4,412 research outputs found
New analytic solutions of the collective Bohr hamiltonian for a beta-soft, gamma-soft axial rotor
New analytic solutions of the quadrupole collective Bohr hamiltonian are
proposed, exploiting an approximate separation of the beta and gamma variables
to describe gamma-soft prolate axial rotors. The model potential is a sum of
two terms: a beta-dependent term taken either with a Coulomb-like or a
Kratzer-like form, and a gamma-dependent term taken as an harmonic oscillator.
In particular it is possible to give a one parameter paradigm for a beta-soft,
gamma-soft axial rotor that can be applied, with a considerable agreement, to
the spectrum of 234U.Comment: (Dipartimento di Fisica ``G.Galilei'' and INFN, via Marzolo 8,
I-35131 Padova, Italy) 10 pages, 3 figure
Center clusters in the Yang-Mills vacuum
Properties of local Polyakov loops for SU(2) and SU(3) lattice gauge theory
at finite temperature are analyzed. We show that spatial clusters can be
identified where the local Polyakov loops have values close to the same center
element. For a suitable definition of these clusters the deconfinement
transition can be characterized by the onset of percolation in one of the
center sectors. The analysis is repeated for different resolution scales of the
lattice and we argue that the center clusters have a continuum limit.Comment: Table added. Final version to appear in JHE
Critical Droplets and Phase Transitions in Two Dimensions
In two space dimensions, the percolation point of the pure-site clusters of
the Ising model coincides with the critical point T_c of the thermal transition
and the percolation exponents belong to a special universality class. By
introducing a bond probability p_B<1, the corresponding site-bond clusters keep
on percolating at T_c and the exponents do not change, until
p_B=p_CK=1-exp(-2J/kT): for this special expression of the bond weight the
critical percolation exponents switch to the 2D Ising universality class. We
show here that the result is valid for a wide class of bidimensional models
with a continuous magnetization transition: there is a critical bond
probability p_c such that, for any p_B>=p_c, the onset of percolation of the
site-bond clusters coincides with the critical point of the thermal transition.
The percolation exponents are the same for p_c<p_B<=1 but, for p_B=p_c, they
suddenly change to the thermal exponents, so that the corresponding clusters
are critical droplets of the phase transition. Our result is based on Monte
Carlo simulations of various systems near criticality.Comment: Final version for publication, minor changes, figures adde
Stable and Efficient Structures for the Content Production and Consumption in Information Communities
Real-world information communities exhibit inherent structures that
characterize a system that is stable and efficient for content production and
consumption. In this paper, we study such structures through mathematical
modelling and analysis. We formulate a generic model of a community in which
each member decides how they allocate their time between content production and
consumption with the objective of maximizing their individual reward. We define
the community system as "stable and efficient" when a Nash equilibrium is
reached while the social welfare of the community is maximized. We investigate
the conditions for forming a stable and efficient community under two
variations of the model representing different internal relational structures
of the community. Our analysis results show that the structure with "a small
core of celebrity producers" is the optimally stable and efficient for a
community. These analysis results provide possible explanations to the
sociological observations such as "the Law of the Few" and also provide
insights into how to effectively build and maintain the structure of
information communities.Comment: 21 page
Enhanced excitation of Giant Pairing Vibrations in heavy-ion reactions induced by weakly-bound projectiles
The use of radioactive ion beams is shown to offer the possibility to study
collective pairing states at high excitation energy, which are not usually
accessible with stable projectiles because of large energy mismatch. In the
case of two-neutron stripping reactions induced by 6He, we predict a population
of the Giant Pairing Vibration in 208Pb or 116Sn with cross sections of the
order of a millibarn, dominating over the mismatched transition to the ground
state.Comment: 6 pages, 4 figure
Commentary: The case for caution in predicting scientists’ future impact
We stress-test the career predictability model proposed by Acuna et al.
[Nature 489, 201-202 2012] by applying their model to a longitudinal career
data set of 100 Assistant professors in physics, two from each of the top 50
physics departments in the US. The Acuna model claims to predict h(t+\Delta t),
a scientist's h-index \Delta t years into the future, using a linear
combination of 5 cumulative career measures taken at career age t. Here we
investigate how the "predictability" depends on the aggregation of career data
across multiple age cohorts. We confirm that the Acuna model does a respectable
job of predicting h(t+\Delta t) up to roughly 6 years into the future when
aggregating all age cohorts together. However, when calculated using subsets of
specific age cohorts (e.g. using data for only t=3), we find that the model's
predictive power significantly decreases, especially when applied to early
career years. For young careers, the model does a much worse job of predicting
future impact, and hence, exposes a serious limitation. The limitation is
particularly concerning as early career decisions make up a significant
portion, if not the majority, of cases where quantitative approaches are likely
to be applied.Comment: 2 pages, 1 figur
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