593 research outputs found
Quantum hair and the string-black hole correspondence
We consider a thought experiment in which an energetic massless string probes
a "stringhole" (a heavy string lying on the correspondence curve between
strings and black holes) at large enough impact parameter for the regime to be
under theoretical control. The corresponding, explicitly unitary, -matrix
turns out to be perturbatively sensitive to the microstate of the stringhole:
in particular, at leading order in , it depends on a projection of the
stringhole's Lorentz-contracted quadrupole moment. The string-black hole
correspondence is therefore violated if one assumes quantum hair to be
exponentially suppressed as a function of black-hole entropy. Implications for
the information paradox are briefly discussed.Comment: 11 pages, no figures, typos corrected, discussion and references
added, version accepted for publication in Class. and Quantum Gravit
Gravitational Radiation from Massless Particle Collisions
We compute classical gravitational bremsstrahlung from the gravitational
scattering of two massless particles at leading order in the (center of mass)
deflection angle . The calculation,
although non-perturbative in the gravitational constant, is surprisingly simple
and yields explicit formulae --in terms of multidimensional integrals-- for the
frequency and angular distribution of the radiation. In the range , the GW spectrum behaves like , is confined to cones of angular sizes (around the deflected particle
trajectories) ranging from to , and exactly
reproduces, at its lower end, a well-known zero-frequency limit.
At the radiation is confined to cones of angular size of
order resulting in a scale-invariant
() spectrum. The total efficiency in GW production is dominated
by this "high frequency" region and is formally logarithmically divergent in
the UV. If the spectrum is cutoff at the limit of validity of our
approximations (where a conjectured bound on GW power is also saturated), the
fraction of incoming energy radiated away turns out to be at leading logarithmic accuracy.Comment: Reference to related work added. Version accepted by Classical &
Quantum Gravit
A supersymmetric matrix model: II. Exploring higher-fermion-number sectors
Continuing our previous analysis of a supersymmetric quantum-mechanical
matrix model, we study in detail the properties of its sectors with fermion
number F=2 and 3. We confirm all previous expectations, modulo the appearance,
at strong coupling, of {\it two} new bosonic ground states causing a further
jump in Witten's index across a previously identified critical 't Hooft
coupling . We are able to elucidate the origin of these new SUSY
vacua by considering the limit and a strong coupling
expansion around it.Comment: 14 pages, 4 figure
Emerging Hawking-like Radiation from Gravitational Bremsstrahlung Beyond the Planck Scale
We argue that, as a consequence of the graviton's spin-2, its bremsstrahlung
in trans-planckian-energy () gravitational scattering at small
deflection angle can be nicely expressed in terms of helicity-transformation
phases and their transfer within the scattering process. The resulting spectrum
exhibits deeply sub-planckian characteristic energies of order (reminiscent of Hawking radiation), a suppressed fragmentation region, and
a reduced rapidity plateau, in broad agreement with recent classical estimates.Comment: 5 pages, 3 figures. Comments and references added. Typos corrected.
Fig. 3 modified. Title modified. Same as published articl
A note on C-Parity Conservation and the Validity of Orientifold Planar Equivalence
We analyze the possibility of a spontaneous breaking of C-invariance in gauge
theories with fermions in vector-like - but otherwise generic - representations
of the gauge group. QCD, supersymmetric Yang-Mills theory, and orientifold
field theories, all belong to this class. We argue that charge conjugation is
not spontaneously broken as long as Lorentz invariance is maintained.
Uniqueness of the vacuum state in pure Yang-Mills theory (without fermions) and
convergence of the expansion in fermion loops are key ingredients. The fact
that C-invariance is conserved has an interesting application to our proof of
planar equivalence between supersymmetric Yang-Mills theory and orientifold
field theory on R4, since it allows the use of charge conjugation to connect
the large-N limit of Wilson loops in different representations.Comment: 9 pages, LaTex. v2: minor changes, accepted to Phys.Lett.
Dimensionally reduced SYM at large-: an intriguing Coulomb approximation
We consider the light-cone (LC) gauge and LC quantization of the dimensional
reduction of super Yang Mills theory from four to two dimensions. After
integrating out all unphysical degrees of freedom, the non-local LC Hamiltonian
exhibits an explicit supersymmetry. A further SUSY-preserving
compactification of LC-space on a torus of radius , allows for a large-
numerical study where the smooth large- limit of physical quantities can be
checked. As a first step, we consider a simple, yet quite rich, "Coulomb
approximation" that maintains an subgroup of the original
supersymmetry and leads to a non-trivial generalization of 't Hooft's model
with an arbitrary --but conserved-- number of partons. We compute numerically
the eigenvalues and eigenvectors both in momentum and in position space. Our
results, so far limited to the sectors with 2, 3 and 4 partons, directly and
quantitatively confirm a simple physical picture in terms of a string-like
interaction with the expected tension among pairs of nearest-neighbours along
the single-trace characterizing the large- limit. Although broken by our
approximation, traces of the full supersymmetry are still
visible in the low-lying spectrum.Comment: 30 pages, 13 figures, Footnote page 3 replaced, Note Added at the
end, 4 References adde
Observation angles, Fermi coordinates, and the Geodesic-Light-Cone gauge
We show that the angular directions locally measured by a static geodesic
observer in a generic cosmological background and expressed in the system of
Fermi Normal Coordinates always coincide with those expressed in the
Geodesic-Light-Cone (GLC) gauge, up to a local transformation which exploits
the residual gauge freedom of the GLC coordinates. This is not the case for
other gauges - like, for instance, the synchronous and longitudinal gauge -
commonly used in the context of observational cosmology. We also make an
explicit proposal for the GLC gauge-fixing condition that ensures a full
identification of its angles with the observational ones.Comment: 14 pages, version accepted for publication on JCA
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