η’ beam asymmetry at threshold using the BGO-OD experiment

The unexpected nodal structure of the beam asymmetry recently reported by the GRAAL collaboration in η′ photoproduction very close to threshold could be explained by a previously unobserved narrow resonance. The BGO-OD experiment is ideally suited to verify this measurement via the detection of forward going charged particles which in the threshold region of interest allows the identification of the reaction γp → η′p solely based on the proton going in the forward direction. This yields unprecedented statistics if, in the missing mass analysis of the η′ meson, the background can be sufficiently well controlled. Preliminary results using a linearly polarised photon beam are shown. The reaction γp → η′p was identified in the BGO forward spectrometer, with simulated data used to seperate signal and background.


Introduction
As can be seen in figure 1 the η beam asymmetry has already been measured by the GRAAL experiment [1]. There an unexpected nodal structure was observed in the Θ CMS η (polar angle of the η in the centre of mass frame) dependance of the beam asymmetry that vanishes rapidly with higher beam energies. The polarised cross section for the reaction can be written as the unpolarised cross section including an additional term as shown in equation 1.
P γ E γ is the photon beam polarisation, Σ E γ , Θ CMS η the beam asymmetry and φ η the azimuthal angle of the η . An expansion of Σ E γ , Θ CMS η in partial waves can be done, leading to the expression shown in equation 2-4 [2]. This * e-mail: alef@physik.uni-bonn.de is valid for a truncation at l max = 3 (F-waves).
For the lowest order (k=0) this results in a sin 2 (Θ) modulation of Σ which is not observed in the data. k=1 yields the observed sin 2 (Θ) · cos (Θ) modulation. The corresponding coefficient a 1 consists of interference terms between S-F, P-D and D-F-wave. The observed behaviour can be explained by a narrow resonance close to threshold. This is further supported by results from the Bonn-Gatchina partial wave analysis group which show that including a narrow resonance (D 13 (1900)) with a mass of M η p = 1900 ± 1 MeV and a width of Γ η p < 3 MeV close to threshold much more accuratly describes the experimental data [3], see figure 2. EtaMAID can describe the observed behaviour as well us-   ing a narrow S 11 (1900) resonance [2]. Experimentally the determination of the beam asymmetry is done by extracting the normalised η yields for two perpendicular polarisation planes (+ and -), substracting the two and dividing out the sum to remove detector inefficiencies. This is shown in equation 5.
From this the beam asymmetry can be calculated by applying a cos(2φ η ) fit to the φ η distribution of A in different cos Θ CMS η bins.

The BGO-OD experiment at ELSA
The BGO-OD experiment is located at the ELSA [4] (Electron-Stretcher-Accelerator) facility at the physics institute of the University of Bonn. ELSA is a three stage accelerator which can boost electrons up to 3.2 GeV. The electrons are produced via a thermionic gun, from which they are first accelerated using a linear accelerator. Subsequently they enter a booster synchroton, where they are accelerated up to 1.6 GeV. Several runs of the synchroton are used to fill the stretcher ring, where the electrons reach their final energy of up to 3.2 GeV. From there they are fed to one of the two hadron physics experiments (BGO-OD and Crystal Barrel) or into an area for detector tests. At BGO-OD (see figure 3) the electron beam enters the goniometer tank at which point bremsstrahlung is produced via a thin copper or diamond radiator. The copper radiator is employed to produce an unpolarised photon beam whereas the diamond radiator is used to create a linearly polarised photon beam through coherent bremsstrahlung. Using a magnet and a hodoscope (the tagging system) the post-bremsstrahlung electron's energy can be determined.

Analysis of γp → η p
Due to the Lorentz boost, protons will be boosted in a forward direction close to threshold, thus entering the forward spectrometer, where their momentum and direction can be determined. No other particles are reconstructed in this analysis, resulting in high statistics at the expense of a significant background. After identifying the proton, the mising mass, m miss , to the proton can be calculated as shown in 6.  subsequently binned as shown in table 1.

η number extraction
In figure 4 the η peak is clearly visible above background. This is not the case anymore if the data is split up into the aforementioned bins (see table 1) as can be seen in figure  5.
The missing mass spectra of several simulated reactions are fit simultaneously to the missing mass spectrum of the real data, which makes it possible to determine the contributions of the different reactions to the total spectrum 1 .
To ensure that there is an η contribution in the real data, the fits have been performed with and without the η included. Figure 5 shows two example bins to demonstrate that this channel is needed for a proper fit.

Preliminary beam asymmetry results
Having extracted the η yields from the different bins, the beam asymmetry can be calculated. Ideally the polarisation is equal for both polarisation planes, however the polarisation degree varies slightly with orientation and time, so a weighted mean, P γ E γ , ± , is determined for each orientation and divided out, see equation 7-9.
By fitting a cos(2φ η ) toÃ the beam asymmetry can be extracted. The particle numbers are carefully flux normalised. Figure 6 shows the fits to the φ η distribution ofÃ in the 1 The fits were performed using the RooFit library of Root [5].

Conclusion
It has been shown, that the reaction γp → η p can be identified at the BGO-OD experiment using only proton identification in the forward spectrometer and fitting the resulting missing mass spectrum. This technique yields high statistics, but results in a large amount of background. Despite this, the data can be well decribed in the chosen energy range close to threshold with several simulated reaction channels. These fits are challenging due to the similar nature of signal and background and work is still ongoing. Furthermore systematic checks on the signal/background extraction will be performed in the future. A very preliminary first look at the resulting beam asymmetry is also given.  Figure 5. Fits in two different bins without the η (top two plots) and the same bins with the η included (bottom two plots). The black data points are the real data, the total fit is shown in solid red, the η p contribution in dotted red, 2πp in purple, 3πp in light blue, ηπ 0 p in green, ηπ 0 π 0 p in dark purple, ωp in dark blue and π 0 p in brown. Note that ωp and π 0 p only contribute in the most backward cos Θ CMS η bins.