1,123 research outputs found
Dijet production as a centrality trigger for p-p collisions at CERN LHC
We demonstrate that a trigger on hard dijet production at small rapidities
allows to establish a quantitative distinction between central and peripheral
collisions in pbar-p and p-p collisions at Tevatron and LHC energies. Such a
trigger strongly reduces the effective impact parameters as compared to minimum
bias events. This happens because the transverse spatial distribution of hard
partons (x >~ 10^{-2}) in the proton is considerably narrower than that of soft
partons, whose collisions dominate the total cross section. In the central
collisions selected by the trigger, most of the partons with x >~ 10^{-2}
interact with a gluon field whose strength rapidly increases with energy. At
LHC (and to some extent already at Tevatron) energies the strength of this
interaction approaches the unitarity ('black-body') limit. This leads to
specific modifications of the final state, such as a higher probability of
multijet events at small rapidities, a strong increase of the transverse
momenta and depletion of the longitudinal momenta at large rapidities, and the
appearance of long-range correlations in rapidity between the forward/backward
fragmentation regions. The same pattern is expected for events with production
of new heavy particles (Higgs, SUSY). Studies of these phenomena would be
feasible with the CMS-TOTEM detector setup, and would have considerable impact
on the exploration of the physics of strong gluon fields in QCD, as well as the
search for new particles at LHC.Comment: 17 pages, Revtex 4, 14 EPS figures. Expanded discussion of some
points, added 3 new figures and new references. Included comment on
connection with cosmic ray physics near the GZK cutoff. To appear in Phys Rev
High dissolved organic carbon release by benthic cyanobacterial mats in a Caribbean reef ecosystem
Benthic cyanobacterial mats (BCMs) are increasing in abundance on coral reefs worldwide. However, their impacts on biogeochemical cycling in the surrounding water and sediment are virtually unknown. By measuring chemical fluxes in benthic chambers placed over sediment covered by BCMs and sediment with BCMs removed on coral reefs in Curaçao, Southern Caribbean, we found that sediment covered by BCMs released 1.4 and 3.5 mmol C m<sup>-2</sup> h<sup>-1</sup> of dissolved organic carbon (DOC) during day and night, respectively. Conversely, sediment with BCMs removed took up DOC, with day and night uptake rates of 0.9 and 0.6 mmol C m<sup>-2</sup> h<sup>-1</sup>. DOC release by BCMs was higher than reported rates for benthic algae (turf and macroalgae) and was estimated to represent 79% of the total DOC released over a 24 h diel cycle at our study site. The high nocturnal release of DOC by BCMs is most likely the result of anaerobic metabolism and degradation processes, as shown by high respiration rates at the mat surface during nighttime. We conclude that BCMs are significant sources of DOC. Their increased abundance on coral reefs will lead to increased DOC release into the water column, which is likely to have negative implications for reef health
Ground state properties of exotic nuclei near Z=40 in the relativistic mean-field theory,
Study of the ground-state properties of Kr, Sr and Zr isotopes has been
performed in the framework of the relativistic mean field (RMF) theory using
the recently proposed relativistic parameter set NL-SH. It is shown that the
RMF theory provides an unified and excellent description of the binding
energies, isotope shifts and deformation properties of nuclei over a large
range of isospin in the Z=40 region. It is observed that the RMF theory with
the force NL-SH is able to describe the anomalous kinks in isotope shifts in Kr
and Sr nuclei, the problem which has hitherto remained unresolved. This is in
contrast with the density-dependent Skyrme Hartree-Fock approach which does not
reproduce the behaviour of the isotope shifts about shell closure. On the Zr
chain we predict that the isotope shifts exhibit a trend similar to that of the
Kr and Sr nuclei. The RMF theory also predicts shape coexistence in heavy Sr
isotopes. Several dramatic shape transitions in the isotopic chains are shown
to be a general feature of nuclei in this region. A comparison of the
properties with the available mass models shows that the results of the RMF
theory are generally in accord with the predictions of the finite-range droplet
model.Comment: 24 pages Latex, 7 figures (available upon request), Nuclear Physics A
(in press)
Elastic and quasi-elastic and scattering in the Dipole Model
We have in earlier papers presented an extension of Mueller's dipole cascade
model, which includes sub-leading effects from energy conservation and running
coupling as well as colour suppressed saturation effects from pomeron loops via
a ``dipole swing''. The model was applied to describe the total and diffractive
cross sections in and collisions, and also the elastic cross
section in scattering.
In this paper we extend the model to describe the corresponding quasi-elastic
cross sections in , namely the exclusive production of vector mesons
and deeply virtual compton scattering. Also for these reactions we find a good
agrement with measured cross sections. In addition we obtain a reasonable
description of the -dependence of the elastic and quasi-elastic
cross sections
Light Nuclei near Neutron and Proton Drip Lines in the Relativistic Mean-Field Theory
We have made a detailed study of the ground-state properties of nuclei in the
light mass region with atomic numbers Z=10-22 in the framework of the
relativistic mean-field (RMF) theory. The nonlinear model with
scalar self-interaction has been employed. The RMF calculations have been
performed in an axially deformed configuration using the force NL-SH. We have
considered nuclei about the stability line as well as those close to proton and
neutron drip lines. It is shown that the RMF results provide a good agreement
with the available empirical data. The RMF predictions also show a reasonably
good agreement with those of the mass models. It is observed that nuclei in
this mass region are found to possess strong deformations and exhibit shape
changes all along the isotopic chains. The phenomenon of the shape coexistence
is found to persist near the stability line as well as near the drip lines. It
is shown that the magic number N=28 is quenched strongly, thus enabling the
corresponding nuclei to assume strong deformations. Nuclei near the neutron and
proton drip lines in this region are also shown to be strongly deformed.Comment: 49 pages Latex, 12 postscript figures, to appear in Nuclear Physics
Asymmetric nuclear matter:the role of the isovector scalar channel
We try to single out some qualitative new effects of the coupling to the
-isovector-scalar meson introduced in a minimal way in a
phenomenological hadronic field theory. Results for the equation of state
() and the phase diagram of asymmetric nuclear matter () are
discussed. We stress the consistency of the -coupling introduction in a
relativistic approach. New contributions to the slope and curvature of the
symmetry energy and the neutron-proton effective mass splitting appear
particularly interesting. A more repulsive for neutron matter at high
baryon densities is expected. Effects on new critical properties of warm ,
mixing of mechanical and chemical instabilities and isospin distillation, are
also presented. The influence is mostly on the {\it isovectorlike}
collective response.
The results are largely analytical and this makes the physical meaning quite
transparent. Implications for nuclear structure properties of drip-line nuclei
and for reaction dynamics with Radioactive Beams are finally pointed out.Comment: 12 pages, 10 Postscript figure
Quasi-stationary regime of a branching random walk in presence of an absorbing wall
A branching random walk in presence of an absorbing wall moving at a constant
velocity undergoes a phase transition as the velocity of the wall
varies. Below the critical velocity , the population has a non-zero
survival probability and when the population survives its size grows
exponentially. We investigate the histories of the population conditioned on
having a single survivor at some final time . We study the quasi-stationary
regime for when is large. To do so, one can construct a modified
stochastic process which is equivalent to the original process conditioned on
having a single survivor at final time . We then use this construction to
show that the properties of the quasi-stationary regime are universal when
. We also solve exactly a simple version of the problem, the
exponential model, for which the study of the quasi-stationary regime can be
reduced to the analysis of a single one-dimensional map.Comment: 2 figures, minor corrections, one reference adde
Limiting fragmentation in hadron-hadron collisions at high energies
Limiting fragmentation in proton-proton, deuteron-nucleus and nucleus-nucleus
collisions is analyzed in the framework of the Balitsky-Kovchegov equation in
high energy QCD. Good agreement with experimental data is obtained for a wide
range of energies. Further detailed tests of limiting fragmentation at RHIC and
the LHC will provide insight into the evolution equations for high energy QCD.Comment: 28 pages, 10 figures (2 new figures, text slightly expanded, and some
additional references
Theoretical study of the two-proton halo candidate Ne including contributions from resonant continuum and pairing correlations
With the relativistic Coulomb wave function boundary condition, the energies,
widths and wave functions of the single proton resonant orbitals for Ne
are studied by the analytical continuation of the coupling constant (ACCC)
approach within the framework of the relativistic mean field (RMF) theory.
Pairing correlations and contributions from the single-particle resonant
orbitals in the continuum are taken into consideration by the resonant
Bardeen-Cooper-Schrieffer (BCS) approach, in which constant pairing strength is
used. It can be seen that the fully self-consistent calculations with NL3 and
NLSH effective interactions mostly agree with the latest experimental
measurements, such as binding energies, matter radii, charge radii and
densities. The energy of 2s orbital is slightly higher than that
of orbital, and the occupation probability of the
2s orbital is about 20%, which are in accordance with the
shell model calculation and three-body model estimation
Donor states in modulation-doped Si/SiGe heterostructures
We present a unified approach for calculating the properties of shallow
donors inside or outside heterostructure quantum wells. The method allows us to
obtain not only the binding energies of all localized states of any symmetry,
but also the energy width of the resonant states which may appear when a
localized state becomes degenerate with the continuous quantum well subbands.
The approach is non-variational, and we are therefore also able to evaluate the
wave functions. This is used to calculate the optical absorption spectrum,
which is strongly non-isotropic due to the selection rules. The results
obtained from calculations for Si/SiGe quantum wells allow us to
present the general behavior of the impurity states, as the donor position is
varied from the center of the well to deep inside the barrier. The influence on
the donor ground state from both the central-cell effect and the strain arising
from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure
- âŠ