Review of two-photon exchange in electron scattering

We review the role of two-photon exchange (TPE) in electron–hadron scattering, focusing in particular on hadronic frameworks suitable for describing the low and moderate Q² region relevant to most experimental studies. We discuss the effects of TPE on the extraction of nucleon form factors and their role in the resolution of the proton electric to magnetic form factor ratio puzzle. The implications of TPE on various other observables, including neutron form factors, electroproduction of resonances and pions, and nuclear form factors, are summarized. Measurements seeking to directly identify TPE effects, such as through the angular dependence of polarization observables, nonlinear ε contributions to the cross sections, and via e⁺p to e⁻p cross section ratios, are also outlined. In the weak sector, we describe the role of TPE and γZ interference in parity-violating electron scattering, and assess their impact on the extraction of the strange form factors of the nucleon and the weak charge of the proton.     

Model-independent properties of two-photon exchange in elastic electron-proton scattering

We derive from general symmetry properties of the hadron electromagnetic interaction, such as C-invariance and crossing symmetry, the general characteristics of two-photon exchange in electron-proton elastic scattering. We show that the presence of this mechanism destroys the linearity of the Rosenbluth separation.PACS: 25.30.Bf Elastic electron scattering - 13.40.-f Electromagnetic processes and properties - 13.60.-r Photon and charged-lepton interactions with hadrons - 13.88. + e Polarization in interactions and scattering     

Delta resonance contribution to two-photon exchange in electron-proton scattering

We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate Delta resonance. The Delta two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and Delta two-photon exchange corrections has an angular dependence compatible with both the polarization-transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.     

Determination of two-photon exchange amplitudes from elastic electron-proton scattering data

Using the available cross-section and polarization data for elastic electron-proton scattering, we provide an extraction of the two-photon exchange amplitudes at a common value of four-momentum transfer, around Q ² = 2.5 GeV^2. This analysis also predicts the e ⁺ p/e ^- p elastic-scattering cross-section ratio, which will be measured by forthcoming experiments.     

Radiative corrections to two-photon exchange in elastic electron-proton and positron-proton scattering

The radiative corrections to the differential elastic electron (positron)—proton scattering cross section is determined in the fourth order in perturbation theory by quantum electrodynamics methods. The contribution from two-photon exchange is determined in the static approximation with subsequent recalculation for the center of mass system and with allowance for recoil of the proton. The electromagnetic structure of the proton is taken into account.Kamsk Polytechnic Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 87–94, June, 1993.     

EPR experiment and two-photon interferometry: Report of a two-photon interference experiment

After a very brief review of the historical EPR experiments, this paper reports a new two-photon interference type EPR experiment. A two-photon state was generated by optical parametric down conversion. Pairs of light quanta with degenerate frequency but divergent directions of propagation were sent to two independent Michelson interferometers. First- and second-order interference effects were studied. Different from other reports, we observed that the second-order interference visibility vanished when the optical path difference of the interferometers were much less than the coherence length of the pumping laser beam. However, we also observed that the second-order interference behaved differently depending on whether the interferometers were set at equal or different optical path differences.Presented at the International Workshop on Squeezed and Correlated States, Moscow, December 3–7, 1990.     

Partonic calculation of the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer

We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the two-photon exchange amplitude to the generalized parton distributions which appear in hard exclusive processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2-photon correction does reproduce the Rosenbluth cross sections.     

Rozhdestvenski hooks in two-photon parametric light scattering

The interference of spontaneous parametric radiation was observed from two nonlinear crystals separated by a macroscopic layer of a linear substance and excited by a common pump beam. In the presence of strong dielectric dispersion in the layer, the frequency-angular spectra display features analogous to the Rozhdestvenski hooks. The hook wavelength corresponds to the compensation of group velocity dispersions in the nonlinear crystals and the layer.     

Quark effects, meson-exchange currents and background in the d(e, e′p)Δ reaction

We have studied in detail the cross section for the d(e, e′p)Δ reaction leading to the emission of a fast nucleon and a Δ at rest, which has been advocated as a tool to investigate quark effects in nuclei. We find that ordinary meson-exchange currents mechanisms dominate the quark-exchange effects in the region of excitation of the Δ nd could be competitive at higher energies. Furthermore, at these higher energies, the small cross sections for the quark signal, together with the presence of a background about one order of magnitude bigger than the quark signal, make in any case the extraction of information about quark-exchange currents effects extraordinarily difficult.     

Particle exchange in K+p scattering

Recent phase-shift analyses of K⁺p scattering have shown many features of particle exchange. In exotic channels, particle exchange appears to play a more prominent role than in non-exotic channels where exchange are obscured by direct channel resonances. In the present paper particle exchange has been investigated in the momentum range 0.9 to 1.5 GeV/c, using a phase-shift analysis in which coupling constants were available parameters.     

Out-of-focus fluorescence collection in two-photon microscopy of scattering media

Imaging depth in two-photon microscopy is ultimately limited by the out-of-focus fluorescence background which is collected indistinctly with the useful in-focus signal in scattering samples. We report on a complete Monte Carlo analysis of the collection process in two-photon imaging of turbid media. Epifluorescence collection efficiency of the microscope is shown to vary significantly from point to point inside the scattering medium, to the detriment of the in-focus signal and in favor to the background, lowering the signal-to-background ratio in the image. Moreover we found that this ratio is almost independent of the collecting path field of view for situations where the background overcomes the signal. Assuming that the out-of-focus background can be subtracted to the image, the signal-to-noise ratio in two-photon microscopy is forecast to benefit from enlarging the collection field of view, with a gain roughly proportional to this enlargement for deep imaging. Asymptotic behaviors of the Monte Carlo simulations are quantitatively interpreted from ballistic and diffusive approximations.     

Effect of two-photon saturation on ordinary Raman scattering

The excitation profile of ordinary Raman scattering under steady-state excitation conditions and the time-resolved emission spectrum of ordinary Raman scattering under transient excitation conditions undergo considerable changes when the excitation frequency ω approaches $\text{1}\text{2}$ ω_mn, where ω_mn is the resonance frequency of a two-photon transition from the ground state |n to an excited state |m of a molecule. The appearance of ghost peaks, dips or dispersion-like features centred at ω ? $\text{1}\text{2}$ ω_mn in the excitation profile and of coherence effects such as Rabi nutations with unusual time-dependence in the time-resolved spectrum are predicted.     

Solitons in stimulated Raman scattering and resonant two-photon propagation

We are starting from equations of motion describing both stimulated Raman scattering and resonant two-photon absorption and emission processes. Application of the Estabrook-Wahlquist method leads to a system of differential equations whose integrability conditions are the original equations. This system is used as the starting point for applying the inverse scattering method. Implicit N-soliton formulae and explicit one-soliton formulae are derived. Possible applications under specified experimental conditions are discussed.     

Reinterpretation of two-photon interference experiment using advanced field

The recent two-photon interference experiments by Wang, Zou, and Mandel, with and without a time-dependent light switch introduced into one of interferometer arms, indicate the collapse of photon waves along empty arms, not travelled by the detected photon, at a critical moment on the light cone backwards from the detected photon. It is proposed to interpret the Schrödinger's retarded wave equation as an operator acting on the advanced field satisfying the final condition of the experiment. In this time-symmetrical formulation the advanced field of the detected particle guides the retarded wave from the particle source not to enter empty arms, and the critical moment is interpreted as the time of the passage of the advanced field through the light switch. By changing the relative optical lengths of interferometer arms and observing the independence of the result of the experiment on the relative position of the detectors, we could conceive of the unarousal of the empty wave destined to collapse.     

Tensor exchange in meson-baryon scattering

Amplitudes for A₂ quantum number exchange in K^±N scattering are determined at p_L = 3, 4 and 6 GeV using new K^±N CEX differential cross-section data supplemented by sum rule estimates of polarizations. Amplitudes for ? quantum number exchange are calculated from πN scattering by SU(3) octet symmetry. This is justified by K^±N FESR, which furthermore are used to resolve ambiguities in the analysis. Comparison with other reactions involving charge and hyperchange exchange shows reasonable overall consistency between data, SU(3), and the tensor amplitudes. The phase of the A₂s-channel helicity flip amplitude is well described by a Regge pole term with trajectory displaced downwards relative to that appropriate for the ? flip amplitude. This is shown to be the main mechanism contributing to the difference between differential cross sections for the K^±N CEX processes, connected by line reversal. It is suggested that this mechanism may persist at higher energies. The A₂ non-flip amplitude does not have the standard peripheral form.     

Particle exchange in KN scattering

Particle exchange is shown to explain much of the physics of KN scattering at low energy. In particular K⁺p data is used to extract the coupling constants of the exchanged particles which are shown to be well determined.     

Complete description of polarization effects in e + e- pair productionby a photon in the field of a strong laser wave

We consider production of a e ⁺ e ^- pair by a high-energy photon in the field of a strong laser wave. A probability of this process for circularly or linearly polarized laser photons and for arbitrary polarization of all other particles is calculated. We obtain the complete set of functions which describe such a probability in a compact invariant form. Besides, we discuss in some detail the polarization effects in the kinematics relevant to the problem of conversion at and colliders.Received: 13 December 2004, Published online: 4 February 2005     

High transparency of a two-photon scattering channel in triple-barrier structures

An analytical solution of the Schrödinger equation with open boundary conditions in all scattering channels has been found for asymmetric triple-barrier resonance-tunneling structures with thin high barriers. This solution describes resonance transitions between three quantum levels in a high rf electric field. It is found that, under certain conditions, most electrons incident on the upper resonance level can emit two photons and leave a structure through the lower level without intermediate interaction with phonons. The structure appears to be almost absolutely transparent in a wide range of the rf field amplitude. This behavior fundamentally distinguishes the multiphoton scattering process from previously considered single-photon scattering processes and the quantum efficiency of such transitions can be twice as high as the maximum quantum efficiency of the transitions between neighboring levels and can reach a value of 160% in the limiting case.     

Elastic electron scattering in an intense field and two-photon emission

The elastic scattering amplitude, taking into account the mass correction to the propagation function, is found for electrons interacting with an intense electromagnetic field characteristic of non-linear quantum electrodynamics. The probability for two-photon emission e → e2γ, the mass correction to the probability for one-photon emission and the mass correction to the anomalous field-dependent magnetic moment of the electron are found with the aid of the amplitude.