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Strangeness Photoproduction at Threshold Energies

This study investigates Hyperon photoproduction and a new method of K+ meson detection at threshold energies. Cross-section measurements and the search for rare S0 decays are also discussed. The Crystal Ball detector at the MAMI-C facility is used for experimental measurements.

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Strangeness Photoproduction at Threshold Energies

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  1. Strangeness Photoproduction At Threshold Energies T.C. Jude D.I. Glazier, D.P. Watts The University of Edinburgh

  2. Strangeness Photoproduction At Threshold Energies • Hyperon photoproduction • Crystal Ball detector at the MAMI-C facility • A new method of K+ meson detection • Cross section measurements • Search for rare S0 decays • Future plans

  3. g + p g K+ + L p + p- (~ 64%) n + p0 (~ 36%) 2g g + p g K+ + S0 g + L Hyperon Photoproduction • Important for investigations of nucleon resonances. Predicted nucleon resonances which have not been observed experimentally may couple to strange decay channels [1] • A crucial test of QCD based chiral perturbation theories in the strange quark sector [1] S Capstick and W Roberts, Phys. Rev. D58, 074011 (1998) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  4. Eg [GeV] Cross-Section Measurements • No detailed measurements of g p g K+L at threshold • Significant discrepancies exist between cross-sections measured at JLab [2] and ELSA [3] • Fits to these data sets reveal large differences in the roles of nucleon resonances when describing the photoproduction process [4] The large angular coverage and intense photon beam over the range ~ 0.9 to 1.4 GeV make the Crystal Ball ideal for strangeness measurements at threshold [2] R. Bradford et al., Phys Rev. C 73, 035202 (2006) [3] K.H. Glander et al., Eur Phys. J. A 19, 251 (2004) [4] T. Mart arXiv:0803.0601v1 [nucl-th] (2008) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  5. The Crystal Ball (Edinburgh PID and target in the centre) TAPS Glasgow Photon Tagger Beam direction Experimental Apparatus • The Crystal Ball detector - 672 NaI crystals covering ~93% of 4p steradians • Edinburgh PID - 24 Plastic Scintillators parallel to the beam • TAPS- Segmented BaF2 detector. Used as a forward wall for the Crystal Ball T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  6. Experimental Apparatus • The Crystal Ball detector - 672 NaI crystals covering ~93% of 4p steradians • Edinburgh PID - 24 Plastic Scintillators parallel to the beam • TAPS- Segmented BaF2 detector. Used as a forward wall for the Crystal Ball T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  7. First cluster from K+ < 3ns Secondary cluster from K+gm++ nmdecay > 10ns K+ meson detection in segmented calorimeters • Identify the K+ decay within the crystals of the Crystal Ball and TAPS • Eliminates the need of expensive magnetic spectrometers K+ m++nm ( ~ 63%) Mean lifetime of K+ ~ 12 ns p++p0 (~ 21%) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  8. (a) Energy of the Second Cluster (a) Time difference between first and second clusters T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  9. 2D cut on K+ loci in the DE-E analysis T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  10. Detection of S0 decay (simulation) (simulation) • Measure missing masses from K+g final states • Select the photon giving closest missing mass to the L mass • Further 2D cuts on the photon polar angle and energy in the lab frame T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  11. Detection of S0 decay Photon energy in the S0 rest frame (after 2D cuts) Simulation Real Data From the K+ events identified (in simulation): ~ 40% of K+S0 events correctly identified by detection of the decay photon ~ 10% of K+L events misidentified (Still in progress) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  12. Fitting to K+ Missing Mass Plots • Fit To K+ missing mass plots to measureg(p,K+)L & g(p,K+)S0contributions For determining K+ yield for particular energy and polar angle ranges, integrate over the Lambda peak (blue) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  13. K+ cluster size < 3, punch through K+ rejected K+ cluster size < 3 K+ cluster size < 3, punch through K+ and events with photon from S0 decay rejected Fitting to K+ Missing Mass Plots • Further cuts on K+ cluster size (< 3) and K+ punch through energy (< 340 MeV & reconstructed from polar angle) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  14. g(p,K+)LCross Section Measurements • July 2007 data, ~36500 K+ events that pass cuts • 350 MeV Crystal Ball energy sum, Multiplicity 2+ Target pressure ~ 70.548 kgm-3 ~ 4.2179 x 1028 protons m-3 Target length = 0.048 m r = 2.0246 x 1027 protons m-2 ~ 0.6 Detection Efficiency: 20 milliong(p,K+)Levents with phase space generation and A2 GEANT4 simulation. Approximately 10% Next Slide: Black points: this data Blue Points: JLab data (2006)[2] Red Points: ELSA data (2004)[3] T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  15. Preliminary Preliminary Preliminary Preliminary Black points: this data, Blue Points: JLab data, Red Points: SAPHIR data T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  16. Cross sections for forward angles (K+ centre of mass polar angle < 760) • Approaching region of cross section discrepancy with previous data Preliminary • Future beam times : greater proportion of K+ in forward directions (change of electronic triggers) • Reduce statistical error by ~2 T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  17. Branching ratio of the decay:S0g e+e-L First measured in the 1960s to measure relative parity between S0 and L[5] Accepted branching ratio < 0.5% • Select e+ and e- from 2D graphical cuts in DE-E analysis • For multiple events, select combinations giving the best missing mass of L (from K+, e+ and e- momenta) • Boost into S0 rest frame and sum the energies of the e+ and e- [5] H. Courant et. al. Phys Rev Lett 10, 409 (1963) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  18. Branching ratio of the decay:S0g e+e-L First measured in the 1960s to measure relative parity between S0 and L[5] Accepted branching ratio < 0.5% g p g K+S0simulation with branching ratio for decayS0g e+e-L set at 0.5% Red Fit:S0g e+e-L contributions Blue Fit: Background fromgp g K+Landgp g K+S0 Total S0g e+e decay Background [5] H. Courant et. al. Phys Rev Lett 10, 409 (1963) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  19. Branching ratio of the decay:S0g e+e-L S0g e+e decay Sim: Total Background Real Data: Total S0g e+e decay Background T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  20. Future Plans • More data: resolve g(p,K+)L cross sections at an unprecedented resolution • Increased beam energy: measure the L(1405) structure • Circular polarised beam: measure polarisation observables, CXand CZat threshold Asymmetry of L decay distribution with circular polarised beam Preliminary 1.0 – 1.2 GeV 1.0 – 1.2 GeV q [deg] cos( q ) T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  21. Missing mass from K+ Photoproduction for beam energies < 1.05 GeV (peak observed at the L mass) Very preliminary! Energy [MeV] Future Plans • Deuterium target: study the reaction: g(n(ps),K+)S- T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  22. Future Plans • Strangeness photoproduction with transversely polarised target [6] • Measure the beam-target polarisation observables E and G (longitudinal pol. target), and H and F (transversely pol. target) • Sensitive to missing nucleon resonances, for example D13(1900) E G [6] A. Thomas, Helicity Dependence of Meson Photoproduction on the Proton, 13th Crystal Ball Meeting, Mainz, Germany, March 2009 KAON-MAID T. C. Jude Strangeness Photoproduction At Threshold Energies NSTAR, Beijing, April 2009

  23. Strangeness Photoproduction At Threshold Energies T.C. Jude D.I. Glazier, D.P. Watts The University of Edinburgh

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