167 research outputs found
Level Splitting in Association with the Multiphoton Bloch-Siegert Shift
We present a unitary equivalent spin-boson Hamiltonian in which terms can be
identified which contribute to the Bloch-Siegert shift, and to the level
splittings at the anticrossings associated with the Bloch-Siegert resonances.
First-order degenerate perturbation theory is used to develop approximate
results in the case of moderate coupling for the level splitting.Comment: 8 pages, 2 figure
Δ(1232)-Resonance in the Hydrogen Spectrum
The electromagnetic excitation of the Δ(1232)-resonance plays an appreciable role in the Lamb shift and hyperfine structure of muonic and electronic hydrogen. Its effect appears at the subleading order O(α5), together with other proton-polarizability contributions from forward two-photon exchange. We use the large-Nc relations for the nucleon-to-delta transition form factors to compute the effect of the Δ(1232) in the hydrogen spectrum. We pay particular attention to a subtile difference between predictions based on a direct calculation of the two-photon exchange (or Compton scattering amplitudes) (Faustov et al. in Phys At Nucl 62:2099, 1999) and predictions based on the Δ(1232)-production photoabsorption cross sections (Buchmann in Can J Phys 87:773–783, 2009). The mismatch is explained by studying the dispersion relations for tree-level Compton scattering off the proton in more details
Multiphoton Bloch-Siegert shifts and level-splittings in spin-one systems
We consider a spin-boson model in which a spin 1 system is coupled to an
oscillator. A unitary transformation is applied which allows a separation of
terms responsible for the Bloch-Siegert shift, and terms responsible for the
level splittings at anticrossings associated with Bloch-Siegert resonances.
When the oscillator is highly excited, the system can maintain resonance for
sequential multiphoton transitions. At lower levels of excitation, resonance
cannot be maintained because energy exchange with the oscillator changes the
level shift. An estimate for the critical excitation level of the oscillator is
developed.Comment: 14 pages, 3 figure
An Efficient Opportunistic Cooperative Diversity Protocol for IEEE 802.11 Networks
Opportunistic cooperation promises to enhance the user experience when streaming media over wireless devices by improving wireless network reliability at the link level. This paper presents DAFMAC, an efficient cooperative diversity partner selection algorithm for IEEE 802.11 devices. Simulation results show DAFMAC provides a significantly higher transmission reliability in poor channel conditions than traditional ARQ techniques without modifying the device hardware. Further analysis shows the low overhead of DAFMAC makes it highly competitive with other proposed cooperative retransmission mechanisms in an ad-hoc network
Analytic Performance Model for State-Based MAC Layer Cooperative Retransmission Protocols
© 2015 IEEE. Cooperative retransmission can significantly improve link reliability over lossy and time-varying wireless links. However, comparing retransmission protocols is challenging, and generally requires simplistic assumptions specific to each protocol. In this paper, we develop a general model to evaluate cooperative retransmission protocols with distributed, slot-based contention algorithms. Specifically, we propose to calculate the relay time-out probabilities at a MAC time-slot scale, formulate retransmission outcomes as functions of the time-out probabilities, and derive the probability of a retransmission process for every data frame. We also propose a Markov extension of our model to characterise the dependency between retransmissions of multiple frames. This enables our model to analyse continuous retransmissions of successive frames. Validated by QualNet simulations, our model can analytically predict the probabilities of cooperative retransmissions with an accuracy of ± 1%. As a result, direct comparisons between cooperative retransmission protocols become tangible, without implementing the full protocol in a state-based simulator
DIRECTIONAL SQUARE FUNCTIONS
QuantitativeformulationsofFefferman’scounterexamplefortheballmultiplierare naturally linked to square function estimates for conical and directional multipliers. In this ar- ticle we develop a novel framework for these square function estimates, based on a directional embedding theorem for Carleson sequences and multi-parameter time-frequency analysis tech- niques. As applications we prove sharp or quantified bounds for Rubio de Francia type square functions of conical multipliers and of multipliers adapted to rectangles pointing along di- rections. A suitable combination of these estimates yields a new and currently best-known logarithmic bound for the Fourier restriction to an -gon, improving on previous results of A. Córdoba. Our directional Carleson embedding extends to the weighted setting, yielding previ- ously unknown weighted estimates for directional maximal functions and singular integrals
Impact of NNLO QED corrections on lepton-proton scattering at MUSE
We present the complete next-to-next-to-leading order (NNLO) pure pointlike QED corrections to lepton-proton scattering, including three-photon-exchange contributions, and investigate their impact in the case of the MUSE experiment. These corrections are computed with no approximation regarding the energy of the emitted photons and taking into account lepton-mass effects. We contrast the NNLO QED corrections to known next-to-leading order corrections, where we include the elastic two-photon exchange (TPE) through a simple hadronic model calculation with a dipole ansatz for the proton electromagnetic form factors. We show that, in the low-momentum-transfer region accessed by the MUSE experiment, the improvement due to more sophisticated treatments of the TPE, including inelastic TPE, is of similar if not smaller size than some of the NNLO QED corrections. Hence, the latter have to be included in a precision determination of the low-energy proton structure from scattering data, in particular for electron-proton scattering. For muon-proton scattering, the NNLO QED corrections are considerably smaller
Impact of NNLO QED corrections on lepton-proton scattering at MUSE
We present the complete next-to-next-to-leading order (NNLO) pure pointlike
QED corrections to lepton-proton scattering, including three-photon-exchange
contributions, and investigate their impact in the case of the MUSE experiment.
These corrections are computed with no approximation regarding the energy of
the emitted photons and taking into account lepton-mass effects. We contrast
the NNLO QED corrections to known next-to-leading order corrections, where we
include the elastic two-photon exchange (TPE) through a simple hadronic model
calculation with a dipole ansatz for the proton electromagnetic form factors.
We show that, in the low-momentum-transfer region accessed by the MUSE
experiment, the improvement due to more sophisticated treatments of the TPE,
including inelastic TPE, is of similar if not smaller size than some of the
NNLO QED corrections. Hence, the latter have to be included in a precision
determination of the low-energy proton structure from scattering data, in
particular for electron-proton scattering. For muon-proton scattering, the NNLO
QED corrections are considerably smaller.Comment: Article to be submitted to the EPJ A Topical Collection: Radiative
Corrections: From Medium to High Energy Experiments. 23 pages, 9 figure
Impact of NNLO QED corrections on lepton-proton scattering at MUSE
We present the complete next-to-next-to-leading order (NNLO) pure pointlike QED corrections to lepton-proton scattering, including three-photon-exchange contributions, and investigate their impact in the case of the MUSE experiment. These corrections are computed with no approximation regarding the energy of the emitted photons and taking into account lepton-mass effects. We contrast the NNLO QED corrections to known next-to-leading order corrections, where we include the elastic two-photon exchange (TPE) through a simple hadronic model calculation with a dipole ansatz for the proton electromagnetic form factors. We show that, in the low-momentum-transfer region accessed by the MUSE experiment, the improvement due to more sophisticated treatments of the TPE, including inelastic TPE, is of similar if not smaller size than some of the NNLO QED corrections. Hence, the latter have to be included in a precision determination of the low-energy proton structure from scattering data, in particular for electron-proton scattering. For muon-proton scattering, the NNLO QED corrections are considerably smaller
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