290 research outputs found
Heavy-Quark Form Factors and Threshold Cross Section at O(\alpha_S^2)
During the last year, analytic expressions for the two-loop QCD corrections
to the form factors for the vector, axial-vector, scalar and pseudo-scalar
vertices involving a pair of heavy quarks, , were calculated. The
results are valid for arbitrary momentum transfer and mass of the heavy quarks.
These form factors have a number of applications, including anomalous
couplings, the cross section, and the
forward-backward asymmetry of heavy quarks. Here the threshold
cross section is presented with some new second order axial vector
contributions.Comment: 5 pages. Presented at the International Europhysics Conference on
High Energy Physics (HEP2005), July 21-27 2005, Lisboa, Portuga
Prediction of stable walking for a toy that cannot stand
Previous experiments [M. J. Coleman and A. Ruina, Phys. Rev. Lett. 80, 3658
(1998)] showed that a gravity-powered toy with no control and which has no
statically stable near-standing configurations can walk stably. We show here
that a simple rigid-body statically-unstable mathematical model based loosely
on the physical toy can predict stable limit-cycle walking motions. These
calculations add to the repertoire of rigid-body mechanism behaviors as well as
further implicating passive-dynamics as a possible contributor to stability of
animal motions.Comment: Note: only corrections so far have been fixing typo's in these
comments. 3 pages, 2 eps figures, uses epsf.tex, revtex.sty, amsfonts.sty,
aps.sty, aps10.sty, prabib.sty; Accepted for publication in Phys. Rev. E.
4/9/2001 ; information about Andy Ruina's lab (including Coleman's, Garcia's
and Ruina's other publications and associated video clips) can be found at:
http://www.tam.cornell.edu/~ruina/hplab/index.html and more about Georg
Bock's Simulation Group with whom Katja Mombaur is affiliated can be found at
http://www.iwr.uni-heidelberg.de/~agboc
Tactile Sensors Based on Conductive Polymers
This paper presents results from a selection of tactile sensors that have been designed and fabricated. These sensors are based on a common approach that consists in placing a sheet of piezoresistive material on the top of a set of electrodes. We use a thin film of conductive polymer as the piezoresistive mate¬rial. Specifically, a conductive water-based ink of this polymer is deposited by spin coating on a flexible plastic sheet, giving it a smooth, homogeneous and conducting thin film. The main interest in this procedure is that it is cheap and it allows the fabrication of flexible and low cost tactile sensors. In this work we present results from sensors made using two technologies. Firstly, we have used a flexible Printed Circuit Board (PCB) technology to fabricate the set of electrodes and addressing tracks. The result is a simple, flexible tactile sensor. In addition to these sensors on PCB, we have proposed, designed and fabricated sensors with screen printing technology. In this case, the set of electrodes and addressing tracks are made by printing an ink based on silver nanoparticles. The intense characterization provides us insights into the design of these tactile sensors.This work has been partially funded by the spanish government under contract TEC2006-12376-C02
Two-Loop QCD Corrections to the Heavy Quark Form Factors: Axial Vector Contributions
We consider the Z Q Qbar vertex to second order in the QCD coupling for an
on-shell massive quark-antiquark pair and for arbitrary momentum transfer of
the Z boson. We present closed analytic expressions for the two
parity-violating form factors of that vertex at the two-loop level in QCD,
excluding the contributions from triangle diagrams. These form factors are
expressed in terms of 1-dimensional harmonic polylogarithms of maximum weight
4.Comment: 57 pages, 5 figures. All the results in Section 6 of the paper are
available in an electronic form in the file formulas.in
Two-Loop QCD Corrections to the Heavy Quark Form Factors: Anomaly Contributions
We present closed analytic expressions for the order triangle
diagram contributions to the matrix elements of the singlet and non-singlet
axial vector currents between the vacuum and a quark-antiquark state. We have
calculated these vertex functions for arbitrary momentum transfer and for four
different sets of internal and external quark masses. We show that both the
singlet and non-singlet vertex functions satisfy the correct chiral Ward
identities. Using the exact expressions for the finite axial vector form
factors, we check the quality and the convergence of expansions at production
threshold and for asymptotic energies.Comment: 24 pages, 6 figure
Chemical bath deposition of textured and compact zinc oxide thin films on vinyl-terminated polystyrene brushes
In this study we investigated the influence of an organic polystyrene brush on the deposition of ZnO thin films under moderate conditions. On a non-modified SiOx surface, island growth is observed, whereas the polymer brush induces homogeneous film growth. A chemical modification of the polystyrene brushes during the mineralization process occurs, which enables stronger interaction between the then polar template and polar ZnO crystallites in solution. This may lead to oriented attachment of the crystallites so that the observed (002) texture arises. Characterization of the templates and the resulting ZnO films were performed with ζ-potential and contact angle measurements as well as scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). Infrared spectroscopy (IR) measurements were used to investigate the polystyrene brushes before and after modification
Two-Parton Contribution to the Heavy-Quark Forward-Backward Asymmetry in NNLO QCD
Forward-backward asymmetries, , are important observables for the
determination of the neutral-current couplings of heavy quarks in inclusive
heavy quark production, . In view of the
measurement perspectives on at a future linear collider, precise
predictions of are required for massive quarks. We compute the
contribution of the final state to to order \as^2 in
the QCD coupling. We provide general formulae, and we show that this
contribution to is infrared-finite. We evaluate these two-parton
contributions for and quarks on and near the resonance, and for
quarks above threshold. Moreover, near the threshold we obtain, by
expanding in the heavy-quark velocity , an expression for to order \as^2 and NNLL in . This quantity is equal, to this
order in , to the complete forward-backward asymmetry .Comment: latex, 26 pages, 2 tables, 17 figure
Two-loop QCD Corrections to the Heavy Quark Form Factors: the Vector Contributions
We present closed analytic expressions of the electromagnetic vertex form
factors for heavy quarks at the two-loop level in QCD for arbitrary momentum
transfer. The calculation is carried out in dimensional regularization. The
electric and magnetic form factors are expressed in terms of 1-dimensional
harmonic polylogarithms of maximum weight 4.Comment: 79 pages, 3 figures. Version accepted by Nucl. Phys. B. All the
results in Section 6 of the paper are available in an electronic form in the
file formulas.in
Relaxation Methods for Mixed-Integer Optimal Control of Partial Differential Equations
We consider integer-restricted optimal control of systems governed by
abstract semilinear evolution equations. This includes the problem of optimal
control design for certain distributed parameter systems endowed with multiple
actuators, where the task is to minimize costs associated with the dynamics of
the system by choosing, for each instant in time, one of the actuators together
with ordinary controls. We consider relaxation techniques that are already used
successfully for mixed-integer optimal control of ordinary differential
equations. Our analysis yields sufficient conditions such that the optimal
value and the optimal state of the relaxed problem can be approximated with
arbitrary precision by a control satisfying the integer restrictions. The
results are obtained by semigroup theory methods. The approach is constructive
and gives rise to a numerical method. We supplement the analysis with numerical
experiments
Annealing-Dependent Magnetic Depth Profile in Ga[1-x]Mn[x]As
We have studied the depth-dependent magnetic and structural properties of
as-grown and optimally annealed Ga[1-x]Mn[x]As films using polarized neutron
reflectometry. In addition to increasing total magnetization, the annealing
process was observed to produce a significantly more homogeneous distribution
of the magnetization. This difference in the films is attributed to the
redistribution of Mn at interstitial sites during the annealing process. Also,
we have seen evidence of significant magnetization depletion at the surface of
both as-grown and annealed films.Comment: 5 pages, 3 figure
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