41,129 research outputs found
Graviton propagator from background-independent quantum gravity
We study the graviton propagator in euclidean loop quantum gravity, using the
spinfoam formalism. We use boundary-amplitude and group-field-theory
techniques, and compute one component of the propagator to first order, under a
number of approximations, obtaining the correct spacetime dependence. In the
large distance limit, the only term of the vertex amplitude that contributes is
the exponential of the Regge action: the other terms, that have raised doubts
on the physical viability of the model, are suppressed by the phase of the
vacuum state, which is determined by the extrinsic geometry of the boundary.Comment: 6 pages. Substantially revised second version. Improved boundary
state ansat
Searching for Very High Energy Emission from Pulsars Using the High Altitude Water Cherenkov (HAWC) Observatory
There are currently over 160 known gamma-ray pulsars. While most of them are
detected only from space, at least two are now seen also from the ground. MAGIC
and VERITAS have measured the gamma ray pulsed emission of the Crab pulsar up
to hundreds of GeV and more recently MAGIC has reported emission at
TeV. Furthermore, in the Southern Hemisphere, H.E.S.S. has detected the Vela
pulsar above 30 GeV. In addition, non-pulsed TeV emission coincident with
pulsars has been detected by many groups, including the Milagro Collaboration.
These GeV-TeV observations open the possibility of searching for
very-high-energy (VHE, > 100GeV) pulsations from gamma-rays pulsars in the HAWC
field of view.Comment: Presented at the 34th International Cosmic Ray Conference (ICRC2015),
The Hague, The Netherlands. See arXiv:1508.03327 for all HAWC contribution
Gluon Saturation and Black Hole Criticality
We discuss the recent proposal in hep-th/0611312 where it was shown that the
critical anomalous dimension associated to the onset of non-linear effects in
the high energy limit of QCD coincides with the critical exponent governing the
radius of the black hole formed in the spherically symmetric collapse of a
massless scalar field. We argue that a new essential ingredient in this mapping
between gauge theory and gravity is continuous self-similarity, not present in
the scalar field case but in the spherical collapse of a perfect fluid with
barotropic equation of state. We identify this property with geometric scaling,
present in DIS data at small values of Bjorken x. We also show that the
Choptuik exponent in dimension five tends to the QCD critical value in the
traceless limit of the energy momentum tensor.Comment: Talk given at 12th International Conference on Elastic and
Diffractive Scattering: Forward Physics and QCD, Hamburg, DESY, Germany,
21-25 May 200
Integrable theories and loop spaces: fundamentals, applications and new developments
We review our proposal to generalize the standard two-dimensional flatness
construction of Lax-Zakharov-Shabat to relativistic field theories in d+1
dimensions. The fundamentals from the theory of connections on loop spaces are
presented and clarified. These ideas are exposed using mathematical tools
familiar to physicists. We exhibit recent and new results that relate the
locality of the loop space curvature to the diffeomorphism invariance of the
loop space holonomy. These result are used to show that the holonomy is abelian
if the holonomy is diffeomorphism invariant.
These results justify in part and set the limitations of the local
implementations of the approach which has been worked out in the last decade.
We highlight very interesting applications like the construction and the
solution of an integrable four dimensional field theory with Hopf solitons, and
new integrability conditions which generalize BPS equations to systems such as
Skyrme theories. Applications of these ideas leading to new constructions are
implemented in theories that admit volume preserving diffeomorphisms of the
target space as symmetries. Applications to physically relevant systems like
Yang Mills theories are summarized. We also discuss other possibilities that
have not yet been explored.Comment: 64 pages, 8 figure
Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders
Using the Density Matrix Renormalization Group and two-leg ladders, we
investigate an electronic two-orbital Hubbard model including plaquette
diagonal hopping amplitudes. Our goal is to search for regimes where charges
added to the undoped state form pairs, presumably a precursor of a
superconducting state.For the electronic density , i.e. the undoped
limit, our investigations show a robust antiferromagnetic ground
state, as in previous investigations. Doping away from and for large
values of the Hund coupling , a ferromagnetic region is found to be stable.
Moreover, when the interorbital on-site Hubbard repulsion is smaller than the
Hund coupling, i.e. for in the standard notation of multiorbital Hubbard
models, our results indicate the coexistence of pairing tendencies and
ferromagnetism close to . These results are compatible with previous
investigations using one dimensional systems. Although further research is
needed to clarify if the range of couplings used here is of relevance for real
materials, such as superconducting heavy fermions or pnictides, our theoretical
results address a possible mechanism for pairing that may be active in the
presence of short-range ferromagnetic fluctuations.Comment: 8 pages, 4 Fig
A Simple Algebraic Derivation of the Covariant Anomaly and Schwinger Term
An expression for the curvature of the "covariant" determinant line bundle is
given in even dimensional space-time. The usefulness is guaranteed by its
prediction of the covariant anomaly and Schwinger term. It allows a parallel
derivation of the consistent anomaly and Schwinger term, and their covariant
counterparts, which clarifies the similarities and differences between them. In
particular, it becomes clear that in contrary to the case for anomalies, the
difference between the consistent and covariant Schwinger term can not be
extended to a local form on the space of gauge potentials.Comment: 16 page
Revised Pulsar Spindown
We address the issue of electromagnetic pulsar spindown by combining our
experience from the two limiting idealized cases which have been studied in
great extent in the past: that of an aligned rotator where ideal MHD conditions
apply, and that of a misaligned rotator in vacuum. We construct a spindown
formula that takes into account the misalignment of the magnetic and rotation
axes, and the magnetospheric particle acceleration gaps. We show that near the
death line aligned rotators spin down much slower than orthogonal ones. In
order to test this approach, we use a simple Monte Carlo method to simulate the
evolution of pulsars and find a good fit to the observed pulsar distribution in
the P-Pdot diagram without invoking magnetic field decay. Our model may also
account for individual pulsars spinning down with braking index n < 3, by
allowing the corotating part of the magnetosphere to end inside the light
cylinder. We discuss the role of magnetic reconnection in determining the
pulsar braking index. We show, however, that n ~ 3 remains a good approximation
for the pulsar population as a whole. Moreover, we predict that pulsars near
the death line have braking index values n > 3, and that the older pulsar
population has preferentially smaller magnetic inclination angles. We discuss
possible signatures of such alignment in the existing pulsar data.Comment: 8 pages, 7 figures; accepted to Ap
Interacting social processes on interconnected networks
We propose and study a model for the interplay between two different
dynamical processes --one for opinion formation and the other for decision
making-- on two interconnected networks and . The opinion dynamics on
network corresponds to that of the M-model, where the state of each agent
can take one of four possible values (), describing its level of
agreement on a given issue. The likelihood to become an extremist ()
or a moderate () is controlled by a reinforcement parameter .
The decision making dynamics on network is akin to that of the
Abrams-Strogatz model, where agents can be either in favor () or against
() the issue. The probability that an agent changes its state is
proportional to the fraction of neighbors that hold the opposite state raised
to a power . Starting from a polarized case scenario in which all agents
of network hold positive orientations while all agents of network have
a negative orientation, we explore the conditions under which one of the
dynamics prevails over the other, imposing its initial orientation. We find
that, for a given value of , the two-network system reaches a consensus
in the positive state (initial state of network ) when the reinforcement
overcomes a crossover value , while a negative consensus happens
for . In the phase space, the system displays a
transition at a critical threshold , from a coexistence of both
orientations for to a dominance of one orientation for
. We develop an analytical mean-field approach that gives an
insight into these regimes and shows that both dynamics are equivalent along
the crossover line .Comment: 25 pages, 6 figure
Clustering in disordered ferromagnets: The Curie temperature in diluted magnetic semiconductors
We theoretically investigate impurity correlation and magnetic clustering
effects on the long-range ferromagnetic ordering in diluted magnetic
semiconductors, such as , using
analytical arguments and direct Monte Carlo simulations. We obtain an analytic
formula for the ferromagnetic transition temperature which becomes
asymptotically exact in the strongly disordered, highly dilute (i.e. small )
regime. We establish that impurity correlations have only small effects on
with the neutrally correlated random disorder producing the nominally
highest . We find that the ferromagnetic order is approached from the
high temperature paramagnetic side through a random magnetic clustering
phenomenon consistent with the percolation transition scenario.Comment: 5 pages, 4 figure
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