Study of the reactions at photon energies up to 2.6 GeV††thanks: This work is supported in part by the Deutsche Forschungsgemeinschaft (SPP KL 980/2-3) and the SFB/TR16
The reactions were studied with the SAPHIR detector using a tagged photon beam at the electron stretcher facility ELSA in Bonn. The decays and were fully reconstructed. Reaction cross sections were measured as a function of the photon energy from threshold up to GeV with considerably improved statistics compared to a previous bubble chamber measurement. The cross sections rise monotonously with increasing photon energy. The two-particle mass distributions of and show substantial production of resonant states.
pacs:13.30.-aDecays of baryons and 14.20.Jnhyperons
Photon-induced reactions on nucleons at low energies are commonly used to study the excitation of baryonic resonances. A review of baryon spectroscopy, its aims and its achievements can be found elsewhere klempt (). Searches for such resonances were carried out in the SAPHIR experiment analysing the reactions Glan03b (), Goers99 (), Lawall05 (), wu (), barthomega (), barthphi (), and etaprime (). The measurements presented here extend this search to the reactions where strangeness-zero resonant states might contribute.
The data analysis is based on 180 million triggered events which were taken with the magnetic multiparticle detector SAPHIR Schwille () at the GeV electron stretcher facility ELSA Hillert () using a tagged photon beam which covered the photon energy range from threshold (of the reactions considered here) to GeV. A detailed description of the experiment is given elsewhere Glan03b (); barthomega ().
The data are available via internet111http://saphir.physik.uni-bonn.de/saphir/publications.
2 Event reconstruction and event selection
The kinematical reconstruction of the reactions with , and of with either or , was based on the measurements of the photon energy in the tagging system and of the three-momenta of the charged particles in the final states reconstructed in the drift chamber system. The topology of the events is sketched in fig. 1.
In the first step, the primary vertex was searched by combining pairs out of the three tracks extrapolated into the target region. The pair with the best matching was accepted. Then the hypotheses were tested by a kinematical fit which used the photon energy and the reconstructed momenta of and . The hypothesis with the better fit probability were tested by a kinematical fit which used the photon energy and the reconstructed momenta of the particles defining the primary vertex. The fit determined the 3-momentum of the which allowed us to reconstruct its track downstream of the accepted primary vertex. In the next step, the decay vertex was calculated as intercept of the track with the extrapolated track of the third charged particle. The decay hypotheses and / were tested at the decay vertex by carrying out corresponding kinematical fits, and the complete reaction was tested by simultaneous fits at both, the primary and the decay vertex. Finally, time-of-flight (TOF) measurements carried out in the range of the geometrical acceptance of the scintillator hodoscopes were used to reject background from other reactions. For it was required that the mass assignments, obtained from TOF measurements for the positively charged particles, had a value below GeV. This cut removed events with a proton in the final state. For it was required that the mass assignments were consistent with the mass values from the fit.
At this stage, the sample of events identified as with still contained substantial background from events due to the reaction . The final states of these reactions have proton and in common, and the identification of pions and kaons is not unique because of the limited time resolution of the time-of-flight (TOF) measurement and the restriction of geometrical acceptance of the scintillator hodoscopes.
Figure 2 shows the proton angular distribution in the rest system with respect to the momentum of in the laboratory system. For comparison, the same distribution of Monte-Carlo simulated events due to is shown which passed the same selection cuts. The peak in the data at low angles is qualitatively described by the simulated background. An angular cut was applied (vertical line) to remove most of this background contribution.
In the next step, the decay time of was calculated using the track length and the 3-momentum of the . The distributions are shown in figs. 4 and 4 together with those of Monte-Carlo simulated events. The residual background seen at large decay times is due to secondary reactions in target and central drift chamber. It is subtracted in the final background substraction.
The strong excess of events at small decay times indicates background from other reactions which accumulates at small times. Due to the limited resolution this is expected if all charged particles originate from the same production vertex. In order to reduce this background, events with s were removed. For events with decay, another cut was applied for large decay times. The cut s removed events in a region where further background is visible.
The data sets, obtained after the selection cuts, contained 4429 events from the reaction and 11267 events from , with 5080 of the decaying into and 6187 into . Background contributions which were not removed by the selection cuts described above were estimated by Monte-Carlo simulations and finally subtracted (section 6).
3 Acceptance of the events
The acceptance was determined by simulating events in the SAPHIR setup for the reactions according to phase space with propagation of and subsequent decays and or , respectively. Charged particles in the final states were tracked through the drift chamber system taking into account the magnetic field and multiple scattering in all materials. Simulated events were processed like real events through the event reconstruction and selection procedures. The total acceptance accounted for the trigger efficiency of the data taking periods, the event reconstruction efficiency and the data reduction according to the event selection cuts. The mean acceptance was of the order of for and for . The acceptance of the latter reaction was lower because it includes the efficiency of the TOF measurements for both decay modes and, in addition, the cut in the angular distribution of decays (see section 2).
4 Background from other reactions
Background was estimated by generating events according to phase space for the reactions listed in table 1. The events were processed through reconstruction and selection criteria as real events. The background event samples obtained were normalised according to the photon flux.
The errors in the background estimate are dominated by a constant value of 10% due to the model dependence of the event simulation and the uncertainty of the background cross sections. This error and the statistical errors were added in quadrature. For , , the reactions and contribute on average with about , and all reactions together with to the observed total cross section (see section 6). For , , the reaction contributes on average with about and the total background adds up to of the observed cross section.
5 and mass distributions
The invariant mass distributions for the system for the reactions and are shown in figs. 5 and 6. Both, and distributions show a peak structure in the mass range of and and another pronounced peak in the mass range of . Figure 7 shows the mass distribution for events assigned to the reaction . The peak at 890 MeV indicates production.
From the observed resonance peaks it can be concluded that substantial parts of both reaction cross sections are due to intermediate two-body resonant states. The production was studied in detail and is presented in a separate paper Lambda1520 ().
6 Reaction cross sections
Cross sections were determined as a function of the photon energy for both reactions, in case of separately for both decay modes.
The cross sections of the reactions were measured in the photon energy range from threshold to 2.6 GeV. They rise monotonously up to values of about 0.3 b for and about 0.8 b for . Regarding the hitherto existing data an evident improvement concerning the energy resolution and the total errors is achieved. No indications are found for narrow structures in the total cross sections, nor strong threshold enhancements as seen, e. g., in Krusche:1995nv (), Glan03b (); bleckmann (); clas (), or barthomega (). The and mass spectra show pronounced peak structures, indicating that a substantial part of the cross sections is due to two-body intermediate states. The intermediate state is investigated in a separate paper.
We would like to thank the technical staff of the ELSA machine group for their invaluable contributions to the experiment. We gratefully acknowledge the support by the Deutsche Forschungsgemeinschaft in the framework of the Schwerpunktprogramm “Investigation of the hadronic structure of nucleons and nuclei with electromagnetic probes” (SPP 1034 KL 980/2-3) and the Sonderforschungsbereich SFB/TR16 (“Subnuclear Structure of Matter”).
- (1) E. Klempt, J.-M. Richard, Rev. Mod. Phys. 82, 1095â1153 (2010).
- (2) K.-H. Glander et al., Eur. Phys. J. A 19, 251 (2004).
- (3) S. Goers et al., Phys. Lett. B 464, 331 (1999).
- (4) R. Lawall et al., Eur. Phys. J. A 24, 275 (2005).
- (5) C. Wu et al.: Eur. Phys. J. A 23 (2) (2005).
- (6) J. Barth et al.: Eur. Phys. J. A 18 117-127 (2003).
- (7) J. Barth et al.: Eur. Phys. J. A 17 2, 269-274 (2003).
- (8) R. Plötzke et al.: Phys. Lett. B 444 555-562 (1998).
- (9) W. J. Schwille et al., Nucl. Instr. Meth. A 344, 470 (1994).
- (10) W. Hillert, Eur. Phys. J. A28, 139 (2006).
- (11) F. W. Wieland et al., preceding paper.
- (12) R. Erbe et al. (ABBHHM), Physical Review 188, 2060 (1969).
- (13) S. Goers, doctoral thesis, Bonn University (1999), BONN-IR-1999-09.
- (14) B. Krusche et al., Phys. Rev. Lett. 74, 3736 (1995).
- (15) A. Bleckmann et al., Z. Phys. 239, 1 (1970).
- (16) R. Bradford et al., Phys. Rev. C 73, 035202 (2006).