56 research outputs found
Boltzmann hierarchy for interacting neutrinos I: formalism
Starting from the collisional Boltzmann equation, we derive for the first
time and from first principles the Boltzmann hierarchy for neutrinos including
interactions with a scalar particle. Such interactions appear, for example, in
majoron-like models of neutrino mass generation. We study two limits of the
scalar mass: (i) An extremely massive scalar whose only role is to mediate an
effective 4-fermion neutrino-neutrino interaction, and (ii) a massless scalar
that can be produced in abundance and thus demands its own Boltzmann hierarchy.
In contrast to, e.g., the first-order Boltzmann hierarchy for
Thomson-scattering photons, our interacting neutrino/scalar Boltzmann
hierarchies contain additional momentum-dependent collision terms arising from
a non-negligible energy transfer in the neutrino-neutrino and neutrino-scalar
interactions. This necessitates that we track each momentum mode of the phase
space distributions individually, even if the particles were massless.
Comparing our hierarchy with the commonly used -parameterisation, we find no formal correspondence between the two
approaches, which raises the question of whether the latter parameterisation
even has an interpretation in terms of particle scattering. Lastly, although we
have invoked majoron-like models as a motivation for our study, our treatment
is in fact generally applicable to all scenarios in which the neutrino and/or
other ultrarelativistic fermions interact with scalar particles.Comment: 44 pages, 14 figures; included scalar Boltzmann hierarchy in the
massless case and plots of integral kernels; accepted by JCA
Transitions of tethered polymer chains: A simulation study with the bond fluctuation lattice model
A polymer chain tethered to a surface may be compact or extended, adsorbed or
desorbed, depending on interactions with the surface and the surrounding
solvent. This leads to a rich phase diagram with a variety of transitions. To
investigate these transitions we have performed Monte Carlo simulations of a
bond-fluctuation model with Wang-Landau and umbrella sampling algorithms in a
two-dimensional state space. The simulations' density of states results have
been evaluated for interaction parameters spanning the range from good to poor
solvent conditions and from repulsive to strongly attractive surfaces. In this
work, we describe the simulation method and present results for the overall
phase behavior and for some of the transitions. For adsorption in good solvent,
we compare with Metropolis Monte Carlo data for the same model and find good
agreement between the results. For the collapse transition, which occurs when
the solvent quality changes from good to poor, we consider two situations
corresponding to three-dimensional (hard surface) and two-dimensional (very
attractive surface) chain conformations, respectively. For the hard surface, we
compare tethered chains with free chains and find very similar behavior for
both types of chains. For the very attractive surface, we find the
two-dimensional chain collapse to be a two-step transition with the same
sequence of transitions that is observed for three-dimensional chains: a
coil-globule transition that changes the overall chain size is followed by a
local rearrangement of chain segments.Comment: 17 pages, 12 figures, to appear in J. Chem. Phy
On the fluctuations of jamming coverage upon random sequential adsorption on homogeneous and heterogeneous media
The fluctuations of the jamming coverage upon Random Sequential Adsorption
(RSA) are studied using both analytical and numerical techniques. Our main
result shows that these fluctuations (characterized by )
decay with the lattice size according to the power-law . The exponent depends on the dimensionality of
the substrate and the fractal dimension of the set where the RSA process
actually takes place () according to .This
theoretical result is confirmed by means of extensive numerical simulations
applied to the RSA of dimers on homogeneous and stochastic fractal substrates.
Furthermore, our predictions are in excellent agreement with different previous
numerical results.
It is also shown that, studying correlated stochastic processes, one can
define various fluctuating quantities designed to capture either the underlying
physics of individual processes or that of the whole system. So, subtle
differences in the definitions may lead to dramatically different physical
interpretations of the results. Here, this statement is demonstrated for the
case of RSA of dimers on binary alloys.Comment: 20 pages, 8 figure
Interacting neutrinos in cosmology: exact description and constraints
We consider the impact of neutrino self-interactions described by an
effective four-fermion coupling on cosmological observations. Implementing the
exact Boltzmann hierarchy for interacting neutrinos first derived in
[arxiv:1409.1577] into the Boltzmann solver CLASS, we perform a detailed
numerical analysis of the effects of the interaction on the cosmic microwave
background (CMB) anisotropies, and compare our results with known
approximations in the literature. While we find good agreement between our
exact approach and the relaxation time approximation used in some recent
studies, the popular -parameterisation fails to reproduce the correct scale dependence of
the CMB temperature power spectrum. We then proceed to derive constraints on
the effective coupling constant using currently available
cosmological data via an MCMC analysis. Interestingly, our results reveal a
bimodal posterior distribution, where one mode represents the standard
CDM limit with , and the other
a scenario in which neutrinos self-interact with an effective coupling constant
.Comment: 25 pages, 11 figures; accepted for publication in JCA
Conformational dynamics and internal friction in homopolymer globules: equilibrium vs. non-equilibrium simulations
We study the conformational dynamics within homopolymer globules by solvent-implicit Brownian dynamics simulations. A strong dependence of the internal chain dynamics on the Lennard-Jones cohesion strength ε and the globule size N [subscript G] is observed. We find two distinct dynamical regimes: a liquid-like regime (for ε ε[subscript s] with slow internal dynamics. The cohesion strength ε[subscript s] of this freezing transition depends on N G . Equilibrium simulations, where we investigate the diffusional chain dynamics within the globule, are compared with non-equilibrium simulations, where we unfold the globule by pulling the chain ends with prescribed velocity (encompassing low enough velocities so that the linear-response, viscous regime is reached). From both simulation protocols we derive the internal viscosity within the globule. In the liquid-like regime the internal friction increases continuously with ε and scales extensive in N [subscript G] . This suggests an internal friction scenario where the entire chain (or an extensive fraction thereof) takes part in conformational reorganization of the globular structure.American Society for Engineering Education. National Defense Science and Engineering Graduate Fellowshi
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