1,764 research outputs found
Irrational behavior of algebraic discrete valuations
We study algebraic discrete valuations dominating normal local domains of
dimension two. We construct a family of examples to show that the
Hilbert-Samuel function of the associated graded ring of the valuation can fail
to be asymptotically of the form: quasi-polynomial plus a bounded function. We
also show that the associated multiplicity can be irrational, or even
transcendental
Dirty Weyl semimetals: Stability, phase transition and quantum criticality
We study the stability of three-dimensional incompressible Weyl semimetals in
the presence of random quenched charge impurities. Combining numerical analysis
and scaling theory we show that in the presence of sufficiently weak randomness
(i) Weyl semimetal remains stable, while (ii) double-Weyl semimetal gives rise
to compressible diffusive metal where the mean density of states at zero energy
is finite. At stronger disorder, Weyl semimetal undergoes a quantum phase
transition and enter into a metallic phase. Mean density of states at zero
energy serves as the order parameter and displays single-parameter scaling
across such disorder driven quantum phase transition. We numerically determine
various exponents at the critical point, which appear to be insensitive to the
number of Weyl pairs. We also extract the extent of the quantum critical regime
in disordered Weyl semimetal and the phase diagram of dirty double Weyl
semimetal at finite energies.Comment: 5 pages and 5 figures (Supplementary: 6 pages and 5 figure):
Published version, added discussion, new results and reference
Relic density and PAMELA events in a heavy wino dark matter model with Sommerfeld effect
In a wino LSP scenario the annihilation cross section of winos
gravitationally bound in galaxies can be boosted by a Sommerfeld enhancement
factor which arises due to the ladder of exchanged W bosons between the initial
states. The boost factor obtained can be in the range S ~ 10^4 if the mass is
close to the resonance value of M ~ 4 TeV. In this paper we show that if one
takes into account the Sommerfeld enhancement in the relic abundance
calculation then the correct relic density is obtained for 4 TeV wino mass due
to the enhanced annihilation after their kinetic decoupling. At the same time
the Sommerfeld enhancement in the \chi \chi --> W^+ W^- annihilation channel is
sufficient to explain the positron flux seen in PAMELA data without
significantly exceeding the observed antiproton signal. We also show that (e^-
+ e^+) and gamma ray signals are broadly compatible with the Fermi-LAT
observations. In conclusion we show that a 4 TeV wino DM can explain the
positron and antiproton fluxes observed by PAMELA and at the same time give a
thermal relic abundance of CDM consistent with WMAP observations.Comment: 24 pages, 12 figures, 1 table; title corrected in arxiv metadat
Coherent network analysis for continuous gravitational wave signals in a pulsar timing array: Pulsar phases as extrinsic parameters
Supermassive black hole binaries are one of the primary targets for
gravitational wave searches using pulsar timing arrays. Gravitational wave
signals from such systems are well represented by parametrized models, allowing
the standard Generalized Likelihood Ratio Test (GLRT) to be used for their
detection and estimation. However, there is a dichotomy in how the GLRT can be
implemented for pulsar timing arrays: there are two possible ways in which one
can split the set of signal parameters for semi-analytical and numerical
extremization. The straightforward extension of the method used for continuous
signals in ground-based gravitational wave searches, where the so-called pulsar
phase parameters are maximized numerically, was addressed in an earlier paper
(Wang et al. 2014). In this paper, we report the first study of the performance
of the second approach where the pulsar phases are maximized semi-analytically.
This approach is scalable since the number of parameters left over for
numerical optimization does not depend on the size of the pulsar timing array.
Our results show that, for the same array size (9 pulsars), the new method
performs somewhat worse in parameter estimation, but not in detection, than the
previous method where the pulsar phases were maximized numerically. The origin
of the performance discrepancy is likely to be in the ill-posedness that is
intrinsic to any network analysis method. However, scalability of the new
method allows the ill-posedness to be mitigated by simply adding more pulsars
to the array. This is shown explicitly by taking a larger array of pulsars.Comment: 30 pages, 11 figures, revised version, published in Ap
A coherent method for the detection and estimation of continuous gravitational wave signals using a pulsar timing array
The use of a high precision pulsar timing array is a promising approach to
detecting gravitational waves in the very low frequency regime ( Hz) that is complementary to the ground-based efforts (e.g., LIGO,
Virgo) at high frequencies ( Hz) and space-based ones (e.g.,
LISA) at low frequencies ( Hz). One of the target sources for
pulsar timing arrays are individual supermassive black hole binaries that are
expected to form in galactic mergers. In this paper, a likelihood based method
for detection and estimation is presented for a monochromatic continuous
gravitational wave signal emitted by such a source. The so-called pulsar terms
in the signal that arise due to the breakdown of the long-wavelength
approximation are explicitly taken into account in this method. In addition,
the method accounts for equality and inequality constraints involved in the
semi-analytical maximization of the likelihood over a subset of the parameters.
The remaining parameters are maximized over numerically using Particle Swarm
Optimization. Thus, the method presented here solves the monochromatic
continuous wave detection and estimation problem without invoking some of the
approximations that have been used in earlier studies.Comment: 33 pages, 10 figures, submitted to Ap
Higgsino Dark Matter in Nonuniversal Gaugino Mass Models
We study two simple and well motivated nonuniversal gaugino mass models,
which predict higgsino dark matter. One can account for the observed dark
matter relic density along with the observed Higgs boson mass of ~ 125 GeV over
a large region of the parameter space of each model, corresponding to higgsino
mass of ~ 1 TeV. In each case this parameter region covers the gluino mass
range of 2-3 TeV, parts of which can be probed by the 14 TeV LHC experiments.
We study these model predictions for LHC in brief and for dark matter detection
experiments in greater detail.Comment: 35 pages, 11 figures, pdflatex, new references and a few relevant
decay branching ratios added in two tables. Version to appear in Phys Rev
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