2,375 research outputs found
Gravitational collapse with tachyon field and barotropic fluid
A particular class of space-time, with a tachyon field, \phi, and a
barotropic fluid constituting the matter content, is considered herein as a
model for gravitational collapse. For simplicity, the tachyon potential is
assumed to be of inverse square form i.e., V(\phi) \sim \phi^{-2}. Our purpose,
by making use of the specific kinematical features of the tachyon, which are
rather different from a standard scalar field, is to establish the several
types of asymptotic behavior that our matter content induces. Employing a
dynamical system analysis, complemented by a thorough numerical study, we find
classical solutions corresponding to a naked singularity or a black hole
formation. In particular, there is a subset where the fluid and tachyon
participate in an interesting tracking behaviour, depending sensitively on the
initial conditions for the energy densities of the tachyon field and barotropic
fluid. Two other classes of solutions are present, corresponding respectively,
to either a tachyon or a barotropic fluid regime. Which of these emerges as
dominant, will depend on the choice of the barotropic parameter, \gamma.
Furthermore, these collapsing scenarios both have as final state the formation
of a black hole.Comment: 18 pages, 7 figures. v3: minor changes. Final version to appear in
GR
The Heat Kernel on AdS_3 and its Applications
We derive the heat kernel for arbitrary tensor fields on S^3 and (Euclidean)
AdS_3 using a group theoretic approach. We use these results to also obtain the
heat kernel on certain quotients of these spaces. In particular, we give a
simple, explicit expression for the one loop determinant for a field of
arbitrary spin s in thermal AdS_3. We apply this to the calculation of the one
loop partition function of N=1 supergravity on AdS_3. We find that the answer
factorizes into left- and right-moving super Virasoro characters built on the
SL(2, C) invariant vacuum, as argued by Maloney and Witten on general grounds.Comment: 46 pages, LaTeX, v2: Reference adde
Exact Multiplicities in the Three-Anyon Spectrum
Using the symmetry properties of the three-anyon spectrum, we obtain exactly
the multiplicities of states with given energy and angular momentum. The
results are shown to be in agreement with the proper quantum mechanical and
semiclassical considerations, and the unexplained points are indicated.Comment: 16 pages plus 3 postscript figures, Kiev Institute for Theoretical
Physics preprint ITP-93-32
Particle Collisions on Stringy Black Hole Background
The collision of two particles in the background of a Sen black hole is
studied. With the equations of motion of the particles, the center-of-mass
energy is investigated when the collision takes place at the horizon of a Sen
black hole. For an extremal Sen black hole, we find that the center-of-mass
energy will be arbitrarily high with two conditions: (1) spin and (2)
one of the colliding particles has the critical angular momentum
. For a nonextremal Sen black hole, we show that, in order to
obtain an unlimited center-of-mass energy, one of the colliding particles
should have the critical angular momentum ( is
the radius of the outer horizon for a nonextremal black hole). However, a
particle with the angular momentum could not approach the
black hole from outside of the horizon through free fall, which implies that
the collision with arbitrarily high center-of-mass energy could not take place.
Thus, there is an upper bound of the center-of-mass energy for the nonextremal
black hole. We also obtain the maximal center-of-mass energy for a
near-extremal black hole and the result implies that the Planck-scale energy is
hard to be approached. Furthermore, we also consider the back-reaction effects.
The result shows that, neglecting the gravitational radiation, it has a weak
effect on the center-of-mass energy. However, we argue that the maximum allowed
center-of-mass energy will be greatly reduced to below the Planck-scale when
the gravitational radiation is included.Comment: 17 pages, 4 figures, published versio
Non Abelian Geometrical Tachyon
We investigate the dynamics of a pair of coincident D5 branes in the
background of NS5 branes. It has been proposed by Kutasov that the system
with a single probing D-brane moving radially in this background is dual to the
tachyonic DBI action for a non-BPS Dp brane. We extend this proposal to the
non-abelian case and find that the duality still holds provided one promotes
the radial direction to a matrix valued field associated with a non-abelian
geometric tachyon and a particular parametrization for the transverse scalar
fields is chosen. The equations of motion of a pair of coincident D5 branes
moving in the NS5 background are determined. Analytic and numerical solutions
for the pair are found in certain simplified cases in which the U(2) symmetry
is broken to corresponding to a small transverse separation
of the pair. For certain range of parameters these solutions describe periodic
motion of the centre of mass of the pair 'bouncing off' a finite sized throat
whose minimum size is limited by the D5 branes separation.Comment: 18 pages, 2 figures, PdfLatex: references added.accepted for
publication in JHE
Kerr-CFT From Black-Hole Thermodynamics
We analyze the near-horizon limit of a general black hole with two commuting
killing vector fields in the limit of zero temperature. We use black hole
thermodynamics methods to relate asymptotic charges of the complete spacetime
to those obtained in the near-horizon limit. We then show that some
diffeomorphisms do alter asymptotic charges of the full spacetime, even though
they are defined in the near horizon limit and, therefore, count black hole
states. We show that these conditions are essentially the same as considered in
the Kerr/CFT corresponcence. From the algebra constructed from these
diffeomorphisms, one can extract its central charge and then obtain the black
hole entropy by use of Cardy's formula.Comment: 19 pages, JHEP3, no figures. V2: References added, small typos fixe
Beyond Logarithmic Corrections to Cardy Formula
As shown by Cardy modular invariance of the partition function of a given
unitary non-singular 2d CFT with left and right central charges c_L and c_R,
implies that the density of states in a microcanonical ensemble, at excitations
Delta and Delta-bar and in the saddle point approximation, is
\rho_0(\Delta,\bar\Delta;c_L, c_R)=c_L c_R
\exp(2\pi\sqrt{{c_L\Delta}/{6}})\exp(2\pi\sqrt{{c_R\bar\Delta}/{6}}). In this
paper, we extend Cardy's analysis and show that in the saddle point
approximation and up to contributions which are exponentially suppressed
compared to the leading Cardy's result, the density of states takes the form
\rho(\Delta,\bar\Delta; c_L,c_R)= f(c_L\Delta)
f(c_R\bar\Delta)\rho_0(\Delta,\bar\Delta; c_L, c_R), for a function f(x) which
we specify. In particular, we show that (i) \rho (\Delta,\bar\Delta; c_L, c_R)
is the product of contributions of left and right movers and hence, to this
approximation, the partition function of any modular invariant, non-singular
unitary 2d CFT is holomorphically factorizable and (ii) \rho(\Delta,\bar\Delta;
c_L, c_R)/(c_Lc_R) is only a function of and . In
addition, treating \rho(\Delta,\bar\Delta; c_L, c_R) as the density of states
of a microcanonical ensemble, we compute the entropy of the system in the
canonical counterpart and show that the function f(x) is such that the
canonical entropy, up to exponentially suppressed contributions, is simply
given by the Cardy's result \ln\rho_0(\Delta,\bar\Delta; c_L, c_R).Comment: 30 pages, no figures; v2: minor improvements, one reference added,
v3: minor corrections to match the published versio
Quantum networks reveal quantum nonlocality
The results of local measurements on some composite quantum systems cannot be
reproduced classically. This impossibility, known as quantum nonlocality,
represents a milestone in the foundations of quantum theory. Quantum
nonlocality is also a valuable resource for information processing tasks, e.g.
quantum communication, quantum key distribution, quantum state estimation, or
randomness extraction. Still, deciding if a quantum state is nonlocal remains a
challenging problem. Here we introduce a novel approach to this question: we
study the nonlocal properties of quantum states when distributed and measured
in networks. Using our framework, we show how any one-way entanglement
distillable state leads to nonlocal correlations. Then, we prove that
nonlocality is a non-additive resource, which can be activated. There exist
states, local at the single-copy level, that become nonlocal when taking
several copies of it. Our results imply that the nonlocality of quantum states
strongly depends on the measurement context.Comment: 4 + 3 pages, 4 figure
Mental health morbidity among people subject to immigration detention in the UK: a feasibility study
Aims: The UK has one of the largest systems of immigration detention in Europe.. Those detained include asylum-seekers and foreign national prisoners, groups with a higher prevalence of mental health vulnerabilities compared with the general population. In light of little published research on the mental health status of detainees in immigration removal centres (IRCs), the primary aim of this study was to explore whether it was feasible to conduct psychiatric research in such a setting. A secondary aim was to compare the mental health of those seeking asylum with the rest of the detainees.
Methods: Cross-sectional study with simple random sampling followed by opportunistic sampling. Exclusion criteria included inadequate knowledge of English and European Union nationality. Six validated tools were used to screen for mental health disorders including developmental disorders like Personality Disorder, Attention Deficit Hyperactivity Disorder (ADHD), Autistic Spectrum Disorder (ASD) and Intellectual Disability, as well as for needs assessment. These were the MINI v6, SAPAS, AQ-10, ASRS, LDSQ and CANFOR. Demographic data were obtained using a participant demographic sheet. Researchers were trained in the use of the screening battery and inter-rater reliability assessed by joint ratings.
Results: A total of 101 subjects were interviewed. Overall response rate was 39%. The most prevalent screened mental disorder was depression (52.5%), followed by personality disorder (34.7%) and post-traumatic stress disorder (20.8%). 21.8% were at moderate to high suicidal risk. 14.9 and 13.9% screened positive for ASD and ADHD, respectively. The greatest unmet needs were in the areas of intimate relationships (76.2%), psychological distress (72.3%) and sexual expression (71.3%). Overall presence of mental disorder was comparable with levels found in prisons. The numbers in each group were too small to carry out any further analysis.
Conclusion: It is feasible to undertake a psychiatric morbidity survey in an IRC. Limitations of the study include potential selection bias, use of screening tools, use of single-site study, high refusal rates, the lack of interpreters and lack of women and children in study sample. Future studies should involve the in-reach team to recruit participants and should be run by a steering group consisting of clinicians from the IRC as well as academics
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