857 research outputs found
The Gerasimov-Drell-Hearn sum rule and the single-pion photoproduction multipole E0+ close to threshold
The long-standing discrepancy between the Gerasimov-Drell-Hearn sum rule and
the analysis of pion photoproduction multipoles is greatly diminished by use of
s-wave multipoles that are in accord with the predictions of chiral
perturbation theory and describe the experimental data in the threshold region.
The remaining difference may be due to contributions of channels with more
pions and/or heavier mesons whose contributions to the sum rule remain to be
investigated by a direct measurement of the photoabsorption cross sections.Comment: 9 pages, latex, 1 figure, to appear in Phys. Rev.
Dispersion relations and the spin polarizabilities of the nucleon
A forward dispersion calculation is implemented for the spin polarizabilities
of the proton and the neutron. These polarizabilities
are related to the spin structure of the nucleon at low energies and are
structure-constants of the Compton scattering amplitude at . In the absence of a direct experimental measurement of these
quantities, a dispersion calculation serves the purpose of constraining the
model building, and of comparing with recent calculations in heavy baryon
chiral perturbation theory.Comment: 14 pages Latex, 1 ps figur
Cosmic Strings Stabilized by Fermion Fluctuations
We provide a thorough exposition of recent results on the quantum
stabilization of cosmic strings. Stabilization occurs through the coupling to a
heavy fermion doublet in a reduced version of the standard model. The study
combines the vacuum polarization energy of fermion zero-point fluctuations and
the binding energy of occupied energy levels, which are of the same order in a
semi-classical expansion. Populating these bound states assigns a charge to the
string. Strings carrying fermion charge become stable if the Higgs and gauge
fields are coupled to a fermion that is less than twice as heavy as the top
quark. The vacuum remains stable in the model, because neutral strings are not
energetically favored. These findings suggest that extraordinarily large
fermion masses or unrealistic couplings are not required to bind a cosmic
string in the standard model.Comment: Based on talk by HW at QFEXT 11 (Benasque, Spain), 15p, uses
ws-ijmpcs.cls (incl
Bounds for mixing time of quantum walks on finite graphs
Several inequalities are proved for the mixing time of discrete-time quantum
walks on finite graphs. The mixing time is defined differently than in
Aharonov, Ambainis, Kempe and Vazirani (2001) and it is found that for
particular examples of walks on a cycle, a hypercube and a complete graph,
quantum walks provide no speed-up in mixing over the classical counterparts. In
addition, non-unitary quantum walks (i.e., walks with decoherence) are
considered and a criterion for their convergence to the unique stationary
distribution is derived.Comment: This is the journal version (except formatting); it is a significant
revision of the previous version, in particular, it contains a new result
about the convergence of quantum walks with decoherence; 16 page
Generalized polarizabilities and electroexcitation of the nucleon
Generalized nucleon polarizabilities for virtual photons can be defined in
terms of electroproduction cross sections as function of the 4-momentum
transfer . In particular, the sum of the generalized electric and magnetic
polarizabilities and the spin polarizability can
be expressed by virtual photon absorption cross sections integrated over the
excitation energy. These quantities have been calculated within the framework
of the recently developed unitary isobar model for pion photo- and
electroproduction on the proton, which describes the available experimental
data up to an excitation energy of about 1 GeV. Our results have been compared
to the predictions of chiral perturbation theory.Comment: 21 pages, latex, 14 figure
Spectral shift function for operators with crossed magnetic and electric fields
We obtain a representation formula for the derivative of the spectral shift
function related to the operators and . We establish a limiting absorption principle
for and an estimate for
, provided , where $Q =
(D_x - By)^2 + D_y^2 + V(x,y).
Impact of dynamical chiral symmetry breaking on meson structure and interactions
We provide a glimpse of recent progress in meson physics made via QCD's
Dyson-Schwinger equations with: a perspective on confinement and dynamical
chiral symmetry breaking (DCSB); a pre'cis on the physics of in-hadron
condensates; results for the masses of the \pi, \sigma, \rho, a_1 mesons and
their first-radial excitations; and an illustration of the impact of DCSB on
the pion form factor.Comment: 6 pages, 3 figures, 1 table. Contribution to Proceedings of the 11th
International Workshop on Meson Production, Properties and Interaction,
Uniwersytet Jagiellonski, Instytut Fizyki, Krakow, Poland, 10-15 June 201
- …