11 research outputs found
Dilepton Production from AGS to SPS Energies within a Relativistic Transport Approach
We present a nonperturbative dynamical study of production in
proton-nucleus and nucleus-nucleus collisions from AGS to SPS energies on the
basis of the covariant transport approach HSD. For p + Be reactions the
dilepton yield for invariant masses GeV is found to be dominated
by the decays of the and mesons at all energies
from 10 -- 450 GeV. For nucleus-nucleus collisions, however, the dilepton yield
shows an additional large contribution from , and channels. Systematic studies are presented for the 'free' meson mass
scenario in comparison to a 'dropping' meson mass scenario at finite baryon
density. We find that for 'dropping' meson masses the invariant dilepton mass
range 0.35 GeV 0.65 GeV is increased in comparison to the 'free'
meson mass scenario and that the data of the CERES-collaboration for
nucleus-nucleus collisions can be described much better within the 'dropping'
mass scheme. We study in detail the contributions from the various dilepton
channels as a function of the transverse momentum and rapidity of the lepton
pair as well as a function of the charged particle multiplicity. Furthermore,
various direct photon channels for S + Au at 200 GeV/u are computed and found
to be well below the upper bounds measured by the WA80-collaboration.Comment: 38 pages, LaTeX, including 19 postscript figures, to appear in Nucl.
Phys.
Dilepton production and -scaling at BEVALAC/SIS energies
We present a dynamical study of production in C + C and Ca + Ca
collisions at BEVALAC/SIS energies on the basis of the covariant transport
approach HSD employing momentum-dependent -meson spectral functions that
include the pion modifications in the nuclear medium as well as the
polarization of the -meson due to resonant scattering. We find
that the experimental data from the DLS collaboration cannot be described
within the -meson spectral function approach. A dropping -mass
scenario leads to a good reproduction of the DLS dilepton data, however,
violates the -scaling of and spectra as observed by the
TAPS collaboration as well as photoproduction on nuclei.Comment: 35 pages, ReVTeX, including 11 postscript figures, UGI-97-06, Nucl.
Phys. A, in pres
Snapshots in time: precise correlations of peat-based proxy climate records in Scotland using mid-Holocene tephras
Mid-Holocene tephra layers have been located and geochemically analysed from seven ombrotrophic bogs in Scotland. The tephras found in these Scottish peats occur as stratigraphically discrete horizons, not visible to the naked eye, and originate from volcanic activity in Iceland. Identifying and geochemically typing the tephra layers can enable precise correlations between sites on regional scales, depending on the spatial extent of the airfall events. Dispersal of both the Glen Garry and Hekla-4 tephras is excellent over Scotland, enabling snapshot views of the past at the time of these tephra depositions. Palaeoecological analyses have also been undertaken on these bogs, providing detailed reconstructions of past changes in bog vegetation and surface wetness, a proxy for past climates. Correlations between the ombrotrophic bogs at the time of the Glen Garry tephra revealed significant differences between the proxy climate records in the north and the south of Scotland, suggesting asynchronous changes between northern Scotland and the rest of Great Britain
Sensing small-scale human activity in the palaeoecological record: fine spatial resolution pollen analyses from Glen Affric, northern Scotland
This paper examines the importance of palynological site selection criteria, speci"cally basin size, for the detection of vegetation mosaics and small- or local-scale human activity within a spatially diverse, mosaic landscape. Using a site selection strategy which recognizes landscape patchiness, pollen analyses from three small peat basins (10-56 m diameter) in an open, exposed upland valley (greater than 250 m OD) provide records which are sensitive to local vegetation mosaics and small-scale, localized agriculture. The results indicate c. 4000 14C years (4400 cal. years) of land use, with spatial and temporal variations in the valley. Contrasts between the sequences suggest that local pollen production remains an important component of the pollen rain deposited in small peat basins, even in open environments; this is especially true of palynological‘agricultural indicators'. By comparison, sites with regional pollen source areas underestimate the spatial diversity of the upland landscape, and are insensitive to small-scale human activity in an environment where the fragmentary distribution of soils suitable for agriculture favoured a small-scale, dispersed pattern of farming. It is therefore essential to match the spatial resolution of pollen records with the grain size or scale of variations in the environment under investigation in order to sense the scale of mosaics in vegetation and agriculture within patchy landscapes