985 research outputs found
Reality of Complex Affine Toda Solitons
There are infinitely many topological solitons in any given complex affine
Toda theories and most of them have complex energy density. When we require the
energy density of the solitons to be real, we find that the reality condition
is related to a simple ``pairing condition.'' Unfortunately, rather few soliton
solutions in these theories survive the reality constraint, especially if one
also demands positivity. The resulting implications for the physical
applicability of these theories are briefly discussed.Comment: LaTeX, 15 pages, UBTH-049
Causality violation, gravitational shockwaves and UV completion
The effective actions describing the low-energy dynamics of QFTs involving
gravity generically exhibit causality violations. These may take the form of
superluminal propagation or Shapiro time advances and allow the construction of
"time machines", i.e. spacetimes admitting closed non-spacelike curves. Here,
we discuss critically whether such causality violations may be used as a
criterion to identify unphysical effective actions or whether, and how,
causality problems may be resolved by embedding the action in a fundamental, UV
complete QFT. We study in detail the case of photon scattering in an
Aichelburg-Sexl gravitational shockwave background and calculate the phase
shifts in QED for all energies, demonstrating their smooth interpolation from
the causality-violating effective action values at low-energy to their
manifestly causal high-energy limits. At low energies, these phase shifts may
be interpreted as backwards-in-time coordinate jumps as the photon encounters
the shock wavefront, and we illustrate how the resulting causality problems
emerge and are resolved in a two-shockwave time machine scenario. The
implications of our results for ultra-high (Planck) energy scattering, in which
graviton exchange is modelled by the shockwave background, are highlighted.Comment: 42 pages, 15 figures, updated reference
The AdS(5)xS(5) Semi-Symmetric Space Sine-Gordon Theory
The generalized symmetric space sine-Gordon theories are a series of
1+1-integrable field theories that are classically equivalent to superstrings
on symmetric space spacetimes F/G. They are formulated in terms of a
semi-symmetric space as a gauged WZW model with fermions and a potential term
to deform it away from the conformal fixed point. We consider in particular the
case of PSU(2,2|4)/Sp(2,2)xSp(4) which corresponds to AdS(5)xS(5). We argue
that the infinite tower of conserved charges of these theories includes an
exotic N=(8,8) supersymmetry that is realized in a mildy non-local way at the
Lagrangian level. The supersymmetry is associated to a double central extension
of the superalgebra psu(2|2)+psu(2|2) and includes a non-trivial R symmetry
algebra corresponding to global gauge transformations, as well as 2-dimensional
spacetime translations. We then explicitly construct soliton solutions and show
that they carry an internal moduli superspace CP(2|1)xCP(2|1) with both bosonic
and Grassmann collective coordinates. We show how to semi-classical quantize
the solitons by writing an effective quantum mechanical system on the moduli
space which takes the form of a co-adjoint orbit of SU(2|2)xSU(2|2). The
spectrum consists of a tower of massive states in the short, or atypical,
symmetric representations, just as the giant magnon states of the string world
sheet theory, although here the tower is truncated.Comment: 39 pages, references adde
Causality, renormalizability and ultra-high energy gravitational scattering
The amplitude A(s,t) for ultra-high energy scattering can be found in the
leading eikonal approximation by considering propagation in an Aichelburg-Sexl
gravitational shockwave background. Loop corrections in the QFT describing the
scattered particles are encoded for energies below the Planck scale in an
effective action which in general exhibits causality violation and Shapiro time
advances. In this paper, we use Penrose limit techniques to calculate the full
energy dependence of the scattering phase shift Theta_scat(hat_s},, where the
single variable hat_s = Gs/m^2 b^(d-2) contains both the CM energy s and impact
parameter b, for a range of scalar QFTs in d dimensions with different
renormalizability properties. We evaluate the high-energy limit of
Theta_scat(hat_s) and show in detail how causality is related to the existence
of a well-defined UV completion. Similarities with graviton scattering and the
corresponding resolution of causality violation in the effective action by
string theory are briefly discussed.Comment: 23 page
A New and Elementary CP^n Dyonic Magnon
We show that the dressing transformation method produces a new type of dyonic
CP^n magnon in terms of which all the other known solutions are either
composites or arise as special limits. In particular, this includes the
embedding of Dorey's dyonic magnon via an RP^3 subspace of CP^n. We also show
how to generate Dorey's dyonic magnon directly in the S^n sigma model via the
dressing method without resorting to the isomorphism with the SU(2) principle
chiral model when n=3. The new dyon is shown to be either a charged dyon or
topological kink of the related symmetric-space sine-Gordon theories associated
to CP^n and in this sense is a direct generalization of the soliton of the
complex sine-Gordon theory.Comment: 21 pages, JHEP3, typos correcte
Solitons in Two--Dimensional Topological Field Theories
We consider a class of supersymmetric non--unitary theories in
two--dimensional Minkowski spacetime which admit classical solitonic solutions.
We show how these models can be twisted into a topological sector whose
energy--momentum tensor is a BRST commutator. There is an infinite number of
degrees of freedom associated to the zero modes of the solitons. As explicit
realizations of such models we discuss the BRST quantization of a system of
free fields, while in the interacting case we study complexified twisted
Toda theories.Comment: 14 pages, IFUM 441/F
An integrable deformation of the AdS5Ă—S5superstring
The S-matrix on the world-sheet theory of the string in AdS5 x S5 has
previously been shown to admit a deformation where the symmetry algebra is
replaced by the associated quantum group. The case where q is real has been
identified as a particular deformation of the Green-Schwarz sigma model. An
interpretation of the case with q a root of unity has, until now, been lacking.
We show that the Green-Schwarz sigma model admits a discrete deformation which
can be viewed as a rather simple deformation of the F/F_V gauged WZW model,
where F=PSU(2,2|4). The deformation parameter q is then a k-th root of unity
where k is the level. The deformed theory has the same equations-of-motion as
the Green-Schwarz sigma model but has a different symplectic structure. We show
that the resulting theory is integrable and has just the right amount of
kappa-symmetries that appear as a remnant of the fermionic part of the original
gauge symmetry. This points to the existence of a fully consistent deformed
string background.Comment: 23 pages, improved and expanded discussion of metric and B fiel
The Effect of Gravitational Tidal Forces on Renormalized Quantum Fields
The effect of gravitational tidal forces on renormalized quantum fields
propagating in curved spacetime is investigated and a generalisation of the
optical theorem to curved spacetime is proved. In the case of QED, the
interaction of tidal forces with the vacuum polarization cloud of virtual e^+
e^- pairs dressing the renormalized photon has been shown to produce several
novel phenomena. In particular, the photon field amplitude can locally increase
as well as decrease, corresponding to a negative imaginary part of the
refractive index, in apparent violation of unitarity and the optical theorem.
Below threshold decays into e^+ e^- pairs may also occur. In this paper, these
issues are studied from the point of view of a non-equilibrium initial-value
problem, with the field evolution from an initial null surface being calculated
for physically distinct initial conditions and for both scalar field theories
and QED. It is shown how a generalised version of the optical theorem, valid in
curved spacetime, allows a local increase in amplitude while maintaining
consistency with unitarity. The picture emerges of the field being dressed and
undressed as it propagates through curved spacetime, with the local
gravitational tidal forces determining the degree of dressing and hence the
amplitude of the renormalized quantum field. These effects are illustrated with
many examples, including a description of the undressing of a photon in the
vicinity of a black hole singularity.Comment: 76 pages, jheppub.sty, 10 figures, small corrections. arXiv admin
note: text overlap with arXiv:1006.014
Bound States of the q-Deformed AdS5 x S5 Superstring S-matrix
The investigation of the q deformation of the S-matrix for excitations on the
string world sheet in AdS5 x S5 is continued. We argue that due to the lack of
Lorentz invariance the situation is more subtle than in a relativistic theory
in that the nature of bound states depends on their momentum. At low enough
momentum |p|<E the bound states transform in the anti-symmetric representation
of the super-algebra symmetry and become the solitons of the Pohlmeyer reduced
theory in the relativistic limit. At a critical momentum |p|=E they become
marginally unstable, and at higher momenta the stable bound states are in the
symmetric representation and become the familiar magnons in the string limit as
q->1. This subtlety fixes a problem involving the consistency of crossing
symmetry with the relativistic limit found in earlier work. With mirror
kinematics, obtained after a double Wick rotation, the bound state structure is
simpler and there are no marginally unstable bound states.Comment: 25 page
Non-perturbative effects in the BMN limit of N=4 supersymmetric Yang-Mills
One-instanton contributions to the correlation functions of two
gauge-invariant single-trace operators in N=4 SU(N) Yang-Mills theory are
studied in semi-classical approximation in the BMN limit. The most
straightforward examples involve operators with four bosonic impurities. The
explicit form for the correlation functions, which determine the anomalous
dimensions, follows after integration over the large number of bosonic and
fermionic moduli. Our results demonstrate that the instanton contributions
scale appropriately in the BMN limit. We find impressive agreement with the
D-instanton contributions to mass matrix elements of the dual plane-wave IIB
superstring theory, obtained in a previous paper. Not only does the dependence
on the scaled coupling constants match, but the dependence on the mode numbers
of the states is also in striking agreement.Comment: 52 pages, no figures, latex; V2: minor change
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