4,701 research outputs found
Electromagnetic selection rules in the triangular alpha-cluster model of 12C
After recapitulating the procedure to find the bands and the states occurring
in the alpha-cluster model of C in which the clusters
are placed at the vertexes of an equilateral triangle, we obtain the selection
rules for electromagnetic transitions. While the alpha cluster structure leads
to the cancellation of E1 transitions, the approximations carried out in
deriving the roto-vibrational hamiltonian lead to the disappearance of M1
transitions. Furthermore, although in general the lowest active modes are E2,
E3, and M2, M3, , the cancellation of M2, M3 and M5
transitions between certain bands also occurs, as a result of the application
of group theoretical techniques drawn from molecular physics. These
implications can be very relevant for the spectroscopic analysis of
-ray spectra of C
The egalitarian effect of search engines
Search engines have become key media for our scientific, economic, and social
activities by enabling people to access information on the Web in spite of its
size and complexity. On the down side, search engines bias the traffic of users
according to their page-ranking strategies, and some have argued that they
create a vicious cycle that amplifies the dominance of established and already
popular sites. We show that, contrary to these prior claims and our own
intuition, the use of search engines actually has an egalitarian effect. We
reconcile theoretical arguments with empirical evidence showing that the
combination of retrieval by search engines and search behavior by users
mitigates the attraction of popular pages, directing more traffic toward less
popular sites, even in comparison to what would be expected from users randomly
surfing the Web.Comment: 9 pages, 8 figures, 2 appendices. The final version of this e-print
has been published on the Proc. Natl. Acad. Sci. USA 103(34), 12684-12689
(2006), http://www.pnas.org/cgi/content/abstract/103/34/1268
Electric and magnetic response to the continuum for A=7 isobars in a dicluster model
Mirror isobars Li and Be are investigated in a dicluster model. The
magnetic dipole moments and the magnetic dipole response to the continuum are
calculated in this framework. The magnetic contribution is found to be small
with respect to electric dipole and quadrupole excitations even at
astrophysical energies, at a variance with the case of deuteron. Energy
weighted molecular sum rules are evaluated and a formula for the molecular
magnetic dipole sum rule is found which matches the numerical calculations.
Cross-sections for photo-dissociation and radiative capture as well as the
S-factor for reactions of astrophysical significance are calculated with good
agreement with known experimental data.Comment: Accepted in EPJ
The electron screening puzzle and nuclear clustering
Accurate measurements of nuclear reactions of astrophysical interest within,
or close to, the Gamow peak, show evidence of an unexpected effect attributed
to the presence of atomic electrons in the target. The experiments need to
include an effective "screening" potential to explain the enhancement of the
cross sections at the lowest measurable energies. Despite various theoretical
studies conducted over the past 20 years and numerous experimental
measurements, a theory has not yet been found that can explain the cause of the
exceedingly high values of the screening potential needed to explain the data.
In this letter we show that instead of an atomic physics solution of the
"electron screening puzzle", the reason for the large screening potential
values is in fact due to clusterization effects in nuclear reactions, in
particular for reaction involving light nuclei.Comment: 6 pages, 2 figures, accepted for publication in Physics Letters
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
Pairing in the continuum: the quadrupole response of the Borromean nucleus 6He
The ground state and low-lying continuum states of 6He are found within a
shell model scheme, in a basis of two-particle states built out of continuum
p-states of the unbound 5He nucleus, using a simple pairing contact-delta
interaction. This accounts for the Borromean character of the bound ground
state, revealing its composition. We investigate the quadrupole response of the
system and we put our calculations into perspective with the latest
experimental results. The calculated quadrupole strength distribution
reproduces the narrow 2+ resonance, while a second wider peak is found at about
3.9 MeV above the g.s. energy.Comment: 5 pages, 5 figure
Electric multipole response of the halo nucleus He
The role of different continuum components in the weakly-bound nucleus He
is studied by coupling unbound spd-waves of He by means of simple pairing
contact-delta interaction. The results of our previous investigations in a
model space containing only p-waves, showed the collective nature of the ground
state and allowed the calculation of the electric quadrupole transitions. We
extend this simple model by including also sd-continuum neutron states and we
investigate the electric monopole, dipole and octupole response of the system
for transitions to the continuum, discussing the contribution of different
configurations.Comment: 22 pages, 10 figure
On the Consensus Threshold for the Opinion Dynamics of Krause-Hegselmann
In the consensus model of Krause-Hegselmann, opinions are real numbers
between 0 and 1 and two agents are compatible if the difference of their
opinions is smaller than the confidence bound parameter \epsilon. A randomly
chosen agent takes the average of the opinions of all neighbouring agents which
are compatible with it. We propose a conjecture, based on numerical evidence,
on the value of the consensus threshold \epsilon_c of this model. We claim that
\epsilon_c can take only two possible values, depending on the behaviour of the
average degree d of the graph representing the social relationships, when the
population N goes to infinity: if d diverges when N goes to infinity,
\epsilon_c equals the consensus threshold \epsilon_i ~ 0.2 on the complete
graph; if instead d stays finite when N goes to infinity, \epsilon_c=1/2 as for
the model of Deffuant et al.Comment: 15 pages, 7 figures, to appear in International Journal of Modern
Physics C 16, issue 2 (2005
The Sznajd Consensus Model with Continuous Opinions
In the consensus model of Sznajd, opinions are integers and a randomly chosen
pair of neighbouring agents with the same opinion forces all their neighbours
to share that opinion. We propose a simple extension of the model to continuous
opinions, based on the criterion of bounded confidence which is at the basis of
other popular consensus models. Here the opinion s is a real number between 0
and 1, and a parameter \epsilon is introduced such that two agents are
compatible if their opinions differ from each other by less than \epsilon. If
two neighbouring agents are compatible, they take the mean s_m of their
opinions and try to impose this value to their neighbours. We find that if all
neighbours take the average opinion s_m the system reaches complete consensus
for any value of the confidence bound \epsilon. We propose as well a weaker
prescription for the dynamics and discuss the corresponding results.Comment: 11 pages, 4 figures. To appear in International Journal of Modern
Physics
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