15,041 research outputs found
Birth of Closed Strings and Death of Open Strings during Tachyon Condensation
The tremendous progress achieved through the study of black holes and branes
suggests that their time dependent generalizations called Spacelike branes
(S-branes) may prove similarly useful. An example of an established approach to
S-branes is to include a string boundary interaction and we first summarize
evidence for the death of open string degrees of freedom for the homogeneous
rolling tachyon on a decaying brane. Then, we review how to extract the flat
S-brane worldvolumes describing the homogeneous rolling tachyon and how large
deformations correspond to creation of lower dimensional strings and branes.
These S-brane worldvolumes are governed by S-brane actions which are on equal
footing to D-brane actions, since they are derived by imposing conformality on
the string worldsheet, as well as by analyzing fluctuations of time dependent
tachyon configurations. As further examples we generalize previous solutions of
the S-brane actions so as to describe multiple decaying and nucleating closed
fundamental strings. Conceptually S-brane actions are therefore different from
D-brane actions and can provide a description of time dependent strings/branes
and possibly their interactions.Comment: 15 pages, 7 eps figures; invited review for Modern Physics Letters A,
including new solutions for S-brane actions. v2 published version, minor
typos correcte
Understanding/unravelling carotenoid excited singlet states.
Carotenoids are essential light-harvesting pigments in natural photosynthesis. They absorb in the blue–green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and thus expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet–singlet excitation energy transfer, and carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. The photochemistry and photophysics of carotenoids have often been interpreted by referring to those of simple polyene molecules that do not possess any functional groups. However, this may not always be wise because carotenoids usually have a number of functional groups that induce the variety of photochemical behaviours in them. These differences can also make the interpretation of the singlet excited states of carotenoids very complicated. In this article, we review the properties of the singlet excited states of carotenoids with the aim of producing as coherent a picture as possible of what is currently known and what needs to be learned
The Concentration-Density Relation of Galaxies in Las Campanas Redshift Survey
We report the results of the evaluation of the ``concentration-density''
relation of galaxies in the local universe, taking advantage of the very large
and homogeneous data set available from the Las Campanas Redshift Survey
(Shectman et al. 1996). This data set consists of galaxies inhabiting the
entire range of galactic environments, from the sparsest field to the densest
clusters, thus allowing us to study environmental variations without combining
multiple data sets with inhomogeneous characteristics. Concentration is
quantified by the automatically-measured concentration index , which is a
good measure of a galaxy's bulge-to-disk ratio. The environment of the sample
galaxies is characterized both by the three-space local galaxy density and by
membership in groups and clusters. We find that the distribution of C in galaxy
populations varies both with local density and with cluster/group membership:
the fraction of centrally-concentrated galaxies increases with local galaxy
density, and is higher in clusters than in the field. A comparison of the
concentration-local density relation in clusters and the field shows that the
two connect rather smoothly at the intermediate density regime, implying that
the apparent cluster/field difference is only a manifestation of the variation
with the local density. We conclude that the structure of galaxies is
predominantly influenced by the local density and not by the broader
environments characterized by cluster/field memberships.Comment: 11 pages, 4 figures, ApJ in press, uses psfig.st
Moving NRQCD for B Form Factors at High Recoil
We derive the continuum and lattice tree-level moving NRQCD (mNRQCD) through
order 1/m^2. mNRQCD is a generalization of NRQCD for dealing with hadrons with
nonzero velocity u_mu. The quark's total momentum is written as P^mu=Mu^mu+k^mu
where k^mu << Mu^mu is discretized and Mu^mu is treated exactly. Radiative
corrections to couplings on the lattice are discussed. mNRQCD is particularly
useful for calculating B->pi and B->D form factors since errors are similar at
low and high recoil.Comment: 3 pages, 1 figure, Lattice2002(heavyquark
Physics at SuperB
Flavour will play a crucial role in understanding physics beyond the Standard
Model. Progress in developing a future programme to investigate this central
area of particle physics has recently passed a milestone, with the completion
of the conceptual design report for SuperB, a very high luminosity, asymmetric
e+e- collider. This article summarizes the important role of SuperB in
understanding new physics in the LHC era.Comment: 4 pages, 2 figures. To appear in the proceedings of the International
Europhysics Conference on High Energy Physics (EPS-HEP2007), Manchester,
England, 19-25 July 200
Neutron-Rich Nuclei in Heaven and Earth
An accurately calibrated relativistic parametrization is introduced to
compute the ground state properties of finite nuclei, their linear response,
and the structure of neutron stars. While similar in spirit to the successful
NL3 parameter set, it produces an equation of state that is considerably softer
-- both for symmetric nuclear matter and for the symmetry energy. This
softening appears to be required for an accurate description of several
collective modes having different neutron-to-proton ratios. Among the
predictions of this model are a symmetric nuclear-matter incompressibility of
K=230 MeV and a neutron skin thickness in 208Pb of Rn-Rp=0.21 fm. Further, the
impact of such a softening on the properties of neutron stars is as follows:
the model predicts a limiting neutron star mass of Mmax=1.72 Msun, a radius of
R=12.66 km for a ``canonical'' M=1.4 Msun neutron star, and no (nucleon) direct
Urca cooling in neutrons stars with masses below M=1.3 Msun.Comment: 4 pages, 3 tables, and no figure
Laser-induced microexplosion confined in a bulk of silica: formation of nanovoids
We report on the nanovoid formation inside synthetic silica, viosil, by single femtosecond pulses of 30–100nJ energy, 800nm wavelength, and 180fs duration. It is demonstrated that the void is formed as a result of shock and rarefaction waves at pulse power much lower than the threshold of self-focusing. The shock-compressed region around the nanovoid is demonstrated to have higher chemical reactivity. This was used to reveal the extent of the shock-compressed region by wet etching. Application potential of nanostructuring of dielectrics is discussed
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