20,693 research outputs found
QCD at high baryon density in a random matrix model
A high density diquark phase seems to be a generic feature of QCD. If so it
should also be reproduced by random matrix models. We discuss a specific one in
which the random matrix elements of the Dirac operator are supplemented by a
finite chemical potential and by non-random elements which model the formation
of instanton-anti-instanton molecules. Comparing our results to those found in
a previous investigation by Vanderheyden and Jackson we find additional support
for our starting assumption, namely that the existence of a high density
diquark phase is common to all QCD-like models.Comment: 16 pages, 4 figures, final version to appear in Eur.Phys.J.
Quark condensate in two-flavor QCD
We compute the condensate in QCD with two flavors of dynamical fermions using
numerical simulation. The simulations use overlap fermions, and the condensate
is extracted by fitting the distribution of low lying eigenvalues of the Dirac
operator in sectors of fixed topological charge to the predictions of Random
Matrix Theory.Comment: revtex, 18 pages, 4 postscript figures. V.2, the published version,
corrects an error for the shape facto
Reweighting towards the chiral limit
We propose to perform fully dynamical simulations at small quark masses by
reweighting in the quark mass. This approach avoids some of the technical
difficulties associated with direct simulations at very small quark masses. We
calculate the weight factors stochastically, using determinant breakup and low
mode projection to reduce the statistical fluctuations. We find that the weight
factors fluctuate only moderately on nHYP smeared dynamical Wilson-clover
ensembles, and we could successfully reweight 16^4, (1.85fm)^4 volume
configurations from m_q = 20MeV to m_q = 5MeV quark masses, reaching the
epsilon-regime. We illustrate the strength of the method by calculating the low
energy constant F from the epsilon-regime pseudo-scalar correlator.Comment: 17 pages, 8 figure
Phylogenetics of Cucumis (Cucurbitaceae)
Background: Melon, Cucumis melo, and cucumber, C. sativus, are among the most widely cultivated crops worldwide. Cucumis, as traditionally conceived, is geographically centered in Africa, with C. sativus and C. hystrix thought to be the only Cucumis species in Asia. This taxonomy forms the basis for all ongoing Cucumis breeding and genomics efforts. We tested relationships among Cucumis and related genera based on DNA sequences from chloroplast gene, intron, and spacer regions (rbcL, matK, rpl20-rps12, trnL, and trnL-F), adding nuclear internal transcribed spacer sequences to resolve relationships within Cucumis.
Results: Analyses of combined chloroplast sequences (4,375 aligned nucleotides) for 123 of the 130 genera of Cucurbitaceae indicate that the genera Cucumella, Dicaelospermum, Mukia, Myrmecosicyos, and Oreosyce are embedded within Cucumis. Phylogenetic trees from nuclear sequences for these taxa are congruent, and the combined data yield a well-supported phylogeny. The nesting of the five genera in Cucumis greatly changes the natural geographic range of the genus, extending it throughout the Malesian region and into Australia. The closest relative of Cucumis is Muellerargia, with one species in Australia and Indonesia, the other in Madagascar. Cucumber and its sister species, C. hystrix, are nested among Australian, Malaysian, and Western Indian species placed in Mukia or Dicaelospermum and in one case not yet formally described. Cucumis melo is sister to this Australian/Asian clade, rather than being close to African species as previously thought. Molecular clocks indicate that the deepest divergences in Cucumis, including the split between C. melo and its Australian/Asian sister clade, go back to the mid-Eocene.
Conclusion: Based on congruent nuclear and chloroplast phylogenies we conclude that Cucumis comprises an old Australian/Asian component that was heretofore unsuspected. Cucumis sativus evolved within this Australian/Asian clade and is phylogenetically far more distant from C. melo than implied by the current morphological classification
The Diffuse Source at the Center of LMC SNR 0509-67.5 is a Background Galaxy at z = 0.031
Type Ia supernovae (SNe Ia) are well-known for their use in the measurement
of cosmological distances, but our continuing lack of concrete knowledge about
their progenitor stars is both a matter of debate and a source of systematic
error. In our attempts to answer this question, we presented unambiguous
evidence that LMC SNR 0509-67.5, the remnant of an SN Ia that exploded in the
Large Magellanic Cloud 400 +/- 50 years ago, did not have any point sources
(stars) near the site of the original supernova explosion, from which we
concluded that this particular supernova must have had a progenitor system
consisting of two white dwarfs (Schaefer & Pagnotta 2012). There is, however,
evidence of nebulosity near the center of the remnant, which could have been
left over detritus from the less massive WD, or could have been a background
galaxy unrelated to the supernova explosion. We obtained long-slit spectra of
the central nebulous region using GMOS on Gemini South to determine which of
these two possibilities is correct. The spectra show H-alpha emission at a
redshift of z = 0.031, which implies that the nebulosity in the center of LMC
SNR 0509-67.5 is a background galaxy, unrelated to the supernova.Comment: 2 figures, accepted for publication in Ap
Improving meson two-point functions by low-mode averaging
Some meson correlation functions have a large contribution from the low lying
eigenmodes of the Dirac operator. The contribution of these eigenmodes can be
averaged over all positions of the source. This can improve the signal in these
channels significantly. We test the method for meson two-point functions.Comment: Talk given at Lattice2004(spectrum), Fermilab, June 21-26, 200
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