K^{0}\pi^{0}\Sigma^{+} and K^{*0}\Sigma^{+} photoproduction off the proton

and photoproduction off the proton

M. Nanova II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   J. C. S. Bacelar    B. Bantes    O. Bartholomy    D. Bayadilov    R. Beck    Y.A. Beloglazov    R. Castelijns    V. Crede    H. Dutz    A. Ehmanns    D. Elsner    K. Essig    R. Ewald    I. Fabry    K. Fornet-Ponse    M. Fuchs    Ch. Funke    R. Gothe    R. Gregor II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   A. B. Gridnev    E. Gutz    P. Hoffmeister    I. Horn    I. Jaegle    J. Junkersfeld    H. Kalinowsky    S. Kammer    V. Kleber    Frank Klein    Friedrich Klein    E. Klempt    M. Konrad    M. Kotulla II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   B. Krusche    M. Lang    J. Langheinrich    H. Löhner    I.V. Lopatin    J. Lotz    S. Lugert II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   D. Menze    J. G. Messchendorp    T. Mertens    V. Metag II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   C. Morales    D.V. Novinski    R. Novotny II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   M. Ostrick    L. M. Pant II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   H. van Pee    M. Pfeiffer II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   A. Radkov    A. Roy II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   S. Schadmand II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   Ch. Schmidt    H. Schmieden    B. Schoch    S. V. Shende    V. Sokhoyan    A. Süle    V. V. Sumachev    T. Szczepanek    U. Thoma II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   D. Trnka II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   R. Varma II. Physikalisches Institut, Universität Gießen, Germany
Kernfysisch Versneller Institut, Groningen, The Netherlands
Physikalisches Institut, Universität Bonn, Germany
Helmholtz-Institut für Strahlen- u. Kernphysik, Universität Bonn, Germany
Petersburg Nuclear Physics Institute, Gatchina, Russia
Department of Physics, Florida State University, Tallahassee, FL, USA
Physikalisches Institut, Universität Basel, Switzerland
Present address: Forschunszentrum Jülich, Germany
Present address: University of South Carolina, Columbia, SC, USA
Present address: Universität Mainz, Germany
Present address: BARC, Mumbai, India
Present address: I.I.T. Powai, Mumbai, India
Present address: Universität Münster, Germany
   D. Walther    Ch. Weinheimer       Ch. Wendel
(The CBELSA/TAPS Collaboration)
Mariana.Nanova@physik.uni-giessen.de
Received: date / Revised version: date
Abstract

The exclusive reactions and , leading to the p 4 final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5 GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4 geometrical acceptance. Differential and total cross sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction suggesting dominant -channel production. and higher lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction indicates a -channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.

pacs:
13.60.LeMeson production and 25.20.LjPhotoproduction reactions and 14.20.JnHyperons

1 Introduction

The internal structure of the nucleon is reflected in the rich pattern of baryon resonances. The number of experimentally observed resonances is much smaller than predicted from theory Capstick (). This is often referred to as the ’missing’ resonance problem. Baryon resonances have often large widths and overlap largely, which makes the study of the excited states particularly difficult. It is possible to overcome this problem by looking at specific decay channels. Up to now most existing data are based on elastic scattering experiments. If the hypothesis is correct that the missing states are unobserved because they couple weakly to the decay channel, it may be possible to establish some of these missing states in other channels. Some of the resonances are predicted to decay into final states with strange particle pairs, coupling strongly to and  Capstick1 (). Strangeness production experiments will therefore be an important tool to establish ’missing’ resonances and their properties or to disprove their existence. Recently new measurements of differential and total cross sections of hyperon photoproduction have been reported by the SAPHIR lawall (), LEPS sumi (), CBELSA/TAPS ralph () and CLAS brad (); brad1 () collaborations. A partial wave analysis provided a good description of these data by introduction of a new state with quantum numbers and two solutions for the mass - either at 1885 MeV or at 1970 MeV  anis1 (); anis2 ().
Higher mass nucleon resonances could favour decays into . Therefore vector meson photoproduction can be used to search for nucleon resonances which couple strongly to the channel, where denotes a hyperon  Capstick2 (). On the other hand, photoproduction of shares elements in common with other strangeness production reactions, such as and or , which lead to or to and resonance excitations with different couplings. The investigation of higher lying hyperon resonances will provide more information about the baryon resonances in meson-hyperon decay channels and help to understand their contribution as background to photoproduction in the reaction .
The cross sections for these reaction channels are small and their experimental identification is difficult. Therefore their investigation became only feasible when high quality photon beam facilities combined with 4 high resolution detectors became available. Here we present the experimental data of (1189) photoproduction off the proton by analysing:
(1)
This reaction contains the following isobars:
(2)
(3)
(4)
where the (892) decays into and the decays into . The contribution of a non-resonant (n.r.) pair in (4) occurs together with a (1189).
Recent theoretical studies of photoproduction in the channel  oset () have been performed using a chiral unitary approach for meson-baryon scattering in the energy range close to 1700 MeV, below the threshold for production. The theoretical model is based on the assumption that the (1700) is excited and decays into (1385) or (1232). It has been applied to calculate the cross sections of the reactions and  oset1 (). The main conclusion is that the mechanism of both reactions is similar - going through the production of the , which is dynamically generated with strong couplings to the and . The current data will be compared with the predictions of this model for the reaction .
Quark model predictions for photoproduction via nucleon resonance excitations in the channels and were presented in Ref. zhao (). In the model, resonances are treated as genuine quark states. There are only two free parameters corresponding to the vector and tensor couplings which depend on the quark mass. Using this approach the cross sections for production have been predicted based on SU(3) symmetry and quark coupling parameters extracted from non-strange production like meson production. The assumption of t-channel exchange in this model leads to strong forward peaking of at higher energies ( 2 GeV).
Another theoretical prediction for a -channel exchange dominated reaction mechanism in photoproduction involves the assumption that the scalar (800) meson may play an important role  Oh (). It was demonstrated that exchange could describe the reaction mechanism in production, but for the production the contribution from the (800) meson could be substantial. The results of the CLAS collaboration have also been compared to this theoretical prediction and are in good agreement. Nevertheless, the open question here is the controversial structure of the (800) meson. We will compare our data to this model too.
Experimentally, photoproduction has been studied with the SAPHIR detector, in the reaction  wieland (). The was reconstructed from where the measured charged particles are and proton. The differential cross sections show a forward peaking of the meson. The measured total cross section is 0.35 b at 2.2 GeV incident photon energy.
In recent studies of photoproduction by the CLAS collaboration in the reaction  Hleiq (), the was reconstructed from the detected particles and ; the was treated as missing particle. The angular distributions are forward peaked; good agreement with the quark model of  zhao () was achieved after a slight adjustment of the vector and tensor couplings to the nucleon. The small enhancement of the cross section at backward angles has been interpreted as effect of - and -channel resonances that couple to . The production of higher hyperon resonances , decaying into or , has overlapping kinematics with production leading to a background for the channel .
Our data presented here will be compared with predictions of the available theoretical models and with the published experimental results. The reactions (1)-(4) have been identified via the neutral decay channels, which exclude the contamination from hyperon resonances subsequently decaying via production. The contribution from higher hyperon resonances, decaying into , provide an important background to the reaction (2). We will show evidence for identified higher hyperon states. For photon energies above 1850 MeV, production is dominated by reaction (2). Thus it is important to identify the contribution against the leading contribution, since they are both leading to the same final state.
This paper is organized as follows: In Section 2 we describe the experiment. Section 3 provides the analysis method and event reconstruction. Section 4 shows how to reconstruct and remove the p events which have the same final state and represent a considerable background to the reactions of interest. In Section 5 we discuss the reconstruction of , and . The differential and total cross sections are given and discussed in Section 6. The paper is summarised in Section 7.

Figure 1: Sketch of the experimental setup CBELSA/TAPS. The electron beam enters from the left side, hits the radiator and produces bremsstrahlung. The photons are energy tagged and hit the LH target, which is in the center of the Crystal Barrel detector. The TAPS detector is placed just after the CB and serves as a forward wall of the CB. Charged particles leaving the target are identified in the inner scintillating-fibre detector and in the plastic scintillators in front of each BaF crystal in TAPS. A photon counter for the flux determination ( intensity monitor) is placed further downstream.

2 The experiment

Data have been taken with the detector systems Crystal Barrel (CB) aker92 () and TAPS Novotny1 (); Gabler1 () at the 3.5 GeV electron stretcher facility ELSA Husmann (); Hillert (). The detector setup is shown schematically in figure 1. Electrons extracted from ELSA with energy hit a primary radiation target, a thin copper or diamond crystal, and produce bremsstrahlung  Elsner (). The tagging system consists of 480 scintillating fibers and 14 partly overlapping scintillator bars. It provides the corresponding energy of the photons () from the deflection of the scattered electrons in a magnetic field. Photons were tagged in the energy range from 0.5 GeV up to 2.9 GeV for an incoming electron energy of 3.2 GeV. The total tagged photon intensity was about in this energy range. The energy resolution varied between 2 MeV for the high photon energies and 25 MeV for the low photon energies at an electron beam energy of 3.2 GeV. The part of the beam that did not produce any bremsstrahlung photons is deflected by the magnet as well. Since the electrons have retained their full energy the curvature of their track is smaller and they pass over the tagger into a beam dump.
At the end of the beam line a erenkov detector consisting of 9 lead glass crystals has been installed (fig. 1) which measures those photons that pass through the target without undergoing an interaction. The information provided from this detector has been used for the photon flux determination (see section 6.1).
The Crystal Barrel detector, a photon calorimeter consisting of 1290 CsI(Tl) crystals (16 radiation lengths), covered the complete azimuthal angle and the polar angle from to . The liquid hydrogen target in the center of the CB (5 cm in length, 3 cm in diameter) has been surrounded by a scintillating fibre-detector to detect charged particles  suft (). The CB has been combined with a forward detector - the TAPS calorimeter - consisting of 528 hexagonal BaF crystals (12 ), covering polar angles between and and the complete 2 azimuthal angle. In front of each BaF module a 5 mm thick plastic scintillator has been mounted for the identification of charged particles. The combined CB/TAPS detector covered 99% of the full 4 solid angle. The high granularity of this system makes it very well suited for the detection of multi-photon final states.
The first level trigger was derived from TAPS, requiring either one or two hits above different thresholds. The second level trigger was based on a fast cluster recognition (FACE) logic, providing the number of clusters in the Crystal Barrel. For part of the data the minimal number of hits in FACE was one, otherwise at least two hits were requested, which did not introduce any bias for the channels analyzed here.

Figure 2: (a): Invariant mass M(p) against M() in the energy range 2000-2150 MeV, after applying the kinematic fit. The peak is clearly seen at 547 MeV. The resolution () in M() is 9 MeV. (b): After eliminating the events with M() 600 MeV, a peak at M() = 896 MeV and M(p) = 1189 MeV is observed, showing correlated production.

3 Event reconstruction and event selection

The events due to the reactions and were reconstructed from the measured eight photons and the proton in the final channel. Only events containig exactly nine clusters - eight neutral and one additional charged hit - were selected. The charged clusters were identified by using the plastic scintillators in front of the TAPS detector and the fibre detector in the CB. In order to reduce the background, a cut in a missing mass spectrum derived from the identified eight photons was applied. The cut selected events in the region of the nucleon mass; the width of the missing mass cut varied as a function of the incident photon energy (40 MeV at = 1 GeV to 120 MeV at = 2.6 GeV). Events which survived this cut were kinematically fitted to the hypothesis . The procedure is described in detail in Pee (). Measurements of the deposited energy and the direction of the photons in the CB and TAPS calorimeters were used in the fit. For the proton, only the two angles of its trajectory were used, its energy was calculated, since the fit is overconstrained. The constraints applied in this analysis are energy and momentum conservation and the invariant masses of the pions.

Figure 3: Confidence level distributions for the kinematic fit to the hypothesis for experimental data (left) and the corresponding Monte Carlo simulation (right). The dashed verticle line shows the applied cut on the confidence level.

The confidence level distribution of the fit is shown in fig. 3. Above 20 the distribution is flat in both data and Monte Carlo events. The combination of pairs yielding the highest confidence level was taken as the correct one. Events for which the kinematic fit yielded a confidence level of less than 10 were removed from the data. In order to eliminate time accidental background a prompt coincidence between a photon in TAPS and an electron in the tagger was required. Random time coincidences were subtracted, using events outside the prompt time coincidence window. More details of this procedure can be found in  david ().

4 The reaction

An important competing channel, leading to the same final state is . Calculations oset1 () predicted 30 times larger cross section in comparison to the reactions with production. Experimentally, the cross section for reaction was determined to 4horn (). Events with one and three ’s from -decay were selected. The two-dimensional plot of the invariant mass versus that of the 3 system as a result of the 4 possible combinations is shown in fig. 2a. A vertical band for around 547 MeV can be seen. The invariant mass distribution within the band exhibits a strong peak due to the intermediate state. The results from the analysis of the channel will be published separately. In the present analysis events due to were removed by a cut M() 600 MeV (fig. 2b). After these cuts, 9500 events remain for the analysis of strangeness production.

Figure 4: Projections of fig. 2b on the y-axis (a) and (b) on the x-axis with a cut of 1160-1220 MeV on the p invariant mass spectrum (see fig. 2). The incoming photon energies are between 2000 and 2150 MeV. The solid curve shows the fit of the signal and the background. There are 340 events assigned to and 100 events due to . A resolution of 15 MeV ( ) for is given by the fit. The invariant mass spectrum from all combinations of 4-events, cutting on the invariant mass spectrum from 1160-1220 MeV, is shown in (c). The solid curve shows the fit to the background and the signal. The fit yields a mass resolution of 10 MeV (). (a) and (c) yield agree within 10% .

5 The yield of hyperon , and mesons

From the final state, the , , and , are reconstructed. The p and 3 combinations are shown in fig. 2b under the condition that the invariant mass of the 2 from the 3 on the x-axis should be: 470 MeV 520 MeV. This cut has been applied to reconstruct the and which require kaons to be selected (reactions (2), (3) and (4)). A peak at = 896 MeV and = 1189 MeV shows correlated production. Figures  4a and  4b present the projections on the and invariant mass axes, respectively. The spectrum in 4a is fitted by a combination of polynomial and Gaussian functions and shows a clear peak at 11892.0 MeV, corresponding to . The fit yields a resolution =153.1 MeV. The invariant mass spectrum in  4b, a projection on the x-axis of the two-dimensional plot in fig. 2b with a cut on in 1160-1220 MeV, shows a peak around 896 MeV, corresponding to . Also after applying the cuts a considerable background remains. The background in this spectrum is very complex and is discussed later in this section. The invariant mass from six different combinations is shown in fig. 4c. All combinations have been taken into account requiring that one of the other two ’s and a proton have an invariant mass between 1160 and 1220 MeV, because the kaons are always produced with the . The peak at 496 MeV corresponds to mesons from one of the reactions (2), (3) or (4). Higher lying hyperon states could contribute to production via: , where could be or higher lying states decaying into (1189). The threshold for the excitation of (1385) is =1400 MeV. The reactions on the proton with neutral mesons only, such as , excludes ’s as intermediate states. Requiring the invariant mass to be close to the mass, namely between 1160-1220 MeV, figure 5(top) shows a plot of versus for incident photon energies of 2000-2300 MeV. The cut for this plot is important to reconstruct the reaction (3), where decays in . The vertical band around 500 MeV on the x-axis is from the events (cf. figure 4c). The projection onto the y-axis, with a cut on between 470 and 520 MeV is plotted in fig. 5(bottom), which corresponds to a cut on the invariant mass. This cut is shown with solid vertical lines. The non background (shaded area in fig. 5) can be determined from side band cuts - shown with the dashed vertical lines on figure 5(top). Left and right from the kaon peak, in the invariant mass range 445-470 MeV and 520-545 MeV, the y-projection of the two-dimensional plot(fig. 5) shows no peak (shaded area on fig. 5(bottom). The full spectrum is fitted with a polynomial background and six Breit-Wigner resonance shapes representing the (1385), (1460), (1560), (1620), (1660), (1670) with parameters given by  pdg (). The position and width of the resonances are taken from  pdg () and the strengths of the corresponding Breit Wigner are taken as free parameters. As it can be seen the polynomial background (dashed line on fig. 5(bottom)) is in good agreement with the background which we got from the sidebands (shaded area). Apart from the (1385) no detailed information on the other resonances can be extracted due to overlap. The estimated production cross section of the resonance is around 0.70.3 at 1.85 GeV, which is of the same order as observed for the channel in  wieland (); guo ().

Figure 5: (top) The invariant mass versus the invariant mass , requiring to be between 1160-1220 MeV. The incident photon energy is 2000-2300 MeV. The lines show the cut on invariant mass for the y-projection. (bottom) The projection onto axis. The fit function is composed of a polynomial background and Breit-Wigner resonance shapes including (1385), (1460), (1560), (1620), (1660) and (1670) resonances with parameters given by pdg (). The shaded area represents the background in the sideband region of , as marked by the dashed lines on the 2-dimensional plot above.

Monte Carlo studies have been performed to understand the background  me (). We assumed that the main contribution to the background in spectra is mainly caused by 4 sequential resonance decays and other channels leading to the same final state. The production of higher lying hyperon resonances is hereby of particular importance. Since the final particles produced by and are the same, higher lying hyperon states contribute to the spectra as well. To investigate this contribution we have simulated the reaction and the population of higher resonances which decay into (1189). The 3 invariant mass calculated from the decay pions and the additional pion from the decay contributes to the background in the spectra as shown in figure 6. The contribution of (1385) and higher resonances to the background has been normalized to the experimentally observed yields. The background below the signal is composed of the decay contributions and a 3 body phase space part. The full fit of the experimental spectrum, shown in figure 6, has been done using the signal including the simulated combinatorial background, the background from , the 3-body phase space and adjusting their relative magnitudes.

(MeV) (without )
1400-1500 164 185 - -
1500-1700 7711 7013 - -
1700-1850 9216 9519 - -
1850-2000 24434 15239 9220 3010
2000-2150 34035 24040 10020 11125
2150-2300 24034 14637 9415 3010
2300-2500 29038 18140 10916 1610
sum 129973 90281 39536 18730
Table 1: , , and counts in different incident photon energy bins (determined by integration of the respective angular distributions)

The numbers of the identified , , and are listed in Table 1 for different photon energy bins. The statistical error of the cross section data has been estimated by , where S are the counts in the signal and B are the counts in the background underneath the signal.

Figure 6: (Color online) The experimental invariant mass (full black points) for the incident photon energy 2000-2150 MeV. The full fit is shown (red solid curve). The background is described by the contributions from higher states (dashed-dotted curve, and the 3-body phase space (dotted curve). The signal shape, including the combinatorial background, is given by the simulation (histogram).
Figure 7: Total detection efficiency for the reactions (open circles) and (full circles) as a function of the center-of-mass angle of the for different incident photon energy bins.

6 Results and discussion

6.1 Absolute reaction cross sections

This section describes how the absolute reaction cross sections are determined. The essential ingredients are the reaction yields, the detection efficiency of the individual final states and the photon flux. Due to the almost 4 coverage, the detection efficiency for the hyperon final states is practically independent of the production angle. This is illustrated for both reactions and in fig. 7, where the total efficiency is shown, including geometrical acceptance and the detector efficiency. The total efficiency was determined with a GEANT-based Monte Carlo simulation. The simulated events were evenly distributed over the available phase space and analyzed using the same event selection criteria, kinematic fit, applied cuts, thresholds and trigger conditions as for the experimental data. The resulting efficiency varies slightly between 7-10% with the incoming photon energy; its angular dependence is very small.
Uncertainties in the reconstruction of hyperons and vector mesons have been studied. By varying the fit conditions in order to achieve a consistent description of the background in different kinematical regions, an error of 3% -15% is deduced.
The photon flux through the target is determined by counting the photons reaching the intensity detector in coincidence with electrons registered in the tagger system. This provided an absolute normalization for all measurements. The main point is the accurate determination of the efficiency of the tagging system, as defined by the probability to identify the corresponding photon in the photon beam for each detected electron in the tagger. The detector has almost 100% photon detection efficiency. By comparing the number of electrons in the tagging system in coincidence with the number of counts in the detector, the tagging efficiency has been determined to vary between 64 to 74 % for different beamtimes. The systematic uncertainty in the cross sections, caused by the photon flux determination has been checked by measurements of known reactions, such as , and is estimated to be 5%-15% depending on the photon energy.

6.2 Differential cross section

The differential cross sections are calculated from the number of events identified in the respective channel using:

(1)

() are the counts of () determined in different angle and energy bins as described in Sect. 5.
() is the efficiency determined as described in Sect. 6.1;
is the number of primary photons in the respective energy bin determined as described in Sect. 6.1;
is the target area density for the LH target used in this experiment;
is the angle bin width of the angular distributions.
() is the branching ratio of the reaction () respectively; ’s were identified via their decay into 2 which has a relative branching ratio of 98.798%.

Figure 8: Differential cross section /d for . Hereby, the contribution from final state has been removed. In the range of energies below the threshold of production the theoretical band for the angular distribution of  doering () is the shaded area on the first picture. The systematic errors are shown in the boxes on the bottom of each picture.
Figure 9: (Color online) Differential cross section (full circles). The empty squares are the CLAS experimental data Hleiq (). The solid curve represents the theoretical calculations for the reaction in zhao (); zhao_up () with parameters and . The dashed and dotted curves denote the calculations in  Oh () for model II and I respectively. The dashed-dotted curve represents calculations in zhao_up () with free parameters and . The grey band on the botom represents the systematic uncertainty.

The angular distributions for the reactions and are shown in figures  8 and  9, respectively. The results are plotted as a function of and as a function of respectively. For incident photon energies higher than 1850 MeV, above the threshold for the production, the cross section for the has been extracted from the difference between and yields, i.e. .
In the energy range 1500-1850 MeV, below the production threshold, the differential cross section is almost flat. These measurements are in good agreement with the theoretical prediction of  doering (), based on the model of  oset1 (), as shown in the first picture in figure 8. The flat angular distribution indicates dominant s-channel production which is a genuine prediction of a chiral dynamical calculation based on the dominance of the in the entrance channel, plus the coupling of this resonance to . For the energy region 1500-1850 MeV, where the production is energetically not possible, the reconstructed events could come from a resonance which decays subsequently in and . For energies higher than 1850 MeV, an additional contribution from the reaction is possible. These two contributions can be separated experimentally through the mass spectrum which exhibits a sharp peak at the position of the meson (fig. 6). The observed counts in the peak at different bins of are used for the differential cross section determination (fig. 9 full circles).
The differential cross section d/d for the reaction shows a rise in the forward direction when plotted vs the production angle (fig. 9). Production of the meson via -channel exchange seems to play an important role in the reaction dynamics. These results are compared with the updated calculations from the model in  zhao_up (), using the free parameters and , which describe the universal couplings for the vector and tensor part in the quark vector-meson interaction. These parameters are different from those given in  zhao (), which were derived from meson photoproduction based on SU(3) symmetry. Our data are compared to the experimental data from the CLAS collaboration Hleiq () which also show an enhancement in the forward direction. At backward angles we do not observe any rise in the cross section. There is a discrepancy to the CLAS data for the forward angle bins and at incident photon energies higher than 2150 MeV. Our experimental data are consistently higher at angles with in the lower two pictures in fig. 9.
A comparison to another theoretical model assuming (800) meson exchange  Oh () is also shown. The theoretical curves of the so-called model II provide a reasonable agreement with the experimental data which is not the case for model I. The main difference between models I and II of ref. Oh () is in the form and the strength of the form factor of the (800) meson.

Figure 10: Differential cross sections for the reaction as a function of the incident photon energy for the angular bins shown in fig. 9. Our experimental data (full circles) are compared to the CLAS data (empty squares) Hleiq (). The solid curve refers to theoretical prediction for the reaction in zhao_up ().
Figure 11: Differential cross section /dt for . For small values of momentum transfer the data are fitted by an exponential function .

The energy dependence of the differential cross sections for the reaction is presented in fig. 10 for six angular bins and compared with the experimental data from CLAS. The peaking in the forward direction is more pronounced than in the CLAS data as it can be seen in the lowest two pictures in figure 10.

6.3 t-Scaling

The forward peaking of the cross section suggests that there is a contribution to the reaction mechanism from -channel exchange. To test this idea the differential cross section d/d as function of was converted to d/ as a function of . The variable is the Mandelstam invariant that gives the 4-momentum squared of the exchange particles. Since the momentum transfer is limited by kinematics, lies between and , given by:

(2)

The cross section d/ is shown in fig. 11. The straight lines represent fits of in the region below 1.0 of . The slope parameter has a negative value. We plot the slope parameter as a function of the incident photon energy in fig. 12. It can be seen that the slope parameter rises with the photon energy. This is an indication for an increasing contribution to production via -channel exchange as predicted in zhao ().

Figure 12: The slope parameter of as a function of the beam energy . The error bars include the uncertainties on the slope fits shown in fig. 11.

6.4 Total cross section

The total cross sections for the and reactions are shown in fig. 13.

Figure 13: (Color online) The energy dependence of the total cross sections for (empty circles) and (filled circles). The shaded area represents a band for the predicted values from the theoretical calculations of the reaction in  oset1 (). The solid curve refers to theoretical prediction for the reaction in zhao (). The dashed and dotted curves are the calculations with meson exchange Oh () in the -channel, model II and model I, respectively. The systematic errors are shown as an error band on the bottom.

The cross sections agree within errors with those determined by integrating over the differential cross sections. The experimental cross section for the channel are in reasonable agreement with theoretical predictions of reference zhao () with the assumption of a -channel exchange.
The predicted total cross section for in  oset1 () is given by a band (the shaded area in fig. 13). The good agreement between theory and experiment suggests that the dynamics used in the model is reasonable for the corresponding energy region. The cross section for rises with increasing photon energy. Above 1850 MeV the dominating channel for production is obviously . It is about factor 2 larger than the cross section of the reaction which includes excited hyperon decays into the .

7 Summary

We have reported measurements of differential and total cross sections for the and reactions. The experimental data have been compared with the available theoretical predictions. At low incident photon energies below the production threshold ( 1850 MeV) only the channel is energetically possible, exhibiting a flat angular distribution, dominated by s-channel production which is the prediction based on the dominance of the , subsequently decaying into .
For energies above 1850 MeV the production is mainly associated with production. The angular distributions for the reaction show a rise in the forward direction of the vector meson which indicates a -channel exchange contribution to the reaction mechanism. It is, however, not yet possible to make an explicit conclusion about the exchanged particle, due to the significant contribution to the channel from the production.
The presented data provide valuable confirmation of the theoretical predictions concerning the reaction mechanism of the and reactions. and higher resonances have been observed in the decay channel. On the basis of existing data we have estimated the production cross section of the resonance. A corresponding value for the cross section of the higher states photoproduction can not be given because of the overlap of these resonances. Further experiments are required to study the higher resonances. Polarisation experiments will be needed to clarify details of the strangeness production process.

8 Acknowledgements

We thank the accelerator group of ELSA as well as the technicians of the participating institutions for their outstanding support. We acknowledge illuminating discussion with Kenneth Hicks on details of the data analysis. Useful discussions with E. Oset and M. Döring and also Q. Zhao on the theoretical interpretation of the data are also highly acknowledged. This work was supported by Deutsche Forschungsgemeinschaft SFB/TR16 and Schweizerischer Nationalfond.

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