Strangeness photoproduction at the BGO-OD experiment

Transcription

1 Strangeness photoproduction at the BGO-OD experiment Tom Jude On behalf of the BGO-OD Collaboration University of Bonn Supported by the DFG / SFB tr-16 NSTAR 215 Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

2 The BGO-OD experiment at ELSA Energy tagged bremsstrahlung photon beam up to 3 GeV BGO calorimeter (central region) & Forward Spectrometer combination High momentum resolution, excellent charged and neutral particle ID γ monitoring Open Dipole magnet MRPC BGO ball MWPC scintillator barrel Beam dump Tagger ARGUS Møller Tagger magnet ToF ToF ToF ToF Goniometer Drift chambers SciFi2 detector MOMO Tom Jude (University of Bonn) silicon Cryotarget, LH2 or LD2 Strangeness photoproduction at BGO-OD NSTAR / 16

3 Incident photon beam parameters ELSA e beam 3.5 GeV, BGO-OD experiment - Tagged γ beam 3 GeV Current 2 na (ideal for tagged photon beam), 1 na upgrade planned Linearly polarised γ beam - coherent bremsstrahlung using a diamond radiator Circularly polarised γ beam - ELSA achieves full spin rotation maximum polarisation at E e 1.7 GeV (W 2 GeV) Energy region of unresolved/missing resonances K Y thresholds ω and η thresholds Non-understood bump structure in φ photoproduction Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

4 [MeV] p X γγ Missing mass: γ Analysis overview (Preliminary Data) Example of recent commissioning data (preliminary calibrations) E e = 2.9 GeV, linearly polarised beam Neutral meson reconstruction in the BGO Two γ invariant mass [MeV] β Charged particle ID in the Forward Spec Momentum (p) [MeV] Counts Fit to π peak Mean = 137 MeV σ = 15 MeV Fit to η peak Mean = 528 MeV σ = 31 MeV Two γ invariant mass [MeV] Counts Mη M ω M η Missing mass: γp px [MeV] Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

5 Strangeness Photoproduction - Physics Motivation Acceptance at forward angles with BGO-OD - ideal for dominantly t-channel exchange mechanisms Identify final states of mixed charge - investigation of Y resonances, eg K Λ(145) K π Σ K π γλ K γγγpπ Differential cross sections & polarisation observables K Λ, K Σ K Σ states K Σ K Λ (deuteron target) Intense production beam times starting now Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

6 Previous γp K Σ measurements R. Ewald et al.,phys. Lett. B 713 (212) 18 (CBELSA/TAPS Collaboration) Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

7 Previous γp K Σ measurements Cusp structure due to K subthreshold production rescattering to π & K? Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

8 Previous γp K Σ measurements The 1st beam asymmetry measurement R. Ewald et.al, PLB 738 (214) 268 (CBELSA/TAPS Collaboration) 1.5 Black line - Bonn-Gatchina solution (BG211-2) Black dashed - BG211-2m (includes this Σ data & CLAS P data) Red line - Kaon-MAID Red dashed- Kaon-MAID modified (K t-channel switched off) BGO-OD Proposal Extend the range of Σ over the cusp structure (17-18 MeV) Identify charged & neutral decay modes (higher stats.) K π π, Γ = 31% K π π, Γ = 69% Σ pπ, Γ = 52% Σ nπ, Γ = 48%.5 Σ γ cm cos θk = cm cos θk = cm cos θ K =.4 cm cos θk =. cm cos θ K = Photon beam energy [MeV] Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

9 B. S. Zou, Eur. Phys. J. A 35 (28) 325 & Jia-Jun Wu, S. Dulat & B. S. Zou, Phys. Rev. D 8 (29) 1753 Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16 Strangeness photoproduction with BGO-OD γp K Λ - cover important range at forward angles & disputed peak structure at W = 19 MeV Bydzovsky and D. Skoupil, arxiv: v1 (212) Proceedings of SNP12 Many Y states poorly established & CQM do not explain hyperon spectrum Hadronic molecules - predict new Y states, eg Σ close to Σ(1385)

10 K identification in the BGO Time delayed, K weak decay within the crystals of the BGO ball T.C Jude, D.I. Glazier, D.P. Watts, PLB, 735 (214) 112 Lifetime 12 ns, 2 main decay modes: K µ ν µ K π π νµ BGO crystal 3ns Κ 16ns 19ns µ 18ns Decay sub cluster Incident sub cluster 1 Simulated data Experimental data Fitted K lifetime 11 ns Energy of decay sub cluster [MeV] Time between incident and decay sub cluster [ns] Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

11 Identifying the Σ Λγ decay Boost all neutral particles in the BGO into missing hyperon rest frame If K Σ : Missing mass from K γ(lab frame) = M Λ = 1116 MeV (Y axis) γ energy (hyperon rest frame) = M Σ M Λ = 77 MeV (X axis) γ Missing mass from K Experimental data γ Missing mass from K Simulated γp K Λ 8 8 γ Missing mass from K γ energy in hyperon rest frame [MeV] Simulated γp K Σ Counts γ energy in hyperon rest frame [MeV] Experimental data Simulated K Λ Simulated K Σ γ energy in hyperon rest frame [MeV] γ energy in hyperon rest frame [MeV]

12 Missing hyperon masses from K detection in the BGO Data subset where Σ Λγ is tagged Σ Λγ detection efficiency from sim. ( 6%, K Λ false eff. < 1%) Scale data by efficiency - separate K Λ and K Σ M Λ = 1116 MeV M Σ = 1193 MeV All Data With Σ Λγ Tagged 4 3 All Data Σ Λγ efficiency scaled Missing (hyperon) mass [MeV] Missing (hyperon) mass [MeV] K Σ subtracted With Σ Λγ Tagged K Λ false eff. scaled K Λ subtracted Missing (hyperon) mass [MeV] Missing (hyperon) mass [MeV] Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16 5

13 K identification in the forward spectrometer β determined from momenta measured in the forward spectrometer Characteristic loci of protons & π ± To enhance K signal π mass reconstructed in the BGO BGO total energy deposition < 25 MeV β p [MeV/c] β 1.1 β With Calculated β p [MeV/c] p [MeV/c] Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

14 Missing (Y) mass from K in the forward spectrometer 2 18 No extra selection cuts 12 Energy deposition in the BGO < 25 MeV Missing (hyperon) mass [MeV] Missing (hyperon) mass [MeV] Energy deposition in the BGO < 25 MeV & π reconstructed π reconstructed in the BGO Λ(1116) Σ(1193) Σ(1385) Λ(145) Λ(152) Missing (hyperon) mass [MeV] Missing (hyperon) mass [MeV] Photon tagger hodoscope with x4 energy resolution installed Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

15 Summary & Collaborating Institutions K identification Analysis in preparation High statistics for K Σ polarisation observables Basis for neutral channels, eg γn K Λ K identification ID reaction channels with BGO & forward spectrometer K Y cross sections at forward angles Σ states Ready for high statistics data taking Physikalisches Institut, Bonn, Germany Helmholtz-Institut für Strahlen und Kernphysik, Bonn, Germany Justus-Liebig-Universität Giessen, Germany Institut für Physik, Basel, Switzerland INFN sezione di Pavia, Pavia, Italy The University of Messina,Messina, Italy INFN sezione di Catania, Catania, Italy INFN Roma Tor Vergata, Rome, Italy The University of Rome Tor Vergata, Rome, Italy INFN sezione di Torino, Torino, Italy The University of Torino, Torino, Italy INFN sezione di Roma, Rome, Italy INFN - ISS, Rome, Italy INFN - LNF, Frascati (Rome), Italy National Science Center Kharkov Institute of Physics and Technology, Kharkov, Ukraine Russian Academy of Sciences Institute for Nuclear Research, Moscow, Russia Petersburg Nuclear Physics Institute, Gatchina, Russia The University of Edinburgh, Edinburgh, UK Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16

16 Extra slide Missing mass from forward protons & selecting π /η in the BGO ball Counts All Protons in the forward spectrometer Counts With an η reconstructed in the BGO Counts Missing mass: γp px [MeV] With a π reconstructed in the BGO Missing mass: γp px [MeV] Missing mass: γp px [MeV] Tom Jude (University of Bonn) Strangeness photoproduction at BGO-OD NSTAR / 16