6,937 research outputs found
IUE observations of Herbig-Haro and related objects
The I.U.E. spectra of the Herbig-Haro objectives H-H 1, H-H 2H, H-H 2G and H-H 32 A are considered. In order to obtain a better understanding of the physics of these intriguing objects and of the formation of the ultraviolet continua and emission lines in H-H objects, the UV spectra of the Cohen-Schwartz star, the T Tauri star AS 353A, and of the reflection nebula NGC 1999 (which is illuminated by the young object V 380 Orionis), are also examined
Searching for GeV-scale new Gauge Bosons in QGP thermal dilepton production
In this paper we propose to use the measurement of the thermal Quark-Gluon
Plasma (QGP) dilepton spectra in the Intermediate Mass Region (IMR) of
heavy-ion collisions, as a new method to search for GeV-scale dark gauge bosons
(gamma' or Z'). Such light mediators are a common feature of light (i.e. low
mass) dark matter scenarios, which have been invoked to explain puzzling
signals in dark matter indirect and direct detection experiments. First we show
that a light gamma' or Z' will generate a resonant enhancement of the dilepton
spectrum produced thermally by the QGP, at an energy corresponding to the dark
gauge boson mass. Secondly, using data from the PHENIX experiment, we are able
to set an upper limit on the combined coupling of this new gauge boson to
quarks and leptons (independently of their vectorial or axial nature) chi_q
chi_e < 10^(-3) at the 95% confidence level for a gauge boson mass in the range
1.5 - 2.5 GeV. This result complements previous searches for new light gauge
bosons and probes a new region of the parameter space, particularly interesting
in the case of non-universal couplings to quarks and leptons. Prospects for the
discovery of such a boson by the ALICE collaboration are also discussed.Comment: 11 pages, 4 figure
Casimir effects in graphene systems: unexpected power laws
We present calculations of the zero-temperature Casimir interaction between
two freestanding graphene sheets as well as between a graphene sheet and a
substrate. Results are given for undoped graphene and for a set of doping
levels covering the range of experimentally accessible values. We describe
different approaches that can be used to derive the interaction. We point out
both the predicted power law for the interaction and the actual distance
dependence.Comment: 10 pages,5 figures, conferenc
Global Mapping Function (GMF): A new empirical mapping function based on numerical weather model data
Troposphere mapping functions are used in the analyses of Global Positioning System and Very Long Baseline Interferometry observations to map a priori zenith hydrostatic and wet delays to any elevation angle. Most analysts use the Niell Mapping Function (NMF) whose coefficients are determined from site coordinates and the day of year. Here we present the Global Mapping Function (GMF), based on data from the global ECMWF numerical weather model. The coefficients of the GMF were obtained from an expansion of the Vienna Mapping Function (VMF1) parameters into spherical harmonics on a global grid. Similar to NMF, the values of the coefficients require only the station coordinates and the day of year as input parameters. Compared to the 6-hourly values of the VMF1 a slight degradation in short-term precision occurs using the empirical GMF. However, the regional height biases and annual errors of NMF are significantly reduced with GMF
Investigation of radiation induced hight-temperature embrittlement for ferritic steels in the temperature range of the α-γ-Transformation
Shocking Signals of Dark Matter Annihilation
We examine whether charged particles injected by self-annihilating Dark
Matter into regions undergoing Diffuse Shock Acceleration (DSA) can be
accelerated to high energies. We consider three astrophysical sites where shock
acceleration is supposed to occur, namely the Galactic Centre, galaxy clusters
and Active Galactic Nuclei (AGN). For the Milky Way, we find that the
acceleration of cosmic rays injected by dark matter could lead to a bump in the
cosmic ray spectrum provided that the product of the efficiency of the
acceleration mechanism and the concentration of DM particles is high enough.
Among the various acceleration sources that we consider (namely supernova
remnants (SNRs), Fermi bubbles and AGN jets), we find that the Fermi bubbles
are a potentially more efficient accelerator than SNRs. However both could in
principle accelerate electrons and protons injected by dark matter to very high
energies. At the extragalactic level, the acceleration of dark matter
annihilation products could be responsible for enhanced radio emission from
colliding clusters and prediction of an increase of the anti-deuteron flux
generated near AGNs.Comment: 9 pages, 4 figure
Light spin-1/2 or spin-0 Dark Matter particles
We recall and precise how light spin-0 particles could be acceptable Dark
Matter candidates, and extend this analysis to spin-1/2 particles. We evaluate
the (rather large) annihilation cross sections required, and show how they may
be induced by a new light neutral spin-1 boson U. If this one is vectorially
coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation
cross section into e+e- automatically includes a v_dm^2 suppression factor at
threshold, as desirable to avoid an excessive production of gamma rays from
residual Dark Matter annihilations. We also relate Dark Matter annihilations
with production cross sections in e+e- scatterings. Annihilation cross sections
of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same
expressions. Just as for spin-0, light spin-1/2 Dark Matter particles
annihilating into e+e- could be responsible for the bright 511 keV gamma ray
line observed by INTEGRAL from the galactic bulge.Comment: 10 page
Primordial Black Holes, Hawking Radiation and the Early Universe
The 511 keV gamma emission from the galactic core may originate from a high
concentration () of primordial black holes (PBHs) in the core
each of whose Hawking radiation includes positrons per second.
The PBHs we consider are taken as near the lightest with longevity greater than
the age of the universe (mass kg; Schwarzschild radius
fm). These PBHs contribute only a small fraction of cold dark matter,
. This speculative hypothesis, if confirmed implies
the simultaneous discovery of Hawking radiation and an early universe phase
transition.Comment: 4 Page
- …