5,228 research outputs found
Inclusive searches for squarks and gluinos with the ATLAS detector
Despite the absence of experimental evidence, weak scale supersymmetry
remains one of the best motivated and studied Standard Model extensions. This
report summarises recent ATLAS results on inclusive searches for supersymmetric
squarks and gluinos, including third generation squarks produced in the decay
of gluinos. Results are presented for both R-parity conserving and R-parity
violating scenarios, with final states containing jets with and without missing
transverse momentum, light leptons, taus or photons.Comment: Proceedings for the LHCP 2014 conferenc
Dwarf Spheroidal Satellite Formation in a Reionized Local Group
Dwarf spheroidal satellite galaxies have emerged a powerful probe of
small-scale dark matter clustering and of cosmic reionization. They exhibit
structural and chemical continuity with dwarf irregular galaxies in the field
and with spheroidal galaxies in high-density environments. By combining
empirical constraints derived for star formation at low gas column densities
and metallicities in the local universe with a model for dark matter and
baryonic mass assembly, we provide an analytical description of how the dwarf
spheroidals acquired their stellar content. Their progenitors formed stars
until the gas content, initially reduced from the cosmic average by the thermal
pressure of the reionized intergalactic medium, was finally ram pressure
stripped during the progenitors' accretion on to the host galaxy. Dwarf
spheroidal satellites of differing luminosities seem to share very similar most
massive progenitor histories that reach thresholds for gas cooling by atomic
line emission at epochs at which the Lagrangian volume of the Local Group
should have been reionized. We hypothesize that dwarf spheroidals formed the
bulk of their stars in partially rotationally supported HI disks in a reionized
universe. This model provides an explanation for the "common mass scale"
relation and reproduces the empirical luminosity-size and
luminosity-metallicity relations. Explosive feedback phenomena, such as
outflows driven by the concerted action of supernovae, need not have been
significant in the dwarf spheroidals' formation. We further speculate that the
true pre-reionization fossils should exhibit a structure distinct from that of
the dwarf spheroidals, e.g., in the form of dense isolated or nuclear star
clusters.Comment: 18 pages, 7 figures, MNRAS, in pres
Influence of plasma chemistry on oxygen triplets
The plasma chemistry of fluorocarbon-oxygen-argon
discharges and its influence on prominent oxygen triplets are studied. The oxygen 777 triplet is very important for the measurement of atomic oxygen in low pressure plasmas, since the 777.417 nm spectral line is frequently used for
actinometry.
In this paper we identify changes in the individual 777 triplet lines arising from cascade effects from higher energy levels of oxygen, and from resonant energy transfer from energetic carbon atoms in carbon-rich plasmas. The lower energy levels of three oxygen triplets (544 nm, 616 nm, 645 nm) are the upper states of the 777 triplet. Increased emission intensity from the 544, 616, and 645 triplets result in changes to the relative intensity of
the individual lines of the 777 triplet, and this can lead to errors in using the 777 triplet, e.g. for actinometry. Also, in operational conditions with strong carbon emission (around 601 nm), the relative intensity of the individual oxygen 777 lines is affected. The upper energy levels of these carbon lines is close to the oxygen 777 upper energy levels, suggesting that resonant energy transfer between the carbon and the oxygen is occurring.
The experiments are performed in a commercial semiconductor
dielectric etcher operating with dual rf frequencies of 2 MHz and 27 MHz. Pressure (13-19 Pa), rf power (200-1200 W), and gas mixtures (argon with addmixtures of 5-13% oxygen and C4F8) are typical in application to dielectric etching
Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium
X-ray observations of galaxy cluster merger shocks can be used to constrain
nonthermal processes in the intracluster medium (ICM). The presence of
nonthermal pressure components in the ICM, as well as the shock acceleration of
particles and their escape, all affect shock jump conditions in distinct ways.
Therefore, these processes can be constrained using X-ray surface brightness
and temperature maps of merger shock fronts. Here we use these observations to
place constraints on particle acceleration efficiency in intermediate Mach
number (M ~ 2-3) shocks and explore the potential to constrain the contribution
of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to
ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in
merger shocks discovered in the galaxy clusters A520 (M ~ 2) and 1E 0657-56 (M
~ 3) using a multifluid model comprised of a thermal plasma, a nonthermal
plasma, and a magnetic field. Based on the published X-ray spectroscopic data
alone, we find that the fractional contribution of cosmic rays accelerated in
these shocks is lower than about 10% of the shock downstream pressure. Current
observations do not constrain the fractional contribution of nonthermal
components to the pressure of the undisturbed shock upstream. Future X-ray
observations, however, have the potential to either detect particle
acceleration in these shocks through its effect on the shock dynamics, or to
place a lower limit on the nonthermal pressure contributions in the undisturbed
ICM. We briefly discuss implications for models of particle acceleration in
collisionless shocks and the estimates of galaxy cluster masses derived from
X-ray and Sunyaev-Zel'dovich effect observations.Comment: 10 pages, 4 figures, comments welcom
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