38,679 research outputs found
Collins Fragmentation and the Single Transverse Spin Asymmetry
We study the Collins mechanism for the single transverse spin asymmetry in
the collinear factorization approach. The correspondent twist-three
fragmentation function is identified. We show that the Collins function
calculated in this approach is universal. We further examine its contribution
to the single transverse spin asymmetry of semi-inclusive hadron production in
deep inelastic scattering and demonstrate that the transverse momentum
dependent and twist-three collinear approaches are consistent in the
intermediate transverse momentum region where both apply.Comment: 10 pages, 2 figure
Structured variable selection and estimation
In linear regression problems with related predictors, it is desirable to do
variable selection and estimation by maintaining the hierarchical or structural
relationships among predictors. In this paper we propose non-negative garrote
methods that can naturally incorporate such relationships defined through
effect heredity principles or marginality principles. We show that the methods
are very easy to compute and enjoy nice theoretical properties. We also show
that the methods can be easily extended to deal with more general regression
problems such as generalized linear models. Simulations and real examples are
used to illustrate the merits of the proposed methods.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS254 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
On Primordial Perturbations of Test Scalar Fields
The primordial perturbations of test scalar fields not affecting the
evolution of background may be very interesting since they can be transferred
to the curvature perturbations by some mechanisms, and thus under certain
condition can be responsible for the structure formation of observable
universe. In this brief report we study the primordial perturbations of test
scalar fields in various (super)accelerated expanding backgrounds.Comment: 4 pages, no figures, clarifications and refs. added, typos corrected,
to publish in PR
Moduli and K\"ahler potential in fermionic strings
We study the problem of identifying the moduli fields in fermionic
four-dimensional string models. We deform a free-fermionic model by introducing
exactly marginal operators in the form of Abelian Thirring interactions on the
world-sheet, and show that their couplings correspond to the untwisted moduli
fields. We study the consequences of this method for simple free-fermionic
models which correspond to orbifolds and obtain their moduli
space and K\"ahler potential by symmetry arguments and by direct calculation of
string scattering amplitudes. We then generalize our analysis to more
complicated fermionic structures which arise in constructions of realistic
models corresponding to asymmetric orbifolds, and obtain the moduli space and
K\"ahler potential for this case. Finally we extend our analysis to the
untwisted matter sector and derive expressions for the full K\"ahler potential
to be used in phenomenological applications, and the target space duality
transformations of the corresponding untwisted matter fields.Comment: 27pp Latex text, no figs, CERN-TH.7259/94, CTP-TAMU-14/94 and
ACT-06/9
Hadronic Dijet Imbalance and Transverse-Momentum Dependent Parton Distributions
We compare several recent theoretical studies of the single transverse spin
asymmetry in dijet-correlations at hadron colliders. We show that the results
of these studies are all consistent. To establish this, we investigate in
particular the two-gluon exchange contributions to the relevant initial and
final state interactions in the context of a simplifying model. Overall, the
results confirm that the dijet imbalance obeys at best a non-standard or
"generalized" transverse-momentum-dependent factorization.Comment: 13 pages, 3 figure
Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum
In magnetic materials, skyrmions are nanoscale regions where the orientation
of electron spin changes in a vortex-type manner. Here we show that spin-orbit
coupling in a focused vector beam results in a skyrmion-like photonic spin
distribution of the excited waveguided fields. While diffraction limits the
spatial size of intensity distributions, the direction of the field, defining
photonic spin, is not subject to this limitation. We demonstrate that the
skyrmion spin structure varies on the deep-subwavelength scales down to 1/60 of
light wavelength, which corresponds to about 10 nanometre lengthscale. The
application of photonic skyrmions may range from high-resolution imaging and
precision metrology to quantum technologies and data storage where the spin
structure of the field, not its intensity, can be applied to achieve
deep-subwavelength optical patterns
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