Single π 0-Photoproduction off Quasi-Free Protons and Neutrons

Abstract. We report on the preliminary results of a high statistics measurement of π0photoproduction off quasi-free protons and neutrons from the deuteron with the CB/TAPS setup at the MAMI electron accelerator facility in Mainz. Preliminary differential and total cross sections covering the full angular range and photon energies up to the second and third resonance region have been measured. The comparison of free and quasi-free proton results allows a detailed analysis of possible nuclear effects on the cross section.


Introduction
Meson photoproduction allows a detailed investigation of the excitation spectrum of the nucleon and of the interactions of mesons with nucleons and nuclei.In order to understand the isospin decomposition of the electromagnetic excitations, it is necessary to measure the reaction not only on the proton, but also on the neutron.Since no free neutron target exists, the sole experimental possibility to investigate this subject is the quasi-free photoproduction of mesons off neutrons bound in nuclei, in particular in the deuteron.As a consequence the production cross section will of course be influenced by nuclear effects, such as Fermi motion and final state interactions (FSI).However, such effects can be studied by a comparison of the free to the quasi-free cross section measured in coincidence with recoil protons.

Experimental Setup
The experiment was carried out at the Mainzer Microtron (MAMI) in Mainz, Germany.The MAMI electron beam facility produces a continuous electron beam with energies up to 1.5 GeV.The electron beam is directed onto a copper radiator where it produces a photon beam via the Bremsstrahlungs process that is then impinging on the LD 2 target.The main detectors used in this experiment, providing nearly full angular coverage, are the Crystal Ball calorimeter (CB) surrounding the target and the Two-Arm Photon Spectrometer (TAPS) which is placed as a forward wall.The separation of neutral and charged particles is done with plastic scintillators, either as bars arranged in a cylindrical setup surrounding the target (CB) or as hexagonally shaped vetos (TAPS).

Data Analysis
The production thresholds of the reactions γd → π 0 p(n) and γd → π 0 n(p) are close to ∼ 145 MeV.The present experiment covered incident photon energies from 400 MeV to 1.4 GeV, i.e. the ∆-resonance and the second and third resonance regions of the nucleon.At higher incident photon energies many other reactions compete with single π 0 photoproduction, among them final states where π 0 mesons are emitted together with other decay products.The coherent π 0 -production (∼ 140 MeV) is negligible at a e-mail: manuel.dieterle@unibas.chEPJ Web of Conferences photon energies above 500 MeV and hence does not significantly contribute in the investigated energy range.The main background contributions stem from (in brackets: respective production thresholds) Therefore a sophisticated analysis was necessary in order to isolate the reaction of interest.
As a first step of the analysis, particles had to be distinguished from photons.Charged particles were identified with the help of the veto detectors and (in TAPS) also with time-of-flight versus energy spectra.Neutrons were isolated from photons with time-of-flight, a pulse-shape analysis of the scintillation light of the BaF 2 crystals of the TAPS detector, and (for events with three neutral hits) a χ 2 -analysis, finding the most probable combination of two photons to a π 0 -meson, and assigning the bachelor photon as neutron candidate.As a next step, the π 0 were identified with the standard invariant mass analysis The distributions for different analysis steps is shown on the right-hand side of Fig. 1.The spectrum is already rather free of background after requiring coincidence between the hits in the detectors and removing random events in the photon tagging spectrometer (green area in Fig. 1, right-hand side).The remaining background, which is mostly of after MM Fig. 1.Main identification spectra of the analysis, using the example of the quasi-free inclusive reaction.Lefthand side: missing mass distribution for one specific photon energy and cos(θ CMS π 0 ) bin.Black dots: data, blue line: simulated signal, green line: sum of all simulated background contributions, red line: simulated signal + background).Right-hand side: evolution of the invariant mass distribution.Yellow area: after the χ 2 -test (only in the neutral channel) and requiring coincidence between the individual hits in the detectors, green area: after additionally removing random events in the tagger, Red area: after applying the missing mass cut.combinatorial nature originating from the competing channels, was removed by cuts on the missing mass ∆M = ((E γ + m N − E π 0 ) 2 − (p γ − p π 0 ) 2 ) 1/2 , as illustrated in Fig. 1, left-hand side.The position of the cuts was determined from the result of a Monte Carlo simulation of the detector response for all contributing channels.This procedure was applied for the identification of both quasi-free exclusive reactions γd → π 0 p(n) and γd → π 0 n(p) as well as for the quasi-free inclusive reaction γd → π 0 (NN) for which the π 0 is detected in coincidence with 1 or 0 nucleons.This reaction can be used to demonstrate the quality of the reaction identification, since its cross section should agree with the sum of the cross sections of the two exclusive reactions.

Preliminary Results
The fact that the sum of the two exclusive cross sections (open magenta dots in Fig. 2, left-hand side) is in almost perfect agreement with the measured quasi-free inclusive cross section (black dots in Fig. 2, left-hand side) proves the quality of the reaction identification.Furthermore, the quasi-free inclusive cross section (black dots in Fig. 2, left-hand side) is in good agreement with former results [1].The nearly identical shape of the measured single π 0 total cross sections and the results of the MAID model [2] and the SAID analysis of the previous data [3] (Fig. 2, left and center) demonstrates a correct understanding of the resonance contributions to the different reactions.It also reveals that the contributions to single π 0 photoproduction on the proton (D 13 (1520), F 15 (1680)) are rather different to those on the neutron (D 13 (1520), D 15 (1675)).The discrepancy in absolute magnitude between the measured cross sections and the model results is also observed for the differential cross sections (Fig. 3) and cannot MESON2012 -12th International Workshop on Meson Production, Properties and Interaction be explained by nuclear Fermi motion alone.The fact that folding the theoretical model results with Fermi motion does not overcome this problem reveals the importance of other nuclear effects, such as final state interactions, meson rescattering or others.This discrepancy was also observed in previous experiments [1], [4].The same level of discrepancy was observed in the present experiment where the model results are scaled down (Fig. 2, right-hand side).