DETERMINATION OF THE COSY PROTON BEAM POLARIZATION USING THE WASA DETECTOR ∗

The dynamics of η -meson production and the interaction of η mesons with nucleons can be studied using the (cid:126)pp → ppη reaction via measurements of the analyzing power A y . To this end, we have performed a measurement of the (cid:126)pp → ppη reaction using the WASA-at-COSY detector, which provides large acceptance and is ϕ -symmetric. The experiment was carried out for the beam momenta of 2026 MeV/ c and 2188 MeV/ c . In this article, we present a method and results for the determination of the degree of the beam polarization.

(153) η meson. Up to now, measurements of A y in the pp → ppη reaction were conducted by the COSY-11 and DISTO collaborations [29][30][31][32]. However, these previous experiments suffered from low statistics and limited geometrical acceptance. In order to determine A y more accurately, the axially symmetric WASA detector [38] and the vertically polarized proton beam of COSY have been used to collect a high statistics data sample. An experiment was conducted for the beam momenta of 2026 MeV/c and 2188 MeV/c which correspond to excess energies of 15 MeV and 72 MeV, respectively [35,39]. The trigger was set to select events corresponding to the pp → ppη and pp → pp reactions. Proton-proton elastic scattering provides information for the determination of the polarization and the luminosity. Additionally, to control the effects caused by potential asymmetries in the detector setup the spin orientation was flipped from cycle to cycle.

Extraction of the pp → pp reaction
The first step in the study of the asymmetry A y is the analysis of the reaction pp → pp. Based on the elastic scattering reaction, we have calculated the polarization degree for the whole data sample. In the pp → pp reaction, one of the protons is registered in the Forward Detector, with geometrical acceptance of polar angle θ FD from 3 • to 18 • and the second is registered in the Central Detector. By utilizing ∆E-E method in the Forward Range Hodoscope and the angular correlation of the outgoing protons, we have selected events corresponding to the proton-proton elastic scattering. For further calculations, we have chosen only two θ * FD ranges in the centre of mass: [30 • -34 • ] and [34 • -38 • ], for which A y is available from the EDDA database [41] and for which the number of events was sufficient. For precise background suppression, all events were divided into 36 bins in the azimuthal angle ϕ. Exemplary distribution of the θ * FD vs. θ * CD is shown in Fig. 1.  Figure 2 shows a clear asymmetry between angular distribution obtained for different spin orientations. In order to estimate the background, for each bin in θ * FD , the θ * CD distribution was fitted separately with a fifth-order polynomial, excluding from the fit the range from 130 • to 160 • , as shown in Fig. 3.

Polarization determination
The polarization was determined from elastic scattering events by the following equation where N exp (θ, ϕ) and N exp (θ, ϕ + 180) correspond to a given angular bin after correction for the acceptance, and P denotes the polarization. A y (θ) was calculated based on the results of the EDDA experiment [41]. The procedure is described in details elsewhere [36]. The polarization determined from all experimental data is shown in Fig. 4. The upper panel shows the results for measurements with a polarized beam obtained for both studied beam momenta. The lower panel presents the polarization of the control sample, which has been measured with an unpolarized beam. In the case of the control sample, the measured polarization is consistent with zero, within the statistical uncertainty. Figure 4 shows the results before and after the correction for shifts of the center of the beam-target interaction region from its nominal position [36,37]. The average polarization of all acquired data was calculated analyzing all events together. The results are shown in Table I.

Summary
The polarization of elastic scattering events in the reaction pp → pp has been presented. The polarization differs between the spin modes. However, it is stable between runs for the whole period of data taking. Additionally, the value of the polarization for an unpolarized beam has been measured to be consistent with zero, as expected. A high degree of polarization for both beam momenta gives a very good perspective for the determination of the analyzing power for the pp → ppη reaction.
We acknowledge support by the Polish National Science Center through grant No. 2011/03/B/ST2/01847, and by the FFE grants of the Research Center Jülich.