4,527 research outputs found
N=2 Coset Compactifications with Non-Diagonal Invariants
We consider 4-dimensional string models obtained by tensoring N=2 coset
theories with non-diagonal modular invariants. We present results from a
systematic analysis including moddings by discrete symmetries.Comment: 29 page
D=6, N=1 String Vacua and Duality
We review the structure string vacua with emphasis on the
different connections due to T-dualities and S-dualities. The topics discussed
include: Anomaly cancellation; K3 and orbifold heterotic
compactifications; T-dualities between and
heterotic vacua; non-perturbative heterotic vacua and small instantons; N=2
Type-II/Heterotic duality in D=4 ; F-theory/heterotic duality in D=6; and
heterotic/heterotic duality in six and four dimensions.Comment: 52 pages, plain Latex. To appear in the proceedings of the APCTP
Winter School on Duality, Mt. Sorak (Korea), February 199
Studying Cherrypicking: Substantive and Methodological Reflections
The Cherrypicking projectdeveloped an innovative methodological strategy to assess the consequences of participatory processes. This led to a number of publications on the determinants of the fate of citizens’ proposals, amongst other considerations. The completion of the project marks an opportunity to reflect critically on our methodological choices and the substantive findings from the research. This paperconsiders what we learned from the project and how this relates to on-going debates about methodological strategies to analyze the consequences of participatory processes. To what extent do the methodological choices adopted condition the results reached?What are the theoretical and practical implications of our findings? Is the evidence we uncovered generalizable to different social and political contexts
Building GUTs from strings
We study in detail the structure of Grand Unified Theories derived as the
low-energy limit of orbifold four-dimensional strings. To this aim, new
techniques for building level-two symmetric orbifold theories are presented.
New classes of GUTs in the context of symmetric orbifolds are then constructed.
The method of permutation modding is further explored and SO(10) GUTs with both
or -plets are obtained. SU(5) models are also found through this
method. It is shown that, in the context of symmetric orbifold GUTs,
only a single GUT-Higgs, either a or a , can be present and it always
resides in an order-two untwisted sector. Very restrictive results also hold in
the case of . General properties and selection rules for string GUTs are
described. Some of these selection rules forbid the presence of some particular
GUT-Higgs couplings which are sometimes used in SUSY-GUT model building. Some
semi-realistic string GUT examples are presented and their properties briefly
discussed.Comment: 40 pages, no figures, Late
A class of non-supersymmetric orientifolds
We study type IIB orientifolds on T^{2d}/Z_N with supersymmetry broken by the
compactification. We determine tadpole cancellation conditions including
anti-branes and considering different actions for the parity Omega. Using these
conditions we then obtain the spectrum of tachyons and massless states. Various
examples with N even correspond to type 0B orientifolds.Comment: 49 pages, Late
High directivity fractal boundary microstrip patch antenna
A novel patch antenna with a fractal boundary condition is proposed. Experimental and numerical results corroborate the fact that the fractal characteristic of the perimeter produces localised modes. This property is utilised in the design of a microstrip patch antenna with a measured directivity of 12.7 dB.Peer ReviewedPostprint (published version
Humidity contribution to C_n^2 over a 600m pathlength in a tropical marine environment
We present new optical turbulence structure parameter measurements, C_n^2,
over sea water between La Parguera and Magueyes Island (17.6N 67W) on the
southwest coast of Puerto Rico. The 600 meter horizontal paths were located
approximately 1.5 m and 10 m above sea level. No data of this type has ever
been made available in the literature. Based on the data, we show that the
C_n^2 measurements are about 7 times less compared to equivalent land data.
This strong evidence reinforces our previous argument that humidity must be
accounted for to better ascertain the near surface atmospheric turbulence
effects, which current visible / near infrared C_n^2 bulk models fail to do. We
also explore the generalised fractal dimension of this littoral data and
compare it to our reference land data. We find cases that exhibit monofractal
characteristics, that is to say, the effect of rising temperatures during the
daylight hours upon turbulence are counterbalanced by humidity, leading to a
single characteristic scale for the measurements. In other words, significant
moisture changes in the measurement volume cancels optical turbulence increases
due to temperature rises. Figures available as JPG only.Comment: 7 pages, 4 figures, 1 table, SPIE Photonics West 2007, paper 6457B-2
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