357 research outputs found
Pion emission in 2H, 12C, 27Al, gamma pi+ reactions at threshold
The first data from MAX-lab in Lund, Sweden on pion production in
photonuclear reactions at threshold energies, is presented. The decrease of the
total yield of pi+ in gamma + 12C, 27Al reactions below 200 MeV as well as
differential, dsigma/dOmega, cross sections follow essentially predictions from
an intranuclear cascade model with an attractive potential for pion-nucleus
interaction in its simplest form. Double differential, d2sigma/dOmegadT, cross
sections at 176 MeV show, however, deviations from the model, which call for
refinements of nuclear and Coulomb potentials and possibly also for coherent
pion production mechanisms.Comment: 19 pages, 7 figure
Radio precursors to neutron star binary mergings
We discuss a possible generation of radio bursts preceding final stages of
binary neutron star mergings which can be accompanied by short gamma-ray
bursts. Detection of such bursts appear to be advantageous in the low-frequency
radio band due to a time delay of ten to several hundred seconds required for
radio signal to propagate in the ionized intergalactic medium. This delay makes
it possible to use short gamma-ray burst alerts to promptly monitor specific
regions on the sky by low-frequency radio facilities, especially by LOFAR. To
estimate the strength of the radio signal, we assume a power-law dependence of
the radio luminosity on the total energy release in a magnetically dominated
outflow, as found in millisecond pulsars. Based on the planned LOFAR
sensitivity at 120 MHz, we estimate that the LOFAR detection rate of such radio
transients could be about several events per month from redshifts up to
in the most optimistic scenario. The LOFAR ability to detect such
events would crucially depend on exact efficiency of low-frequency radio
emission mechanism.Comment: 6 pages, 2 figures, Accepted for publication in Astrophysics & Space
Science. Largely extended version of ArXiv:0912.521
Can a supernova be located by its neutrinos?
A future core-collapse supernova in our Galaxy will be detected by several
neutrino detectors around the world. The neutrinos escape from the supernova
core over several seconds from the time of collapse, unlike the electromagnetic
radiation, emitted from the envelope, which is delayed by a time of order
hours. In addition, the electromagnetic radiation can be obscured by dust in
the intervening interstellar space. The question therefore arises whether a
supernova can be located by its neutrinos alone. The early warning of a
supernova and its location might allow greatly improved astronomical
observations. The theme of the present work is a careful and realistic
assessment of this question, taking into account the statistical significance
of the various neutrino signals. Not surprisingly, neutrino-electron forward
scattering leads to a good determination of the supernova direction, even in
the presence of the large and nearly isotropic background from other reactions.
Even with the most pessimistic background assumptions, SuperKamiokande (SK) and
the Sudbury Neutrino Observatory (SNO) can restrict the supernova direction to
be within circles of radius and , respectively. Other
reactions with more events but weaker angular dependence are much less useful
for locating the supernova. Finally, there is the oft-discussed possibility of
triangulation, i.e., determination of the supernova direction based on an
arrival time delay between different detectors. Given the expected statistics
we show that, contrary to previous estimates, this technique does not allow a
good determination of the supernova direction.Comment: 11 pages including 2 figures. Revised version corrects typos, adds
some brief comment
Low-Energy Universality in Atomic and Nuclear Physics
An effective field theory developed for systems interacting through
short-range interactions can be applied to systems of cold atoms with a large
scattering length and to nucleons at low energies. It is therefore the ideal
tool to analyze the universal properties associated with the Efimov effect in
three- and four-body systems. In this "progress report", we will discuss recent
results obtained within this framework and report on progress regarding the
inclusion of higher order corrections associated with the finite range of the
underlying interaction.Comment: Commissioned article for Few-Body Systems, 47 pp, 16 fig
Zero-point vacancies in quantum solids
A Jastrow wave function (JWF) and a shadow wave function (SWF) describe a
quantum solid with Bose--Einstein condensate; i.e. a supersolid. It is known
that both JWF and SWF describe a quantum solid with also a finite equilibrium
concentration of vacancies x_v. We outline a route for estimating x_v by
exploiting the existing formal equivalence between the absolute square of the
ground state wave function and the Boltzmann weight of a classical solid. We
compute x_v for the quantum solids described by JWF and SWF employing very
accurate numerical techniques. For JWF we find a very small value for the zero
point vacancy concentration, x_v=(1.4\pm0.1) x 10^-6. For SWF, which presently
gives the best variational description of solid 4He, we find the significantly
larger value x_v=(1.4\pm0.1) x 10^-3 at a density close to melting. We also
study two and three vacancies. We find that there is a strong short range
attraction but the vacancies do not form a bound state.Comment: 19 pages, submitted to J. Low Temp. Phy
Schwinger Pair Production via Instantons in Strong Electric Fields
In the space-dependent gauge, each mode of the Klein-Gordon equation in a
strong electric field takes the form of a time-independent Schr\"{o}dinger
equation with a potential barrier. We propose that the single- and
multi-instantons of quantum tunneling may be related with the single- and
multi-pair production of bosons and the relative probability for the no-pair
production is determined by the total tunneling probability via instantons. In
the case of a uniform electric field, the instanton interpretation recovers
exactly the well-known pair production rate for bosons and when the Pauli
blocking is taken into account, it gives the correct fermion production rate.
The instanton is used to calculate the pair production rate even in an
inhomogeneous electric field. Furthermore, the instanton interpretation
confirms the fact that bosons and fermions can not be produced by a static
magnetic field only.Comment: RevTex 7 Pages, No figure; Formulae for the production rate in very
strong fields and references added; the final version accepted in Phys. Rev.
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at TeV
The elliptic, , triangular, , and quadrangular, , azimuthal
anisotropic flow coefficients are measured for unidentified charged particles,
pions and (anti-)protons in Pb-Pb collisions at TeV
with the ALICE detector at the Large Hadron Collider. Results obtained with the
event plane and four-particle cumulant methods are reported for the
pseudo-rapidity range at different collision centralities and as a
function of transverse momentum, , out to GeV/.
The observed non-zero elliptic and triangular flow depends only weakly on
transverse momentum for GeV/. The small dependence
of the difference between elliptic flow results obtained from the event plane
and four-particle cumulant methods suggests a common origin of flow
fluctuations up to GeV/. The magnitude of the (anti-)proton
elliptic and triangular flow is larger than that of pions out to at least
GeV/ indicating that the particle type dependence persists out
to high .Comment: 16 pages, 5 captioned figures, authors from page 11, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/186
Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at TeV
The inclusive transverse momentum () distributions of primary
charged particles are measured in the pseudo-rapidity range as a
function of event centrality in Pb-Pb collisions at
TeV with ALICE at the LHC. The data are presented in the range
GeV/ for nine centrality intervals from 70-80% to 0-5%.
The Pb-Pb spectra are presented in terms of the nuclear modification factor
using a pp reference spectrum measured at the same collision
energy. We observe that the suppression of high- particles strongly
depends on event centrality. In central collisions (0-5%) the yield is most
suppressed with at -7 GeV/. Above
GeV/, there is a significant rise in the nuclear modification
factor, which reaches for GeV/. In
peripheral collisions (70-80%), the suppression is weaker with almost independently of . The measured nuclear
modification factors are compared to other measurements and model calculations.Comment: 17 pages, 4 captioned figures, 2 tables, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/284
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
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