Threshold neutral pion electroproduction in heavy baryon chiral perturbation theory

Abstract

We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S-wave multipoles E 0+ and L 0+ is carried out to three orders. There appear several undetermined lowenergy constants. Three are taken from a recent study of the new TAPS threshold π 0 photoproduction data, one is fixed from the proton Dirac form factor and the novel S-wave constants appearing at orders q 4 and q 5 are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared k 2 = −0.1 GeV 2 . The inclusion of a particular set of dimension five operators is forced upon the fit by a soft-pion theorem which severely constrains the momentum dependence of the longitudinal S-wave multipole L 0+ at order q 4 . We give predictions for lower photon virtualities for the various multipoles and differential cross sections. Further improvements are briefly touched upon.1

Precision Measurement of Electroproduction of pi⁰ near Threshold

“…2) the present data agree with the total cross section predictions of ChPT (with LECs determined from earlier MAMI measurements [28,29] Since the quantitative predictions of ChPT depend upon the choice of LECs it remains to be seen whether fitting new LECs to the presentt data or adding more terms to the calculations would improve the agreement between theory and experiment.…”

“…from similar fits to model cross sections from ChPT [28,29], MAID and DMT. Our data include runs for which the beam was not energy locked.…”

Hadron structure at lowQ2

“…The predictions of ChPT for the p waves are in good agreement with the data, whereas P 3 comes out much too small in dispersion theory. Since the latter approach is mainly based on input from the imaginary parts of the multipoles M 1+ and M 1− in the resonance region, this (Bernard et al, 1996c) and the dashed line the result of the phenomenological model MAID (Drechsel et al, 1999). Figure from Merkel et al (2002).…”

“…The figure also shows the contributions of the most important channels: the one-pion channels dominate up to ν ≈ 500 MeV, the two-pion branching ratio becomes comparable in the second resonance region at ν ≈ 700 MeV, and the large η branch of the resonance N * (1535) is hidden in the background. Because the nucleon resonances lie above the one-pion threshold, any separation in a continuous background and (Bernard et al, 1996c), dashed line: MAID (Drechsel et al, 1999), dashed-dotted line: DMT model . Figure from Weis et al (2007). discrete resonances is necessarily model-dependent.…”

Near threshold neutral pion electroproduction on deuterium in chiral perturbation theory

“…In fact, one also has to account for one particular dimension five operator as explained in [10], see also Sect. 4.2. tb ss …”