Two-photon Intermediate State Research Articles

Search for C violation in the decay η → π0e+e− with WASA-at-COSY

We report on the search for the rare decay η→π0e+e− which is of interest to study C violation in the electromagnetic interaction which would indicate contributions from physics beyond the Standard Model, since the allowed decay via a two-photon intermediate state is strongly suppressed. The experiment has been performed using the WASA-at-COSY installation, located at the COSY accelerator of the Forschungszentrum Jülich, Germany. In total 3×107 events of the reaction pd→3Heη have been recorded at an excess energy of Q=59.8MeV. Based on this data set the C parity violating decay η→π0γ⁎→π0e+e− via a single-photon intermediate state has been searched for, resulting in new upper limits of Γ(η→π0e+e−)/Γ(η→π+π−π0)<3.28×10−5 and Γ(η→π0e+e−)/Γ(η→all)<7.5×10−6 (CL=90%), respectively.

The proton size puzzle: experiment vs theory.

Current status of the proton size puzzle from experimental and theoretical points of view is briefly discussed. The interest to these studies is primarily related to experiments conducted by the CREMA collaboration (Charge Radius Experi- ments with Muonic Atoms) with muonic hydrogen and deuterium by methods of laser spectroscopy. As a result a more accurate value of the proton charge radius was found to be rp = 0:84184(67) fm, which is different from the value recommended by CODATA for 7σ. In the second part we discuss recent calculations of the contribution of light pseudoscalar (PS) and axial-vector (AV) mesons to the interaction operator of a muon and a proton in muonic hydrogen atom, with the coupling of mesons to the muon being via two-photon intermediate state. Numerical estimates of the contributions to the hyperfine structure of the spectrum of the S and P levels are presented. It is shown that such contribution to the hyperfine splitting in muonic hydrogen is rather important for a comparison with precise experimental data.

The contribution of pseudoscalar and axial-vector mesons to hyperfine structure of muonic hydrogen

In the framework of the quasipotential method in quantum electrodynamics we calculate the contribution of light pseudoscalar (PS) and axial-vector (AV) mesons to the interaction operator of a muon and a proton in muonic hydrogen atom. The coupling of mesons with the muon is via two-photon intermediate state. The parametrization of the transition form factor of two photons into PS and AV mesons, based on the experimental data on the transition form factors and QCD asymptotics, is used. Numerical estimates of the contributions to the hyperfine structure of the spectrum of the S and P levels are presented. It is shown that such contribution to the hyperfine splitting in muonic hydrogen is rather important for a comparison with precise experimental data.

Long-distance contribution to the forward-backward asymmetry in decaysK+→π+l+l−

The long-distance contribution via the two-photon intermediate state to the forward-backward asymmetries in decays K+ --> pi+ l+ l- (l=e and mu) has been studied within the standard model. In order to evaluate the dispersive part of the K+ --> pi+ gamma* gamma* --> pi+ l+ l- amplitude, we employ a phenomenological form factor to soften the ultraviolet behavior of the transition. It is found that, this long-distance transition, although subject to some theoretical uncertainties, can lead to significant contributions to the forward-backward asymmetries, which could be tested in the future high-precise experiments.

On the electron–positron energy asymmetry in KL→π0e+e−

Motivated by some new experimental and theoretical efforts, we update the theoretical analysis of the electron–positron energy asymmetry, which arises, in KL→π0e+e−, from the two-photon intermediate state in the Standard Model. It is found that the measurement of this asymmetry in future experiments may increase our understanding of the KL,S→π0e+e− decays, which would thus provide some useful information on quark flavor physics. Meanwhile, in the Standard Model the electron–positron energy asymmetry in the decay of KS→π0e+e− is expected to be vanishingly small, therefore the asymmetry in KS→π0e+e− might be a very interesting quantity to explore new physics scenarios.

Long-distance contribution to the muon-polarization asymmetry inK+→π+μ+μ−

We revisit the calculation of the long-distance contribution to the muon-polarization asymmetry \Delta_{LR}, which arises, in K^+\to\pi^+\mu^+\mu^-, from the two-photon intermediate state. The parity-violating amplitude of this process, induced by the local anomalous K^+\pi^-\gamma^*\gamma^* transition, is analysed; unfortunately, one cannot expect to predict its contribution to the asymmetry by using chiral perturbation theory alone. Here we evaluate this amplitude and its contribution to \Delta_{LR} by employing a phenomenological model called the FMV model, in which the utility of the vector and axial-vector resonances exchange is important to soften the ultraviolet behaviour of the transition. We find that the long-distance contribution is of the same order of magnitude as the standard model short-distance contribution.

Asymmetry ine+e¯→K++K−andη→π0+γ+γ

The final-state interaction for the off-shell-photon process $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{K}^{+}+{K}^{\ensuremath{-}}$ is discussed in terms of an "intermediate representation" where one of the photons is on shell. This allows the evaluation of the absorptive part and the soft-photon contribution for $e+\overline{e}\ensuremath{\rightarrow}{K}^{+}+{K}^{\ensuremath{-}}$ from the two-photon intermediate state. The asymmetry due to the interference between the one-photon and the two-photon amplitudes for this process, near the ${f}^{\ensuremath{'}}$-resonance position, is found to be quite large, but goes through zero. Generalization of the analysis of the final-state interaction in the $s$-wave state to SU(3) allows us to predict a decay width $\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}+\ensuremath{\gamma}+\ensuremath{\gamma})\ensuremath{\approx}0.05$ keV, which is in good agreement with the experimental number.

Possible resonant enhancement of the two-photon exchange in electron-proton scattering

We investigate the corrections to electron-proton scattering and proton-antiproton annihilation into an electron-positron pair due to the two-photon intermediate state in a dispersion-theoretic treatment in terms of the proton Compton scattering amplitude. The possible resonances in the annihilation channel which might enhance this contribution are discussed. It turns out in the case of negligible electron mass that up to spin 2 only a 2+ particle can contribute via real Compton scattering, which could give a 10% effect in the cross-section with reasonable choice of the coupling constants. Even if we relax the restriction that the two-photon intermediate state should be real, scalar and pseudoscalar particles do not contribute; however, a 1+ particle might appear.