182 research outputs found
Duality violations and spectral sum rules
We study the issue of duality violations in the VV-AA vacuum polarization
function in the chiral limit. This is done with the help of a model with an
expansion in inverse powers of the number of colors, Nc, allowing us to
consider resonances with a finite width. Due to these duality violations, the
Operator Product Expansion (OPE) and the moments of the spectral function (e.g.
the Weinberg sum rules) do not match at finite momentum, and we analyze this
difference in detail. We also perform a comparative study of many of the
different methods proposed in the literature for the extraction of the OPE
parameters and find that, when applied to our model, they all fare quite
similarly. In fact, the model strongly suggests that a significant improvement
in precision can only be expected after duality violations are included. To
this end, we propose a method to parameterize these duality violations. The
method works quite well for the model, and we hope that it may also be useful
in future determinations of OPE parameters in QCD.Comment: 29 pages, 9 figures, LateX file. Small changes to match journal
versio
Orbital order out of spin disorder: How to measure the orbital gap
The interplay between spin and orbital degrees of freedom in the Mott-Hubbard
insulator is studied by considering an orbitally degenerate superexchange
model. We argue that orbital order and the orbital excitation gap in this model
are generated through the order-from-disorder mechanism known previously from
frustrated spin models. We propose that the orbital gap should show up
indirectly in the dynamical spin structure factor; it can therefore be measured
using the conventional inelastic neutron scattering method
Langevin dynamics of the Lebowitz-Percus model
We revisit the hard-spheres lattice gas model in the spherical approximation
proposed by Lebowitz and Percus (J. L. Lebowitz, J. K. Percus, Phys. Rev.{\
144} (1966) 251). Although no disorder is present in the model, we find that
the short-range dynamical restrictions in the model induce glassy behavior. We
examine the off-equilibrium Langevin dynamics of this model and study the
relaxation of the density as well as the correlation, response and overlap
two-time functions. We find that the relaxation proceeds in two steps as well
as absence of anomaly in the response function. By studying the violation of
the fluctuation-dissipation ratio we conclude that the glassy scenario of this
model corresponds to the dynamics of domain growth in phase ordering kinetics.Comment: 21 pages, RevTeX, 14 PS figure
Charged Particles in a 2+1 Curved Background
The coupling to a 2+1 background geometry of a quantized charged test
particle in a strong magnetic field is analyzed. Canonical operators adapting
to the fast and slow freedoms produce a natural expansion in the inverse square
root of the magnetic field strength. The fast freedom is solved to the second
order.
At any given time, space is parameterized by a couple of conjugate operators
and effectively behaves as the `phase space' of the slow freedom. The slow
Hamiltonian depends on the magnetic field norm, its covariant derivatives, the
scalar curvature and presents a peculiar coupling with the spin-connection.Comment: 22 page
Mildred Dresselhaus and Solid State Pedagogy at MIT
Mildred Dresselhaus is known for her influential research on the physics of carbon. Her wideâranging influence as a physics teacher, although wellâknown to her students, has been less thoroughly examined. Exploring how Dresselhaus grew into her role teaching solid state physics at MIT reveals much about how that subfield evolved
Indirect Searches for -like Resonances at the LHC
We explore the possibility of indirectly observing the effects of -like
particles with electroweak strength couplings in the Drell-Yan channel at the
LHC with masses above the resonance direct search reach. We find that, mostly
due to statistical limitations, this is very unlikely in almost all classes of
models independently of the spin of the resonance. Not unexpectedly, the one
possible exception to this general result is the case of degenerate
Kaluza-Klein (KK) excitations of the photon and that occur in some
extra-dimensional models. In this special case, the strong destructive
interference with the Standard Model (SM) exchanges below the resonance mass
leads to a well-known significant suppression of the cross section and thus
increased sensitivity to this particular new physics scenario.Comment: 18 pages, 6 figs, discussion and reference adde
The Higgs intense--coupling regime in constrained SUSY models and its astrophysical implications
We analyze the Higgs intense--coupling regime, in which all Higgs particles
of the Minimal Supersymmetric Standard Model are light with masses of the same
order and the value of \tb the ratio of vacuum expectation values of the two
Higgs fields is large, in the framework of Supergravity scenarios with
non--universal soft Supersymmetry breaking scalar masses in the Higgs sector.
In particular, we calculate the relic density abundance of the lightest
neutralino candidate for cold dark matter and the rates in direct and indirect
detection at present and future experiments. We first show that while in the
mSUGRA model this regime is disfavored by present data, there are regions in
the parameter space of models with non--universal Higgs masses where it can
occur. We then show that because of the large value of and the
relatively low values of the neutral Higgs boson masses, the cross section for
neutralino--nucleon scattering is strongly enhanced in this regime and would
allow for the observation of a signal in direct detection experiments such as
CDMS--Soudan. The expected sensitivity of gamma--ray detectors like GLAST might
be also sufficient to observe the annihilation of neutralinos in such a regime.Comment: 19 pages, 5 figure
Vortices in Ginzburg-Landau billiards
We present an analysis of the Ginzburg-Landau equations for the description
of a two-dimensional superconductor in a bounded domain. Using the properties
of a special integrability point of these equations which allows vortex
solutions, we obtain a closed expression for the energy of the superconductor.
The role of the boundary of the system is to provide a selection mechanism
for the number of vortices.
A geometrical interpretation of these results is presented and they are
applied to the analysis of the magnetization recently measured on small
superconducting disks. Problems related to the interaction and nucleation of
vortices are discussed.Comment: RevTex, 17 pages, 3 eps figure
Gluon mass generation in the PT-BFM scheme
In this article we study the general structure and special properties of the
Schwinger-Dyson equation for the gluon propagator constructed with the pinch
technique, together with the question of how to obtain infrared finite
solutions, associated with the generation of an effective gluon mass.
Exploiting the known all-order correspondence between the pinch technique and
the background field method, we demonstrate that, contrary to the standard
formulation, the non-perturbative gluon self-energy is transverse
order-by-order in the dressed loop expansion, and separately for gluonic and
ghost contributions. We next present a comprehensive review of several subtle
issues relevant to the search of infrared finite solutions, paying particular
attention to the role of the seagull graph in enforcing transversality, the
necessity of introducing massless poles in the three-gluon vertex, and the
incorporation of the correct renormalization group properties. In addition, we
present a method for regulating the seagull-type contributions based on
dimensional regularization; its applicability depends crucially on the
asymptotic behavior of the solutions in the deep ultraviolet, and in particular
on the anomalous dimension of the dynamically generated gluon mass. A
linearized version of the truncated Schwinger-Dyson equation is derived, using
a vertex that satisfies the required Ward identity and contains massless poles
belonging to different Lorentz structures. The resulting integral equation is
then solved numerically, the infrared and ultraviolet properties of the
obtained solutions are examined in detail, and the allowed range for the
effective gluon mass is determined. Various open questions and possible
connections with different approaches in the literature are discussed.Comment: 54 pages, 24 figure
Neutralino, axion and axino cold dark matter in minimal, hypercharged and gaugino AMSB
Supersymmetric models based on anomaly-mediated SUSY breaking (AMSB)
generally give rise to a neutral wino as a WIMP cold dark matter (CDM)
candidate, whose thermal abundance is well below measured values. Here, we
investigate four scenarios to reconcile AMSB dark matter with the measured
abundance: 1. non-thermal wino production due to decays of scalar fields ({\it
e.g} moduli), 2. non-thermal wino production due to decays of gravitinos, 3.
non-thermal wino production due to heavy axino decays, and 4. the case of an
axino LSP, where the bulk of CDM is made up of axions and thermally produced
axinos. In cases 1 and 2, we expect wino CDM to constitute the entire measured
DM abundance, and we investigate wino-like WIMP direct and indirect detection
rates. Wino direct detection rates can be large, and more importantly, are
bounded from below, so that ton-scale noble liquid detectors should access all
of parameter space for m_{\tz_1}\alt 500 GeV. Indirect wino detection rates via
neutrino telescopes and space-based cosmic ray detectors can also be large. In
case 3, the DM would consist of an axion plus wino admixture, whose exact
proportions are very model dependent. In this case, it is possible that both an
axion and a wino-like WIMP could be detected experimentally. In case 4., we
calculate the re-heat temperature of the universe after inflation. In this
case, no direct or indirect WIMP signals should be seen, although direct
detection of relic axions may be possible. For each DM scenario, we show
results for the minimal AMSB model, as well as for the hypercharged and gaugino
AMSB models.Comment: 29 pages including 13 figure
- âŠ