Recent results from the CBELSA/TAPS experiment at ELSA

In order to gain a better understanding of the dynamics inside the nucleon and of the non-perturbative regime of QCD, the nucleon excitation spectra and the properties of nucleon resonances are investigated. An essential experimental tool to achieve this goal is the study of different photoproduction reactions. Partial wave analyses are performed in order to obtain information about the contributing resonances. A complete experiment is needed to extract the underlying amplitudes unambiguously, which requires the measurement of carefully chosen single and double polarization observables in addition to the unpolarized cross section. The CBELSA/TAPS experiment in Bonn offers the possibility to measure several polarization observables using a linearly or circularly polarized photon beam and with a longitudinally or transversely polarized target. This contribution gives an overview of recently measured polarization observables in different final states. The impact of the new data is discussed.


Introduction
Comparisons of the nucleon or delta excitation spectra to either quark model predictions [1] or latest lattice QCD calculations [2] reveal that the dynamics inside the nucleons are not well understood in the non-perturbative regime of QCD. In particular, more states are predicted than there have been experimentally observed so far. This feature is known as the missing resonances problem. One possible explanation for this could be that most of the states, which were found in the past, were observed in πN scattering experiments. In case some resonances may not couple strongly to the πN, they could have been missed. Therefore, it is crucial to probe the excitation spectra using different production mechanisms such as the photoproduction of mesons. Several facilities around the world have been studying photoproduction reactions like the CLAS collaboration at JLab [3][4][5], the A2 collaboration at MAMI [6], the LEPS collaboration at SPring-8 [7] and the CBELSA/TAPS collaboration at ELSA [8][9][10][11][12][13][14].
The determination of the resonance parameters from the data is a complicated task since the resonances are broad and overlap strongly. Thus, partial wave analyses (PWA) are necessary to disentangle the contributing resonances and their parameters. Different PWA approaches exist e.g. the BnGa [15], the JüBo [16], the SAID [17] or MAID [18] PWA. In order to obtain a unique solution, it is not sufficient to only measure the total unpolarized cross section as it is only sensitive to the squared moduli of the partial wave amplitudes. Using a polarized photon beam, a polarized target or by measuring the recoil nucleon polarization, it is possible to measure different polarization observables. shows all the possible combinations of single and double polarization observables that can be measured in the photoproduction of single pseudoscalar mesons. These polarization observables have a sensitivity to interference terms between different partial waves. Therefore, the polarization observables enhance the possibility to detect contributions from small partial waves, via interference of the small waves with large dominant partial waves.
For more than a decade the main focus of the CBELSA/TAPS experiment in Bonn has been dedicated to acquiring polarization observables for different final states. This contribution reports on the latest measured data and their impact.

Experimental setup
The ELectron Stretcher Accelerator (ELSA) [19] provides unpolarized or longitudinally polarized electrons with an energy of up to 3.2 GeV. The electron beam is delivered either to the BGO-OD [20] or to the CBELSA/TAPS experimental area (see Figure 1). The electrons impinge on a radiator located inside the goniometer tank and produce a photon beam via bremsstrahlung. The scattered electrons are deflected according to their momenta in the magnetic field of the tagging system and detected by scintillation fibers and bars.
A thin diamond crystal is used as radiator in order to have coherent bremsstrahlung. The diamond crystal lattice can be positioned precisely with respect to the electron beam, which allows to control the coherent edge position [21]. A maximal polarization degree of 65% is achieved for a coherent edge position of E γ = 950 MeV and of 35% for E γ = 1850 MeV. Longitudinally polarized electrons in combination with an amorphous radiator are utilized to get EPJ Web of Conferences 241, 01001 (2020) NSTAR 2019 http://doi.org/10.1051/epjconf/202024101001 Table 1. Table lists all possible polarization observables that can be obtained by using a polarized photon beam, a polarized target by measuring the recoil nucleon polarization degree. Some observables can be measured in two different ways, e.g. P can be measured as a single polarization observable or as a double polarization observable with linearly polarized photon beam and a transversely polarized target. All information are taken from [22].
photon beam target recoil nucleon target and recoil circularly polarized photons. Here, a maximal polarization degree of around 65% could be reached for the maximal beam photon energy that is possible to tag for an ELSA energy of E 0 = 2400 MeV. The photons impinge a target which is located in the center of the Crystal Barrel (CB) calorimeter [23]. As target material an unpolarized liquid hydrogen is used to perform measurements on free protons. A longitudinally or transversely polarizable frozen-spin butanol or deuterized butanol target is available for the measurement of polarization observables with polarized protons or neutrons, respectively [24]. A maximal polarization degree of p T = 84% and high relaxation times of around 2000 h were reported for the transversely polarized butanol target of most recent beamtimes, which has been possible due to a collaborative effort between the target group of the CBELSA/TAPS collaboration in Bonn and the target group of the A2 collaboration in Mainz [25]. In addition, a carbon foam target is used to study the unpolarized background contribution of the butanol target. Furthermore, inmedium modification of mesons are investigated by utilizing different solid targets.
The CB calorimeter consists of 1320 CsI(Tl) crystals that cover a large polar angular range from 30 • to 156 • and the full azimuthal angular range. Each crystal is now read out by two avalanche photodiodes instead of the previously used PIN photodiodes. The new readout electronics allows to use the CB in the first level trigger which is crucial for a high trigger efficiency of complete neutral final states, e.g. nπ 0 . The upgrade was successfully completed and new data were taken since December 2017.
In forward direction, the MiniTAPS calorimeter [26] is located, which is made of 216 BaF 2 crystals and readout by photomultipliers in order to gain a fast trigger signal.
A cylindrically formed scintillation fiber detector [27] is located around the target. Together with scintillation plates, that are present in front of all the crystals that fulfill θ < 30 • , the identification of charged particles is possible.

Determination of polarization observables
Since the CBELSA/TAPS experiment is equipped with two electromagnetic calorimeters, it is ideally suited for detecting photons in the final state. Thus, this experiment focuses on reconstructing neutral mesons that decay into two or more photons. Figure 2 shows a typical two photon invariant mass spectrum. It is possible to obtain high statistics data sets with very low background contributions for single meson final states like the pπ 0 and pη final states. In addition, final states like pω or pη can be selected as well. Furthermore, multi-meson final states like pπ 0 π 0 or pπ 0 η have been successfully selected.
As already mentioned, the CBELSA/TAPS experiment provides the possibility to measure with a polarized photon beam and with a polarized target. Thus, the CBELSA/TAPS setup can be used to measure the polarization observables of the beam-target category (see Table  1). The polarized cross section dσ dΩ pol for a single pseudoscalar meson reads [22] where p lin γ and p circ γ are the degree of linearly and circularly polarized photons, p x , p y and p z give the target polarization degree in transverse and longitudinal direction and dσ dΩ 0 is the unpolarized differential cross section.
For the measurement of the beam asymmetry Σ data were taken with a linearly polarized photon beam and a liquid hydrogen target in 2013. Here, the coherent edge settings were chosen at high beam photon energies of E γ = 1750 MeV and E γ = 1850 MeV, where high precision data is scarce for many final states. Using a polarized frozen-spin butanol target instead of the hydrogen target, allows to measure the double polarization observable H. In addition, the polarization observables T and P, whereby P is measured here as a double polarization observable (see Table 1), can be accessed simultaneously as well. Data were taken with this configuration before and after the CBELSA/TAPS upgrade using different coherent edge settings (from E γ = 950 MeV to E γ = 1300 MeV) [10,11]. Furthermore, the double polarization observable G was measured in a separate beam time using a longitudinally polarized butanol target in combination with linearly polarized photons [12,13]. Besides, the  Figure 1. Overview of the CBELSA/TPAS experimental area. The electron beam from ELSA (upper right corner) impinges a thin radiator inside the goniometer tank and a photon beam is produced via bremsstrahlung. The photon energy can be determined with the tagging system that deflects the scattered electrons according to their momenta and detects them via scintillation fibers and bars. The photons hit a target at the center of the Crystal Barrel calorimeter, which is shown in large in the right bottom corner. It is complemented by the MiniTAPS detector in forward direction. The inner detector, which consists of scintillation fibers, and the scintillation plates, that are mounted in front of the forward crystals, are utilized for the identification of charged particles.
helicity asymmetry E can be accessed by using circularly polarized photons and a longitudinally polarized butanol target [8,9]. It is defined as [22] Here, σ 1/2 and σ 3/2 are the spin dependent cross sections for antiparallel or parallel spin configuration of the incoming beam photon and target, respectively. A complication due to the used butanol target is that it does not only contain contributions from the polarizable free hydrogen nuclei but also from the unpolarized bound carbon and oxygen nuclei. The carbon background was studied in detail using additional measurements with a carbon foam target. This allowed the determination of the so-called dilution factor, which gives the amount of polarizable hydrogen nuclei of the selected data. It is given by where N C and N B are the count rates of the selected carbon and butanol data, respectively and s C is a scaling factor that is needed to account mainly for the differences in the photon fluxes during the measurements performed with the carbon and the butanol target. References [9,11,13] demonstrate in detail how the dilution factor can be determined from the data.

Single meson photoproduction: γp → pπ 0
The pπ 0 channel is the most intensely studied photoproduction final state due to its large cross section. It has the largest database for the unpolarized cross section and polarization observables available now, to which the CBELSA/TAPS collaboration contributed significantly. resonance due to its much higher mass [35].
The helicity asymmetry E was determined for a large energy range from E γ = 600 MeV to E γ =2300 MeV and used to determine the spin dependent total cross sections σ 1/2 and σ 3/2 [8,9], which are shown in Figure 4. It is remarkable that already in the second resonance region (E γ = 660 MeV to E γ = 900 MeV) discrepancies exist between data and PWA predictions. The third resonance region (E γ = 900 MeV to E γ = 1200 MeV) is also not well described by the different PWA predictions: while the MAID, SAID-SN11 and BnGa-2011-02 PWA underestimate the σ 1/2 contribution, the JüBo group overestimates it. The description of the data was significantly improved by new fits of the PWA groups.
The results for the polarization observables T, P and H are shown in Figure 5 [10,11]. The data are in good agreement with previously existing data and exceed them in terms of statistics, angular and energy coverage. Furthermore, the observable H has been measured for the first time. The data show a relatively good agreement also to different PWA predictions. After the CB upgrade finished, new data were taken for T, P and H, which are depicted in Figures 6 and 7. The new preliminary data shows a very good agreement to the already published CBELSA/TAPS data. In addition, an overall improvement of the statistical precision could be achieved by a factor of approximately two. Furthermore, the existing database for the polarization observables P and H could be extended to E γ = 1300 MeV.

Single meson photoproduction: γp → pη
The database of the pη final state is not well established in comparison to the pπ 0 final state since the pη cross section is much smaller. Therefore, only a scarce energy and angular coverage exists with larger statistical error bars. Nevertheless, it is prudent to study the pη final state since the η has an isospin of I = 0 and can only couple to N * resonances and therefore provides an isospin filter.
The polarization observables E, G, T, P and H have been determined by the CBELSA/TAPS collaboration for the pη final state as well. Figure 8 shows the results for the double polarization observable G. This new data provides much needed constraints to the pη photoproduction amplitudes since none of the PWAs could predict the angular dependence of G. A new fit of the BnGa PWA including this new data resulted in the precise determination of N * → pη branching ratios [14]. Here, a remarkable result is the new determined branching ratio of 0.33 ± 0.04 for the N(1650) 1 2 − (S 11 ) resonance [14]. This branching ratio was listed as 0.14 − 0.22 [33] in the past and thus, was significantly smaller than the branching ratio for the N(1535) 1 2 − (S 11 ) with 0.42 +0. 13 −0.12 [33], which was topic of intense discussions [36].
The beam asymmetry Σ was determined for the pη final state as well, providing a precise data-set with a large energy and full angular coverage. A Legendre moment analysis was performed here as well, which showed evidence for the pη -cusp in the E 0+ multipole of pη [37,38]. Thus, the unprecedented precision in the new beam asymmetry data puts a reinforced requirement on the PWA groups to implement all relevant singularities correctly into their amplitude parametrizations.

Impact of polarization observables
Using the determined double polarization observables G, E, T, P and H of the pπ 0 final state by the CBELSA/TAPS collaboration, stronger constraints could be imposed on the photoproduction multipoles of the pπ 0 final state by the BnGa, JüBo and SAID groups. As an example the real and imaginary parts of the E 0+ , E 1+ , M 1− and M 2− multipoles are shown in Figure 10. It becomes apparent that the fit error bars of the BnGa PWA decrease by a factor of 2.25 due to the inclusion of these new polarization measurements [10]. In addition, the deviation between the different PWA groups was investigated by taking the variance of all three PWA (BnGa, JüBo and SAID) summed over all pπ 0 photoproduction multipoles up to L = 4 [15]. Here, a clear reduction of the variance was observed due to inclusion of the new polarization data. Thus, the multipoles of the different PWA are slowly converging towards one solution.
Based on the latest polarization data from several collaborations, e.g. A2, CLAS, GRAAL, and CBELSA/TAPS, several resonances have been upgraded regarding their star-rating in the PDG [47].

Summary and Outlook
The CBELSA/TAPS collaboration has measured almost all of the polarization observables of the beam-target category for single and multi-meson final states with high precision, covering a large region in phase-space. The results were used by several PWA groups to constrain their solutions. A comparison of the multipoles shows that the new polarization data helps to reach the goal of   Figure 9. A Dalitz plot is depicted for the reaction γp → pπ 0 π 0 for an energy range of 1800 MeV < E γ < 2200 MeV. The horizontal and vertical bands indicate the intermediate states.  [15,41]). In addition, the MAID (black line) [18], SAID-CM12 (solid green line) [17], SAID-SN11 (green dashed line) [52] and the JüBo-2015 (magenta line) [50] PWA fit solutions are shown as well. Taken from [11].
finding one unique solution. The results have now entered the PDG and the star-rating of several resonances was improved.
The CBELSA/TAPS experiment was successfully upgraded with a new readout electronics for the CB detector. The new data, that has been taken after the upgrade, show promising results with improved statistical precision. The finished detector upgrade allows to measure polarization observables for final states that involve neutral particles exclusively, with a high trigger efficiency. These final states, e.g. nπ 0 , are in particular important since there is only very few data available for these final states.