Forward-backward asymmetry

B. Forward-Backward Asymmetry in Angle-Resolved Chiral Molecule Photoemission 1. A Simple Analogy... 

This forward-backward asymmetry of the photoelectron distribution, expected when a randomly oriented sample of molecular enantiomers is ionized by circularly polarized light, is central to our discussion. The photoelectron angular... 

The second row in Fig. 15 shows examples at the three selected photon energies of the C=0 15 difference spectra obtained for both enantiomers. After normalization by the mean spectrum the asymmetry factor F(54.7°) is plotted along the bottom row. After correction for the cos(54.7°) term arising from the specific experimental geometry the net forward-backward asymmetry, y, can be estimated to reach a peak 15% in the hv = 298.7-eV photoionization. 

The actual eigenstates are equal admixtures of the two unperturbed pure spin states when the field is exactly at the value at which the crossing would have occurred (v,m = 0). Since initially (when the muon stops) the system is in a well defined muon spin state, i.e., one of the two unperturbed pure spin states, the system oscillates at the frequency vT between the muon spin being along and opposite to the field, as implied by Eqs. 10 and 11. Thus, upon time averaging the positron counts the forward-backward asymmetry is reduced. 

Figure 3-3. Forward/backward asymmetry in the c.m. system for the H + C02 channel, from the crossed molecular beams experiments of Casavecchia and coworkers (Alagia et al. 1993). This asymmetry can be related to the lifetime of the HOCO1" intermediate.

Pigure 9 shows an electron-ion covariance map obtained at the same time as Pig. 8b. Por convenience the time-averaged electron and ion TOP spectra are shown alongside the x and y axes. The electron TOP is virtually structureless, the slight forward-backward asymmetry and the peak at 292 ns being due to a small inhomogeneity in the extraction field. The forward electrons are unaffected and a kinetic energy scale is shown for both electrons and protons. In order to ensure that this lack of structure was not the result of poor instrumental resolution, an electron TOP spectrum of Xe was taken under identical conditions. A series of ATI peaks was observed in both the forward and backward direction, separated by about 2 eV. 

In the tunnelling regime, the total ionization might be expected to depend solely on the instantaneous electric field. However, the Corkum model [39] suggests that when two laser frequencies are applied to an atom, certain aspects of the ionization process depend, in addition, on the relative phases of the two fields. Using a mixture of fundamental and second harmonic of the Nd YAG laser, there is recent experimental evidence that the ATI spectra of Kr depend upon the relative phases of the fields [46]. In particular there is a considerable forward-backward asymmetry in the emission of the higher energy electrons. 

S- and P-wave scattering, the DCS is as5mimetric with forward-backward asymmetry. 

AO observed forward-backward asymmetry (of decay positron C( ) energy distribution fimction of emitted positrons... 

There is, however, no universal requirement that there should be symmetry of product distribution with respect to forward and backward of a plane passing through CM and perpendicular to a collision axis.t As will be discussed below, such symmetry is normally found only when an intermediate having a lifetime of many rotations is formed. Direct reactions, i.e., those in which no persistent intermediate is involved, usually display a forward-backward asymmetry. 

Left °Li residue momentum distributions measured following neutron removal from "Li on at 287 MeV/7). Right Angular correlations of the decay neutrons measured relative to an axis defined by the °Li recoil direction as shown in the inset. The points are the experimental data and the histogram is a reconstruction corrected for experimental resolution and acceptance effects. Note the strong forward-backward asymmetry, which reflects interference of the = 0 and 1 final states... 

Comparison of quantities observable in electron-positron collisions of energies near the Z mass with the predictions of the Standard Model of elementary particles (Amsier et al. 2008). Note the high precision of the calculations and the good agreement between theory and experiment. At present the largest deviation is in the electron-positron forward-backward asymmetry and the anomalous magnetic moment of the muon. If a measured quantity has two uncertainties, the first one is the statistical and the second the systematic error... 

Given that sin 9w is close to the value 0.25 that makes Ve = 0,Vft = 0, the effect is exceedingly small. However, the angular dependence of the differential cross-section is sensitive to the axial-vector coupling of the 8md gives rise to a forward-backward asymmetry in the angular... 

The forward-backward asymmetry evident in eqn (8.5.19) will be discussed in connection with the possibility of polarized e+e collisions in Section 8.8. 

The forward -backward asymmetry in the polarised case is thus given... 

Fig. 13.21 shows the forward-backward asymmetry as function of cos 6 as measured by ALEPH (Dydak, 1991). This figure should be compared e.g. with Fig. 8.20. 

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