2,047 research outputs found
Omega and eta meson production in p+p reactions at E_{kin} = 3.5 GeV
We report on the exclusive production of omega and eta mesons in p+p
reactions at 3.5 GeV beam kinetic energy. Production cross sections, angular
distributions and Dalitz plots of both mesons were determined. Moreover, the
relative contribution of the N(1535) resonance in eta production at this energy
was evaluated. We conclude that eta mesons produced via N(1535) exihibit an
isotropic angular distribution, whereas those produced directly show a strong
anisotropic distribition. omega mesons show a slightly anisotropic angular
distribition.Comment: 5 pages, 6 figures, proceedings of the 11th International Workshop on
Meson Production, Properties and Interaction, Krak\'ow, Poland, 10 - 15 June
201
On the muon neutrino mass
During the runs of the PS 179 experiment at LEAR of CERN, we photographed an
event of antiproton-Ne absorption, with a complete pi+ -> mu+ ->e+ chain. From
the vertex of the reaction a very slow energy pi+ was emitted. The pi+ decays
into a mu+ and subsequently the mu+ decays into a positron. At the first decay
vertex a muon neutrino was emitted and at the second decay vertex an electron
neutrino and a muon antineutrino. Measuring the pion and muon tracks and
applying the momentum and energy conservation and using a classical statistical
interval estimator, we obtained an experimental upper limit for the muon
neutrino mass: m_nu < 2.2 MeV at a 90% confidence level. A statistical analysis
has been performed of the factors contributing to the square value of the
neutrino mass limit.Comment: 18 pages, 5 eps figure
The evolution of the spatially-resolved metal abundance in galaxy clusters up to z=1.4
We present the combined analysis of the metal content of 83 objects in the
redshift range 0.09-1.39, and spatially-resolved in the 3 bins (0-0.15,
0.15-0.4, >0.4) R500, as obtained with similar analysis using XMM-Newton data
in Leccardi & Molendi (2008) and Baldi et al. (2012). We use the pseudo-entropy
ratio to separate the Cool-Core (CC) cluster population, where the central gas
density tends to be relatively higher, cooler and more metal rich, from the
Non-Cool-Core systems. The average, redshift-independent, metal abundance
measured in the 3 radial bins decrease moving outwards, with a mean metallicity
in the core that is even 3 (two) times higher than the value of 0.16 times the
solar abundance in Anders & Grevesse (1989) estimated at r>0.4 R500 in CC (NCC)
objects. We find that the values of the emission-weighted metallicity are
well-fitted by the relation at given radius. A
significant scatter, intrinsic to the observed distribution and of the order of
0.05-0.15, is observed below 0.4 R500. The nominal best-fit value of
is significantly different from zero in the inner cluster regions () and in CC clusters only. These results are confirmed also with a
bootstrap analysis, which provides a still significant negative evolution in
the core of CC systems (P>99.9 per cent). No redshift-evolution is observed
when regions above the core (r > 0.15 R500) are considered. A reasonable good
fit of both the radial and redshift dependence is provided from the functional
form , with in CC clusters
and for NCC systems. Our results
represent the most extensive study of the spatially-resolved metal distribution
in the cluster plasma as function of redshift.Comment: 5 pages. Research Note accepted for publication in A&
Numerical simulation and analytical modelling of self-heating in FDSOI MOSFETs down to very deep cryogenic temperatures
Self-heating (SHE) TCAD numerical simulations have been performed, for the
first time, on 30nm FDSOI MOS transistors at extremely low temperatures. The
self-heating temperature rise dTmax and the thermal resistance Rth are computed
as functions of the ambient temperature Ta and the dissipated electrical power
(Pd), considering calibrated silicon and oxide thermal conductivities. The
characteristics of the SHE temperature rise dTmax(Pd) display sub-linear
behavior at sufficiently high levels of dissipated power, in line with standard
FDSOI SHE experimental data. It has been observed that the SHE temperature rise
dTmax can significantly exceed the ambient temperature more easily at very low
temperatures. Furthermore, a detailed thermal analysis of the primary heat
flows in the FDSOI device has been conducted, leading to the development of an
analytical SHE model calibrated against TCAD simulation data. This SHE
analytical model accurately describes the dTmax(Pd) and Rth(Ta) characteristics
of an FDSOI MOS device operating at extreme low ambient temperatures. These
TCAD simulations and analytical models hold great promise for predicting the
SHE and electro-thermal performance of FDSOI MOS transistors against ambient
temperature and dissipated power
Apparent high metallicity in 3-4 keV galaxy clusters: the inverse iron-bias in action in the case of the merging cluster Abell 2028
Recent work based on a global measurement of the ICM properties find evidence
for an increase of the iron abundance in galaxy clusters with temperature
around 2-4 keV up to a value about 3 times larger than that typical of very hot
clusters. We have started a study of the metal distribution in these objects
from the sample of Baumgartner et al. (2005), aiming at resolving spatially the
metal content of the ICM. We report here on a 42ks XMM observation of the first
object of the sample, the cluster Abell 2028. The XMM observation reveals a
complex structure of the cluster over scale of 300 kpc, showing an interaction
between two sub-clusters in cometary-like configurations. At the leading edges
of the two substructures cold fronts have been detected. The core of the main
subcluster is likely hosting a cool corona. We show that a one-component fit
for this region returns a biased high metallicity. This inverse iron bias is
due to the behavior of the fitting code in shaping the Fe-L complex. In
presence of a multi-temperature structure of the ICM, the best-fit metallicity
is artificially higher when the projected spectrum is modeled with a single
temperature component and it is not related to the presence of both Fe-L and
Fe-K emission lines in the spectrum. After accounting for the bias, the overall
abundance of the cluster is consistent with the one typical of hotter, more
massive clusters. We caution the interpretation of high abundances inferred
when fitting a single thermal component to spectra derived from relatively
large apertures in 3-4 keV clusters, because the inverse iron bias can be
present. Most of the inferences trying to relate high abundances in 3-4 keV
clusters to fundamental physical processes will likely have to be revised.Comment: 13 pages, 8 figures.Accepted for publication in Astronomy and
Astrophysycs. Minor changes to match published versio
Bottom-Up Approach to Moduli Dynamics in Heavy Gravitino Scenario : Superpotential, Soft Terms and Sparticle Mass Spectrum
The physics of moduli fields is examined in the scenario where the gravitino
is relatively heavy with mass of order 10 TeV, which is favored in view of the
severe gravitino problem. The form of the moduli superpotential is shown to be
determined, if one imposes a phenomenological requirement that no physical CP
phase arise in gaugino masses from conformal anomaly mediation. This bottom-up
approach allows only two types of superpotential, each of which can have its
origins in a fundamental underlying theory such as superstring. One
superpotential is the sum of an exponential and a constant, which is identical
to that obtained by Kachru et al (KKLT), and the other is the racetrack
superpotential with two exponentials. The general form of soft supersymmetry
breaking masses is derived, and the pattern of the superparticle mass spectrum
in the minimal supersymmetric standard model is discussed with the KKLT-type
superpotential. It is shown that the moduli mediation and the anomaly mediation
make comparable contributions to the soft masses. At the weak scale, the
gaugino masses are rather degenerate compared to the minimal supergravity,
which bring characteristic features on the superparticle masses. In particular,
the lightest neutralino, which often constitutes the lightest superparticle and
thus a dark matter candidate, is a considerable admixture of gauginos and
higgsinos. We also find a small mass hierarchy among the moduli, gravitino, and
superpartners of the standard-model fields. Cosmological implications of the
scenario are briefly described.Comment: 45 pages, 10 figures, typos correcte
The Gravitino-Overproduction Problem in Inflationary Universe
We show that the gravitino-overproduction problem is prevalent among
inflation models in supergravity. An inflaton field \phi generically acquires
(effective) non-vanishing auxiliary field G_\phi, if the K\"ahler potential is
non-minimal. The inflaton field then decays into a pair of the gravitinos. We
extensively study the cosmological constraints on G_\phi for a wide range of
the gravitino mass. For many inflation models we explicitly estimate G_\phi,
and show that the gravitino-overproduction problem severely constrains the
inflation models, unless such an interaction as K = \kappa/2 |\phi|^2 z^2 +
h.c. is suppressed (here z is the field responsible for the supersymmetry
breaking). We find that many of them are already excluded or on the verge of,
if \kappa \sim O(1).Comment: 34 pages, 7 figures. v2: minor correction
TCAD Modeling of High-Field Electron Transport in Bulk Wurtzite GaN: The Full-Band SHE-BTE
Gallium Nitride (GaN) High-Electron Mobility Transistors (HEMTs) actually represent one of the best candidates for medium-high power and radio frequency applications. As they operate at large bias and electric fields, a comprehensive analysis of the high-field transport properties is fundamentals, as hot electrons are expected to play a relevant role for the device reliability. In this perspective, Technology Computer-Aided Design (TCAD) simulations can be a very useful tool for the understanding of the phenomena dominating hot-electron degradation mechanisms. The most-accurate modeling approaches are based on the direct solution of the Boltzmann equation, which is not actually available for the GaN material. In this work, the deterministic solution of the Boltzmann transport equation via the spherical-harmonics expansion (SHE-BTE), as incorporated in a commercial TCAD tool, has been extended to the analysis of GaN electrons. To this purpose, the details of the full-band structure has been derived from DFT calculations as in state-of-art literature works, and the electron density of states, g(E), and group velocity g(E), have been calculated for the SHE-BTE for the first time. In addition to this, an accurate calibration of the total scattering rate accounting for nonpolar acoustic and optical carrier-phonon interaction, Coulomb scattering and impact ionization has been carried out against available Monte Carlo data and experiments. The proposed model is also shown to correctly predict the temperature dependence of the electron impact-ionization coefficient and current density up to breakdown
High efficiency thermionic converter studies
The objective is to improve thermionic converter performance by means of reduced interelectrode losses, greater emitter capabilities, and lower collector work functions until the converter performance level is suitable for out-of-core space reactors and radioisotope generators. Electrode screening experiments have identified several promising collector materials. Back emission work function measurements of a ZnO collector in a thermionic diode have given values less than 1.3 eV. Diode tests were conducted over the range of temperatures of interest for space power applications. Enhanced mode converter experiments have included triodes operated in both the surface ionization and plasmatron modes. Pulsed triodes were studied as a function of pulse length, pulse potential, inert gas fill pressure, cesium pressure, spacing, emitter temperature and collector temperature. Current amplifications (i.e., mean output current/mean grid current) of several hundred were observed up to output current densities of one amp/sq cm. These data correspond to an equivalent arc drop less than 0.1 eV
Remark on the minimal seesaw model and leptogenesis with tri/bi-maximal mixing
We have studied a leptogenesis scenario in the framework of the minimal
seesaw model with tri/bi-maximal mixing. Usually, at least one of the elements
in the Dirac mass matrix is fixed to be zero, for example, we denote it by
. We have pointed out that the absolute value of the CP asymmetry has
several minimums and maximums with non-zero . Thus one can expect that
more rich phenomena, such as an enhanced leptogenesis, are hidden in the space.Comment: 9 pages, 7 figures, uses RevTex4. To appear in Physcal Review
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