Measurement of polarization observables in π⁰ and π⁰π⁰ photoproduction from protons and neutrons at MAMI and ELSA

A detailed understanding of the nucleon and its excitation spectrum is inevitable for the investigation of the underlying strong interaction in the non-perturbative regime. Unfortunately, it is still poorly understood and the agreement between predicted and experimentally observed excited states is still unsatisfactory. For most quantum numbers, many more states are predicted than were experimentally identified. This is known as the problem of missing resonances. It is still unknown whether these discrepancies between theoretical predictions and experiment result from an inappropriate consideration of the internal degrees of freedom of the nucleon in the models or from experimental bias in the measurements. In the past, mostly pion-induced reactions were used to investigate the excitation spectrum of the nucleon. However, such experiments are only sensitive to excited states that couple significantly to Npi, while other states would remain undiscovered. On the other hand, higher excited nucleons might decay via a cascade of intermediate states and with the emission of multiple mesons thereof, such that these states are only accessible using multiple meson production.
In the last few years, several new developments from the theory side in the non-perturbative regime have led to very promising results and are accompanied with a lot of experimental efforts using meson photoproduction, which include measurements of sequential decays via intermediate nucleon resonances. However, the measurements are complicated by the huge amount of broad and overlapping resonances, such that the identification of the individual contributions requires an elaborate partial wave analysis of the measured spectrum. Depending on the meson of interest, the situation can though be simplified. For example, in eta photoproduction (as the eta meson has isospin zero), contributions from isospin 3/2 (Delta) resonances are excluded and only isospin 1/2 (N*) resonances can contribute to the nucleon excitation spectrum. On the other hand, single pi0 photoproduction is complicated by contributions from Delta resonances, but eliminates a lot of background contributions, since photons only couple weakly to neutral mesons. Investigations of pi0pi0 or etapi0 share the mentioned properties, but in addition allow for the investigation of higher lying resonances, R, by a cascade of decays via intermediate resonances, R', and the subsequent emission of meson m1 and meson m2: g+N->R->R'+m1->N+m1+m2.
Experiments on neutrons are especially important, as a distinct isospin decomposition of pion photoproduction requires the measurement on the neutron, which is only feasible with quasi-free neutrons bound in the deuteron. Due to the complication of the reaction and the non-trivial detection of neutrons, such data is so far sparse. In the last years, numerous developments on the experimental side rendered experiments with the coincident neutron detection possible and the availability of deuterated butanol targets even allows for the measurement of observables from polarized neutrons.
In this work, single and double pi0 photoproduction from unpolarized and polarized neutrons bound inside the deuteron was studied throughout the second and third resonance region of the nucleon. The experiments were carried out within the A2 and CBELSA/TAPS Collaborations at the electron accelerator facilities MAMI in Mainz, Germany, and ELSA in Bonn, Germany. Data was taken with circularly polarized tagged photon beams, unpolarized liquid deuterium targets, and longitudinally polarized deuterated butanol targets. The experimental setups mainly comprised of the electromagnetic calorimeters Crystal Ball and TAPS at A2 and Crystal Barrel and MiniTAPS at CBELSA/TAPS, which both almost cover the full solid angle.
The present data revealed significant differences in the production mechanisms of single and double pi0 photoproduction from protons and neutrons. Furthermore, it was found that significant nuclear effects result in a suppression of the single (double) pi0 photoproduction cross section for quasi-free protons of 35% (15%) with respect to the free proton. This indicates that a direct deduction of the results for the free neutron from those of the quasi-free neutron is not possible. Influences from nuclear Fermi motion have been removed by a complete reconstruction of the final state. A correction for the effects from nuclear final state interaction has been established by a comparison of the cross section from quasi-free protons to the results from free protons and was used to estimate the results on the free neutron. Thereby, the correction is based on the assumption that the nuclear effects are similar for protons and neutrons.
For the first time, results for the unpolarized and helicity dependent differential and total cross sections for single and double pi0 photoproduction from free and quasi-free nucleons have been extracted with high statistical quality. The results are compared to the latest available predictions from reaction models and partial-wave analyses that are based on data from other isospin channels. The impact of the present results from single pi0 photoproduction from the free neutron will be demonstrated by the results of a new analysis in the framework of the Bonn-Gatchina coupled-channel partial-wave analysis, which included the present data. A detailed investigation of the contributions from sequential decays to double pi0 photoproduction discloses the different nature of the corresponding cross section on the proton and the neutron.
The first time measurements of unpolarized and polarized observables in single and double pi0 photoproduction from the neutron provide substantial input for the partial-wave analyses and will put stringent limits on the resonance contributions to the excitation spectrum of the nucleon.