Supersymmetric AdS_3, AdS_2 and Bubble Solutions

We present new supersymmetric AdS_3 solutions of type IIB supergravity and AdS_2 solutions of D=11 supergravity. The former are dual to conformal field theories in two dimensions with N=(0,2) supersymmetry while the latter are dual to conformal quantum mechanics with two supercharges. Our construction also includes AdS_2 solutions of D=11 supergravity that have non-compact internal spaces which are dual to three-dimensional N=2 superconformal field theories coupled to point-like defects. We also present some new bubble-type solutions, corresponding to BPS states in conformal theories, that preserve four supersymmetries.Comment: v2: 33 pages, published version in JHE

The Decay of Magnetic Fields in Kaluza-Klein Theory

Magnetic fields in five-dimensional Kaluza-Klein theory compactified on a circle correspond to ``twisted'' identifications of five dimensional Minkowski space. We show that a five dimensional generalisation of the Kerr solution can be analytically continued to construct an instanton that gives rise to two possible decay modes of a magnetic field. One decay mode is the generalisation of the ``bubble decay" of the Kaluza-Klein vacuum described by Witten. The other decay mode, rarer for weak fields, corresponds in four dimensions to the creation of monopole-anti-monopole pairs. An instanton for the latter process is already known and is given by the analytic continuation of the \KK\ Ernst metric, which we show is identical to the five dimensional Kerr solution. We use this fact to illuminate further properties of the decay process. It appears that fundamental fermions can eliminate the bubble decay of the magnetic field, while allowing the pair production of Kaluza-Klein monopoles.Comment: 25 pages, one figure. The discussion of fermions has been revised: We show how fundamental fermions can eliminate the bubble-type instability but still allow pair creation of monopole

On the universal outcome of star-formation: Is there a link between stars and brown-dwarfs?

(abridged) The recent evidence obtained by Briceno et al. that star-formation in Taurus-Auriga (TA) may be producing significantly fewer brown dwarfs (BDs) per star than the ONC is investigated by setting up a realistic model stellar plus BD population and explicitly taking into account a high binary proportion and dynamical evolution in the TA groups and the ONC. The Briceno result is reproduced almost exactly despite an identical IMF in both systems because many BD-BD and star-BD binaries are disrupted in the ONC thus freeing BDs, while the TA groups remain unevolved dynamically. However, the resulting populations do not have the correct star-star, star-BD and expecially BD-BD binary properties, even if a variable BD IMF is allowed for. The conclusion is therefore that BDs need to be added as a separate population which has its own binary properties. Such an extra population can have various origins which are briefly discussed in this contribution but more fully in an associated paper.Comment: MNRAS, accepted, 23 pages, 14 figures, LaTeX, two references adde

Implications of latency data for threshold and nonthreshold models of signal detection

Two detection cum latency models are constructed by combining sets of plausible assumptions about latency with, in the first instance, the model of the. Theory of Signal Detection and, in the second instance, a general threshold model. The aim of this paper is to show that latency statistics can be used to provide sharp tests for distinguishing between the two models. The two statistics studied are the reaction time operating characteristic (RT-ROC) and the plot of average reaction time against response probability (RT-probability curve). It is shown that, under the first detection model, the RT-ROC lies below the ROC derived from detection rates except at the "yes-no" point, and the RT-probability curve is decreasing whereas, under the second model, the RT-ROC coincides with portions of the latter ROC, and decreasing RT-probability curves do not arise naturally.The effect of criterion variability on the ROC is discussed in detail and conditions are given under which the RT-ROC is equivalent to a ROC generated by variable criteria.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34057/1/0000335.pd

Nucleation of PP-Branes and Fundamental Strings

We construct a solution to the low-energy string equations of motion in five dimensions that describes a circular loop of fundamental string exponentially expanding in a background electric $H$-field. Euclideanising this gives an instanton for the creation of a loop of fundamental string in a background $H$-field, and we calculate the rate of nucleation. Solutions describing magnetically charged strings and $p$-branes, where the gauge field comes from Kaluza-Klein reduction on a circle, are also constructed. It is known that a magnetic flux tube in four (reduced) spacetime dimensions is unstable to the pair creation of Kaluza-Klein monopoles. We show that in $(4+p)$ dimensions, magnetic $(p+1)$ ``fluxbranes" are unstable to the nucleation of a magnetically charged spherical $p$-brane. In ten dimensions the instanton describes the nucleation of a Ramond-Ramond magnetically charged six-brane in type IIA string theory. We also find static solutions describing spherical charged $p$-branes or fundamental strings held in unstable equilibrium in appropriate background fields. Instabilities of intersecting magnetic fluxbranes are also discussed.Comment: 28 pages, harvmac (b), reference added, typos correcte

Searching for a Stochastic Background of Gravitational Waves with LIGO

The Laser Interferometer Gravitational-wave Observatory (LIGO) has performed the fourth science run, S4, with significantly improved interferometer sensitivities with respect to previous runs. Using data acquired during this science run, we place a limit on the amplitude of a stochastic background of gravitational waves. For a frequency independent spectrum, the new limit is $\Omega_{\rm GW} < 6.5 \times 10^{-5}$. This is currently the most sensitive result in the frequency range 51-150 Hz, with a factor of 13 improvement over the previous LIGO result. We discuss complementarity of the new result with other constraints on a stochastic background of gravitational waves, and we investigate implications of the new result for different models of this background.Comment: 37 pages, 16 figure

Quantum state preparation and macroscopic entanglement in gravitational-wave detectors

Long-baseline laser-interferometer gravitational-wave detectors are operating at a factor of 10 (in amplitude) above the standard quantum limit (SQL) within a broad frequency band. Such a low classical noise budget has already allowed the creation of a controlled 2.7 kg macroscopic oscillator with an effective eigenfrequency of 150 Hz and an occupation number of 200. This result, along with the prospect for further improvements, heralds the new possibility of experimentally probing macroscopic quantum mechanics (MQM) - quantum mechanical behavior of objects in the realm of everyday experience - using gravitational-wave detectors. In this paper, we provide the mathematical foundation for the first step of a MQM experiment: the preparation of a macroscopic test mass into a nearly minimum-Heisenberg-limited Gaussian quantum state, which is possible if the interferometer's classical noise beats the SQL in a broad frequency band. Our formalism, based on Wiener filtering, allows a straightforward conversion from the classical noise budget of a laser interferometer, in terms of noise spectra, into the strategy for quantum state preparation, and the quality of the prepared state. Using this formalism, we consider how Gaussian entanglement can be built among two macroscopic test masses, and the performance of the planned Advanced LIGO interferometers in quantum-state preparation