4,234 research outputs found
Power Spectra in a Zero-Range Process on a Ring: Total Occupation Number in a Segment
We study the dynamics of density fluctuations in the steady state of a
non-equilibrium system, the Zero-Range Process on a ring lattice. Measuring the
time series of the total number of particles in a \emph{segment} of the
lattice, we find remarkable structures in the associated power spectra, namely,
two distinct components of damped-oscillations. The essential origin of both
components is shown in a simple pedagogical model. Using a more sophisticated
theory, with an effective drift-diffusion equation governing the stochastic
evolution of the local particle density, we provide reasonably good fits to the
simulation results. The effects of altering various parameters are explored in
detail. Avenues for improving this theory and deeper understanding of the role
of particle interactions are indicated.Comment: 21 pages, 15 figure
Critical phase in non-conserving zero-range processes and equilibrium networks
Zero-range processes, in which particles hop between sites on a lattice, are
closely related to equilibrium networks, in which rewiring of links take place.
Both systems exhibit a condensation transition for appropriate choices of the
dynamical rules. The transition results in a macroscopically occupied site for
zero-range processes and a macroscopically connected node for networks.
Criticality, characterized by a scale-free distribution, is obtained only at
the transition point. This is in contrast with the widespread scale-free
real-life networks. Here we propose a generalization of these models whereby
criticality is obtained throughout an entire phase, and the scale-free
distribution does not depend on any fine-tuned parameter.Comment: 4 pages, 4 figure
An exactly solvable dissipative transport model
We introduce a class of one-dimensional lattice models in which a quantity,
that may be thought of as an energy, is either transported from one site to a
neighbouring one, or locally dissipated. Transport is controlled by a
continuous bias parameter q, which allows us to study symmetric as well as
asymmetric cases. We derive sufficient conditions for the factorization of the
N-body stationary distribution and give an explicit solution for the latter,
before briefly discussing physically relevant situations.Comment: 7 pages, 1 figure, submitted to J. Phys.
Dark-ages Reionization & Galaxy Formation Simulation VIII. Suppressed growth of dark matter halos during the Epoch of Reionization
We investigate how the hydrostatic suppression of baryonic accretion affects
the growth rate of dark matter halos during the Epoch of Reionization. By
comparing halo properties in a simplistic hydrodynamic simulation in which gas
only cools adiabatically, with its collisionless equivalent, we find that halo
growth is slowed as hydrostatic forces prevent gas from collapsing. In our
simulations, at the high redshifts relevant for reionization (between
and ), halos that host dwarf galaxies () can be reduced by up to a factor of 2 in mass due to the
hydrostatic pressure of baryons. Consequently, the inclusion of baryonic
effects reduces the amplitude of the low mass tail of the halo mass function by
factors of 2 to 4. In addition, we find that the fraction of baryons in dark
matter halos hosting dwarf galaxies at high redshift never exceeds
of the cosmic baryon fraction. When implementing baryonic processes, including
cooling, star formation, supernova feedback and reionization, the suppression
effects become more significant with further reductions of to
60\%. Although convergence tests suggest that the suppression may become weaker
in higher resolution simulations, this suppressed growth will be important for
semi-analytic models of galaxy formation, in which the halo mass inherited from
an underlying N-body simulation directly determines galaxy properties. Based on
the adiabatic simulation, we provide tables to account for these effects in
N-body simulations, and present a modification of the halo mass function along
with explanatory analytic calculations.Comment: 17 pages, 11 figures; Updated to match the published version. Two
changes in Figures 1 and 3 in order to 1) correct bin sizes of the 10^8 and
10^8.5 Msol bins for NOSN_NOZCOOL_NoRe (was 0.5, should be 0.25); 2) include
stellar mass in baryon fraction (was missed in Fig. 3). Quantitative
description of Fig. 3 changed slightly in Section 2.2. All other results and
conclusions remain unchange
Dark-ages reionization and galaxy formation simulation--VII. The sizes of high-redshift galaxies
We investigate high-redshift galaxy sizes using a semi-analytic model
constructed for the Dark-ages Reionization And Galaxy-formation Observables
from Numerical Simulation project. Our fiducial model, including strong
feedback from supernovae and photoionization background, accurately reproduces
the evolution of the stellar mass function and UV luminosity function. Using
this model, we study the size--luminosity relation of galaxies and find that
the effective radius scales with UV luminosity as at --. We show that recently discovered very luminous
galaxies at (Bowler et al. 2016) and (Oesch et al. 2016)
lie on our predicted size--luminosity relations. We find that a significant
fraction of galaxies at will not be resolved by JWST, but GMT will have
the ability to resolve all galaxies in haloes above the atomic cooling limit.
We show that our fiducial model successfully reproduces the redshift evolution
of average galaxy sizes at . We also explore galaxy sizes in models
without supernova feedback. The no-supernova feedback models produce galaxy
sizes that are smaller than observations. We therefore confirm that supernova
feedback plays an important role in determining the size--luminosity relation
of galaxies and its redshift evolution during reionization.Comment: 10 pages, 4 figures, Accepted for publication in MNRA
Kink stability, propagation, and length scale competition in the periodically modulated sine-Gordon equation
We have examined the dynamical behavior of the kink solutions of the
one-dimensional sine-Gordon equation in the presence of a spatially periodic
parametric perturbation. Our study clarifies and extends the currently
available knowledge on this and related nonlinear problems in four directions.
First, we present the results of a numerical simulation program which are not
compatible with the existence of a radiative threshold, predicted by earlier
calculations. Second, we carry out a perturbative calculation which helps
interpret those previous predictions, enabling us to understand in depth our
numerical results. Third, we apply the collective coordinate formalism to this
system and demonstrate numerically that it accurately reproduces the observed
kink dynamics. Fourth, we report on a novel occurrence of length scale
competition in this system and show how it can be understood by means of linear
stability analysis. Finally, we conclude by summarizing the general physical
framework that arises from our study.Comment: 19 pages, REVTeX 3.0, 24 figures available from A S o
Dark-ages reionization & galaxy formation simulation IV: UV luminosity functions of high-redshift galaxies
In this paper we present calculations of the UV luminosity function from the
Dark-ages Reionization And Galaxy-formation Observables from Numerical
Simulations (DRAGONS) project, which combines N-body, semi-analytic and
semi-numerical modelling designed to study galaxy formation during the Epoch of
Reionization. Using galaxy formation physics including supernova feedback, the
model naturally reproduces the UV LFs for high-redshift star-forming galaxies
from through to . We investigate the luminosity--star
formation rate (SFR) relation, finding that variable SFR histories of galaxies
result in a scatter around the median relation of -- dex depending on
UV luminosity. We find close agreement between the model and observationally
derived SFR functions. We use our calculated luminosities to investigate the
luminosity function below current detection limits, and the ionizing photon
budget for reionization. We predict that the slope of the UV LF remains steep
below current detection limits and becomes flat at
. We find that () per cent of the total
UV flux at () has been detected above an observational limit of
, and that galaxies fainter than
are the main source of ionizing photons for
reionization. We investigate the luminosity--stellar mass relation, and find a
correlation for galaxies with that has the form
, in good agreement with observations, but
which flattens for fainter galaxies. We determine the luminosity--halo mass
relation to be , finding that
galaxies with reside in host dark matter haloes of
at , and that this mass decreases
towards high redshift.Comment: 17 pages, 14 figures, Accepted for publication in MNRA
Leptonic universality breaking in Upsilon decays as a probe of new physics
In this work we examine the possible existence of new physics beyond the
standard model which could modify the branching fractions of the leptonic
(mainly tauonic) decays of bottomonium vector resonances below the
threshold. The decay width is factorized as the product of two pieces: a) the
probability of an intermediate pseudoscalar color-singlet state
(coupling to the dominant Fock state of the Upsilon via a magnetic dipole
transition) and a soft (undetected) photon; b) the annihilation width of the
pair into two leptons, mediated by a non-standard CP-odd Higgs boson
of mass about 10 GeV, introducing a quadratic dependence on the lepton mass in
the partial width. The process would be unwittingly ascribed to the
leptonic channel thereby (slightly) breaking lepton universality. A possible
mixing of the pseudoscalar Higgs and bottomonium resonances is also considered.
Finally, several experimental signatures to check out the validity of the
conjecture are discussed.Comment: LaTeX, 22 pages, 2 EPS figure
Chaos and unpredictability in evolutionary dynamics in discrete time
A discrete-time version of the replicator equation for two-strategy games is
studied. The stationary properties differ from that of continuous time for
sufficiently large values of the parameters, where periodic and chaotic
behavior replace the usual fixed-point population solutions. We observe the
familiar period-doubling and chaotic-band-splitting attractor cascades of
unimodal maps but in some cases more elaborate variations appear due to
bimodality. Also unphysical stationary solutions have unusual physical
implications, such as uncertainty of final population caused by sensitivity to
initial conditions and fractality of attractor preimage manifolds.Comment: 4 pages, 4 figure
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