4,850 research outputs found
A unique parametrization of the shapes of secondary dilepton spectra observed in central heavy-ion collisions at CERN-SPS energies
A unique parametrization of secondary (thermal) dilepton yields in heavy-ion
experiments at CERN-SPS is proposed. This parametrization resembles a thermal
annihilation rate. This is inspired by the observation that lepton
pair production rates are quantitatively similar, whether expressed in a
hadronic or partonic basis. Adding the thermal yield and the background
contributions (hadronic cocktail, Drell-Yan, correlated semileptonic decays of
open charm) the spectral shapes of the CERES/NA45, NA38, NA50 and HELIOS/3 data
from experiments with lead and sulfur beams can be well described.Comment: 23 pages including figures (new version: only new output format
The interaction of dolomite surfaces with metal impurities: a computer simulation study
This study investigates the behaviour of selected, morphologically important surfaces of dolomite (CaMg(CO3)2), using computational modelling techniques. Interatomic potential methods have been used to examine impurity substitution at cationic sites in these surfaces. Environmentally prevalent cations were studied to this end, namely Ni21, Co21, Zn21, Fe21, Mn21 and Cd21, all of which are also found as end-member carbonate minerals. Solid?solution substitution was investigated and showed that Cd and Mn will substitute from their end-member carbonate phase at either dolomite cation site. Mn is found to preferentially substitute at Mg sites, in agreement with experimental findings. For Ni21, Co21 and Zn21, the magnitude of substitution energies is approximately equal for all surfaces, with the exception of the (1014) surface. However, for the larger cations, a far greater disparity in substitution energies is observed. At a stepped surface, analogous substitutions were performed and it was found that substitution energies for all impurity cations were reduced, indicating that uptake is more viable during growth. The predominant surface, the (1014), was solvated with a monolayer of water in order to investigate the influence of hydration on substitution energetics. The addition of water changes the relative preference for substitution of the different cations. Under aqueous conditions, the substitution energy is determined by three competing factors, the relative importance of which cannot be predicted without this type of computational investigation
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H I 21-cm Cosmology and the Bispectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays
New massively redundant low frequency arrays allow for a novel investigation
of closure relations in interferometry. We employ commissioning data from the
Hydrogen Epoch of Reionization Array to investigate closure quantities in this
densely packed grid array of 14m antennas operating at 100 MHz to 200 MHz. We
investigate techniques that utilize closure phase spectra for redundant triads
to estimate departures from redundancy for redundant baseline visibilities. We
find a median absolute deviation from redundancy in closure phase across the
observed frequency range of about 4.5deg. This value translates into a
non-redundancy per visibility phase of about 2.6deg, using prototype
electronics. The median absolute deviations from redundancy decrease with
longer baselines. We show that closure phase spectra can be used to identify
ill-behaved antennas in the array, independent of calibration. We investigate
the temporal behavior of closure spectra. The Allan variance increases after a
one minute stride time, due to passage of the sky through the primary beam of
the transit telescope. However, the closure spectra repeat to well within the
noise per measurement at corresponding local sidereal times (LST) from day to
day. In future papers in this series we will develop the technique of using
closure phase spectra in the search for the HI 21cm signal from cosmic
reionization
Large mass dileptons from the passage of jets through quark gluon plasma
We calculate the emission of large mass dileptons originating from the
annihilation of quark jets passing through quark gluon plasma. Considering
central collisions of heavy nuclei at SPS, RHIC and LHC energies, we find that
the yield due to the jet-plasma interaction gets progressively larger as the
collision energy increases. We find it to be negligible at SPS energies, of the
order of the Drell-Yan contribution and much larger than the normal thermal
yield at RHIC energies and up to a factor of ten larger than the Drell-Yan
contribution at LHC energies. An observation of this new dilepton source would
confirm the occurrence of jet-plasma interactions and of conditions suitable
for jet-quenching to take place.Comment: 9 pages, 11 figures; references added, improved calculation,
conclusions unchange
Graphenylene, a unique two-dimensional carbon network with nondelocalized cyclohexatriene units
Over many years chemists have established the general principle that two-dimensional chemical structures constructed with pure sp2-carbon atoms will definitely form an aromatic system with delocalized electron density. However, based on a recently proposed chemical structure, graphenylene, this rule may finally be broken. Herein, we predict the properties of a new two-dimensional sp2- carbon network known as graphenylene, which is the first example of a non-delocalized sp2-carbon structure composed of cyclohexatriene units with two quite distinct C–C bonds within a C6 ring. In addition, theoretical calculations demonstrate that graphenylene has periodic pores of 3.2 Å in diameter and is a semiconductor with a narrow direct band gap, making it promising for various applications, such as electronic devices and efficient hydrogen separation. This study provides a new perspective on carbon allotropes, leading to a better understanding of [N]phenylene based organic frameworks, as well as clarifying the relationship between benzene and cyclohexatriene
Off-shell effects in dilepton production from hot interacting mesons
The production of dielectrons in reactions involving a_1 mesons and pions is
studied. We compare results obtained with different phenomenological
Lagrangians that have been used in connection with hadronic matter and finite
nuclei. We insist on the necessity for those interactions to satisfy known
empirical properties of the strong interaction. Large off-shell effects in
dielectron production are found and some consequences for the interpretation of
heavy ion data are outlined. We also compare with results obtained using
experimentally-extracted spectral functions.Comment: 14 pages, LaTeX2e, 2 figure
Crystal structure of Cu-Sn-In alloys around the {\eta} phase field studied by neutron diffraction
The study of the Cu-Sn-In ternary system has become of great importance in
recent years, due to new environmental regulations forcing to eliminate the use
of Pb in bonding technologies for electronic devices. A key relevant issue
concerns the intermetallic phases which grow in the bonding zone and are
determining in their quality and performance. In this work, we focus in the
{\eta}-phase (Cu2In or Cu6Sn5) that exists in both end binaries and as a
ternary phase. We present a neutron diffraction study of the constitution and
crystallography of a series of alloys around the 60 at.% Cu composition, and
with In contents ranging from 0 to 25 at.%, quenched from 300\degreeC. The
alloys were characterized by scanning electron microscopy, probe microanalysis
and high-resolution neutron diffraction. The Rietveld refinement of neutron
diffraction data allowed to improve the currently available model for site
occupancies in the hexagonal {\eta}-phase in the binary Cu-Sn as well as in
ternary alloys. For the first time, structural data is reported in the ternary
Cu-Sn-In {\eta}-phase as a function of composition, information that is of
fundamental technological importance as well as valuable input data for ongoing
modelisations of the ternary phase diagram.Comment: 8 pages, 10 figure
Photons from axial-vector radiative decay in a hadron gas
Strange and non-strange axial-vector meson radiative decays contribute to
photon production in hadron gas. One- and two-hadron radiative decay modes of
, and are studied. At 200 MeV
temperature and for a narrow range in photon energies they contribute more to
the net thermal photon production rate than ,
or . They provide
significant contribution to the rate for photon energies as high as 1.5--2.0
GeV. For higher energies they are less important.Comment: 10 pages + 7 figures uuencoded in separate file, MSUCL-92
Dilepton-tagged jets in relativistic nucleus-nucleus collisions: A case study
We study the A+B -> l+ l- + jet +X process in nucleus-nucleus collisions at
relativistic energies. The dilepton as well as the jet will pass through the
matter produced in such collisions. The recoiling dilepton will carry
information about the kinematical features of the jet, and will thus prove to
be a very effective tool in isolating in-medium effects such as energy-loss and
fragmentation function modifications. We estimate the contributions due to
correlated charm and bottom decay and we identify a window where they are small
as compared to pairs from the NLO Drell-Yan process.Comment: 7 pages, 9 figures Two figures modified, references adde
A New Phase of Matter: Quark-Gluon Plasma Beyond the Hagedorn Critical Temperature
I retrace the developments from Hagedorn's concept of a limiting temperature
for hadronic matter to the discovery and characterization of the quark-gluon
plasma as a new state of matter. My recollections begin with the transformation
more than 30 years ago of Hagedorn's original concept into its modern
interpretation as the "critical" temperature separating the hadron gas and
quark-gluon plasma phases of strongly interacting matter. This was followed by
the realization that the QCD phase transformation could be studied
experimentally in high-energy nuclear collisions. I describe here my personal
effort to help develop the strangeness experimental signatures of quark and
gluon deconfinement and recall how the experimental program proceeded soon to
investigate this idea, at first at the SPS, then at RHIC, and finally at LHC.
As it is often the case, the experiment finds more than theory predicts, and I
highlight the discovery of the "perfectly" liquid quark-gluon plasma at RHIC. I
conclude with an outline of future opportunities, especially the search for a
critical point in the QCD phase diagram.Comment: To appear in {\em Melting Hadrons, Boiling Quarks} by Rolf Hagedorn
and Johan Rafelski (editor), Springer Publishers, 2015 (open access
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