Search results for "Amplitude"
showing 10 items of 1169 documents
Structure analysis of the virtual Compton scattering amplitude at low energies
1998
We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the $ep\to e'p'\gamma$ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon.
Lateral diffusion estimation in fully depleted thick CCD using flat field image analysis
2011
Abstract In thick fully depleted CCDs charge carrier transport from the back window to the gates is accompanied by charge diffusion. Lateral diffusion smooths out density variations of the incoming photon flux by redistributing charges spatially. This creates short range positive correlations in recorded amplitudes. Pixel-to-pixel amplitude variations can also be caused by pixel size and quantum efficiency variations. Pixel size variations result in short range negative correlations. Our study shows that the characteristic diffusion width can be extracted from flat field data. The study was performed on fully depleted, thick CCDs produced in a technology study for the Large Synoptic Survey …
Compton Scattering by the Proton
2001
Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV < E_\gamma < 800 MeV and photon scattering angles 30^0 < \theta^{lab}_\gamma < 150^0, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The data are in good agreement with the dispersion theory based on the SAID-SM99K parameterization of photo-meson amplitudes. From the subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to the multipole E2/M1 ratio EMR(340 MeV) =(-1.6 \pm 0.4(stat+syst) \pm 0.2(model)%. From all data below 455 MeV the proton's backward spin polarizability was determined to be …
The first dedicated Virtual Compton Scattering experiment at MAMI
1999
We measured the absolute cross sections for photon electro-production off the proton, ep to ep gamma , with the high resolution spectrometers at MAMI at momentum transfer q=600 MeV/c and photon polarization epsilon =0.62. We covered the momentum range for the outgoing real photon q'=33/111 MeV/c. From the extracted virtual Compton scattering amplitude we deduce values for two structure functions related to the generalized polarizabilities of the proton.
Compton scattering by the proton through θcms = 75° and 90° in the Δ-resonance region
1997
Abstract Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θcms = 75° and θcms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the reso…
Threshold electro production of neutral pions at low q2 off the proton and off the deuteron
2000
Abstract The A1 Collaboration performs an extend program to measure threshold production of the neutral pion as test of Heavy Baryon Chiral Perturbation Theory. Starting with direct threshold measurements to extract the s wave amplitudes at threshold we continued with out of plane measurements with polarized beam to access the small p wave amplitudes. Coherent production off the deuteron is the first attempt to access the neutron amplitude in electro production.
On the effect of excited states in lattice calculations of the nucleon axial charge
2017
Excited-state contamination is one of the dominant uncertainties in lattice calculations of the nucleon axial-charge, $g_A$. Recently published results in leading-order chiral perturbation theory (ChPT) predict the excited-state contamination to be independent of the nucleon interpolator and positive. However, empirical results from numerical lattice calculations show negative contamination (downward curvature), indicating that present-day calculations are not in the regime where the leading-order ChPT predictions apply. In this paper we show that, under plausible assumptions, one can reproduce the behavior of lattice correlators by taking into account final-state $N \pi$ interactions, in p…
Breit-Wigner formalism for non-Abelian theories
2003
The consistent description of resonant transition amplitudes within the framework of perturbative field theories necessitates the definition and resummation of off-shell Green's functions, which must respect several crucial physical requirements. In particular, the generalization of the usual Breit-Wigner formalism in a non-Abelian context constitutes a highly non-trivial problem, related to the fact that the conventionally defined Green's functions are unphysical. We briefly review the main field-theoretical difficulties arising when attempting to use such Green's functions outside the confines of a fixed order perturbative calculation, and explain how this task has been successfully accom…
A SIMULTANEOUS ANALYSIS FOR THE ΔI = 1/2 RULE, DIRECT CP VIOLATION, $K^0 -\bar K^0$ AND $B^0 -\bar B^0$ MIXINGS
1992
To have a consistent picture, a simultaneous analysis for the K → ππ isospin amplitudes A0 and A2, the ΔI = 1/2 rule, CP-violating ε′/ε, and [Formula: see text] and [Formula: see text] mixings within the standard model is made by employing the recent reanalyzed values of the parameters and relating the penguin matrix elements involved in the K → ππ decays to the known matrix elements of the ΔI = 1/2 operator (Q2 − Q1) and ΔI = 3/2 operator (Q2 + 2Q1). The allowed values for all the relevant quantities are calculated in detail. A general bound for top quark mass mt, CP phase δ and ε′/ε is found: for δ in the second quadrant, 84 GeV ≤ mt ≤ 250 GeV , δ ≤ 173° and 1 × 10−4 < ε′/ε ≤ 1 × 10−2…
Structure of Λ(1405) and chiral dynamics
2005
We report on a recent theoretical work on the structure of the Λ(1405) resonance within a chiral unitary approach, in which the resonance is dynamically generated in meson-baryon scattering. Studying the analytic structure of the scattering amplitude, we have found that there are two poles lying around energies of Λ(1405) with different widths and couplings to the meson-baryon states. We discuss reactions to conform the double pole structure in experiment and elastic K − p scattering at low energies.