12,490 research outputs found
Excited nucleons with chirally improved fermions
We study positive and negative parity nucleons on the lattice using the
chirally improved lattice Dirac operator. Our analysis is based on a set of
three operators chi_i with the nucleon quantum numbers but in different
representations of the chiral group and with different diquark content. We use
a variational method to separate ground state and excited states and determine
the mixing coefficients for the optimal nucleon operators in terms of the
chi_i. We clearly identify the negative parity resonances N(1535) and N(1650)
and their masses agree well with experimental data. The mass of the observed
excited positive parity state is too high to be interpreted as the Roper state.
Our results for the mixing coefficients indicate that chiral symmetry is
important for N(1535) and N(1650) states. We confront our data for the mixing
coefficients with quark models and provide insights into the physics of the
nucleon system and the nature of strong decays.Comment: Tables added, small modifications in the tex
QCD phase diagram and charge fluctuations
We discuss the phase structure and fluctuations of conserved charges in two
flavor QCD. The importance of the density fluctuations to probe the existence
of the critical end point is summarized. The role of these fluctuations to
identify the first order phase transition in the presence of spinodal phase
separation is also discussed.Comment: 8 pages, 8 figures, plenary talk given at the 19th International
Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006
(QM 2006), Shanghai, China, 14-20 Nov 200
Gamma-Ray Burster Counterparts: HST Blue and Ultraviolet Data
The surest solution of the Gamma Ray Burst (GRB) mystery is to find an
unambiguous low-energy quiescent counterpart. However, to date no reasonable
candidates have been identified in the x-ray, optical, infrared, or radio
ranges. The Hubble Space Telescope (HST) has now allowed for the first deep
ultraviolet searches for quiescent counterparts. This paper reports on
multiepoch ultraviolet searches of five GRB positions with HST. We found no
sources with significant ultraviolet excesses, variability, parallax, or proper
motion in any of the burst error regions. In particular, we see no sources
similar to that proposed as a counterpart to the GRB970228. While this negative
result is disappointing, it still has good utility for its strict limits on the
no-host-galaxy problem in cosmological models of GRBs. For most cosmological
models (with peak luminosity 6X10^50 erg/s), the absolute B magnitude of any
possible host galaxy must be fainter than -15.5 to -17.4. These smallest boxes
for some of the brightest bursts provide the most critical test, and our limits
are a severe problem for all published cosmological burst models.Comment: 15 pages, 2 ps figures, accepted for publication in the Astrophysical
Journa
Visualizing elements of Sha[3] in genus 2 jacobians
Mazur proved that any element xi of order three in the Shafarevich-Tate group
of an elliptic curve E over a number field k can be made visible in an abelian
surface A in the sense that xi lies in the kernel of the natural homomorphism
between the cohomology groups H^1(k,E) -> H^1(k,A). However, the abelian
surface in Mazur's construction is almost never a jacobian of a genus 2 curve.
In this paper we show that any element of order three in the Shafarevich-Tate
group of an elliptic curve over a number field can be visualized in the
jacobians of a genus 2 curve. Moreover, we describe how to get explicit models
of the genus 2 curves involved.Comment: 12 page
Spatial heterogeneity of air pollution statistics in Europe
Air pollution is one of the leading causes of death globally, and continues to have a detrimental effect on our health. In light of these impacts, an extensive range of statistical modelling approaches has been devised in order to better understand air pollution statistics. However, the time-varying statistics of different types of air pollutants are far from being fully understood. The observed probability density functions (PDFs) of concentrations depend very much on the spatial location and on the pollutant substance. In this paper, we analyse a large variety of data from 3544 different European monitoring sites and show that the PDFs of nitric oxide (NO), nitrogen dioxide ([Formula: see text] ) and particulate matter ([Formula: see text] and [Formula: see text] ) concentrations generically exhibit heavy tails and are asymptotically well approximated by q-exponential distributions with a given width parameter [Formula: see text] . We observe that the power-law parameter q and the width parameter [Formula: see text] vary widely for the different spatial locations. For each substance, we find different patterns of parameter clouds in the [Formula: see text] plane. These depend on the type of pollutants and on the environmental characteristics (urban/suburban/rural/traffic/industrial/background). This means the effective statistical physics description of air pollution exhibits a strong degree of spatial heterogeneity
Higher-order ratios of baryon number cumulants
The relevance of higher order cumulants of net baryon number fluctuations for
the analysis of freeze-out and critical conditions in heavy-ion collisions at
LHC and RHIC is addressed. The sign structure of the higher order cumulants in
the vicinity of the chiral crossover temperature might be a sensitive probe and
may allow to elucidate their relation to the QCD phase transition. We calculate
ratios of generalized quark-number susceptibilities to high orders in three
flavor QCD-like models and investigate their sign structure close to the chiral
crossover line.Comment: presented at the International Conference "Critical Point and Onset
of Deconfinement - CPOD 2011", Wuhan, November 7-11, 2011; version to appear
in Cent. Eur. J. Phy
Baryon spectroscopy with spatially improved quark sources
We study baryons on the lattice with a special focus on excited states. For
that purpose we construct several interpolators which differ in their Dirac
structure. These interpolators are built from Jacobi smeared quarks with
different widths in order to allow for operators with improved spatial
wavefunctions. We compute all cross correlations and use the variational method
to determine which combinations of operators have best overlap with ground and
excited states. Our approach yields promising results for the spin-1/2 baryons:
nucleon, sigma, xi and lambda. For the spin-3/2 baryons, delta and omega, we
obtain results which are consistent with results of other groups.Comment: LaTeX [PoS], 6 pages, 4 figures, talk presented at Lattice 2005
(Hadron spectrum and quark masses
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