159 research outputs found
A 0-dimensional counter-example to rooting?
We provide an example of a 0-dimensional field theory where rooting does not
work.Comment: 3 pages; Physics Letters B (2010
Hadronic Vacuum Polarization Contribution to g-2 from the Lattice
We give a short description of the present situation of lattice QCD
simulations. We then focus on the computation of the anomalous magnetic moment
of the muon using lattice techniques. We demonstrate that by employing improved
observables for the muon anomalous magnetic moment, a significant reduction of
the lattice error can be obtained. This provides a promising scenario that the
accuracy of lattice calculations can match the experimental errors.Comment: Proceedings of the International Workshop on e+e- collisions from Phi
to Psi (PHIPSI11), Sep. 19-22, 2011, BINP, Novosibirsk, Russi
Calculation of fermion loops for and nucleon scalar and electromagnetic form factors
The exact evaluation of the disconnected diagram contributions to the
flavor-singlet pseudoscalar meson mass, the nucleon sigma term and the nucleon
electromagnetic form factors, is carried out utilizing GPGPU technology with
the NVIDIA CUDA platform. The disconnected loops are also computed using
stochastic methods with several noise reduction techniques. Various dilution
schemes as well as the truncated solver method are studied. We make a
comparison of these stochastic techniques to the exact results and show that
the number of noise vectors depends on the operator insertion in the fermionic
loop.Comment: Version accepted for publication in Comp. Phys. Commun. References
added. 13 pages, 12 figure
Light flavor baryon spectrum with higher order hyperfine interactions
We study the spectrum of light flavor baryons in a quark-model framework by
taking into account the order hyperfine interactions
due to two-gluon exchange between quarks. The calculated spectrum agree better
with the experimental data than the results from hyperfine interactions with
only one-gluon exchange. It is also shown that two-gluon exchange hyperfine
interactions bring a significantly improved correction to the Gell-Mann--Okubo
mass formula. Two-gluon exchange corrections on baryon excitations (including
negative parity baryons) are also briefly discussed.Comment: 31 latex pages, final version in journal publicatio
Filtered overlap: speedup, locality, kernel non-normality and Z_A~1
We investigate the overlap operator with a UV filtered Wilson kernel. The
filtering leads to a better localization of the operator even on coarse
lattices and with the untuned choice . Furthermore, the axial-vector
renormalization constant is much closer to 1, reducing the mismatch with
perturbation theory. We show that all these features persist over a wide range
of couplings and that the details of filtering prove immaterial. We investigate
the properties of the kernel spectrum and find that the kernel non-normality is
reduced. As a side effect we observe that for certain applications of the
filtered overlap a speed-up factor of 2-4 can be achieved.Comment: 30 pp, 23 fig
Identification of microRNA-mRNA functional interactions in UVB-induced senescence of human diploid fibroblasts
BACKGROUND: Cellular senescence can be induced by a variety of extrinsic stimuli, and sustained exposure to sunlight is a key factor in photoaging of the skin. Accordingly, irradiation of skin fibroblasts by UVB light triggers cellular senescence, which is thought to contribute to extrinsic skin aging, although molecular mechanisms are incompletely understood. Here, we addressed molecular mechanisms underlying UVB induced senescence of human diploid fibroblasts. RESULTS: We observed a parallel activation of the p53/p21(WAF1) and p16(INK4a)/pRb pathways. Using genome-wide transcriptome analysis, we identified a transcriptional signature of UVB-induced senescence that was conserved in three independent strains of human diploid fibroblasts (HDF) from skin. In parallel, a comprehensive screen for microRNAs regulated during UVB-induced senescence was performed which identified five microRNAs that are significantly regulated during the process. Bioinformatic analysis of miRNA-mRNA networks was performed to identify new functional mRNA targets with high confidence for miR-15a, miR-20a, miR-20b, miR-93, and miR-101. Already known targets of these miRNAs were identified in each case, validating the approach. Several new targets were identified for all of these miRNAs, with the potential to provide new insight in the process of UVB-induced senescence at a genome-wide level. Subsequent analysis was focused on miR-101 and its putative target gene Ezh2. We confirmed that Ezh2 is regulated by miR-101 in human fibroblasts, and found that both overexpression of miR-101 and downregulation of Ezh2 independently induce senescence in the absence of UVB irradiation. However, the downregulation of miR-101 was not sufficient to block the phenotype of UVB-induced senescence, suggesting that other UVB-induced processes induce the senescence response in a pathway redundant with upregulation of miR-101. CONCLUSION: We performed a comprehensive screen for UVB-regulated microRNAs in human diploid fibroblasts, and identified a network of miRNA-mRNA interactions mediating UVB-induced senescence. In addition, miR-101 and Ezh2 were identified as key players in UVB-induced senescence of HDF
Up, down, strange and charm quark masses with N-f=2+1+1 twisted mass lattice QCD
We present a lattice QCD calculation of the up, down, strange and charm quark masses performed using the gauge configurations produced by the European Twisted Mass Collaboration with N-f = 2 + 1 + 1 dynamical quarks, which include in the sea, besides two light mass degenerate quarks, also the strange and charm quarks with masses close to their physical values. The simulations are based on a unitary setup for the two light quarks and on a mixed action approach for the strange and charm quarks. The analysis uses data at three values of the lattice spacing and pion masses in the range 210-450 MeV, allowing for accurate continuum limit and controlled chiral extrapolation. The quark mass renormalization is carried out non-perturbatively using the RI'-MOM method. The results for the quark masses converted to the (MS) over bar scheme are: m(ud) (2 GeV) = 3.70(17) MeV, m(s)(2 GeV) = 99.6(4.3) MeV and m(c)(m(c)) = 1.348(46) GeV. We obtain also the quark mass ratios m(s)/m(ud) = 26.66(32) and m(c)/m(s) = 11.62(16). By studying the mass splitting between the neutral and charged kaons and using available lattice results for the electromagnetic contributions, we evaluate m(u)/m(d) = 0.470(56), leading to m(u) = 2.36(24) MeV and m(d) = 5.03(26) MeV
An analysis of the nucleon spectrum from lattice partially-quenched QCD
The chiral extrapolation of the nucleon mass, M_n, is investigated using data
coming from 2-flavour partially-quenched lattice simulations. A large sample of
lattice results from the CP-PACS Collaboration is analysed using the leading
one-loop corrections, with explicit corrections for finite lattice spacing
artifacts. The extrapolation is studied using finite range regularised chiral
perturbation theory. The analysis also provides a quantitative estimate of the
leading finite volume corrections. It is found that the discretisation,
finite-volume and partial quenching effects can all be very well described in
this framework, producing an extrapolated value of M_n in agreement with
experiment. Furthermore, determinations of the low energy constants of the
nucleon mass's chiral expansion are in agreement with previous methods, but
with significantly reduced errors. This procedure is also compared with
extrapolations based on polynomial forms, where the results are less
encouraging.Comment: Now includes calculation of low energy constants of nucleon mass
chiral extrapolation. 32 pages, 10 figures. Version accepted for publication
Hadron Structure in Lattice QCD
Recent progress in hadron structure calculations within lattice QCD is
reviewed. Results on key observables such as the axial charge, the quark
momentum fraction and the spin content of the nucleon are discussed with focus
on open issues. Lattice QCD studies of the
transition as well as the form factors are also presented.Comment: 18 pages. Lecture presented at the Erice School on Nuclear Physics
2011: "From Quarks and Gluons to Hadrons and Nuclei", organized by A.
Faessler and J. Wambach, 16- 24, Sept. 2011, Erice-Sicily, Ital
Effects of non-perturbatively improved dynamical fermions in QCD at fixed lattice spacing
We present results for the static inter-quark potential, lightest glueballs,
light hadron spectrum and topological susceptibility using a non-perturbatively
improved action on a lattice at a set of values of the bare
gauge coupling and bare dynamical quark mass chosen to keep the lattice size
fixed in physical units ( fm). By comparing these measurements with a
matched quenched ensemble, we study the effects due to two degenerate flavours
of dynamical quarks. With the greater control over residual lattice spacing
effects which these methods afford, we find some evidence of charge screening
and some minor effects on the light hadron spectrum over the range of quark
masses studied (). More substantial differences between
quenched and unquenched simulations are observed in measurements of topological
quantities.Comment: 53 pages, LaTeX/RevTeX, 16 eps figures; corrected clover action
expression and various typos, no results change
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