1,274 research outputs found
Delays in Open String Field Theory
We study the dynamics of light-like tachyon condensation in a linear dilaton
background using level-truncated open string field theory. The equations of
motion are found to be delay differential equations. This observation allows us
to employ well-established mathematical methods that we briefly review. At
level zero, the equation of motion is of the so-called retarded type and a
solution can be found very efficiently, even in the far light-cone future. At
levels higher than zero however, the equations are not of the retarded type. We
show that this implies the existence of exponentially growing modes in the
non-perturbative vacuum, possibly rendering light-like rolling unstable.
However, a brute force calculation using exponential series suggests that for
the particular initial condition of the tachyon sitting in the false vacuum in
the infinite light-cone past, the rolling is unaffected by the unstable modes
and still converges to the non-perturbative vacuum, in agreement with the
solution of Hellerman and Schnabl. Finally, we show that the growing modes
introduce non-locality mixing present with future, and we are led to conjecture
that in the infinite level limit, the non-locality in a light-like linear
dilaton background is a discrete version of the smearing non-locality found in
covariant open string field theory in flat space.Comment: 48 pages, 14 figures. v2: References added; Section 4 augmented by a
discussion of the diffusion equation; discussion of growing modes in Section
4 slightly expande
Is the bump significant? An axion-search example
Many experiments in physics involve searching for a localized excess over
background expectations in an observed spectrum. If the background is known and
there is Gaussian noise, the amount of excess of successive observations can be
quantified by the runs statistic taking care of the look-elsewhere effect. The
distribution of the runs statistic under the background model is known
analytically but the computation becomes too expensive for more than about a
hundred observations. This work demonstrates a principled high-precision
extrapolation from a few dozen up to millions of data points. It is most
precise in the interesting regime when an excess is present. The method is
verified for benchmark cases and successfully applied to real data from an
axion search. The code that implements our method is available at
https://github.com/fredRos/runs .Comment: 18 pages, 8 figures. v2 fixes arxiv's parsing of the URL in the
abstrac
Heavy ion measurement on LDEF
A stack of CR-39 and Kodak CN track detectors was exposed on the NASA satellite LDEF and recovered after almost six years in space. The quick look analysis yielded heavy ion tracks on a background of low energy secondaries from proton interaction. The detected heavy ions show a steep energy spectrum which indicates a radiation belt origin
Measurement of low energy cosmic rays aboard Spacelab-1
In December 1983 the first Spacelab mission was launched for a duration of 10 days. Aboard was the Kiel experiment Isotopic Stack designed for measurement of heavy cosmic ray nuclei with nuclear charge equal to or greater than 3 and energies up to some 100MeV/nuc. One part of the stack was rotated in well defined steps registered by an angle encoder to receive information on impact times of the nuclei. Using this time resolving system geomagnetically forbidden particles can be detected. The chemical composition and energy spectra of mainly CNO particles are examined using a rotated 300 microns m thick CR-39 foil beneath a fixed 100 microns m thick Kodak-Cellulose Nitrate foil. About 600 sq cm have been scanned yielding nearly 100 nuclear tracks within an energy range of approximately 8 to 30 MeV/nuc. The calibration is done by means of a postflight irradiation with 410 MeV/nuc Fe-56 at Berkeley Laboratory, California, USA. Relative abundances and energy spectra are presented
Extracting Angular Observables without a Likelihood and Applications to Rare Decays
Our goal is to obtain a complete set of angular observables arising in a
generic multi-body process. We show how this can be achieved without the need
to carry out a likelihood fit of the angular distribution to the measured
events. Instead, we apply the method of moments that relies both on the
orthogonality of angular functions and the estimation of integrals by Monte
Carlo techniques. The big advantage of this method is that the joint
distribution of all observables can be easily extracted, even for very few
events. The method of moments is shown to be robust against mismodeling of the
angular distribution. Our main result is an explicit algorithm that accounts
for systematic uncertainties from detector-resolution and acceptance effects.
Finally, we present the necessary process-dependent formulae needed for direct
application of the method to several rare decays of interest.Comment: 13 pages, 4 figure
Histone post-translational modifications in preimplantation mouse embryos and their role in nuclear architecture
In mammals, epigenetic markers are globally rearranged after fertilization: while gametes carry special epigenetic signatures and a unique nuclear organization, they attain embryo-specific patterns after fertilization. This “reprogramming” is promoted by the intimate contact between the parental inherited genomes and the oocyte cytoplasm over the first cell cycles of development. Interestingly, histone post-translational modifications (PTMs) are among the factors involved in this reprogramming.During the last few years, many studies focusing on epigenetic modifications have indeed shown that, immediately after fertilization, different histone PTM profiles create an asymmetry between the two parental genomes, although both parental genomes undergo global hyperacetylation and hypomethylation. Thereafter, histone PTMs reprogramming goes on (Beaujean et al., MRD 2014). It is hypothesized that this PTMs reprogramming is required for the embryonic genome activation (EGA). Recently, we for example put forward the importance of the PRC1 complex that binds H3K27me3, for proper EGA and development beyond the two-cell stage (Posfai et al., 2012). By the stage of implantation (blastocyst stage) two cell subpopulations forms: an outer layer of epithelial trophectoderm cells (TE) and the inner cell mass (ICM) located eccentrically within the blastocoelic cavity. Remarkably, some histone PTMs have been found to differ between the ICM vs. TE and to correlate with specific gene expression in each of these cell types (Dahl et al., 2010; Vermilyea et al., 2009).On the other hand, it is well known that diverse parts of the genome have different types of chromatin configuration depending on their function (centromeric and telomeric heterochromatin for instance). Interestingly, the mouse embryo presents a unique organization of the peri-centromeric heterochromatin that locates around the nucleoli. This configuration is rapidly acquired in the maternal pronucleus and more progressively in the paternal one (Martin et al., 2006; Aguirre-Lavin et al., 2012), probably due to the specific epigenetic marks present only in the paternal chromatin. During the 2-cell stage, dissociation of pericentromeric heterochromatin from nucleoli begins, concomitantly with the major phase of embryonic genome activation, although the importance of this remodeling is not yet well understood. Remarkably, it however seems that transcripts generated by pericentromeric satellite repeats are involved in this event and that interference with this phenomenon results in developmental arrest (Probst et al., 2010; Santenard et al., 2010; Fulka & Langerova 2014).Altogether, it suggests that histone PTMs may be closely correlated with the formation of a transcriptionally active or repressive state during early embryonic development and that they can modify chromatin organization and nuclear architecture during mouse embryonic development. It should also be mentioned that knock-outs of several histone modification enzymes have underlined the importance of PTMs during preimplantation development
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