25,829 research outputs found
Active nematic gels as active relaxing solids
I put forward a continuum theory for active nematic gels, defined as fluids
or suspensions of orientable rodlike objects endowed with active dynamics, that
is based on symmetry arguments and compatibility with thermodynamics. The
starting point is our recent theory that models (passive) nematic liquid
crystals as relaxing nematic elastomers. The interplay between viscoelastic
response and active dynamics of the microscopic constituents is naturally taken
into account. By contrast with standard theories, activity is not introduced as
an additional term of the stress tensor, but it is added as an external
remodeling force that competes with the passive relaxation dynamics and drags
the system out of equilibrium. In a simple one-dimensional channel geometry, we
show that the interaction between non-uniform nematic order and activity
results in either a spontaneous flow of particles or a self-organization into
sub-channels flowing in opposite directions
Viscoelastic nematodynamics
Nematic liquid crystals exhibit both crystal-like and fluid-like features. In
particular, the propagation of an acoustic wave shows an unexpected occurrence
of some of the solid-like features at the hydrodynamic level, namely, the
frequency-dependent anisotropy of sound velocity and acoustic attenuation. The
non-Newtonian behavior of nematics also emerges from the frequency-dependent
viscosity coefficients. To account for these phenomena, we put forward a
viscoelastic model of nematic liquid crystals, and we extend our previous
theory to fully include the combined effects of compressibility, anisotropic
elasticity and dynamic relaxation, at any shear rate. The low-frequency limit
agrees with the compressible Ericksen-Leslie theory, while at intermediate
frequencies the model correctly captures the relaxation mechanisms underlying
finite shear and bulk elastic moduli. We show that there are only four
relaxation times allowed by the uniaxial symmetry.Comment: 9 pages, 3 figure
Perturbative renormalization of the first moment of structure functions for domain-wall QCD
Using the domain-wall formulation of lattice fermions, we have computed the
one-loop renormalization factors of one-link operators which measure the first
nontrivial moment of the unpolarized, polarized and transversity structure
functions, in the flavor nonsinglet sector. The knowledge of these factors is
necessary in order to extract physical numbers from domain-wall Monte Carlo
simulations of parton distributions.
We have automated the perturbative calculations by developing suitable FORM
codes. The results show that in many instances the total renormalization
factors are almost equal to one, and that hence the corresponding operators
are, for the appropriate values of the Dirac mass and the coupling ,
practically unrenormalized.Comment: REVTeX 4, 12 pages, 1 figure; changes in the final paragraphs of
sections 1 and 5 concerning comparisons with previous results, plus
correction of minor typos; final version, accepted for publication in
Physical Review
Constraining the Z' Mass in 331 Models using Direct Dark Matter Detection
We investigate a so-called 331 extension of the Standard Model gauge sector
which accommodates neutrino masses and where the lightest of the new neutral
fermions in the theory is a viable particle dark matter candidate. In this
model, processes mediated by the additional gauge boson set both
the dark matter relic abundance and the scattering cross section off of nuclei.
We calculate with unprecedented accuracy the dark matter relic density,
including the important effect of coannihilation across the heavy fermion
sector, and show that indeed the candidate particle has the potential of having
the observed dark matter density. We find that the recent LUX results put very
stringent bounds on the mass of the extra gauge boson, ~TeV, independently of the dark matter mass. We also comment on regime where
our bounds on the mass may apply to generic 331-like models, and
on implications for LHC phenomenology.Comment: 11 pages, 7 figures. Accepted for publicatio
Large deviation principles for the Ewens-Pitman sampling model
Let be the number of blocks with frequency in the exchangeable
random partition induced by a sample of size from the Ewens-Pitman sampling
model. We show that, as tends to infinity, satisfies a
large deviation principle and we characterize the corresponding rate function.
A conditional counterpart of this large deviation principle is also presented.
Specifically, given an initial sample of size from the Ewens-Pitman
sampling model, we consider an additional sample of size . For any fixed
and as tends to infinity, we establish a large deviation principle for the
conditional number of blocks with frequency in the enlarged sample, given
the initial sample. Interestingly, the conditional and unconditional large
deviation principles coincide, namely there is no long lasting impact of the
given initial sample. Potential applications of our results are discussed in
the context of Bayesian nonparametric inference for discovery probabilities.Comment: 30 pages, 2 figure
An S4 model for quarks and leptons with maximal atmospheric angle
We consider a model for quark and lepton masses and mixings based on S4
flavor symmetry. The model contains six Higgs doublets where three of them give
mass to the leptons and the other three gives mass to the quarks. Charged
fermion and quark masses arise from renormalizable interactions while neutrino
Majorana masses are generated through effective dimension five Weinberg
operator. From the study of the minimization of the scalar potential we found a
residual mu-tau symmetry in the neutrino sector predicting zero reactor angle
and maximal atmospheric angle and for the quark sector we found a four-zero
texture. We give a fit of the mass hierarchies and mixing angles in the quark
sector.Comment: some misprinting corrected, one reference and one commment added,
version to be published on Phys. Rev.
- …