1,569 research outputs found
Modeling user navigation
This paper proposes the use of neural networks as a tool for studying navigation within virtual worlds. Results indicate that the network learned to predict the next step for a given trajectory. The analysis of hidden layer shows that the network was able to differentiate between two groups of users identified on the basis of their performance for a spatial task. Time series analysis of hidden node activation values and input vectors suggested that certain hidden units become specialised for place and heading, respectively. The benefits of this approach and the possibility of extending the methodology to the study of navigation in Human Computer Interaction applications are discussed
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The significance of insecure attachment and disorganization in the development of children's externalizing behavior: A meta-analytic study
This study addresses the extent to which insecure and disorganized attachments increase risk for externalizing problems using meta-analysis. From 69 samples (N = 5,947), the association between insecurity and externalizing problems was significant, d = 0.31 (95% CI: 0.23, 0.40). Larger effects were found for boys (d = 0.35), clinical samples (d = 0.49), and from observation-based outcome assessments (d = 0.58). Larger effects were found for attachment assessments other than the Strange Situation. Overall, disorganized children appeared at elevated risk (d = 0.34, 95% CI: 0.18, 0.50), with weaker effects for avoidance (d = 0.12, 95% CI: 0.03, 0.21) and resistance (d = 0.11, 95% CI: â0.04, 0.26). The results are discussed in terms of the potential significance of attachment for mental health
Shadowing of gluons in perturbative QCD: A comparison of different models
We investigate the different perturbative QCD-based models for nuclear
shadowing of gluons. We show that in the kinematic region appropriate to RHIC
experiment, all models give similar estimates for the magnitude of gluon
shadowing. At scales relevant to LHC, there is a sizable difference between
predictions of the different models.Comment: 11 pages, 4 figure
Dilepton production from the Color Glass Condensate
We consider dilepton production in high energy proton-nucleus (and very
forward nucleus-nucleus) collisions. Treating the target nucleus as a Color
Glass Condensate and describing the projectile proton (nucleus) as a collection
of quarks and gluons as in the parton model, we calculate the differential
cross section for dilepton production in quark-nucleus scattering and show that
it is very sensitive to the saturation scale characterizing the target nucleus.Comment: 9 pages LaTeX document, 1 postscript figur
Thermalized Displaced and Squeezed Number States in Coordinate Representation
Within the framework of thermofield dynamics, the wavefunctions of the
thermalized displaced number and squeezed number states are given in the
coordinate representation. Furthermore, the time evolution of these
wavefunctions is considered by introducing a thermal coordinate representation,
and we also calculate the corresponding probability densities, average values
and variances of position coordinate, which are consistent with results in the
literature.Comment: 12 pages, no figures, Revtex. v3: substantially revise
Chiral dynamics and the growth of the nucleon's gluonic transverse size at small x
We study the distribution of gluons in transverse space in the nucleon at
moderately small x (~10^{-2}). At large transverse distances (impact
parameters) the gluon density is generated by the 'pion cloud' of the nucleon,
and can be calculated in terms of the gluon density in the pion. We investigate
the large-distance behavior in two different approaches to chiral dynamics: i)
phenomenological soft-pion exchange, ii) the large-N_c picture of the nucleon
as a classical soliton of the pion field, which corresponds to degenerate N and
Delta states. The large-distance contributions from the 'pion cloud' cause a
\~20% increase in the overall transverse size of the nucleon if x drops
significantly below M_pi/M_N. This is in qualitative agreement with the
observed increase of the slope of the t-dependence of the J/psi photoproduction
cross section at HERA compared to fixed-target energies. We argue that the glue
in the pion cloud could be probed directly in hard electroproduction processes
accompanied by 'pion knockout', gamma^* + N -> gamma (or rho, J/psi) + pi + N',
where the transverse momentum of the emitted pion is large while that of the
outgoing nucleon is restricted to values of order M_pi.Comment: 20 pages, revtex4, 10 eps figure
Saturation and parton level Cronin effect: enhancement vs suppression of gluon production in p-A and A-A collisions
We note that the phenomenon of perturbative saturation leads to transverse
momentum broadening in the spectrum of partons produced in hadronic collisions.
This broadening has a simple interpretation as parton level Cronin effect for
systems in which saturation is generated by the "tree level" Glauber-Mueller
mechanism. For systems where the broadening results form the nonlinear QCD
evolution to high energy, the presence or absence of Cronin effect depends
crucially on the quantitative behavior of the gluon distribution functions at
transverse momenta kt outside the so called scaling window. We discuss the
relation of this phenomenon to the recent analysis by Kharzeev-Levin-McLerran
of the momentum and centrality dependence of particle production in
nucleus-nucleus collisions at RHIC.Comment: 22 pages LaTex, 7 eps-figures, discussion of evolved gluon
distribution revised significantl
Simulating non-Markovian stochastic processes
We present a simple and general framework to simulate statistically correct
realizations of a system of non-Markovian discrete stochastic processes. We
give the exact analytical solution and a practical an efficient algorithm alike
the Gillespie algorithm for Markovian processes, with the difference that now
the occurrence rates of the events depend on the time elapsed since the event
last took place. We use our non-Markovian generalized Gillespie stochastic
simulation methodology to investigate the effects of non-exponential
inter-event time distributions in the susceptible-infected-susceptible model of
epidemic spreading. Strikingly, our results unveil the drastic effects that
very subtle differences in the modeling of non-Markovian processes have on the
global behavior of complex systems, with important implications for their
understanding and prediction. We also assess our generalized Gillespie
algorithm on a system of biochemical reactions with time delays. As compared to
other existing methods, we find that the generalized Gillespie algorithm is the
most general as it can be implemented very easily in cases, like for delays
coupled to the evolution of the system, where other algorithms do not work or
need adapted versions, less efficient in computational terms.Comment: Improvement of the algorithm, new results, and a major reorganization
of the paper thanks to our coauthors L. Lafuerza and R. Tora
The initial energy density of gluons produced in very high energy nuclear collisions
In very high energy nuclear collisions, the initial energy of produced gluons
per unit area per unit rapidity, , is equal to , where is proportional to the gluon density per unit
area of the colliding nuclei. For an SU(2) gauge theory, we perform a
non--perturbative numerical computation of the function . It
decreases rapidly for small but varies only by %, from
to , for a wide range 35.36--296.98 in , including the range relevant for collisions at RHIC and LHC. Extrapolating
to SU(3), we estimate the initial energy per unit rapidity for Au-Au collisions
in the central region at RHIC and LHC.Comment: 11 pages, Latex, 3 figures; revised version-includes additional
numerical data; reference adde
Eikonal Evolution and Gluon Radiation
We give a simple quantum mechanical formulation of the eikonal propagation
approximation, which has been heavily used in recent years in problems
involving hadronic interactions at high energy. This provides a unified
framework for several approaches existing in the literature. We illustrate this
scheme by calculating the total, elastic, inelastic and diffractive DIS cross
sections, as well as gluon production in high energy hadronic collisions. From
the q-qbar-g-component of the DIS cross sections, we straightforwardly derive
low x evolution equations for inelastic and diffractive DIS distribution
functions. In all calculations, we provide all order 1/N corrections to the
results existing in the literature.Comment: 40 pages, LaTeX, 3 eps-figures, typos corrected, to be published in
PR
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