12,788 research outputs found
Lower-dimensional pure-spinor superstrings
We study to what extent it is possible to generalise Berkovits' pure-spinor
construction in d=10 to lower dimensions. Using a suitable definition of a
``pure'' spinor in d=4,6, we propose models analogous to the d=10 pure-spinor
superstring in these dimensions. Similar models in d=2,3 are also briefly
discussed.Comment: 17 page
An Introduction to the Covariant Quantization of Superstrings
We give an introduction to a new approach to the covariant quantization of
superstrings. After a brief review of the classical Green--Schwarz superstring
and Berkovits' approach to its quantization based on pure spinors, we discuss
our covariant formulation without pure spinor constraints. We discuss the
relation between the concept of grading, which we introduced to define vertex
operators, and homological perturbation theory, and we compare our work with
recent work by others. In the appendices, we include some background material
for the Green-Schwarz and Berkovits formulations, in order that this
presentation be self contained.Comment: LaTex, 23 pp. Contribution to the Proceedings of the Workshop in
String Theory, Leuven 2002, some references added and a comment on ref. [16
The Algebraic Method
Combining the effect of an intermediate renormalization prescription (zero
momentum subtraction) and the background field method (BFM), we show that the
algebraic renormalization procedure needed for the computation of radiative
corrections within non-invariant regularization schemes is drastically
simplified. The present technique is suitable for gauge models and, here, is
applied to the Standard Model. The use of the BFM allows a powerful
organization of the counterterms and avoids complicated Slavnov-Taylor
identities. Furthermore, the Becchi-Rouet-Stora-Tyutin (BRST) variation of
background fields plays a special role in disentangling Ward-Takahashi
identities (WTI) and Slavnov-Taylor identities (STI). Finally, the strategy to
be applied to physical processes is exemplified for the process .Comment: Latex, 38 page
Ethylene production and quality in 1-Methylcyclopropene treated 'Abbé Fètel'pears after storage in Dynamically Controlled Atmosphere
This research studies the ethylene production rate (EP) and quality in 1-MCP treated ‘Abbé Fètel’ pears after storage in DCA compared to NA and CA. 1-MCP treated (300 ppb) and control fruit were stored at -0.5°C in NA, CA (2 kPa O2 + 0.7 kPa CO2) and DCA (0.7 kPa O2 + 0.3 kPa CO2). After 4 and 6 months storage, fruit were held up to 7 d at 20°C. Skin colour, firmness and EP were measured during shelf life and the incidence of disorders after 7 d. 1-MCP treatment drastically reduced EP, which began to recover after 7 d at 20°C, except for DCA stored pears. In control fruit, NA stored pears showed the highest EP. 1-MCP treated fruit were the greenest at the end of shelf-life, especially after CA and DCA. Control fruit stored in DCA and in CA were greener than NA both at 1 d and 7 d of shelf life. Pears treated with 1-MCP did not soften during shelf life, while in control fruit firmness decreased from about 40 N to about 15-20 N, whatever the storage atmosphere. 1-MCP treatment prevented soft and superficial scald and internal breakdown, independently of storage atmosphere. DCA prevented superficial scald in control fruit, while it increased internal browning and breakdown in control and 1-MCP treated pears. No differences were found for soft scald incidence between control DCA and CA stored fruit. The highest percentage of sound fruit was found in NA stored 1-MCP treated pears, and the lowest in control fruit stored in DC
Status and perspectives of the μ+ → e+γ decay search with the MEG experiment
The MEG experiment aims at evidence of new physics beyond the Standard Model by searching for charged lepton flavour violation with the μ+ → e+γ decay. This experiment will improve the present experimental limit by two orders of magnitude in the upcoming three years. Novel detectors were developed for this measurement as well as multiple and redundant calibration methods which are mandatory to constantly monitor the performances of the apparatus. The status of the MEG experiment is reviewed and the future perspectives are discussed
Minimal Supergravity and the supersymmetry of Arnold-Beltrami Flux branes
In this paper we study some properties of the newly found Arnold-Beltrami
flux-brane solutions to the minimal supergravity. To this end we first
single out the appropriate Free Differential Algebra containing both a gauge
-form and a gauge -form : then we
present the complete rheonomic parametrization of all the generalized
curvatures. This allows us to identify two-brane configurations with
Arnold-Beltrami fluxes in the transverse space with exact solutions of
supergravity and to analyze the Killing spinor equation in their background. We
find that there is no preserved supersymmetry if there are no additional
translational Killing vectors. Guided by this principle we explicitly construct
Arnold-Beltrami flux two-branes that preserve , and of the
original supersymmetry. Two-branes without fluxes are instead BPS states and
preserve supersymmetry. For each two-brane solution we carefully study
its discrete symmetry that is always given by some appropriate crystallographic
group . Such symmetry groups are transmitted to the
gauge theories on the brane world--volume that occur in the gauge/gravity
correspondence. Furthermore we illustrate the intriguing relation between gauge
fluxes in two-brane solutions and hyperinstantons in topological
sigma-models.Comment: 56 pages, LaTeX source, 8 jpg figures, typos correcte
Pure Spinor Superstrings on Generic type IIA Supergravity Backgrounds
We derive the Free Differential Algebra for type IIA supergravity in 10
dimensions in the string frame. We provide all fermionic terms for all
curvatures. We derive the Green-Schwarz sigma model for type IIA superstring
based on the FDA construction and we check its invariance under kappa-symmetry.
Finally, we derive the pure spinor sigma model and we check the BRST
invariance. The present derivation has the advantage that the resulting sigma
model is constructed in terms of the superfields appearing in the FDA and
therefore one can directly relate a supergravity background with the
corresponding sigma model. The complete explicit form of the BRST
transformations is given and some new pure spinor constraints are obtained.
Finally, the explicit form of the action is given.Comment: 31 pp. no figures, latex, some modifications at pag 21, a missing
term in 4.51 corrected. Discussion on BRST symmetry improve
The formation of massive primordial stars in the presence of moderate UV backgrounds
Radiative feedback from populations II stars played a vital role in early
structure formation. Particularly, photons below the Lyman limit can escape the
star forming regions and produce a background ultraviolet (UV) flux which
consequently may influence the pristine halos far away from the radiation
sources. These photons can quench the formation of molecular hydrogen by
photo-detachment of . In this study, we explore the impact of such
UV radiation on fragmentation in massive primordial halos of a few times ~M. To accomplish this goal, we perform high resolution
cosmological simulations for two distinct halos and vary the strength of the
impinging background UV field in units of . We further make use of
sink particles to follow the evolution for 10,000 years after reaching the
maximum refinement level. No vigorous fragmentation is observed in UV
illuminated halos while the accretion rate changes according to the thermal
properties. Our findings show that a few 100-10, 000 solar mass protostars are
formed when halos are irradiated by at and
suggest a strong relation between the strength of UV flux and mass of a
protostar. This mode of star formation is quite different from minihalos, as
higher accretion rates of about M/yr are observed by
the end of our simulations. The resulting massive stars are the potential
cradles for the formation of intermediate mass black holes at earlier cosmic
times and contribute to the formation of a global X-ray background.Comment: Submitted to APJ, comments are welcome. High resolution copy is
available at http://www.astro.physik.uni-goettingen.de/~mlatif/IMBHs_apj.pd
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