119,128 research outputs found
Difficulties in probing density dependent symmetry potential with the HBT interferometry
Based on the updated UrQMD transport model, the effect of the symmetry
potential energy on the two-nucleon HBT correlation is investigated with the
help of the coalescence program for constructing clusters, and the CRAB
analyzing program of the two-particle HBT correlation. An obvious non-linear
dependence of the neutron-proton (or neutron-neutron) HBT correlation function
() at small relative momenta on the stiffness factor of the
symmetry potential energy is found: when , the
increases rapidly with increasing , while it starts to saturate if
. It is also found that both the symmetry potential energy
at low densities and the conditions of constructing clusters at the late stage
of the whole process influence the two-nucleon HBT correlation with the same
power.Comment: 11 pages, 4 figure
Multiscale change-point segmentation: beyond step functions.
Modern multiscale type segmentation methods are known to detect multiple change-points with high statistical accuracy, while allowing for fast computation. Underpinning (minimax) estimation theory has been developed mainly for models that assume the signal as a piecewise constant function. In this paper, for a large collection of multiscale segmentation methods (including various existing procedures), such theory will be extended to certain function classes beyond step functions in a nonparametric regression setting. This extends the interpretation of such methods on the one hand and on the other hand reveals these methods as robust to deviation from piecewise constant functions. Our main finding is the adaptation over nonlinear approximation classes for a universal thresholding, which includes bounded variation functions, and (piecewise) Holder functions of smoothness order 0 < alpha <= 1 as special cases. From this we derive statistical guarantees on feature detection in terms of jumps and modes. Another key finding is that these multiscale segmentation methods perform nearly (up to a log-factor) as well as the oracle piecewise constant segmentation estimator (with known jump locations), and the best piecewise constant approximants of the (unknown) true signal. Theoretical findings are examined by various numerical simulations
A model comparison of resonance lifetime modifications, a soft equation of state and non-Gaussian effects on correlations at FAIR/AGS energies
HBT correlations of pairs at FAIR/AGS energies are investigated
by using the UrQMD transport model and the CRAB analyzing program. Three
different possible sources (treatment of resonance lifetimes, a soft equation
of state and non-Gaussian effects) to understand the HBT puzzle are
investigated. Firstly, we find that different treatments of the resonance decay
time can not resolve the HBT time-related puzzle, however it can modify the HBT
radii at low transverse momenta to some extent to explain the data slightly.
Secondly, with a soft equation of state with momentum dependence, the measured
transverse momentum dependent HBT radii and ratio can be described
fairly well. Thirdly, non-Gaussian effects are visible in the calculated
correlation function. Using the Edgeworth expansion, one finds that the
non-Gaussian effect is strongest in the longitudinal direction and weakest in
the sideward direction.Comment: 18 pages, 6 figures. To be published in J.Phys.
Resonance model study of kaon production in baryon baryon reactions for heavy ion collisions
The energy dependence of the total kaon production cross sections in baryon
baryon ( and ) collisions are studied in the resonance model, which
is a relativistic, tree-level treatment. This study is the first attempt to
complete a systematic, consistent investigation of the elementary kaon
production reactions for both the pion baryon and baryon baryon reactions. Our
model suggests that the magnitudes of the isospin-averaged total cross sections
for the and ( or )
reactions are almost equal at energies up to about 200 MeV above threshold.
However, the magnitudes for the reactions become about 6 times
larger than those for the reactions at energies about 1 GeV above
threshold. Furthermore, the magnitudes of the isospin-averaged total cross
sections for the reactions turn out to be comparable to
those for the reactions at invariant collision energies
about 3.1 GeV, and about 5 to 10 times larger at invariant collision
energies about 3.5 GeV. The microscopic cross sections are parametrized in all
isospin channels necessary for the transport model studies of kaon production
in heavy ion collisions. These cross sections are then applied in the
relativistic transport model to study the sensitivity to the underlying
elementary kaon production cross sections.Comment: Latex, 47 pages, 23 postscript figures. Typos in the published
version, which informed as errata to the editor, are corrected for the use of
simulation cod
Elliptic flow in heavy ion collisions near the balance energy
The proton elliptic flow in collisions of Ca on Ca at energies from 30 to 100
MeV/nucleon is studied in an isospin-dependent transport model. With increasing
incident energy, the elliptic flow shows a transition from positive to negative
flow. Its magnitude depends on both the nuclear equation of state (EOS) and the
nucleon-nucleon scattering cross section. Different elliptic flows are obtained
for a stiff EOS with free nucleon-nucleon cross sections and a soft EOS with
reduced nucleon-nucleon cross sections, although both lead to vanishing
in-plane transverse flow at the same balance energy. The study of both in-plane
and elliptic flows at intermediate energies thus provides a means to extract
simultaneously the information on the nuclear equation of state and the
nucleon-nucleon scattering cross section in medium.Comment: 6 pages, 2 figure
Medium modifications of the nucleon-nucleon elastic cross section in neutron-rich intermediate energy HICs
Several observables of unbound nucleons which are to some extent sensitive to
the medium modifications of nucleon-nucleon elastic cross sections in
neutron-rich intermediate energy heavy ion collisions are investigated. The
splitting effect of neutron and proton effective masses on cross sections is
discussed. It is found that the transverse flow as a function of rapidity, the
as a function of momentum, and the ratio of halfwidths of the
transverse to that of longitudinal rapidity distribution are very
sensitive to the medium modifications of the cross sections. The transverse
momentum distribution of correlation functions of two-nucleons does not yield
information on the in-medium cross section.Comment: 14 pages, 5 figure
Rank-frequency relation for Chinese characters
We show that the Zipf's law for Chinese characters perfectly holds for
sufficiently short texts (few thousand different characters). The scenario of
its validity is similar to the Zipf's law for words in short English texts. For
long Chinese texts (or for mixtures of short Chinese texts), rank-frequency
relations for Chinese characters display a two-layer, hierarchic structure that
combines a Zipfian power-law regime for frequent characters (first layer) with
an exponential-like regime for less frequent characters (second layer). For
these two layers we provide different (though related) theoretical descriptions
that include the range of low-frequency characters (hapax legomena). The
comparative analysis of rank-frequency relations for Chinese characters versus
English words illustrates the extent to which the characters play for Chinese
writers the same role as the words for those writing within alphabetical
systems.Comment: To appear in European Physical Journal B (EPJ B), 2014 (22 pages, 7
figures
- âŠ