Measurement of π0π+/− photoproduction off the deuteron and dbutanol targets

Abstract. Recent experiments using the Crystal Ball/TAPS setup at the MAMI accelerator in Mainz, Germany continue to study the properties and the excitation spectrum of the nucleon with meson photoproduction. Electromagnetic excitations of the proton and neutron are essential for understanding their isospin decomposition. The electromagnetic coupling of photons to protons is different than that of neutrons in certain states. Cross-section data alone is not sufficient for separating resonances, whereas polarization observables play a crucial role being essential in disentangling the contributing resonant and non-resonant amplitudes. Preliminary results of the polarization observable E of double π production off an unpolarized (LD2) and polarized (dButanol) target are shown with comparison to predictions of recent analyses.


Introduction
Quark models describe the behavior of quarks in nucleons at medium energies. Meson photoproduction allows to investigate the excited states (resonances) of nucleons. Unfortunately, many states are overlapping and cannot be easily differentiated from each other and many have been predicted, but not yet detected [1]. The states that have not yet been observed could be missing because they have not been seen experimentally until now or do not exist at all. Most earlier experiments were preformed with pion beams and some resonances might couple less strongly to pions and couple more strongly to rare channels and have circumvented detection. Most results still arise from experiments on the proton, which does not allow for as much information regarding the isospin structure of the electromagnetic transitions. Therefore, advances in experiments on the neutron can provide an integral piece to the understanding of the nucleon spectrum. Unfortunately, it is not possible to perform measurements on the free nuetron, but a deuterated butanol target has allowed for the possibility to study spin effects with quasi-free neutrons.
We have preliminary results for the contribution of different channels. Final results need of course a more profound analysis including interference effects etc. A possible component from the D 13 → Nρ decay could be important for the understanding of photoproduction of nuclei because in-medium modifications of the ρ would then also effect the D 13 .
From the outcomes of the figure below, there was discussion whether there is a significant contribution from D 13 → Nρ because due to the predicted in-medium modifications of the ρ this could modify the D 13 shape. One of the motivations for the present experiments with * e-mail: debdeep.ghosal@unibas.ch In the above left figure, much of the bump in the second resonance region is due to double pion production. The right figure above is also related to an old problem seen in total photoabsorption where the second resonance bump is completely suppressed for heavier nuclei. [2] proton and deuteron targets is the extraction of a much more precise D 13 → Nρ branching ratio for proton and neutron targets.
For years, cross section data has been used to learn about the nucleon spectrum. However, cross section data alone is not enough to distinguish the broad overlapping resonances. Polarization observables can provide understanding of these overlapping resonances by discovering more information about the complex helicity amplitudes, which describe the interaction between photon beams and nucleons. These amplitudes are fully determined when a complete set of measurements is performed and give rise to the cross section, complemented by polarization observables including beam, target, and recoil asymmetries. Here the observable E (longitudinally polarized target and circularly polarized photon beam) will be discussed.
The relationship between the cross section and polarization observable E can be written in terms of the helicity of beam and target as follows: (2) where σ 1/2 is the cross section of events with anti-parallel beam and target polarizations with total spin 1/2 and σ 3/2 is the cross section of events with parallel beam with total spin of 3/2.

Experimental setup
The measurements were performed at the MAMI-C accelerator in Mainz, Germany [3]. A longitudinally polarized electron beam of energy 1557 MeV and polarization degree of 80% is delivered into the A2 experimental Hall. Circularly polarized photons are produced from a radiator and energy tagged using the Glasgow-Mainz photon tagger [4] with energies between 470 and 1450 MeV. The targets consists of a deuterated butanol material (dButanol) cooled to a low temperature with the deuterons either transversally or longitudinally polarized up to 80%. The target is surrounded by a cylindrical particle identification detector (PID) made up of 24 plastic scintillator strips that each cover 15 • in the azimuthal angle. The PID is then surrounded by a multi-wire poroportional chamber (MWPC) and the MWPC is surrounded by the spherical Crystal Ball (CB) calorimeter [5]. The CB consists of 672 NaI(Tl) crystals and covers 20 • to 160 • in the polar angle. In the forward direction, a hexagonal two arm photon 10 spectrometer (TAPS) built from 72 PbWO 4 (two innermost rings) and 366 BaF 2 crystals (remaining rings) is present. A veto wall is present in front of TAPS is used for particle identification. The combination of the CB and TAPS provides an almost 4 μπ acceptance in the center of mass fram with a high angular and energy resolution.

Event selection
Using information from the detectors, events are collected and then selected based on the number of charged or neutral hits. Neutral mesons are identified via a χ 2 test, which tries to the find the best combination of photon clusters for the meson invariant mass. To eliminate accidental conincident tagger photons, coincidence time cuts are applied and random background subtraction was performed. To separate the background from the signal, kinematic cuts were applied for each W and cosθ bin.
The analysis of γp(n) → π 0 π + n(n) requires the decay photons of the π 0 meson to be detected as well as the π + and the recoil neutron. Therefore, all events with 3 neutral clusters and 1 charged cluster are selected. A dE-E analysis was used to identify the charged pion. [6] The main source of background in this reaction comes from the following reaction that has the same final state: γp(n) → ηπ + n.
The analysis of γn(p) → π 0 π − p(p) requires the decay photons of the π 0 meson to be detected as well as the π − and the recoil proton. Therefore, all events with 2 neutral clusters and 2 charged clusters are selected. [6] Preliminary results regarding comparison plots of efficiency and total cross sections in terms of W (center of mass energy) for liquid deuterium (LD 2 ) and d-Butanol targets are shown in Fig. 2.

E observable extraction
Different versions were used to extract the final E asymmetry. Version-1 refers to where the asymmetry was normalized with twice the unpolarized cross section σ 0 , which was measured with a liquid deuterium target, and does not need to utilize carbon background subtraction since the unpolarized background cancels in the difference of the two helicity states (σ δ = σ 1/2 − σ 3/2 ). Version-2 refers to the normalization of the numerator using the sum of the two spin configurations (σ = σ 1/2 + σ 3/2 ). Here, the background from unpolarized carbon and oxygen nuclei inside the target have to be subtracted.
For the determination of the carbon background dedicated beam-times were carried out, with the same experimental conditions as for the dButanol, but with a carbon target of identical density. For a proper background subtraction, the relative contributions of carbon and oxygen nuclei to the dButanol, also called dilution factor, has to be determined. This was done by comparing missing mass distributions of dButanol data, carbon data and deuterium data. If the individual data sets are properly normalized, the spectra of carbon and deuterium should add up to the dButanol. Missing mass distributions are especially suited for this comparison, as contributions from different nuclei are well separated, due to different Fermi momenta.
Preliminary results of E observable for γp → π 0 π + n and γn → π 0 π − p channels are shown in Fig. 3.  Figure 2. Detection efficiency and total cross section comparison for γp(n) → π 0 π + n(n) (above row) and γn(p) → π 0 π − p(p) (bottom row).For efficiency spectra:Green symbols for unpolarized liquid deuterium target, Red symbols for polarized dButanol target with negative helicity state and Blue symbols for polarized dButanol target with positive helicity state; For cross section plots: Red symbols for unpolarized liquid deuterium target, Blue symbols for polarized dButanol target with negative helicity state and Green symbols for polarized dButanol target with positive helicity state.   Figure 3. E-observable plot in terms of W(center of mass energy) for γp → π 0 π + n (Left plot) and for γn → π 0 π − p (right plot); Blue triangles are for the method with carbon subtraction and the open circles correspond to the direct method.

Conclusion and outlook
For both of the interested channels, there is almost vanishing E-observable asymmetry for both in terms of photon energy and center of mass energy (only later is presented here because of page limitation). Extracted results from the two different methods (version-1 and version- 2) also agree mostly with each other. Further investigation on background subtraction and energy sum correction is under process for the final results. Data from further beamtimes with dButanol target is to be analyzed.