Mach Cones Induced by Fast Partons in a Perturbative Quark-Gluon Plasma
We derive the space-time distribution of energy and momentum deposited by a
fast parton traversing a weakly coupled quark-gluon plasma by treating the fast
part on as the source of an external color field perturbing the medium. We then
use our result as a source term for the linearized hydrodynamical equations of
the medium. We show that the solution contains a sonic Mach cone and a
dissipative wake if the parton moves at a supersonic speed.Comment: Final version accepted for publicatio
Explicit generating functional for pions and virtual photons
We construct the explicit one-loop functional of chiral perturbation theory
for two light flavours, including virtual photons. We stick to contributions
where 1 or 2 mesons and at most one photon are running in the loops. With the
explicit functional at hand, the evaluation of the relevant Green functions
boils down to performing traces over the flavour matrices. For illustration, we
work out the pi+ pi- -> pi0 pi0 scattering amplitude at threshold at order p^4,
e^2p^2.Comment: 20 pages, 2 figures; version accepted for publication, minor
typographical changes, acknowledgments adde
Bringing the power of dynamic languages to hardware control systems
Hardware control systems are normally programmed using high-performance languages like C or C++ and increasingly also Java. All these languages are strongly typed and compiled which brings usually good performance but at the cost of a longer development and testing cycle and the need for more programming expertise. Dynamic languages which were long thought to be too slow and not powerful enough for control purposes are, thanks to modern powerful computers and advanced implementation techniques, fast enough for many of these tasks. We present examples from the LHCb Experiment Control System (ECS), which is based on a commercial SCADA software. We have successfully used Python to integrate hardware devices into the ECS. We present the necessary lightweight middle-ware we have developed, including examples for controlling hardware and software devices. We also discuss the development cycle, tools used and compare the effort to traditional solutions
Electromagnetic corrections in the anomaly sector
Chiral perturbation theory in the anomaly sector for is extended to
include dynamical photons, thereby allowing a complete treatment of isospin
breaking. A minimal set of independent chiral lagrangian terms is determined
and the divergence structure is worked out. There are contributions from
irreducible and also from reducible one-loop graphs, a feature of ChPT at order
larger than four. The generating functional is non-anomalous at order ,
but not necessarily at higher order in . Practical applications to
and to the amplitudes are considered. In
the latter case, a complete discussion of the corrections beyond current
algebra is presented including quark mass as well as electromagnetic effects.Comment: 26 pages, 3 figure
The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance
C+ is a key species in the interstellar medium but its 158 {\mu}m fine
structure line cannot be observed from ground-based telescopes. Current models
of fluorine chemistry predict that CF+ is the second most important fluorine
reservoir, in regions where C+ is abundant. We detected the J = 1-0 and J = 2-1
rotational lines of CF+ with high signal-to-noise ratio towards the PDR and
dense core positions in the Horsehead. Using a rotational diagram analysis, we
derive a column density of N(CF+) = (1.5 - 2.0) \times 10^12 cm^-2. Because of
the simple fluorine chemistry, the CF+ column density is proportional to the
fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H =
(0.6 - 1.5) \times 10^-8. Photochemical models indicate that CF+ is found in
the layers where C+ is abundant. The emission arises in the UV illuminated skin
of the nebula, tracing the outermost cloud layers. Indeed, CF+ and C+ are the
only species observed to date in the Horsehead with a double peaked line
profile caused by kinematics. We therefore propose that CF+, which is
detectable from the ground, can be used as a proxy of the C+ layers.Comment: Accepted to A&A, 4 pages, 4 figures, 2 table
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-
Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6-0.4 (W31C)
We report the detection of strong absorption by interstellar hydrogen
fluoride along the sight-line to the submillimeter continuum source G10.6-0.4
(W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, to
observe the 1232.4763 GHz J=1-0 HF transition in the upper sideband of the Band
5a receiver. The resultant spectrum shows weak HF emission from G10.6-0.4 at
LSR velocities in the range -10 to -3 km/s, accompanied by strong absorption by
foreground material at LSR velocities in the range 15 to 50 km/s. The spectrum
is similar to that of the 1113.3430 GHz 1(11)-0(00) transition of para-water,
although at some frequencies the HF (hydrogen fluoride) optical depth clearly
exceeds that of para-H2O. The optically-thick HF absorption that we have
observed places a conservative lower limit of 1.6E+14 cm-2 on the HF column
density along the sight-line to G10.6-0.4. Our lower limit on the HF abundance,
6E-9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts for
between ~ 30 and 100% of the fluorine nuclei in the gas phase along this
sight-line. This observation corroborates theoretical predictions that -
because the unique thermochemistry of fluorine permits the exothermic reaction
of F atoms with molecular hydrogen - HF will be the dominant reservoir of
interstellar fluorine under a wide range of conditions.Comment: Accepted for publication in Astronomy and Astrophysics (Herschel
special issue). This revised version corrects a typographic error in the HTML
abstract, in which the lower limit on the HF abundance (should be 6E-9) was
previously misstated. The abstract in the PDF version is correct and the
latter has not been modifie
ISO observations of far-infrared rotational emission lines of water vapor toward the supergiant star VY Canis Majoris
We report the detection of numerous far-infrared emission lines of water
vapor toward the supergiant star VY Canis Majoris. A 29.5 - 45 micron grating
scan of VY CMa, obtained using the Short Wavelength Spectrometer (SWS) of the
Infrared Space Observatory (ISO) at a spectral resolving power of approximately
2000, reveals at least 41 spectral features due to water vapor that together
radiate a total luminosity ~ 25 solar luminosities. In addition to pure
rotational transitions within the ground vibrational state, these features
include rotational transitions within the (010) excited vibrational state. The
spectrum also shows the doublet Pi 1/2 (J=5/2) <-- doublet Pi 3/2 (J=3/2) OH
feature near 34.6 micron in absorption. Additional SWS observations of VY CMa
were carried out in the instrument's Fabry-Perot mode for three water
transitions: the 7(25)-6(16) line at 29.8367 micron, the 4(41)-3(12) line
31.7721 micron, and the 4(32)-3(03) line at 40.6909 micron. The higher spectral
resolving power of approximately 30,000 thereby obtained permits the line
profiles to be resolved spectrally for the first time and reveals the "P Cygni"
profiles that are characteristic of emission from an outflowing envelope.Comment: 11 pages (inc. 2 figures), LaTeX, uses aaspp4.sty, accepted for
publication in ApJ Letter
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