Photoproduction of π0 mesons off protons and neutrons in the second and third nucleon resonance regions

Abstract

Background: Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets.Purpose: Study experimentally single π 0 photoproduction off quasi-free nucleons from the deuteron. Investigate nuclear effects by a comparison of the results for free protons and quasi-free protons. Use the quasi-free neutron data (corrected for nuclear effects) to test the predictions of reaction models and partial wave analysis (PWA) for γn → nπ 0 derived from the analysis of the other isospin channels.Methods: High statistics angular distributions and total cross sections for the photoproduction of π 0 mesons off the deuteron with coincident detection of recoil nucleons have been measured for the first time. The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator for photon energies between 0.45 GeV and 1.4 GeV, using an almost 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion.Results: Significant effects from final state interactions (FSI) were observed for participant protons in comparison to free proton targets (between 30% and almost 40%). The data in coincidence with recoil neutrons were corrected for such effects under the assumption that they are identical for participant protons and neutrons. Reaction model predictions and PWA for γn → nπ 0 , based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data. Conclusions:The results demonstrate clearly the importance of a mesurement of the fully neutral final state for the isospin decmposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. The results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood.The photoproduction of mesons is a prime tool for the study of the excitation spectrum of the nucleon, which is a major testing ground for the properties of the strong interaction in the non-perturbative regime. The pion is the lightest meson and has a strong coupling to many nucleon excited states. Although recent years have provided new photoproduction data for many different final states, pion scattering and photoproduction of pions are still central to most analyses which aim to identify and characterize the excited states of nucleons. Many theoretical frameworks are employed to extract this information. They include the SAID multipole analysis [1,2], the MAID unitary isobar model [3,4], the Dubna-Mainz-Taipei (DMT) dynamical model [5], the Bonn-Gatchi...

First measurement of helicity-dependent cross sections in π0η photoproduction from quasi-free nucleons

“…However, there are some interesting experimental results from the comparison of pho-toproduction reactions off free protons and quasi-free protons bound in the deuteron. As mentioned above, differential cross sections for ηπ production are significantly different for free and quasi-free protons, cross sections for single π 0 production are even more different in some energy regions [36,38], also the results for pion pairs differ up to 20%, while effects in η production are insignificant. This means that FSI effects on absolute cross sections are strongly reaction dependent.…”

“…All detector modules were calibrated for their energy and timing response as discussed in detail in [38]. Background from random coincidences with the tagger was subtracted as discussed in [49].…”

“…In the first step of particle identification, hits in the calorimeter were classified as 'charged' or 'neutral' depending on the response of the PID and the CPV. Subsequently, for hits in TAPS, pulse-shape analysis (PSA) and time-of-flight versus energy analysis were used for the separation of photons from protons and neutrons as in [24,38]. The only remaining ambiguity was that photons and neutrons in the CB cannot be distinguished event-byevent (see e.g.…”

Extraction of γn→πN observables from deuteron-target data

“…Extracting (un)polarized cross sections for pion photoproduction off the neutron, γn → π − p and γn → π 0 n, from the deuteron-target data, d(γ, π − )pp and d(γ, π 0 )pn, is an important task at photon facilities such as Jefferson Laboratory (JLab) [1][2][3][4] and MAMI [5][6][7][8][9][10][11], forming a base for studying the baryon spectroscopy. A commonly used procedure of extracting the γ-n cross sections is to apply a certain set of kinematical cuts to the deuteron data and assume that the selected events are from single-nucleon quasi-free processes.…”

“…For an accurate extraction, however, one may wonder what corrections are needed (or not) to account for final state interaction (FSI) effects remaining in the selected events. In the MAMI analysis [7,11], the FSI corrections for γn → π 0 n cross sections were assumed to be the same as those for γp → π 0 p, which still needs a validation. Also, the validity of the applied cuts is a question.…”