87 research outputs found
RPA calculations with Gaussian expansion method
The Gaussian expansion method (GEM) is extensively applied to the
calculations in the random-phase approximation (RPA). We adopt the
mass-independent basis-set that has been tested in the mean-field calculations.
By comparing the RPA results with those obtained by several other available
methods for Ca isotopes, using a density-dependent contact interaction and the
Woods-Saxon single-particle states, we confirm that energies, transition
strengths and widths of their distribution are described by the GEM bases to
good precision, for the , and collective states. The GEM is
then applied to the self-consistent RPA calculations with the finite-range
Gogny D1S interaction. The spurious center-of-mass motion is well separated
from the physical states in the response, and the energy-weighted sum
rules for the isoscalar transitions are fulfilled reasonably well. Properties
of low-energy transitions in Ca are argued in some detail.Comment: 30 pages including 12 figure
Semiclassical evaluation of average nuclear one and two body matrix elements
Thomas-Fermi theory is developed to evaluate nuclear matrix elements averaged
on the energy shell, on the basis of independent particle Hamiltonians. One-
and two-body matrix elements are compared with the quantal results and it is
demonstrated that the semiclassical matrix elements, as function of energy,
well pass through the average of the scattered quantum values. For the one-body
matrix elements it is shown how the Thomas-Fermi approach can be projected on
good parity and also on good angular momentum. For the two-body case the
pairing matrix elements are considered explicitly.Comment: 15 pages, REVTeX, 6 ps figures; changed conten
Combinatorial Level Densities from a Microscopic Relativistic Structure Model
A new model for calculating nuclear level densities is investigated. The
single-nucleon spectra are calculated in a relativistic mean-field model with
energy-dependent effective mass, which yields a realistic density of
single-particle states at the Fermi energy. These microscopic single-nucleon
states are used in a fast combinatorial algorithm for calculating the
non-collective excitations of nuclei. The method, when applied to magic and
semi-magic nuclei, such as Ni, Sn and Pb, reproduces the
cumulative number of experimental states at low excitation energy, as well as
the s-wave neutron resonance spacing at the neutron binding energy.
Experimental level densities above 10 MeV are reproduced by multiplying the
non-collective level densities by a simple vibrational enhancement factor.
Problems to be solved in the extension to open-shell nuclei are discussedComment: 22 pages, 5 figures, revised version, to appear in Nucl. Phys.
Flares In Long And Short Gamma Ray Bursts
The many similarities between the prompt emission pulses in gamma ray bursts
(GRBs) and X-ray flares during the fast decay and afterglow phases of GRBs
suggest a common origin. In the cannonball (CB) model of GRBs, this common
origin is mass accretion episodes of fall-back matter on a newly born compact
object. The prompt emission pulses are produced by a bipolar jet of highly
relativistic plasmoids (CBs) ejected in the early, major episodes of mass
accretion. As the accretion material is consumed, one may expect the engine's
activity to weaken. X-ray flares ending the prompt emission and during the
afterglow phase are produced in such delayed episodes of mass accretion. The
common engine, environment and radiation mechanisms (inverse Compton scattering
and synchrotron radiation) produce their observed similarities. Flares in both
long GRBs and short hard gamma ray bursts (SHBs) can also be produced by
bipolar ejections of CBs following a phase transition in compact objects due to
loss of angular momentum and/or cooling. Optical flares, however, are mostly
produced in collisions of CBs with massive stellar winds/ejecta or with density
bumps along their path. In this paper we show that the master formulae of the
CB model of GRBs and SHBs, which reproduce very well their prompt emission
pulses and their smooth afterglows, seem to reproduce also very well the
lightcurves and spectral evolution of the prominent X-ray and optical flares
that are well sampled.Comment: Added new section on optical flares, 3 tables and 5(x6) multiple
figures comparing observations and theor
The diverse broad-band light-curves of Swift GRBs reproduced with the cannonball model
Two radiation mechanisms, inverse Compton scattering (ICS) and synchrotron
radiation (SR), suffice within the cannonball (CB) model of long gamma ray
bursts (LGRBs) and X-ray flashes (XRFs) to provide a very simple and accurate
description of their observed prompt emission and afterglows. Simple as they
are, the two mechanisms and the burst environment generate the rich structure
of the light curves at all frequencies and times. This is demonstrated for 33
selected Swift LGRBs and XRFs, which are well sampled from early until late
time and faithfully represent the entire diversity of the broad-band light
curves of Swift LGRBs and XRFs. Their prompt gamma-ray and X-ray emission is
dominated by ICS of `glory' light. During their fast decline phase, ICS is
taken over by SR, which dominates their broad-band afterglow. The pulse shape
and spectral evolution of the gamma-ray peaks and the early-time X-ray flares,
and even the delayed optical `humps' in XRFs, are correctly predicted. The
`canonical' and non-canonical X-ray light curves and the chromatic behaviour of
the broad-band afterglows are well reproduced. In particular, in canonical
X-ray light curves, the initial fast decline and rapid softening of the prompt
emission, the transition to the plateau phase, the subsequent gradual
steepening of the plateau to an asymptotic power-law decay, and the transition
from chromatic to achromatic behaviour of the light curves agrees well with
those predicted by the CB model. The Swift early-time data on XRF 060218 are
inconsistent with a black-body emission from a shock break-out through a
stellar envelope. Instead, they are well described by ICS of glory light by a
jet breaking out from SN2006aj.Comment: Accepted for publication in The Astrophysical Journal. 63 pages, 10
(multiple) figure
Identifying networks in social media: The case of #Grexit
We examine the intensity of ‘#Grexit’ usage in Twitter during a period of economic and financial turbulence. Using a frequency-analysis technique, we illustrate that we can extract detailed information from social media data. This allows us to map the networks of interest as it is reflected in Twitter. Our findings identify high-interest in Grexit from Twitter users in key peripheral countries, core Eurozone members as well as core EU member states outside the Eurozone. Overall, our study presents a useful tool for identifying clusters. This is part of a new research agenda utilising the information extracted from big data available via social media channels
Status of Muon Collider Research and Development and Future Plans
The status of the research on muon colliders is discussed and plans are
outlined for future theoretical and experimental studies. Besides continued
work on the parameters of a 3-4 and 0.5 TeV center-of-mass (CoM) energy
collider, many studies are now concentrating on a machine near 0.1 TeV (CoM)
that could be a factory for the s-channel production of Higgs particles. We
discuss the research on the various components in such muon colliders, starting
from the proton accelerator needed to generate pions from a heavy-Z target and
proceeding through the phase rotation and decay ()
channel, muon cooling, acceleration, storage in a collider ring and the
collider detector. We also present theoretical and experimental R & D plans for
the next several years that should lead to a better understanding of the design
and feasibility issues for all of the components. This report is an update of
the progress on the R & D since the Feasibility Study of Muon Colliders
presented at the Snowmass'96 Workshop [R. B. Palmer, A. Sessler and A.
Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics
(Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].Comment: 95 pages, 75 figures. Submitted to Physical Review Special Topics,
Accelerators and Beam
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