3,841 research outputs found
No Generalized TMD-Factorization in the Hadro-Production of High Transverse Momentum Hadrons
It has by now been established that standard QCD factorization using
transverse momentum dependent parton distribution functions fails in
hadro-production of nearly back-to-back hadrons with high transverse momentum.
The essential problem is that gauge invariant transverse momentum dependent
parton distribution functions cannot be defined with process-independent Wilson
line operators, thus implying a breakdown of universality. This has led
naturally to proposals that a correct approach is to instead use a type of
"generalized" transverse momentum dependent factorization in which the basic
factorized structure is assumed to remain valid, but with transverse momentum
dependent parton distribution functions that contain non-standard, process
dependent Wilson line structures. In other words, to recover a factorization
formula, it has become common to assume that it is sufficient to simply modify
the Wilson lines in the parton correlation functions for each separate hadron.
In this paper, we will illustrate by direct counter-example that this is not
possible in a non-Abelian gauge theory. Since a proof of generalized transverse
momentum dependent factorization should apply generally to any hard
hadro-production process, a single counter-example suffices to show that a
general proof does not exist. Therefore, to make the counter-argument clear and
explicit, we illustrate with a specific calculation for a double spin asymmetry
in a spectator model with a non-Abelian gauge field. The observed breakdown of
generalized transverse momentum dependent factorization challenges the notion
that the role of parton transverse momentum in such processes can be described
using separate correlation functions for each external hadron.Comment: 19 pages, 11 figures, typos fixed and minor explanations added,
version to appear in Physical Review
Continuous production of glycerol by catalytic high pressure hydrogenolysis of sucrose
Several continuous reactor systems have been discussed for the catalytic high pressure hydrogenolysis of sucrose to glycerol. Theoretically and actually, continuous reactors lead to lower glycerol yields than in a batch process. Two continuous stirred tank reactors in cascade constitute a reasonable compromise. An economic evaluation of the sucrose route to glycerol in comparison with other synthetic glycerol processes based on allyl chloride and acrolein suggests that the sucrose process can be competitive if a sales potential is developed for the by-products propane-l,2-diol, ethylene glycol, and a mixture of higher polyhydric alcohols containing tetritol, pentitol, methyl fructoside, and hexitol
Ehrenfest-time dependence of weak localization in open quantum dots
Semiclassical theory predicts that the weak localization correction to the
conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time
exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys.
Rev. B {\bf 54}, 14424 (1996)]. We report numerical simulations of weak
localization in the open quantum kicked rotator that confirm this prediction.
Our results disagree with the `effective random matrix theory' of transport
through ballistic chaotic cavities.Comment: 4 pages, 2 figure
Dijet imbalance in hadronic collisions
The imbalance of dijets produced in hadronic collisions has been used to
extract the average transverse momentum of partons inside the hadrons. In this
paper we discuss new contributions to the dijet imbalance that could complicate
or even hamper this extraction. They are due to polarization of initial state
partons inside unpolarized hadrons that can arise in the presence of nonzero
parton transverse momentum. Transversely polarized quarks and linearly
polarized gluons produce specific azimuthal dependences of the two jets that in
principle are not suppressed. Their effects cannot be isolated just by looking
at the angular deviation from the back-to-back situation, rather they enter jet
broadening observables. In this way they directly affect the extraction of the
average transverse momentum of unpolarized partons that is thought to be
extracted. We discuss appropriately weighted cross sections to isolate the
additional contributions.Comment: 15 pages, 2 figures; revised version, published in Phys. Rev.
Theory of the spin-torque-driven ferromagnetic resonance in a ferromagnet/normal-metal/ferromagnet structure
We present a theoretical analysis of current driven ferromagnetic resonance
in a ferromagnet/normal-metal/ferromagnet tri-layer. This method of driving
ferromagnetic resonance was recently realized experimentally by Tulapurkar et
al. [Nature 438, 339 (2005)] and Sankey et al. [Phys. Rev. Lett. 96, 227601
(2006)]. The precessing magnetization rectifies the alternating current applied
to drive the ferromagnetic resonance and leads to the generation of a dc
voltage. Our analysis shows that a second mechanism to generate a dc voltage,
rectification of spin currents emitted by the precessing magnetization, has a
contribution to the dc voltage that is of approximately equal size for the thin
ferromagnetic films used in the experiment.Comment: 6 pages, 1 figure, final version. Changed title, updated references,
added discussions in section I
Classical limit of transport in quantum kicked maps
We investigate the behavior of weak localization, conductance fluctuations,
and shot noise of a chaotic scatterer in the semiclassical limit. Time resolved
numerical results, obtained by truncating the time-evolution of a kicked
quantum map after a certain number of iterations, are compared to semiclassical
theory. Considering how the appearance of quantum effects is delayed as a
function of the Ehrenfest time gives a new method to compare theory and
numerical simulations. We find that both weak localization and shot noise agree
with semiclassical theory, which predicts exponential suppression with
increasing Ehrenfest time. However, conductance fluctuations exhibit different
behavior, with only a slight dependence on the Ehrenfest time.Comment: 17 pages, 13 figures. Final versio
Impurity-assisted Andreev reflection at a spin-active half-metal-superconductor interface
The Andreev reflection amplitude at a clean interface between a half-metallic
ferromagnet (H) and a superconductor (S) for which the half metal's
magnetization has a gradient perpendicular to the interface is proportional to
the excitation energy and vanishes at [B\'{e}ri
{\em et al.}, Phys.\ Rev.\ B {\bf 79}, 024517 (2009)]. Here we show that the
presence of impurities at or in the immediate vicinity of the HS interface
leads to a finite Andreev reflection amplitude at . This
impurity-assisted Andreev reflection dominates the low-bias conductance of a HS
junction and the Josephson current of an SHS junction in the long-junction
limit.Comment: 12 pages, 2 figure
Supertwistors as Quarks of SU(2,2|4)
The GS superstring on AdS_5 x S^5 has a nonlinearly realized, spontaneously
broken SU(2,2|4) symmetry. Here we introduce a two-dimensional model in which
the unbroken SU(2,2|4) symmetry is linearly realized. The basic variables are
supertwistors, which transform in the fundamental representation of this
supergroup.
The quantization of this supertwistor model leads to the complete oscillator
construction of the unitary irreducible representations of the centrally
extended SU(2,2|4). They include the states of d=4 SYM theory, massless and KK
states of AdS_5 supergravity, and the descendants on AdS_5 of the standard
massive string states, which form intermediate and long supermultiplets. We
present examples of such multiplets and discuss possible states of solitonic
and (p,q) strings.Comment: 12 pages, LaTeX, 1 EPS figur
Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis
A sufficiently large unpolarized current can cause a spin-wave instability in
thin nanomagnets with asymmetric contacts. The dynamics beyond the instability
is understood in the perturbative regime of small spin-wave amplitudes, as well
as by numerically solving a discretized model. In the absence of an applied
magnetic field, our numerical simulations reveal a hierarchy of instabilities,
leading to chaotic magnetization dynamics for the largest current densities we
consider.Comment: 14 pages, 10 figures; revtex
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