102,572 research outputs found

### Generation of scale invariant magnetic fields in bouncing universes

We consider the generation of primordial magnetic fields in a class of
bouncing models when the electromagnetic action is coupled non-minimally to a
scalar field that, say, drives the background evolution. For scale factors that
have the power law form at very early times and non-minimal couplings which are
simple powers of the scale factor, one can easily show that scale invariant
spectra for the magnetic field can arise before the bounce for certain values
of the indices involved. It will be interesting to examine if these power
spectra retain their shape after the bounce. However, analytical solutions for
the Fourier modes of the electromagnetic vector potential across the bounce are
difficult to obtain. In this work, with the help of a new time variable that we
introduce, which we refer to as the ${\rm e}$-${\cal N}$-fold, we investigate
these scenarios numerically. Imposing the initial conditions on the modes in
the contracting phase, we numerically evolve the modes across the bounce and
evaluate the spectra of the electric and magnetic fields at a suitable time
after the bounce. As one could have intuitively expected, though the complete
spectra depend on the details of the bounce, we find that, under the original
conditions, scale invariant spectra of the magnetic fields do arise for
wavenumbers much smaller than the scale associated with the bounce. We also
show that magnetic fields which correspond to observed strengths today can be
generated for specific values of the parameters. But, we find that, at the
bounce, the backreaction due to the electromagnetic modes that have been
generated can be significantly large calling into question the viability of the
model. We briefly discuss the implications of our results.Comment: v1: 19 pages, 5 figures; v2: 20 pages, 5 figures, minor revisions, to
appear in JCA

### Measurement of triple differential photon plus jet cross section by D0

We report on a new measurement of triple differential cross section for the
process ppbar -> photon + jet + X in ppbar collisions at sqrt(s)=1.96 TeV by
the D0 Collaboration at Fermilab based on a dataset corresponding to an
integrated luminosity of 1.1 fb-1.Comment: 3 pages, 1 figure, To appear in the Proceedings of the 2007
Europhysics Conference on High Energy Physics, Manchester, UK. Accepted for
publication in Journal of Physics : Conference Serie

### Adsorption assisted translocation of a chain molecule through a pore in a spherical vesicle

We analyze the free energy for translocation of a polymer from the outside of
a spherical vesicle to the inside. The process is assumed to be driven by the
adsorption of the polymer on the inner surface of the vesicle. We argue that in
the case where the polymer is adsorbed on the outer surface too, the entropic
barrier for translocation is absent. We analyze the adsorption energy and find
the free energy profile for the process. We argue that the motion corresponds
to a polymer crossing a region with a change in free energy per segment. Based
upon our earlier analsis of the behaviour of kinks in such a problem, we
conclude that the translocation can occur with a crossing time $t_{cross}\sim
N$

### Primordial features due to a step in the inflaton potential

Certain oscillatory features in the primordial scalar power spectrum are
known to provide a better fit to the outliers in the cosmic microwave
background data near the multipole moments of $\ell=22$ and 40. These features
are usually generated by introducing a step in the popular, quadratic potential
describing the canonical scalar field. Such a model will be ruled out, if the
tensors remain undetected at a level corresponding to a tensor-to-scalar ratio
of, say, $r\simeq 0.1$. In this work, in addition to the popular quadratic
potential, we investigate the effects of the step in a small field model and a
tachyon model. With possible applications to future datasets (such as PLANCK)
in mind, we evaluate the tensor power spectrum exactly, and include its
contribution in our analysis. We compare the models with the WMAP (five as well
as seven-year), the QUaD and the ACBAR data. As expected, a step at a
particular location and of a suitable magnitude and width is found to improve
the fit to the outliers (near $\ell=22$ and 40) in all these cases. We point
out that, if the tensors prove to be small (say, $r\lesssim 0.01$), the
quadratic potential and the tachyon model will cease to be viable, and more
attention will need to be paid to examples such as the small field models.Comment: 17 pages, 6 figures, Discussion shortened, Version to appear in JCA

### Unravelling Strings at the LHC

We construct LHC signature footprints for four semi-realistic string/$M$
theory vacua with an MSSM visible sector. We find that they all give rise to
limited regions in LHC signature space, and are qualitatively different from
each other for understandable reasons. We also propose a technique in which
correlations of LHC signatures can be effectively used to distinguish among
these string theory vacua. We expect the technique to be useful for more
general string vacua. We argue that further systematic analysis with this
approach will allow LHC data to disfavor or exclude major ``corners'' of
string/$M$ theory and favor others. The technique can be used with limited
integrated luminosity and improved.Comment: 34 pages, 16 figure

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