138 research outputs found
String Phase Transitions in a Strong Magnetic Field
We consider open strings in an external constant magnetic field . For an
(infinite) sequence of critical values of an increasing number of (highest
spin component) states lying on the first Regge trajectory becomes tachyonic.
In the limit of infinite all these states are tachyons (with a common
tachyonic mass) both in the case of the bosonic string and for the
Neveu-Schwarz sector of the fermionic string. This result generalizes to
extended object the same instability which occurs in ordinary non-Abelian gauge
theories. The Ramond states have always positive square masses as is the case
for ordinary QED. The weak field limit of the mass spectrum is the same as for
a field theory with gyromagnetic ratio for all charged spin states.
This behavior suggests a phase transition of the string as it has been argued
for the ordinary electroweak theory.Comment: 8 page
On the Central Charge of Spacetime Current Algebras and Correlators in String Theory on AdS3
Spacetime Virasoro and affine Lie algebras for strings propagating in AdS3
are known to all orders in . The central extension of such algebras is
a string vertex, whose expectation value can depend on the number of long
strings present in the background but should be otherwise state-independent. In
hep-th/0106004, on the other hand, a state-dependent expectation value was
found. Another puzzling feature of the theory is lack of cluster decomposition
property in certain connected correlators. This note shows that both problems
can be removed by defining the free energy of the spacetime boundary conformal
field theory as the Legendre transform of the formula proposed in the
literature. This corresponds to pass from a canonical ensemble, where the
number of fundamental strings that create the background can fluctuate, to a
microcanonical one, where it is fixed.Comment: 10 pages, one minor correction, version published in JHE
Notes on a Cure for Higher-Spin Acausality
We present a Lagrangian describing a massive charged spin-2 field and a
scalar in a constant electromagnetic background, and we provide a consistent
description of the system. The Lagrangian, derived from string field theory
through a suitable dimensional reduction, propagates the correct number of
degrees of freedom within the light cone in any space-time dimension less than
26. We briefly discuss the higher-spin generalization of this construction,
that cures the pathologies of a massive charged particle of arbitrary integer
spin by introducing only finitely many new massive degrees of freedom.Comment: 14 pages, comments and references added, minor reorganization done.
To appear in PR
On a Canonical Quantization of 3D Anti de Sitter Pure Gravity
We perform a canonical quantization of pure gravity on AdS3 using as a
technical tool its equivalence at the classical level with a Chern-Simons
theory with gauge group SL(2,R)xSL(2,R). We first quantize the theory
canonically on an asymptotically AdS space --which is topologically the real
line times a Riemann surface with one connected boundary. Using the "constrain
first" approach we reduce canonical quantization to quantization of orbits of
the Virasoro group and Kaehler quantization of Teichmuller space. After
explicitly computing the Kaehler form for the torus with one boundary component
and after extending that result to higher genus, we recover known results, such
as that wave functions of SL(2,R) Chern-Simons theory are conformal blocks. We
find new restrictions on the Hilbert space of pure gravity by imposing
invariance under large diffeomorphisms and normalizability of the wave
function. The Hilbert space of pure gravity is shown to be the target space of
Conformal Field Theories with continuous spectrum and a lower bound on operator
dimensions. A projection defined by topology changing amplitudes in Euclidean
gravity is proposed. It defines an invariant subspace that allows for a dual
interpretation in terms of a Liouville CFT. Problems and features of the CFT
dual are assessed and a new definition of the Hilbert space, exempt from those
problems, is proposed in the case of highly-curved AdS3.Comment: 61 pages, 7 figures. Minor misprints corrected, text in sections 1.3
and 5.4 clarified; version accepted for publication in JHEP. The first
version was released jointly with arXiv:1508.04079 [hep-th
Ghosts of Critical Gravity
Recently proposed "critical" higher-derivative gravities in are
expected to carry logarithmic representation of the Anti de Sitter isometry
group. In this note, we quantize linear fluctuations of these critical
gravities, which are known to be either identical with linear fluctuations of
Einstein's gravity or else satisfy logarithmic boundary conditions at spacial
infinity. We identify the scalar product uniquely defined by the symplectic
structure implied by the classical action, and show that it does not posses
null vectors. Instead, we show that the scalar product between any two Einstein
modes vanishes, while the scalar product of an Einstein mode with a logarithmic
mode is generically nonzero. This is the basic property of logarithmic
representation that makes them neither unitary nor unitarizable.Comment: v2: typos corrected and slight clarifications. 11 page
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