2,308 research outputs found
Anharmonic Decay of Vibrational States in Amorphous Silicon
Anharmonic decay rates are calculated for a realistic atomic model of
amorphous silicon. The results show that the vibrational states decay on
picosecond timescales and follow the two-mode density of states, similar to
crystalline silicon, but somewhat faster. Surprisingly little change occurs for
localized states. These results disagree with a recent experiment.Comment: 10 pages, 4 Postscript figure
Polarized radio emission from extensive air showers measured with LOFAR
We present LOFAR measurements of radio emission from extensive air showers.
We find that this emission is strongly polarized, with a median degree of
polarization of nearly , and that the angle between the polarization
direction of the electric field and the Lorentz force acting on the particles,
depends on the observer location in the shower plane. This can be understood as
a superposition of the radially polarized charge-excess emission mechanism,
first proposed by Askaryan and the geomagnetic emission mechanism proposed by
Kahn and Lerche. We calculate the relative strengths of both contributions, as
quantified by the charge-excess fraction, for individual air showers. We
find that the measured charge-excess fraction is higher for air showers
arriving from closer to the zenith. Furthermore, the measured charge-excess
fraction also increases with increasing observer distance from the air shower
symmetry axis. The measured values range from for very
inclined air showers at to for almost
vertical showers at . Both dependencies are in qualitative
agreement with theoretical predictions.Comment: 22 pages, 14 figures, accepted for publication in JCA
The radio emission pattern of air showers as measured with LOFAR - a tool for the reconstruction of the energy and the shower maximum
The pattern of the radio emission of air showers is finely sampled with the
Low-Frequency ARray (LOFAR). A set of 382 measured air showers is used to test
a fast, analytic parameterization of the distribution of pulse powers. Using
this parameterization we are able to reconstruct the shower axis and give
estimators for the energy of the air shower as well as the distance to the
shower maximum.Comment: 15 pages, 10 figures, accepted for publication in JCA
A method for high precision reconstruction of air shower Xmax using two-dimensional radio intensity profiles
The mass composition of cosmic rays contains important clues about their
origin. Accurate measurements are needed to resolve long-standing issues such
as the transition from Galactic to extragalactic origin, and the nature of the
cutoff observed at the highest energies. Composition can be studied by
measuring the atmospheric depth of the shower maximum Xmax of air showers
generated by high-energy cosmic rays hitting the Earth's atmosphere. We present
a new method to reconstruct Xmax based on radio measurements. The radio
emission mechanism of air showers is a complex process that creates an
asymmetric intensity pattern on the ground. The shape of this pattern strongly
depends on the longitudinal development of the shower. We reconstruct Xmax by
fitting two-dimensional intensity profiles, simulated with CoREAS, to data from
the LOFAR radio telescope. In the dense LOFAR core, air showers are detected by
hundreds of antennas simultaneously. The simulations fit the data very well,
indicating that the radiation mechanism is now well-understood. The typical
uncertainty on the reconstruction of Xmax for LOFAR showers is 17 g/cm^2.Comment: 12 pages, 10 figures, submitted to Phys. Rev.
Realtime processing of LOFAR data for the detection of nano-second pulses from the Moon
The low flux of the ultra-high energy cosmic rays (UHECR) at the highest
energies provides a challenge to answer the long standing question about their
origin and nature. Even lower fluxes of neutrinos with energies above
eV are predicted in certain Grand-Unifying-Theories (GUTs) and e.g.\ models for
super-heavy dark matter (SHDM). The significant increase in detector volume
required to detect these particles can be achieved by searching for the
nano-second radio pulses that are emitted when a particle interacts in Earth's
moon with current and future radio telescopes.
In this contribution we present the design of an online analysis and trigger
pipeline for the detection of nano-second pulses with the LOFAR radio
telescope. The most important steps of the processing pipeline are digital
focusing of the antennas towards the Moon, correction of the signal for
ionospheric dispersion, and synthesis of the time-domain signal from the
polyphased-filtered signal in frequency domain. The implementation of the
pipeline on a GPU/CPU cluster will be discussed together with the computing
performance of the prototype.Comment: Proceedings of the 22nd International Conference on Computing in High
Energy and Nuclear Physics (CHEP2016), US
Alternative Interpretation of Sharply Rising E0 Strengths in Transitional Regions
It is shown that strong 0+2 -> 0+1 E0 transitions provide a clear signature
of phase transitional behavior in finite nuclei. Calculations using the IBA
show that these transition strengths exhibit a dramatic and robust increase in
spherical-deformed shape transition regions, that this rise matches well the
existing data, that the predictions of these E0 transitions remain large in
deformed nuclei, and that these properties are intrinsic to the way that
collectivity and deformation develop through the phase transitional region in
the model, arising from the specific d-boson coherence in the wave functions,
and that they do not necessarily require the explicit mixing of normal and
intruder configurations from different IBA spaces.Comment: 6 pages, 3 figure
Patterns of the ground states in the presence of random interactions: nucleon systems
We present our results on properties of ground states for nucleonic systems
in the presence of random two-body interactions. In particular we present
probability distributions for parity, seniority, spectroscopic (i.e., in the
laboratory framework) quadrupole moments and clustering in the ground
states. We find that the probability distribution for the parity of the ground
states obtained by a two-body random ensemble simulates that of realistic
nuclei: positive parity is dominant in the ground states of even-even nuclei
while for odd-odd nuclei and odd-mass nuclei we obtain with almost equal
probability ground states with positive and negative parity. In addition we
find that for the ground states, assuming pure random interactions, low
seniority is not favored, no dominance of positive values of spectroscopic
quadrupole deformation, and no sign of -cluster correlations, all in
sharp contrast to realistic nuclei. Considering a mixture of a random and a
realistic interaction, we observe a second order phase transition for the
-cluster correlation probability.Comment: 7 page
Boson-conserving one-nucleon transfer operator in the interacting boson model
The boson-conserving one-nucleon transfer operator in the interacting boson
model (IBA) is reanalyzed. Extra terms are added to the usual form used for
that operator. These new terms change generalized seniority by one unit, as the
ones considered up to now. The results obtained using the new form for the
transfer operator are compared with those obtained with the traditional form in
a simple case involving the pseudo-spin Bose-Fermi symmetry in its limit. Sizeable differences are
found. These results are of relevance in the study of transfer reactions to
check nuclear supersymmetry and in the description of (\beta)-decay within IBA.Comment: 13 pages, 1 table, 0 figures. To be published in Phys. Rev.
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