39,807 research outputs found

### Supersymmetric fluxbrane intersections and closed string tachyons

We consider NS-NS superstring model with several ``magnetic'' parameters
$b_s$ (s=1, ...,N) associated with twists mixing a compact $S^1$ direction with
angles in $N$ spatial 2-planes of flat 10-dimensional space. It generalizes the
Kaluza-Klein Melvin model which has single parameter $b$. The corresponding
U-dual background is a R-R type IIA solution describing an orthogonal
intersection of $N$ flux 7-branes. Like the Melvin model, the NS-NS string
model with $N$ continuous parameters is explicitly solvable; we present its
perturbative spectrum and torus partition function explicitly for the N=2 case.
For generic $b_s$ (above some critical values) there are tachyons in the $S^1$
winding sector. A remarkable feature of this model is that while in the Melvin
N=1 case all supersymmetry is broken, a fraction of it may be preserved for $N
>1$ by making a special choice of the parameters $b_s$. Such solvable NS-NS
models may be viewed as continuous-parameter analogs of non-compact orbifold
models. They and their U-dual R-R fluxbrane counterparts may have some
``phenomenological'' applications. In particular, in N=3 case one finds a
special 1/4 supersymmetric R-R 3-brane background. Putting Dp-branes in flat
twisted NS-NS backgrounds leads to world-volume gauge theories with reduced
amount of supersymmetry. We also discuss possible ways of evolution of unstable
backgrounds towards stable ones.Comment: 26 pages, harvmac. v3: reference added, minor changes in appendi

### Exactly solvable string models of curved space-time backgrounds

We consider a new 3-parameter class of exact 4-dimensional solutions in
closed string theory and solve the corresponding string model, determining the
physical spectrum and the partition function. The background fields (4-metric,
antisymmetric tensor, two Kaluza-Klein vector fields, dilaton and modulus)
generically describe axially symmetric stationary rotating (electro)magnetic
flux-tube type universes. Backgrounds of this class include both the dilatonic
(a=1) and Kaluza-Klein (a=\sqrt 3) Melvin solutions and the uniform magnetic
field solution, as well as some singular space-times. Solvability of the string
sigma model is related to its connection via duality to a simpler model which
is a ``twisted" product of a flat 2-space and a space dual to 2-plane. We
discuss some physical properties of this model (tachyonic instabilities in the
spectrum, gyromagnetic ratios, issue of singularities, etc.). It provides one
of the first examples of a consistent solvable conformal string model with
explicit D=4 curved space-time interpretation.Comment: 54 pages, harvmac (corrected and extended version

### Heterotic strings in a uniform magnetic field

An exact conformal model representing a constant magnetic field background in
heterotic string theory is explicitly solved in terms of free
creation/annihilation operators. The spectrum of physical states is examined
for different possible embeddings of the magnetic U(1) subgroup. We find that
an arbitrarily small magnetic field gives rise to an infinite number of
tachyonic excitations corresponding to charged vector states of the massless
level and to higher level states with large spins and charges.Comment: 25 pages, harvmac (few comments added, misprints corrected

### Urgency is a Non-monotonic Function of Pulse Rate

Magnitude estimation was used to assess the experience of urgency in pulse-train stimuli (pulsed white noise) ranging from 3.13 to 200 Hz. At low pulse rates, pulses were easily resolved. At high pulse rates, pulses fused together leading to a tonal sensation with a clear pitch level. Urgency ratings followed a nonmonotonic (polynomial) function with local maxima at 17.68 and 200 Hz. The same stimuli were also used in response time and pitch scaling experiments. Response times were negatively correlated with urgency ratings. Pitch scaling results indicated that urgency of pulse trains is mediated by the perceptual constructs of speed and pitch

### Spectrophotometric measurements of the vibrational relaxation of CO in shock wave and nozzle expansion-flow environments

Spectrophotometric measurements of vibrational relaxation of CO in shock wave and nozzle expansion-flow environment

### On the effective action of stable non-BPS branes

We study the world-volume effective action of stable non-BPS branes present
in Type II theories compactified on K3. In particular, by exploiting the
conformal description of these objects available in the orbifold limit, we
argue that their world-volume effective theory can be chiral. The resulting
anomalies are cancelled through the usual inflow mechanism provided there are
anomalous couplings, similar to those of BPS branes, to the twisted R-R fields.
We also show that this result is in agreement with the conjectured
interpretation of these non-BPS configurations as BPS branes wrapped on
non-supersymmetric cycles of the K3.Comment: 12 pages, LaTex, no figure

### Hyperbolic Spaces in String and M-Theory

We describe string-theory and $d=11$ supergravity solutions involving
symmetric spaces of constant negative curvature. Many examples of
non-supersymmetric string compactifications on hyperbolic spaces $H_r$ of
finite volume are given in terms of suitable cosets of the form $H_r/\Gamma$,
where $\Gamma$ is a discrete group. We describe in some detail the cases of
the non-compact hyperbolic spaces $F_2$ and $F_3$, representing the fundamental
regions of $H_2$ and $H_3$ under $SL(2,Z)$ and the Picard group, respectively.
By writing $AdS$ as a U(1) fibration, we obtain new solutions where
$AdS_{2p+1}$ gets untwisted by T-duality to ${\bf R}\times SU(p,1)/(SU(p)\times
U(1))$. Solutions with time-dependent dilaton field are also constructed by
starting with a solution with NS5-brane flux over $H_3$. A new class of
non-supersymmetric conformal field theories can be defined via holography.Comment: 17 pages, Latex. Small additions and correction

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