Photoproduction of η Mesons off Light Nuclei Lilian Witthauer Hadron 2011 Munich, June 13 th 2011 2 Collaboration a
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Outline 1 Light η -Mesic Nuclei Motivation Experiment Identification of the Coherent Reaction Results 2 Quasi-Free Photoproduction of η Mesons off 3 He Motivation Identification of the Quasi-Free Reaction Results 3 Conclusion & Outlook Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Light η -Mesic Nuclei (F. Pheron et al.) Do the properties of the strong interaction p allow the formation of meson-nucleus bound η n p states? ◮ Several model predictions: ◮ Bhalerao and Liu (1985): attractive s-wave η N-interaction for A > 12. ◮ Liu and Haider (1986): interaction migth lead to the formation of quasi-bound η -nucleus states for A > 10 ( η -mesic nuclei) ◮ new analyses prefer larger values for the scattering length (above 0.5fm) ➢ existence of very light η -mesic nuclei ( 2 H, 3 H, 3 He, 4 He)? ◮ Quasi-bound states should give rise to an enhancement at threshold of the cross section relative to the expectation for phase space behavior. Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Light η -Mesic Nuclei: Experimental Evidence ➢ hadronic induced reactions: ➢ photon induced reactions: T. Mersmann et al. dp → 3 He η ● (COSY-ANKE) M. Pfeiffer et al. H.-H. Adam et al. dp → 3 He η ▲ Phys. Rev. Lett. 92 (2004) 252001 B. Mayer et al. pd → 3 He η ▼ J. Berger et al. dp → 3 He η ■ Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook MAinzer MIcrotron: Electron Accelerator Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook MAinzer MIcrotron: Electron Accelerator Stage: Turns: Energy: LINAC 3 . 97 MeV − RTM1 18 14 . 9 MeV RTM2 51 180 MeV RTM3 90 855 . 1 MeV Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook MAinzer MIcrotron: Electron Accelerator Stage: Turns: Energy: HDSM 43 1508 MeV Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook MAinzer MIcrotron: Electron Accelerator A2 Experimental Hall Bremsstrahlung Tagging − E tagged E γ = E beam e − e − Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Experiment: Crystall Ball and TAPS ◮ Circularly polarised tagged photon beam (0.45 GeV to 1.4 GeV) ◮ Crystal Ball: Highly segmented sphere made of NaI ◮ PID: Cylinder of scintillation counters surrounds target, charged particle detector ◮ TAPS: Forward wall, BaF 2 & PbWO 4 crystals ◮ 3 He Target: 5 . 3 cm long, density: geometrical exeptance close to 4 π 0 . 069 g/cm 3 Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Identification of γ 3 He → 3 He η (F. Pheron et al.) η→ 2 γ η→ 6 γ 40 20 604 MeV 20 10 ◮ Decay channels: η → 2 γ , η → 6 γ 0 0 ◮ Invariant mass analysis for each energy 612 MeV 30 15 20 10 and meson cm-polar angle 10 5 0 0 ◮ 6 γ : photons combined via χ 2 -test to 3 621 MeV 40 20 counts/4 MeV pairs (best solution for 3 π 0 IM) 20 10 0 0 → 110 MeV - 150 MeV cut 638 MeV 40 20 ◮ use overdetermined kinematics to 20 10 0 0 separate coherent from breakup 60 30 655 MeV 40 20 reactions. → missing energy 20 10 ➢ strong coherent component visible 0 0 60 30 672 MeV ➢ recoil taken by quasi-free nucleon 40 20 20 10 ➢ recoil taken by di-nulceon 0 0 -40 -20 0 20 -40 -20 0 20 40 ∆ E [ MeV ] Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook γ 3 He → 3 He η Cross Section (F. Pheron et al.) 0.3 ◮ agreement between 2 γ and 6 γ η→ 2 γ η→ 6 γ ◮ disagreement between existing models and data 0.2 σ[µ b ] ◮ Tiator et al. : PWIA ◮ Fix and Arenh¨ ovel : 1. PWIA 2. 0.1 Tiator et al. DWIA → strong FSI effects, full Fix et al. Shevchenko et al. four-body model 0 ◮ Shevchenko et al. : 2 examples 600 620 640 660 680 700 E γ [ MeV ] for different FSI modelling → strong threshold effects Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook γ 3 He → 3 He η Cross Section (F. Pheron et al.) 40 604.2 MeV 612.7 MeV 20 σ[µ b ] 0 0.2 621.1 MeV 629.6 MeV 40 20 10 0 0.1 σ / σ PWIA d σ /d Ω[ nb/sr ] 638.1 MeV 646.6 MeV 60 40 20 1 0 600 650 700 0 600 620 640 660 680 700 720 655.0 MeV 663.5 MeV 75 E γ [ MeV ] 50 25 ● average of 2 γ and 6 γ F. Pheron et al. 0 ▼ M. Pfeiffer et al., Phys. Rev. Lett. 92 (2004) 252001. 75 671.9 MeV 680.3 MeV PWIA: 50 ( E γ , q ) = q ( A ) � k ( N ) � 25 · F 2 A ( q 2 ) d σ PWIA d σ elem η γ · d Ω k ( A ) q ( N ) d Ω F 2 p ( q 2 ) 0 γ η -1 -0.5 0 0.5 -1 -0.5 0 0.5 1 cos( Θ * cos( Θ η ) Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook π 0 -p back-to-back (F. Pheron et al.) p p p S 11 η π n p n p n p ���� n p γ 1 0.8 counts*E γ 6 [ a.u. ] 0.6 0.4 0.2 M. Pfeiffer et al., Phys. Rev. Lett. 92 (2004) 252001. Peak is an artefact arising from the complicated background structure of quasi-free π 0 production! 0 400 600 800 1000 E γ [ MeV ] Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Quasi-Free η Photoproduction GRAAL collaboration: narrow structure in the cross section: γ + d → η + n ( p ) 0.8 d σ /d Ω , µ b/str 0.7 1 0.6 0.8 0.5 0.6 0.4 0.3 0.4 0.2 0.2 0.1 0 0 d σ /d Ω , µ b/str 1.5 1.6 1.7 1.8 1.9 1.5 1.6 1.7 1.8 1.9 1 W, GeV 0.8 0.6 0.4 0.2 0 1.5 1.6 1.7 1.8 1.9 W,GeV GRAAL, V.Kuznetsov et al., hep-ex 0606065 Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Quasi-Free η Photoproduction GRAAL collaboration , CBELSA/TAPS collaboration , LNS-Sendai: narrow structure in the cross section: W = 1 . 66 GeV γ + d → η + n ( p ) Γ ≈ 25 ± 12 MeV 0.8 d σ /d Ω , µ b/str 0.7 1 0.6 0.8 0.5 0.6 0.4 0.3 0.4 0.2 0.2 0.1 0 0 d σ /d Ω , µ b/str 1.5 1.6 1.7 1.8 1.9 1.5 1.6 1.7 1.8 1.9 1 W, GeV 0.8 0.6 0.4 0.2 0 1.5 1.6 1.7 1.8 1.9 W,GeV ELSA, I.Jaegl´ e et al. EPJA accepted GRAAL, V.Kuznetsov et al., hep-ex 0606065 Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Nature of this Structre is unknown 1. etaMAID: Large contribution of the D 15 (1675) ➢ high value for the branching ratio of Γ η N / Γ tot = 17% (PDG: Γ η N / Γ ≃ 0 − 1%) (L.Tiator, NSTAR2005) 2. Chiral Soliton model: Predicts the existence of a nonstrange member of the baryon antidecuplet ( P 11 -like state). Photoexcitation of this state is thought to have a bigger coupling to the neutron than to the proton (D.Diakonov et al., arXiv:hep-ph/9703373v2) Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Why on 3 He ? Exclude possibility that the structure could arise from nuclear effects: rescattering of mesons, final state interaction ◮ study it for a nucleon system with different momentum distribution and different neutron/proton ratio: 3 He helium-3 deuterium n p n p p Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
Quasi-Free Photoproduction of η Mesons off 3 He Outline Light η -Mesic Nuclei Conclusion & Outlook Reaction Identification of the Quasi-Free Reaction p ( np ) γ + 3 He → n ( pp ) + η × 10 3 1200 ➢ Reconstruct η out of 2 photons M =134.8 MeV 1000 π with invariant mass technique: � 800 M γγ = 2 E γ 1 E γ 2 (1 − cos( φ γ 1 γ 2 )) M =547.3 MeV Counts [a.u.] η 600 ➢ Clean invariant mass because of × 30 400 other cuts on decay photons: 200 Coincidence cuts, random subtraction... 0 0 100 200 300 400 500 600 700 800 900 1000 Invariant Mass [MeV] Photoproduction of η Mesons off Light Nuclei Lilian Witthauer
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