522 research outputs found
Weyl-Heisenberg Spaces for Robust Orthogonal Frequency Division Multiplexing
Design of Weyl-Heisenberg sets of waveforms for robust orthogonal frequency
division multiplex- ing (OFDM) has been the subject of a considerable volume of
work. In this paper, a complete parameterization of orthogonal Weyl-Heisenberg
sets and their corresponding biorthogonal sets is given. Several examples of
Weyl-Heisenberg sets designed using this parameterization are pre- sented,
which in simulations show a high potential for enabling OFDM robust to
frequency offset, timing mismatch, and narrow-band interference
Adaptive strong-field control of chemical dynamics guided by three-dimensional momentum imaging.
Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the nω+C2D4→C2D3++D interaction is manipulated. Second, isomerization of acetylene (nω+C2H2→C2H22+→CH2++C+) is controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule
The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon
The Gerasimov-Drell-Hearn sum rule is one of several dispersive sum rules
that connect the Compton scattering amplitudes to the inclusive photoproduction
cross sections of the target under investigation. Being based on such universal
principles as causality, unitarity, and gauge invariance, these sum rules
provide a unique testing ground to study the internal degrees of freedom that
hold the system together. The present article reviews these sum rules for the
spin-dependent cross sections of the nucleon by presenting an overview of
recent experiments and theoretical approaches. The generalization from real to
virtual photons provides a microscope of variable resolution: At small
virtuality of the photon, the data sample information about the long range
phenomena, which are described by effective degrees of freedom (Goldstone
bosons and collective resonances), whereas the primary degrees of freedom
(quarks and gluons) become visible at the larger virtualities. Through a rich
body of new data and several theoretical developments, a unified picture of
virtual Compton scattering emerges, which ranges from coherent to incoherent
processes, and from the generalized spin polarizabilities on the low-energy
side to higher twist effects in deep inelastic lepton scattering.Comment: 32 pages, 19 figures, review articl
An Iterative Receiver for OFDM With Sparsity-Based Parametric Channel Estimation
In this work we design a receiver that iteratively passes soft information
between the channel estimation and data decoding stages. The receiver
incorporates sparsity-based parametric channel estimation. State-of-the-art
sparsity-based iterative receivers simplify the channel estimation problem by
restricting the multipath delays to a grid. Our receiver does not impose such a
restriction. As a result it does not suffer from the leakage effect, which
destroys sparsity. Communication at near capacity rates in high SNR requires a
large modulation order. Due to the close proximity of modulation symbols in
such systems, the grid-based approximation is of insufficient accuracy. We show
numerically that a state-of-the-art iterative receiver with grid-based sparse
channel estimation exhibits a bit-error-rate floor in the high SNR regime. On
the contrary, our receiver performs very close to the perfect channel state
information bound for all SNR values. We also demonstrate both theoretically
and numerically that parametric channel estimation works well in dense
channels, i.e., when the number of multipath components is large and each
individual component cannot be resolved.Comment: Major revision, accepted for IEEE Transactions on Signal Processin
Experimental Status of the CKM Matrix
The CKM matrix, VV, relates the quark mass and flavor bases. In the standard model, VV is unitary 3×3, and specified by four arbitrary parameters, including a phase allowing for CPCP violation. We review the experimental determination of VV, including the four parameters in the standard model context. This is an active field; the precision of experimental measurements and theoretical inputs continues to improve. The consistency of the determination with the standard model unitarity is investigated. While there remain some issues the overall agreement with standard model unitarity is good
Short-distance constraints on the hadronic light-by-light contribution to the muon g-2
The aim of this work is to investigate the information available from perturbative QCD to constrain the hadronic light-by-light contribution to and to implement it in a way that is compatible with the dispersive description.
The first regime of interest is the one where three photon virtualities are large. Because the external photon is soft in the kinematics, an operator product expansion in presence of an external electromagnetic field needs to be used. The leading order in this expansion corresponds to the usual massless quark loop. In this work, it is shown that the non-perturbative corrections are small and that the -correction is negative and amounts to about 10\% of the quark loop. The second interesting regime is the one where two photon virtualities are much larger than the third. This leads to the so-called Melnikov-Vainshtein constraint.
A way to satisfy the short-distance constraints using heavy pseudoscalars is then presented. This is done using a large- inspired Regge model for the transition form factors of the radially excited pseudoscalars. The contribution of the heavy pseudoscalars is then matched to the quark loop and its first gluonic correction in order to reduce the model-dependence of our estimation.
With the recent results from Fermilab, the discrepancy between the experimental and theoretical determinations of the anomalous magnetic moment of the muon has been pushed to . In order to reach the -threshold, the experimental and theoretical uncertainties need to be reduced further
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