39,807 research outputs found

    Supersymmetric fluxbrane intersections and closed string tachyons

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    We consider NS-NS superstring model with several ``magnetic'' parameters bsb_s (s=1, ...,N) associated with twists mixing a compact S1S^1 direction with angles in NN spatial 2-planes of flat 10-dimensional space. It generalizes the Kaluza-Klein Melvin model which has single parameter bb. The corresponding U-dual background is a R-R type IIA solution describing an orthogonal intersection of NN flux 7-branes. Like the Melvin model, the NS-NS string model with NN continuous parameters is explicitly solvable; we present its perturbative spectrum and torus partition function explicitly for the N=2 case. For generic bsb_s (above some critical values) there are tachyons in the S1S^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>1N >1 by making a special choice of the parameters bsb_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

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    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

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    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

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    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

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

    On the effective action of stable non-BPS branes

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    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

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    We describe string-theory and d=11d=11 supergravity solutions involving symmetric spaces of constant negative curvature. Many examples of non-supersymmetric string compactifications on hyperbolic spaces HrH_r of finite volume are given in terms of suitable cosets of the form Hr/ΓH_r/\Gamma , where Γ\Gamma is a discrete group. We describe in some detail the cases of the non-compact hyperbolic spaces F2F_2 and F3F_3, representing the fundamental regions of H2H_2 and H3H_3 under SL(2,Z)SL(2,Z) and the Picard group, respectively. By writing AdSAdS as a U(1) fibration, we obtain new solutions where AdS2p+1AdS_{2p+1} gets untwisted by T-duality to R×SU(p,1)/(SU(p)×U(1)){\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 H3H_3. A new class of non-supersymmetric conformal field theories can be defined via holography.Comment: 17 pages, Latex. Small additions and correction