285 research outputs found
Zipf's law in Nuclear Multifragmentation and Percolation Theory
We investigate the average sizes of the largest fragments in nuclear
multifragmentation events near the critical point of the nuclear matter phase
diagram. We perform analytic calculations employing Poisson statistics as well
as Monte Carlo simulations of the percolation type. We find that previous
claims of manifestations of Zipf's Law in the rank-ordered fragment size
distributions are not born out in our result, neither in finite nor infinite
systems. Instead, we find that Zipf-Mandelbrot distributions are needed to
describe the results, and we show how one can derive them in the infinite size
limit. However, we agree with previous authors that the investigation of
rank-ordered fragment size distributions is an alternative way to look for the
critical point in the nuclear matter diagram.Comment: 8 pages, 11 figures, submitted to PR
Near-IR Search for Lensed Supernovae Behind Galaxy Clusters - II. First Detection and Future Prospects
Powerful gravitational telescopes in the form of massive galaxy clusters can
be used to enhance the light collecting power over a limited field of view by
about an order of magnitude in flux. This effect is exploited here to increase
the depth of a survey for lensed supernovae at near-IR wavelengths. A pilot SN
search program conducted with the ISAAC camera at VLT is presented. Lensed
galaxies behind the massive clusters A1689, A1835 and AC114 were observed for a
total of 20 hours split into 2, 3 and 4 epochs respectively, separated by
approximately one month to a limiting magnitude J<24 (Vega). Image subtractions
including another 20 hours worth of archival ISAAC/VLT data were used to search
for transients with lightcurve properties consistent with redshifted
supernovae, both in the new and reference data. The feasibility of finding
lensed supernovae in our survey was investigated using synthetic lightcurves of
supernovae and several models of the volumetric Type Ia and core-collapse
supernova rates as a function of redshift. We also estimate the number of
supernova discoveries expected from the inferred star formation rate in the
observed galaxies. The methods consistently predict a Poisson mean value for
the expected number of SNe in the survey between N_SN=0.8 and 1.6 for all
supernova types, evenly distributed between core collapse and Type Ia SN. One
transient object was found behind A1689, 0.5" from a galaxy with photometric
redshift z_gal=0.6 +- 0.15. The lightcurve and colors of the transient are
consistent with being a reddened Type IIP SN at z_SN=0.59. The lensing model
predicts 1.4 magnitudes of magnification at the location of the transient,
without which this object would not have been detected in the near-IR ground
based search described in this paper (unlensed magnitude J~25). (abridged)Comment: Accepted by AA, matches journal versio
Near-IR search for lensed supernovae behind galaxy clusters: I. Observations and transient detection efficiency
Massive galaxy clusters at intermediate redshift can magnify the flux of
distant background sources by several magnitudes and we exploit this effect to
search for lensed distant supernovae that may otherwise be too faint to be
detected. A supernova search was conducted at near infrared wavelengths using
the ISAAC instrument at the VLT. The galaxy clusters Abell 1689, Abell 1835 and
AC114 were observed at multiple epochs of 2 hours of exposure time, separated
by a month. Image-subtraction techniques were used to search for transient
objects with light curve properties consistent with supernovae, both in our new
and archival ISAAC/VLT data. The limiting magnitude of the individual epochs
was estimated by adding artificial stars to the subtracted images. Most of the
epochs reach 90% detection efficiency at SZ(J) ~= 23.8-24.0 mag (Vega). Two
transient objects, both in archival images of Abell 1689 and AC114, were
detected. The transient in AC114 coincides - within the position uncertainty -
with an X-ray source and is likely to be a variable AGN at the cluster
redshift. The transient in Abell 1689 was found at SZ=23.24 mag, ~0.5 arcsec
away from a galaxy with photometric redshift z=0.6 +/-0.15. The light curves
and the colors of the transient are consistent with a reddened Type IIP
supernova at redshift z=0.59 +/- 0.05. The lensing model of Abell 1689 predicts
~1.4 mag of magnification at the position of the transient, making it the most
magnified supernova ever found and only the second supernova found behind a
galaxy cluster. Our pilot survey has demonstrated the feasibility to find
distant gravitationally magnified supernovae behind massive galaxy clusters.
One likely supernova was found behind Abell 1689, in accordance with the
expectations for this survey, as shown in an accompanying analysis paper.Comment: Language-edited version, 9 pages, 6 figures, accepted by A&
Hydrodynamical instabilities in an expanding quark gluon plasma
We study the mechanism responsible for the onset of instabilities in a chiral
phase transition at nonzero temperature and baryon chemical potential. As a
low-energy effective model, we consider an expanding relativistic plasma of
quarks coupled to a chiral field, and obtain a phenomenological chiral
hydrodynamics from a variational principle. Studying the dispersion relation
for small fluctuations around equilibrium, we identify the role played by
chiral waves and pressure waves in the generation of instabilities. We show
that pressure modes become unstable earlier than chiral modes.Comment: 7 pages, 4 figure
Equation of state of hadron resonance gas and the phase diagram of strongly interacting matter
The equation of state of hadron resonance gas at finite temperature and
baryon density is calculated taking into account finite-size effects within the
excluded volume model. Contributions of known hadrons with masses up to 2 GeV
are included in the zero-width approximation. Special attention is paid to the
role of strange hadrons in the system with zero total strangeness. A density-
dependent mean field is added to guarantee that the nuclear matter has a
saturation point and a liquid-gas phase transition. The deconfined phase is
described by the bag model with lowest order perturbative corrections. The
phase transition boundary is found by using the Gibbs conditions with the
strangeness neutrality constraint. The sensitivity of the phase diagram to the
hadronic excluded volume and to the parametrization of the mean-field is
investigated. The possibility of strangeness-antistrangeness separation in the
mixed phase is analyzed. It is demonstrated that the peaks in the kaon to pion
and lambda to pion multiplicity ratios can be explained by a nonmonotonous
behavior of the strangeness fugacity along the chemical freeze-out line.Comment: 40 pages, 31 figure
A highly magnified supernova at z=1.703 behind the massive galaxy cluster Abell 1689
Our ability to study the most remote supernova explosions, crucial for the
understanding of the evolution of the high-redshift universe and its expansion
rate, is limited by the light collection capabilities of telescopes. However,
nature offers unique opportunities to look beyond the range within reach of our
unaided instruments thanks to the light-focusing power of massive galaxy
clusters. Here we report on the discovery of one of the most distant supernovae
ever found, at redshift, z=1.703. Due to a lensing magnification factor of
4.3\pm0.3, we are able to measure a lightcurve of the supernova, as well as
spectroscopic features of the host galaxy with a precision comparable to what
will otherwise only be possible with future generation telescopes.Comment: 19 pages, 4 figures, 1 table, accepted to ApJ
From QFT to DCC
A quantum field theoretical model for the dynamics of the disoriented chiral
condensate is presented. A unified approach to relate the quantum field theory
directly to the formation, decay and signals of the DCC and its evolution is
taken. We use a background field analysis of the O(4) sigma model keeping
one-loop quantum corrections (quadratic order in the fluctuations). An
evolution of the quantum fluctuations in an external, expanding metric which
simulates the expansion of the plasma, is carried out. We examine, in detail,
the amplification of the low momentum pion modes with two competing effects,
the expansion rate of the plasma and the transition rate of the vacuum
configuration from a metastable state into a stable state.We show the effect of
DCC formation on the multiplicity distributions and the Bose-Einstein
correlations.Comment: 34 pages, 10 figure
GEANT4 : a simulation toolkit
Abstract Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics. PACS: 07.05.Tp; 13; 2
Child Sponsorship as Development Education in the Northern Classroom
This chapter explores the ethical dilemmas, and potential harm done when child sponsorship NGOs market sponsorship to children in school settings. Arguing that child sponsorship functions as a form of development education in the northern classroom, this chapter points to the potential for CS marketing strategies to infantalise and demean the poor, through a well-intentioned lens of paternalism. The chapter calls for greater commitment to global citizenship education in the crowded curriculum of secondary education and provides key questions (after Andreotti, 2012) for NGO marketing staff to consider in their public communication
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