Study of the reactions {\bf\gamma p\rightarrow K^{+}\Sigma^{\pm}\pi^{\mp}} at photon energies up to 2.6 GeVThis work is supported in part by the Deutsche Forschungsgemeinschaft (SPP KL 980/2-3) and the SFB/TR16

Study of the reactions at photon energies up to 2.6 GeVthanks: This work is supported in part by the Deutsche Forschungsgemeinschaft (SPP KL 980/2-3) and the SFB/TR16

I. Schulday Physikalisches Institut der Universität Bonn, Germany presently IFB AG, Köln, Germany Part of doctoral thesis (I. Schulday, doctoral thesis,
Bonn University (2004), Bonn-IR-2004-15), http://saphir.
physik.uni-bonn.de/saphir/thesis.html
   R. Lawall Physikalisches Institut der Universität Bonn, Germany presently TÜV Nord, Hamburg, Germany    J. Barth Physikalisches Institut der Universität Bonn, Germany    K.-H. Glander Physikalisches Institut der Universität Bonn, Germany presently TRW Automotive GmbH, Alfdorf, Germany    S. Goers Physikalisches Institut der Universität Bonn, Germany presently TÜV Nord, Hamburg, Germany    J. Hannappel Physikalisches Institut der Universität Bonn, Germany    N. Jöpen Physikalisches Institut der Universität Bonn, Germany    F. Klein Physikalisches Institut der Universität Bonn, Germany email: klein@physik.uni-bonn.de    E. Klempt Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Germany    D. Menze Physikalisches Institut der Universität Bonn, Germany    E. Paul Physikalisches Institut der Universität Bonn, Germany    W.J. Schwille Physikalisches Institut der Universität Bonn, Germany
Received: date / Revised version: date
Abstract

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

1 Introduction

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.

Figure 1: Topologies of the reactions (top) and (bottom) in the target region. Rectangle and circle represent the target area and the inner layer of the central drift chamber. The track was not measured.

Figure 2: Angular distribution of the decay proton in the rest system for the decay for data and Monte Carlo simulated background of events from the reaction (black area) which passed the selection cuts. The Monte Carlo event sample was normalised to the photon flux. The vertical line indicates the cut which was applied to exclude most of this background.

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.

Figure 3: Decay time distribution of the for data (solid line) and for Monte Carlo simulated events (dashed line), normalised to the data in the accepted area. The vertical line indicates the cut applied to the data.

Figure 4: Top: Decay time distribution of the with decay for data (solid line) and Monte Carlo simulated events (dashed line), normalised to the data in the accepted area. Bottom: Decay time distribution of the with decay for data (solid line) and Monte Carlo simulated events (dashed line), normalised to the data in the accepted area. The vertical lines indicate the cuts applied to the data.

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.

Reaction [] 2-10 5-25 58-30 0-0.3 0-0.7 1.8-0.6 2.3-0.6

Table 1: Reactions and cross sections in the photon energy range considered. The list includes , since these reactions also contaminate each other.

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 ().

Figure 5: invariant mass distribution.

Figure 6: invariant mass distribution.

Figure 7: invariant mass distribution.

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.

Figure 8: Excitation function before background subtraction for and background from other reactions.

Figure 9: Excitation function before background subtraction for the reaction with decay and background from other reactions.

Figure 10: Excitation function before background subtraction for the reaction with decay and background from other reactions.

Figures 8, 9 and 10 show the excitation function before background subtraction with statistical errors together with the total background contributions according to section 4.

The final reaction cross sections were obtained by subtracting the accumulated background cross sections bin-by-bin. They are shown in figs. 11 and 12. The errors were calculated by quadratic addition. Cross sections and errors are quoted in tables 4, 4, and 4, respectively.

Figure 11: Cross section of the reaction as a function of the photon energy after subtraction of background from other reactions in comparison to previous measurements ABBHHM (); SGoers ().

Figure 12: Cross section of the reaction as a function of the photon energy after background subtraction from other reactions in comparison to previous measurements ABBHHM (). For this experiment the cross sections from both decay modes are given separately.

7 Summary

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.

[GeV] [] [] 0.0028 0.0026 0.0290 0.0061 0.1121 0.0095 0.1322 0.0137 0.1597 0.0113 0.1894 0.0152 0.1790 0.0152 0.2322 0.0141 0.2761 0.0177 0.2727 0.0179 0.2865 0.0194 0.2595 0.0300 0.3339 0.0243

Table 3: Total cross sections of the reaction with for 13 bins of , obtained after background subtraction.

[GeV] [] [] 0.0116 0.0076 0.0720 0.0191 0.1644 0.0275 0.2314 0.0419 0.4005 0.0359 0.5377 0.0448 0.7802 0.0559 0.6789 0.0641 0.6861 0.0650 0.8147 0.0726 0.9029 0.0894 0.6359 0.0806 0.7302 0.0868

Table 4: Total cross sections of the reaction with for 13 bins of , obtained after background subtraction.

[GeV] [] 0.0067 0.0113 0.0367 0.0347 0.1075 0.0538 0.3395 0.0496 0.3817 0.0519 0.5983 0.0605 0.7209 0.0662 0.7605 0.0794 0.9128 0.0662 0.9323 0.0816 0.8905 0.1151 0.7702 0.1115 0.9037 0.

Table 2: Total cross sections of the reaction for 13 bins of , obtained after background subtraction.

8 Acknowledgements

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”).

References

  • (1) E. Klempt, J.-M. Richard, Rev. Mod. Phys. 82, 1095–1153 (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).
Comments 0
Request Comment
You are adding the first comment!
How to quickly get a good reply:
  • Give credit where it’s due by listing out the positive aspects of a paper before getting into which changes should be made.
  • Be specific in your critique, and provide supporting evidence with appropriate references to substantiate general statements.
  • Your comment should inspire ideas to flow and help the author improves the paper.

The better we are at sharing our knowledge with each other, the faster we move forward.
""
The feedback must be of minimum 40 characters and the title a minimum of 5 characters
   
Add comment
Cancel
Loading ...
158484
This is a comment super asjknd jkasnjk adsnkj
Upvote
Downvote
""
The feedback must be of minumum 40 characters
The feedback must be of minumum 40 characters
Submit
Cancel

You are asking your first question!
How to quickly get a good answer:
  • Keep your question short and to the point
  • Check for grammar or spelling errors.
  • Phrase it like a question
Test
Test description