31,840 research outputs found
Off-Shell Electromagnetic Form Fators of the Nucleon in Chiral Perturbation Theory
We study the electromagnetic form factors of a nucleon in next-to-leading
order chiral perturbation theory (CPT) in the case where one of the nucleons is
off its mass shell. We calculate the leading nonanalytic contributions to
relevant measures for the off-shell dependence in the limited kinematical range
allowed.Comment: 3 pages LaTeX with worldsci.sty (available by mailing
[email protected] and typing "get worldsci.sty" in the subject line),
invited talk given at the International Symposium on Medium Energy Physics,
Beijing, August 199
Carleman estimates and absence of embedded eigenvalues
Let L be a Schroedinger operator with potential W in L^{(n+1)/2}. We prove
that there is no embedded eigenvalue. The main tool is an Lp Carleman type
estimate, which builds on delicate dispersive estimates established in a
previous paper. The arguments extend to variable coefficient operators with
long range potentials and with gradient potentials.Comment: 26 page
The pion form factor on the lattice at zero and finite temperature
We calculate the electromagnetic form factor of the pion in quenched lattice
QCD. The non-perturbatively improved Sheikoleslami-Wohlert lattice action is
used together with the consistently O(a) improved current. We calculate the
pion form factor for masses down to m_pi = 360 MeV, extract the charge radius,
and extrapolate toward the physical pion mass. In the second part, we discuss
results for the pion form factor and charge radius at 0.93 T_c and compare with
zero temperature results.Comment: Invited talk at the Lightcone 2004 conference, Amsterdam, 16-20
August, 200
Charm production in deep inelastic and diffractive scattering
We consider the production of charm by real and virtual photons. Special
attention is paid to diffractive charm production, which provides information
on the gluonic content of the Pomeron. Our calculations are based on the gluon
distributions of the CKMT-model, which is shown to lead to agreement with the
data on open charm production in deep inelastic scattering. We compare
predictions for diffractive charm production of different models for the
distribution of gluons in the Pomeron. Experiments at HERA should be able to
discriminate between them. Predictions for beauty production in diffractive and
non-diffractive interactions of photons are also given.Comment: 14 pages REVTEX and 24 figures include
A machine learning study to identify spinodal clumping in high energy nuclear collisions
The coordinate and momentum space configurations of the net baryon number in heavy ion collisions that undergo spinodal decomposition, due to a first-order phase transition, are investigated using state-of-the-art machine-learning methods. Coordinate space clumping, which appears in the spinodal decomposition, leaves strong characteristic imprints on the spatial net density distribution in nearly every event which can be detected by modern machine learning techniques. On the other hand, the corresponding features in the momentum distributions cannot clearly be detected, by the same machine learning methods, in individual events. Only a small subset of events can be systematically differ- entiated if only the momentum space information is available. This is due to the strong similarity of the two event classes, with and without spinodal decomposition. In such sce- narios, conventional event-averaged observables like the baryon number cumulants signal a spinodal non-equilibrium phase transition. Indeed the third-order cumulant, the skewness, does exhibit a peak at the beam energy (Elab = 3–4 A GeV), where the transient hot and dense system created in the heavy ion collision reaches the first-order phase transition
A Renormalization Group for Hamiltonians: Numerical Results
We describe a renormalization group transformation that is related to the
breakup of golden invariant tori in Hamiltonian systems with two degrees of
freedom. This transformation applies to a large class of Hamiltonians, is
conceptually simple, and allows for accurate numerical computations. In a
numerical implementation, we find a nontrivial fixed point and determine the
corresponding critical index and scaling. Our computed values for various
universal constants are in good agreement with existing data for
area-preserving maps. We also discuss the flow associated with the nontrivial
fixed point.Comment: 11 Pages, 2 Figures. For future updates, check
ftp://ftp.ma.utexas.edu/pub/papers/koch
Crossing conditions in coupled cluster theory
We derive the crossing conditions at conical intersections between electronic
states in coupled cluster theory, and show that if the coupled cluster Jacobian
matrix is nondefective, two (three) independent conditions are correctly placed
on the nuclear degrees of freedom for an inherently real (complex) Hamiltonian.
Calculations using coupled cluster theory on an
conical intersection in hypofluorous acid illustrate the nonphysical artifacts
associated with defects at accidental same-symmetry intersections. In
particular, the observed intersection seam is folded about a space of the
correct dimensionality, indicating that minor modifications to the theory are
required for it to provide a correct description of conical intersections in
general. We find that an accidental symmetry allowed
intersection in hydrogen sulfide is properly described, showing no artifacts as
well as linearity of the energy gap to first order in the branching plane.Comment: 9 pages and 4 figure
Direct Investigation of Superparamagnetism in Co Nanoparticle Films
A direct probe of superparamagnetism was used to determine the complete
anisotropy energy distribution of Co nanoparticle films. The films were
composed of self-assembled lattices of uniform Co nanoparticles 3 nm or 5 nm in
diameter, and a variable temperature scanning-SQUID microscope was used to
measure temperature-induced spontaneous magnetic noise in the samples. Accurate
measurements of anisotropy energy distributions of small volume samples will be
critical to magnetic optimization of nanoparticle devices and media.Comment: 4 pages, 4 figures. Submitted to Physical Review Letter
Current Induced Excitations in Cu/Co/Cu Single Ferromagnetic Layer Nanopillars
Current-induced magnetic excitations in Cu/Co/Cu single layer nanopillars
(~50 nm in diameter) have been studied experimentally as a function of Co layer
thickness at low temperatures for large applied fields perpendicular to the
layers. For asymmetric junctions current induced excitations are observed at
high current densities for only one polarity of the current and are absent at
the same current densities in symmetric junctions. These observations confirm
recent predictions of spin-transfer torque induced spin wave excitations in
single layer junctions with a strong asymmetry in the spin accumulation in the
leads.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
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