12,933 research outputs found
CP4 miracle: shaping Yukawa sector with CP symmetry of order four
We explore the phenomenology of a unique three-Higgs-doublet model based on
the single CP symmetry of order 4 (CP4) without any accidental symmetries. The
CP4 symmetry is imposed on the scalar potential and Yukawa interactions,
strongly shaping both sectors of the model and leading to a very characteristic
phenomenology. The scalar sector is analyzed in detail, and in the Yukawa
sector we list all possible CP4-symmetric structures which do not run into
immediate conflict with experiment, namely, do not lead to massless or
mass-degenerate quarks nor to insufficient mixing or CP-violation in the CKM
matrix. We show that the parameter space of the model, although very
constrained by CP4, is large enough to comply with the electroweak precision
data and the LHC results for the 125 GeV Higgs boson phenomenology, as well as
to perfectly reproduce all fermion masses, mixing, and CP violation. Despite
the presence of flavor changing neutral currents mediated by heavy Higgs
scalars, we find through a parameter space scan many points which accurately
reproduce the kaon CP-violating parameter as well as oscillation
parameters in K and mesons. Thus, CP4 offers a novel minimalistic
framework for building models with very few assumptions, sufficient predictive
power, and rich phenomenology yet to be explored.Comment: 39 pages, 8 figures, 1 table; v2: expanded discussion, extra
references, matches published versio
Cosmic magnetic fields and dark energy in extended electromagnetism
We discuss an extended version of electromagnetism in which the usual gauge
fixing term is promoted into a physical contribution that introduces a new
scalar state in the theory. This new state can be generated from vacuum quantum
fluctuations during an inflationary era and, on super-Hubble scales, gives rise
to an effective cosmological constant. The value of such a cosmological
constant coincides with the one inferred from observations as long as inflation
took place at the electroweak scale. On the other hand, the new state also
generates an effective electric charge density on sub-Hubble scales that
produces both vorticity and magnetic fields with coherent lengths as large as
the present Hubble horizon.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2010, Granada, Spain, 6-10 September 201
Directed percolation with incubation times
We introduce a model for directed percolation with a long-range temporal
diffusion, while the spatial diffusion is kept short ranged. In an
interpretation of directed percolation as an epidemic process, this
non-Markovian modification can be understood as incubation times, which are
distributed accordingly to a Levy distribution. We argue that the best approach
to find the effective action for this problem is through a generalization of
the Cardy-Sugar method, adding the non-Markovian features into the geometrical
properties of the lattice. We formulate a field theory for this problem and
renormalize it up to one loop in a perturbative expansion. We solve the various
technical difficulties that the integrations possess by means of an asymptotic
analysis of the divergences. We show the absence of field renormalization at
one-loop order, and we argue that this would be the case to all orders in
perturbation theory. Consequently, in addition to the characteristic scaling
relations of directed percolation, we find a scaling relation valid for the
critical exponents of this theory. In this universality class, the critical
exponents vary continuously with the Levy parameter.Comment: 17 pages, 7 figures. v.2: minor correction
An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample
In this paper we present optical and near-infrared finding charts, accurate
astrometry (~1") and single-epoch near-infrared photometry for 371 IRAS
sources, 96% of those included in the so-called Arecibo sample of OH/IR stars
(Eder et al. 1988; Lewis et al. 1990a; Chengalur et al. 1993). The main
photometric properties of the stars in the sample are presented and discussed
as well as the problems found during the process of identification of the
optical/near-infrared counterparts. In addition, we also identify suitable
reference stars in each field to be used for differential photometry purposes
in the future.
We find that 39% of the sources (144 in number) have no optical counterpart,
8 of them being invisible even at near infrared wavelengths. The relative
distribution of sources with and without optical counterpart in the IRAS
two-colour diagram and their characteristic near infrared colours are
interpreted as the consequence of the increasing thickness of their
circumstellar shells. Among the objects not detected at near infrared
wavelengths four non-variable sources are proposed to be heavily obscured
post-AGB stars which have just very recently left the AGB. Eight additional
objects with unusually bright and/or blue near-infrared colours are identified
as candidate post-AGB stars and/or proto-planetary nebulae.Comment: 28 pages, 9 figures, for associated finding charts see:
http://www.edpsciences.org/articles/aa/full/2005/08/aa1709/FINDING_CHARTS/are
cibo_index.htm
Contour dynamics model for electric discharges
A contour dynamics model for electrical discharges is obtained and analyzed.
The model is deduced as the asymptotic limit of the minimal streamer model for
the propagation of electric discharges, in the limit of small electron
diffusion. The dispersion relation for a non planar 2-D discharge is
calculated. The development and propagation of finger-like patterns are studied
and their main features quantified.Comment: 4 pages, 2 fi
Tunable gauge potential for neutral and spinless particles in driven lattices
We present a universal method to create a tunable, artificial vector gauge
potential for neutral particles trapped in an optical lattice. The necessary
Peierls phase of the hopping parameters between neighboring lattice sites is
generated by applying a suitable periodic inertial force such that the method
does not rely on any internal structure of the particles. We experimentally
demonstrate the realization of such artificial potentials, which generate
ground state superfluids at arbitrary non-zero quasi-momentum. We furthermore
investigate possible implementations of this scheme to create tuneable magnetic
fluxes, going towards model systems for strong-field physics
Sharp values for the constants in the polynomial Bohnenblust-Hille inequality
In this paper we prove that the complex polynomial Bohnenblust-Hille constant
for -homogeneous polynomials in is exactly
. We also give the exact value of the real polynomial
Bohnenblust-Hille constant for -homogeneous polynomials in .
Finally, we provide lower estimates for the real polynomial Bohnenblust-Hille
constant for polynomials in of higher degrees.Comment: 16 page
Excited electronic states from a variational approach based on symmetry-projected Hartree--Fock configurations
Recent work from our research group has demonstrated that symmetry-projected
Hartree--Fock (HF) methods provide a compact representation of molecular ground
state wavefunctions based on a superposition of non-orthogonal Slater
determinants. The symmetry-projected ansatz can account for static correlations
in a computationally efficient way. Here we present a variational extension of
this methodology applicable to excited states of the same symmetry as the
ground state. Benchmark calculations on the C dimer with a modest basis
set, which allows comparison with full configuration interaction results,
indicate that this extension provides a high quality description of the
low-lying spectrum for the entire dissociation profile. We apply the same
methodology to obtain the full low-lying vertical excitation spectrum of
formaldehyde, in good agreement with available theoretical and experimental
data, as well as to a challenging model insertion pathway for BeH.
The variational excited state methodology developed in this work has two
remarkable traits: it is fully black-box and will be applicable to fairly large
systems thanks to its mean-field computational cost
Maxwellian Neutron Spectrum generation and Stellar Cross-Section measurements: measurement of the 197Au(n,Îł) MACS
Maxwellian-averaged cross-sections (MACS) are needed as an input for the models
of stellar s- and r-processes nucleosynthesis. MACS can be obtained from activation
measurements, irradiating a sample with the neutron field generated by the 7Li(p,n)7Be reaction
at 1912 keV proton energy. At this energy, the neutron energy spectrum is close (R2≤0.9) to a
Maxwellian one of kT=25 keV. However, it was shown that shaping the energy of the incident
proton beam is possible to generate a neutron field with an energy spectrum much closer to a
real Maxwellian (R2>0.995), therefore avoiding or minimizing corrections in the MACS
calculation. We show a preliminary result of an experiment performed at JRC-IRMM (Geel) to
confirm our method. We have measured the MACS30 (kT=30 keV) of the 197Au(n,Îł) reaction,
at CNA (Seville). We obtained 612 mb, in good agreement with the latest measurements
- …