40 research outputs found
Value of Speed in Public Transit Services
High speed is always a desirable feature of public transportation services. Any measure that increases public transport speeds results in benefits to users in tenns of saved travel time, and benefits to the operator in reduced operating costs and in eventual reduction of fleet size.
At the same time, it is known that some major efforts for increasing speeds, often involving considerable cost (such as increasing maximum technical speed of vehicles) result in relatively small increases in average passenger travel speed. This problem is common for a number of different modes of transportation. An excellent example can be found in air transportaÂtion; increases of aircraft cruising speed are costly and have relatively little impact on the passenger average travel speeds, particularly for short-and medium-haul trips. The same problem is observed with both rapid and surface transit in urban areas. In order to derive more specific results applicable in practice, this research is limited to the latter transportation systems: public transportation in urban areas
Non-critical Heterotic Superstrings in Various Dimensions
We construct heterotic string theories on spacetimes of the form R^{d-1,1}
times N=2 linear dilaton, where d=6,4,2,0. There are two lines of
supersymmetric theories descending from the two supersymmetric ten-dimensional
heterotic theories. These have gauge groups which are lower rank subgroups of
E_{8} times E_{8} and SO(32). On turning on a (2,2) deformation which makes the
two dimensional part a smooth SL_{2}(R)/U(1) supercoset, the gauge groups get
broken further. In the deformed theories, there are non-trivial moduli which
are charged under the surviving gauge group in the case of d=6. We construct
the marginal operators on the worldsheet corresponding to these moduli.Comment: 27 pages, harvmac. v2 reference adde
Notes on Non-Critical Superstrings in Various Dimensions
We study non-critical superstrings propagating in dimensional
Minkowski space or equivalently, superstrings propagating on the
two-dimensional Euclidean black hole tensored with d-dimensional Minkowski
space. We point out a subtlety in the construction of supersymmetric theories
in these backgrounds, and explain how this does not allow a consistent
geometric interpretation in terms of fields propagating on a cigar-like
spacetime. We explain the global symmetries of the various theories by using
their description as the near horizon geometry of wrapped NS5-brane
configurations. In the six-dimensional theory, we present a CFT description of
the four-dimensional moduli space and the global O(3) symmetry. The worldsheet
action invariant under this symmetry contains both the N=2 sine-Liouville
interaction and the cigar metric, thereby providing an example where the two
interactions are naturally present in the same worldsheet lagrangian already at
the non-dynamical level.Comment: 33 pages, harvma
D-branes and SQCD in Non-Critical Superstring Theory
Using exact boundary conformal field theory methods we analyze the D-brane
physics of a specific four-dimensional non-critical superstring theory which
involves the N=2 SL(2)/U(1) Kazama-Suzuki model at level 1. Via the holographic
duality of hep-th/9907178 our results are relevant for D-brane dynamics in the
background of NS5-branes and D-brane dynamics near a conifold singularity. We
pay special attention to a configuration of D3- and D5-branes that realizes N=1
supersymmetric QCD and discuss the massless spectrum and classical moduli of
this setup in detail. We also comment briefly on the implications of this
construction for the recently proposed generalization of the AdS/CFT
correspondence by Klebanov and Maldacena within the setting of non-critical
superstrings.Comment: harvmac, 47 pages, 6 figures; v4 same as v3 due to submission erro
A Farey tale for N=4 dyons
We study exponentially suppressed contributions to the degeneracies of
extremal black holes. Within Sen's quantum entropy function framework and
focusing on extremal black holes with an intermediate AdS3 region, we identify
an infinite family of semi-classical AdS2 geometries which can contribute
effects of order exp(S_0/c), where S_0 is the Bekenstein-Hawking-Wald entropy
and c is an integer greater than one. These solutions lift to the extremal
limit of the SL(2,Z) family of BTZ black holes familiar from the "black hole
Farey tail". We test this understanding in N=4 string vacua, where exact dyon
degeneracies are known to be given by Fourier coefficients of Siegel modular
forms. We relate the sum over poles in the Siegel upper half plane to the Farey
tail expansion, and derive a "Farey tale" expansion for the dyon partition
function. Mathematically, this provides a (formal) lift from Hilbert modular
forms to Siegel modular forms with a pole at the diagonal divisor.Comment: 31 page
Two-dimensional superstrings and the supersymmetric matrix model
We present evidence that the supersymmetric matrix model of Marinari and
Parisi represents the world-line theory of N unstable D-particles in type II
superstring theory in two dimensions. This identification suggests that the
matrix model gives a holographic description of superstrings in a
two-dimensional black hole geometry.Comment: 22 pages, 2 figures; v2: corrected eqn 4.6; v3: corrected appendices
and discussion of vacua, added ref
How Do Black Holes Predict the Sign of the Fourier Coefficients of Siegel Modular Forms?
Single centered supersymmetric black holes in four dimensions have
spherically symmetric horizon and hence carry zero angular momentum. This leads
to a specific sign of the helicity trace index associated with these black
holes. Since the latter are given by the Fourier expansion coefficients of
appropriate meromorphic modular forms of Sp(2,Z) or its subgroup, we are led to
a specific prediction for the signs of a subset of these Fourier coefficients
which represent contributions from single centered black holes only. We
explicitly test these predictions for the modular forms which compute the index
of quarter BPS black holes in heterotic string theory on T^6, as well as in Z_N
CHL models for N=2,3,5,7.Comment: LaTeX file, 17 pages, 1 figur
The Effects of Disorder on the Quantum Hall State
A disorder-averaged Hartree-Fock treatment is used to compute the density of
single particle states for quantum Hall systems at filling factor . It
is found that transport and spin polarization experiments can be simultaneously
explained by a model of mostly short-range effective disorder. The slope of the
transport gap (due to quasiparticles) in parallel field emerges as a result of
the interplay between disorder-induced broadening and exchange, and has
implications for skyrmion localization.Comment: 4 pages, 3 eps figure
Enhanced T-odd P-odd Electromagnetic Moments in Reflection Asymmetric Nuclei
Collective P- and T- odd moments produced by parity and time invariance
violating forces in reflection asymmetric nuclei are considered. The enhanced
collective Schiff, electric dipole and octupole moments appear due to the
mixing of rotational levels of opposite parity. These moments can exceed
single-particle moments by more than two orders of magnitude. The enhancement
is due to the collective nature of the intrinsic moments and the small energy
separation between members of parity doublets. In turn these nuclear moments
induce enhanced T- and P- odd effects in atoms and molecules. First a simple
estimate is given and then a detailed theoretical treatment of the collective
T-, P- odd electric moments in reflection asymmetric, odd-mass nuclei is
presented and various corrections evaluated. Calculations are performed for
octupole deformed long-lived odd-mass isotopes of Rn, Fr, Ra, Ac and Pa and the
corresponding atoms. Experiments with such atoms may improve substantially the
limits on time reversal violation.Comment: 28 pages, Revte