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for the understanding of the excitation spectrum and the properties of baryons photoproduction reactions play an important role . in comparison to single meson photoproduction
the importance of the double - meson photoproduction increases at higher energies due to higher cross - sections .
the investigation of the double - meson final states also allows access to cascading decays of resonances via intermediate states such as the @xmath3 and @xmath4 . according to @xcite for a complete experiment in the two - meson photoproduction 15
independent observables have to be measured .
the polarization observables @xmath0 and @xmath1 , accessible for two - meson final states in the case of linearly polarized photon beam and unpolarized target , were measured with the cbelsa / taps experiment for the reaction @xmath5 @xcite .
data on the reaction @xmath2 with linearly polarized photons and unpolarized target were obtained by the graal experiment , where the observable @xmath6 was measured @xcite . as discussed in @xcite ,
the observable @xmath1 reduces to @xmath6 in a quasi two body consideration .
the cross section for the case of a linearly polarized photon beam and an unpolarized target @xcite can be written as : @xmath7 where @xmath8 is the unpolarized cross - section , @xmath9 is the degree of linear polarization of the incoming photon . in comparison to the single meson photoproduction where the incoming photon and outgoing particles are in the same plane , in the double meson photoproduction an additional plane occurs .
consideration of the @xmath10 distributions in the different intervals of angle @xmath11 , being the angle between production and decay planes ( see figure [ fig : kinematics]a ) gives access to the observables @xmath0 and @xmath1 as function of the angle @xmath11 .
an example of a @xmath10 distribution in a limited @xmath11 range , illustrating the contributions of sine and cosine terms , is shown in figure [ fig : kinematics]b .
fig1.eps    ( 87 , 42)*(a ) *    fig2.eps    ( 84 , 42)*(b ) *    ( 81.5 , -1)*@xmath12 $ ] *
the cbelsa / taps experiment was performed at the electron accelerator elsa in bonn @xcite . linearly polarized photons were produced via coherent bremsstrahlung on a diamond crystal .
the energy of these photons was determined by a scintillator based tagging system .
the tagged photons were impinging on a 5 cm liquid hydrogen target @xcite surrounded by the crystal barrel ( cb ) @xcite and taps @xcite calorimeters .
the cb consisting of 1290 csi(tl ) crystals covers the polar angular range @xmath13 , taps with 528 @xmath14 crystals covers @xmath15 . for charge identification
the target is surrounded by a 3 layer scintillating fiber detector @xcite and each of the taps crystals is equipped with a plastic scintillator in front of it . in this
analysis data with two linear polarization settings were considered reaching a maximum polarization of 49.2% at an energy of 1300 mev and of 38.7 % at 1600 mev .
the reaction @xmath16 has been selected asking for events with four or five clusters in the calorimeters . the events were retained if two of the two particle invariant masses resulted in the pion mass within @xmath17 and the missing mass of the fifth particle was consistent with the proton mass within @xmath18 . in case of five cluster events the missing proton direction had to match with the additional hit in one of the calorimeters . for four cluster events with existing inner detector hit the missing proton direction had to match with this hit . to further reduce the background for four cluster events additional cuts derived from monte carlo simulations taking advantage of the correlation between the momentum and polar angle of the missing proton
were performed @xcite .
after preselection , the data was subjected to a kinematic fit @xcite assuming that the interaction took place in the target center .
the events that exceeded a confidence level ( cl ) of 10% for the @xmath16 hypothesis were retained as long as the cl of @xmath16 did as well exceed the cl for the competing @xmath19 hypothesis .
the proton direction resulting from the fit had to agree with the direction of the proton reconstructed in one of the calorimeters where available .
additionally , cuts on the energy deposited in the calorimeters by the particle selected to be a proton by kinematic fit and on the number of crystals in the clusters produced by protons , were derived from monte carlo simulations .
the background level was determined to be well below 1% .
for the determination of polarization observables the cross - section ( see eq .
[ eq : cs three body ] ) was parametrized as : @xmath20 where p is the degree of the linear polarization .
figure [ fig : is and ic proton ] shows the observables @xmath0 and @xmath1 as function of the angle @xmath11 ( see figure [ fig : kinematics]a ) for the proton recoiling case .
solid points are extracted from the data directly , the open points from the conditions @xmath21 and @xmath22 which result from symmetry and parity constraints @xcite .
good agreement between the two sets indicates absence of strong systematic effects in the data .
also , due to the indistinguishability of @xmath23s , in the proton recoiling case , additional symmetry conditions @xmath24 and @xmath25 occur . in figure
[ fig : is and ic pi ] the data for the pion recoiling case are shown . in the studies of the unpolarized data with bonn - gatchina partial - wave analysis ( pwa )
@xcite @xcite it was found that the @xmath4 decays into @xmath26 in s- and d - waves with similar strength .
also , for the @xmath27 decays into @xmath26 the contribution of d - wave was observed to be stronger than the contribution of s - wave .
these results contradict naive expectations expecting d - wave decay modes to be suppressed due to the orbital angular momentum barrier .
to further investigate the contributions of s- and d - waves in the @xmath28 decay , which were not determined in @xcite due to ambiguous solutions in the pwa , the data on @xmath0 and @xmath1 were compared to pwa solutions with d - wave dominance ( solid ) and s - wave dominance ( dashed ) .
these predictions were produced without including these data in the pwa .
noticeable differences are observed between the two predictions , indicating sensitivity of the pwa to the observables @xmath0 and @xmath1 . from this comparison
one can also conclude that none of the solutions is clearly favored , but that the data provides valuable input for the pwa .
fig3.eps    ( 91.9 , -1.2)*@xmath29 $ ] *    ( -1 , 14)*@xmath30 *    ( -1 , 29)*@xmath31 *    ( 7,33.5)*@xmath32
*    ( 38,33.5)*@xmath33 *    ( 69,33.5)*@xmath34 *    fig4.eps    ( 91.9 , -1.2)*@xmath29 $ ] *    ( -1 , 14.5)*@xmath35 *    ( -1 , 29.5)*@xmath36 *    ( 7,33.5)*@xmath37 *    ( 38,33.7)*@xmath38 *    ( 69,33.7)*@xmath39 *
polarization observables @xmath0 and @xmath1 were measured for the first time in the reaction @xmath40 with the cbelsa / taps experiment .
a comparison of @xmath0 and @xmath1 with predictions of the bonn - gatchina pwa showed that the data will provide new constraints for the pwa .
this work was supported by the deutsche forschungsgemeinschaft ( dfg ) within sfb / tr16 .
to unambiguously identify baryon resonances the measurement of polarization observables is of great importance . with the cbelsa / taps experiment , located at the elsa accelerator facility in bonn , polarization observables have been determined . using a linearly polarized photon beam impinging on a liquid hydrogen target , the polarization observables @xmath0 and @xmath1 for the reaction @xmath2 
 have been determined for the first time .     
 address = helmholtz - institut fr strahlen- und kernphysik der universitt bonn , germany