751 research outputs found
Yang--Mills sphalerons in all even spacetime dimensions , : =3,4
The classical solutions to higher dimensional Yang--Mills (YM) systems, which
are integral parts of higher dimensional Einstein--YM (EYM) systems, are
studied. These are the gravity decoupling limits of the fully gravitating EYM
solutions. In odd spacetime dimensions, depending on the choice of gauge group,
these are either topologically stable or unstable. Both cases are analysed, the
latter numerically only. In even spacetime dimensions they are always unstable,
describing saddle points of the energy, and can be described as {\it
sphalerons}. This instability is analysed by constructing the noncontractible
loops and calculating the Chern--Simons (CS) charges, and also perturbatively
by numerically constructing the negative modes. This study is restricted to the
simplest YM system in spacetime dimensions , which is amply
illustrative of the generic case.Comment: 16 pages, 3 figures ; comments added, to appear in J. Phys.
Searching for physics beyond the Standard Model through the dipole interaction
The magnetic dipole interaction played a central role in the development of
QED, and continued in that role for the Standard Model. The muon anomalous
magnetic moment has served as a benchmark for models of new physics, and the
present experimental value is larger than the standard-model value by more than
three standard deviations. The electric dipole moment (EDM) violates parity
({}) and time-reversal ({}) symmetries, and in the context of the
theorem, the combination of charge conjugation and parity (). Since a new
source of {} violation outside of that observed in the and meson
systems is needed to help explain the baryon asymmetry of the universe,
searches for EDMs are being carried out worldwide on a number of systems. The
standard-model value of the EDM is immeasurably small, so any evidence for an
EDM would signify the observation of new physics. Unique opportunities exist
for EDM searches using polarized proton, deuteron or muon beams in storage
rings. This talk will provide an overview of the theory of dipole moments, and
the relevant experiments. The connection to the transition dipole moment that
could produce lepton flavor violating interactions such as is also mentioned.Comment: Invited Plenary talk at the 19th International Spin Physics
Symposium, Juelic
Ultra-High Energy Neutrino Fluxes: New Constraints and Implications
We apply new upper limits on neutrino fluxes and the diffuse extragalactic
component of the GeV gamma-ray flux to various scenarios for ultra high energy
cosmic rays and neutrinos. As a result we find that extra-galactic top-down
sources can not contribute significantly to the observed flux of highest energy
cosmic rays. The Z-burst mechanism where ultra-high energy neutrinos produce
cosmic rays via interactions with relic neutrinos is practically ruled out if
cosmological limits on neutrino mass and clustering apply.Comment: 10 revtex pages, 9 postscript figure
Near-field interactions between metal nanoparticle surface plasmons and molecular excitons in thin-films: part I: absorption
In this and the following paper (parts I and II, respectively), we systematically study the interactions between surface plasmons of metal nanoparticles (NPs) with excitons in thin-films of organic media. In an effort to exclusively probe near-field interactions, we utilize spherical Ag NPs in a size-regime where far-field light scattering is negligibly small compared to absorption. In part I, we discuss the effect of the presence of these Ag NPs on the absorption of the embedding medium by means of experiment, numerical simulations, and analytical calculations, all shown to be in good agreement. We observe absorption enhancement in the embedding medium due to the Ag NPs with a strong dependence on the medium permittivity, the spectral position relative to the surface plasmon resonance frequency, and the thickness of the organic layer. By introducing a low index spacer layer between the NPs and the organic medium, this absorption enhancement is experimentally confirmed to be a near field effect In part II, we probe the impact of the Ag NPs on the emission of organic molecules by time-resolved and steady-state photoluminescence measurements
Towards a Tetravalent Chemistry of Colloids
We propose coating spherical particles or droplets with anisotropic
nano-sized objects to allow micron-scale colloids to link or functionalize with
a four-fold valence, similar to the sp3 hybridized chemical bonds associated
with, e.g., carbon, silicon and germanium. Candidates for such coatings include
triblock copolymers, gemini lipids, metallic or semiconducting nanorods and
conventional liquid crystal compounds. We estimate the size of the relevant
nematic Frank constants, discuss how to obtain other valences and analyze the
thermal distortions of ground state configurations of defects on the sphere.Comment: Replaced to improve figures. 4 figures Nano Letter
Spectra of PP-Wave Limits of M-/Superstring Theory on AdS_p x S^q Spaces
In this paper we show how one can obtain very simply the spectra of the
PP-wave limits of M-theory over AdS_7(4) x S^4(7) spaces and IIB superstring
theory over AdS_5 x S^5 from the oscillator construction of the Kaluza-Klein
spectra of these theories over the corresponding spaces. The PP-wave symmetry
superalgebras are obtained by taking the number P of ``colors'' of oscillators
to be large (infinite). In this large P limit, the symmetry superalgebra
osp(8*|4) of AdS_7 x S^4 and the symmetry superalgebra osp(8|4,R) of AdS_4 x
S^7 lead to isomorphic PP-wave algebras, which is the semi-direct sum of
su(4|2) with H^(18,16), while the symmetry superalgebra su(2,2|4) of AdS_5 x
S^5 leads to the semi-direct sum of [psu(2|2) + psu(2|2) + u(1)] with H^(16,16)
as its PP-wave algebra [H^(m,n) denoting a super-Heisenberg algebra with m
bosonic and n fermionic generators]. The zero mode spectra of M-theory or IIB
superstring theory in the PP-wave limit corresponds simply to the unitary
positive energy representations of these algebras whose lowest weight vector is
the Fock vacuum of all the oscillators. General positive energy supermultiplets
including those corresponding to higher modes can similarly be constructed by
the oscillator method.Comment: Typos corrected; references added; minor modifications to improve
presentation; 37 pages, LaTeX fil
Diblock copolymers at a homopolymer-homopolymer-interface: a Monte Carlo simulation
The properties of diluted symmetric A-B diblock copolymers at the interface
between A and B homopolymer phases are studied by means of Monte Carlo (MC)
simulations of the bond fluctuation model. We calculate segment density
profiles as well as orientational properties of segments, of A and B blocks,
and of the whole chain. Our data support the picture of oriented ``dumbbells'',
which consist of mildly perturbed A and B Gaussian coils. The results are
compared to a self consistent field theory (SCFT) for single copolymer chains
at a homopolymer interface. We also discuss the number of interaction contacts
between monomers, which provide a measure for the ``active surface'' of
copolymers or homopolymers close to the interface
Quasiparticle Description of the QCD Plasma, Comparison with Lattice Results at Finite T and Mu
We compare our 2+1 flavor, staggered QCD lattice results with a quasiparticle
picture. We determine the pressure, the energy density, the baryon density, the
speed of sound and the thermal masses as a function of T and . For the
available thermodynamic quantities the difference is a few percent between the
results of the two approaches. We also give the phase diagram on the --T
plane and estimate the critical chemical potential at vanishing temperature.Comment: 13 pages, 10 figure
A surface-patterned chip as a strong source of ultracold atoms for quantum technologies
Laser-cooled atoms are central to modern precision measurements. They are also increasingly important as an enabling technology for experimental cavity quantum electrodynamics, quantum information processing and matter–wave interferometry. Although significant progress has been made in miniaturizing atomic metrological devices, these are limited in accuracy by their use of hot atomic ensembles and buffer gases. Advances have also been made in producing portable apparatus that benefits from the advantages of atoms in the microkelvin regime. However, simplifying atomic cooling and loading using microfabrication technology has proved difficult. In this Letter we address this problem, realizing an atom chip that enables the integration of laser cooling and trapping into a compact apparatus. Our source delivers ten thousand times more atoms than previous magneto-optical traps with microfabricated optics and, for the first time, can reach sub-Doppler temperatures. Moreover, the same chip design offers a simple way to form stable optical lattices. These features, combined with simplicity of fabrication and ease of operation, make these new traps a key advance in the development of cold-atom technology for high-accuracy, portable measurement devices
- …