Helicity dependent meson photoproduction on 3 He in the -resonance - PowerPoint PPT Presentation

Helicity dependent meson photoproduction on 3 He in the Δ -resonance region Paolo Pedroni INFN-Sezione di Pavia, Italy For the CBMAMI and A2 collaborations

SUMMARY ⎧ Exp. check of the GDH sum rule � Physics motivations ⎨ Determination of the N * properties ⎩ ⎧ for 150 < E γ < < 500 MeV X r ⎪ r 3 � Results ⎨ γ H e → (Mainz) ppn ⎪ ⎩ N π First data First data � Outlook

Experimental verification of the GDH sum rule � Proposed in 1966 by Gerasimov-Drell-Hearn � Prediction on the absorption of circularly polarized photons by longitudinally polarized nucleons/nuclei photon-spin nucleus-spin photon-spin nucleus-spin σ p σ a spin ∞ = ∫ 2 σ ( E ) − σ ( E ) e p γ a γ 2 2 = 4 π I dv S k Anomalous GDH 2 v M magnetic moment ν thr production threshold (nucleon) ⎧ π ⎨ ν thr = photodisin tegration threshold (nuclei) ⎩

GDH sum rule: � Fundamental check of our knowledge of the γ N interaction The only “weak” hypothesis is the assumption that Compton scattering γ N → γ → γ ’ N’ becomes spin independent when ν → ν → ∞ A violation of this assumption can not be easily explained � Important comparison for photoreaction models � Helicity dependence of partial channels (pion photoproduction) is an essential tool for the study of the baryon resonances (interference terms between different electromagnetic multipoles) � Valid for any system with k ≠ 0 ≠ 0 ( 2 H, 3 He) . “Link” between nuclear and nucleon degrees of freedom

Experimental status r r MAMI-Mainz γ p → X ELSA-Bonn Δ (1232) 2 µ b I GDH GDH (p) = 211 ± (p) = 211 ± 5 ± ± 12 D 13 (1520) F 15 (1680) F 35 (1905) unpolarized c.s. 200 MeV

GDH sum rule: predictions Proton I GDH ( µ b) Neutron I GDH ( µ b) γ γ p → N π 172 172 [164] [164] γ n → N π γ 147 147 [131] [131] γ γ p → → N ππ ππ 94 94 γ n → N ππ γ ππ 82 82 γ γ p → N η -8 γ γ n → N η -6 γ p → γ → K Λ Λ ( Σ ) -4 γ γ n → K Λ ( Σ ) 2 γ p → N ρ ( ω ) 0 γ n → N ρ ( ω ) 2 γ γ Regge contrib. -14 Regge contrib. 20 (E γ > 2 Gev) (E γ > 2 Gev) TOTAL 239 [231] TOTAL 244 [231] GDH 205 GDH 233 N π : SAID-FA07K [MAID07] K Λ ( Σ ) : Sumowidagdo et al., PRC 65,0321002 (02) N ππ ππ : Fix, Arenhoevel EPJA 25, 114 (2005) N η : MAID N ρ : Zhao et al., PRC 65, 032201 (03) Regge : Bianchi-Thomas , PLB 450,439(99)

GDH sum rule on the neutron � No Free neutron target available � Model dependent results from nuclear targets � Our experimental goal: to have a “small” and “realiable” model dependence � Two different (and complementary) targets =) deuteron (data from Mainz –Bonn) =) 3 He (no data up to now) � Measurement of partial channels like r r γ d → π NN r r 3 γ H e → π NNN

GDH sum rule on the neutron n p PWIA approach � 2 H: µ ∼ µ p + µ n ⇒ E γ γ > m π Deut ∼ 0.93 • Ι GDH neutron + 0.93 • Ι GDH proton Ι GDH n p p � 3 He: µ ∼ µ n ⇒ (S-state with ∼ 90% prob.) He3 ∼ 0.87 • Ι GDH neutron -0.026 • Ι GDH proton Ι GDH � 3 He better suited to measure Ι GDH neutron (inclusive method) � 2 H better suited to measure partial reaction channels

Status of the deuteron results r r γ d → X AFS model from Ahrenhoevel, Fix and Schwamb

3 He Experimental set-up � Facility tagged photon facility of the MAMI accelerator in Mainz � Beam circularly polarised photons produced by bremsstrahlung of longitudinally polarised electrons E electron = 525 MeV 150 < E γ < 500 MeV CRYSTAL BALL � Target Polarised 3 He gas First feasibility test � Detector MWPC the large acceptance (93%) Crystal Ball (CB) photon PID spectrometer in combination TAPS with the TAPS detector

3 He polarisation MEOP: Metastability Exchange Optical Pumping B = 0 B ≠ 0 m F = +½ (F = ½) 2 3 P 0 Laser Excited state m F = -½ 1083 nm σ + transition m F = +½ (F = ½) 2 3 S 1 Metastable state m F = -½ RF discharge Polarisation transfer to the 3 He ground 1 1 S 0 state by atomic collisions Ground state r r ( ) ( ) ( ) ( ) J.Krimmer et al., NIMA 648, H e S He S He S H e S 3 3 + 3 1 → 3 3 + 3 1 2 1 2 1 1 0 1 0 35 (2011)

Polarised 3 He gas target Cylindrical cell (gas polarised via MEOP) Length: 20 cm diameter: 6 cm Made of quartz glass (thickness: 2 mm) Titanium entrance and exit windows (50 µ m) provide the necessary gas tightness (4 bar) give long relaxation time ( ∼ 20 hrs) of the gas polarisation 3 He polarisation measurements carried out via NMR in collaboration with PI, Mainz technique; field provided by Helmholtz coils solenoid γ -beam Vacuum chamber Helmholtz coils

Charged Particle Z-Vertex from MWPCs N atoms ∼ 10 21 /cm 2 � ∼ 10 2 times Ti windows less than in a solid/liquid target All charged particle 3 He gas events (P-A) difference

Unpolarised data 3 γ He → X “Inclusive” analysis method (NO partial channel separation) Only hadron counting and empty target subtraction � Extrapolation from quasi-free pion production and MAID cross sections � Extrapolation from Schwamb model for ppn Data from CB detector ONLY Overall Extrapolation is about 5 % of the measured yields Good agreement with previous data

3 γ He → π X σ ( µ b) A. Fix model: 0 800 π X π 0 X � Input: Free γ N → π N 600 amplitudes from MAID 400 � Free Amplitudes 200 embedded inside 3 He wave function σ ( µ b) σ ( µ b) 800 � ± FSI taken into π X π ± X account in an 600 approximate way 400 � As expected , FSI play a bigger role in 200 the π 0 case A. Fix (nuclear model) MAID (free nucleons) First data First data 200 300 400 500 E γ (MeV)

Differential unpolarised 3 0 γ He → π X cross section E γ = 418 MeV E γ = 400 MeV CB π 0 X CB A. Fix A. Fix θ LAB π MAID MAID E γ = 381 MeV E γ = 361 MeV First data First data

Differential unpolarised γ 3 ± He → π X cross section diff_cross_sec_5 diff_cross_sec_6 b) b) 120 E γ = 418 MeV E γ = 400 MeV µ µ ( ( 100 Ω Ω 100 /d /d σ σ d d 80 80 60 60 40 40 20 20 0 0 CB π ± X 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 CB ( ) θ ( ° ) θ ° θ LAB π A. Fix A. Fix diff_cross_sec_7 diff_cross_sec_8 b) b) MAID MAID E γ = 381 MeV E γ = 361 MeV 120 µ µ 90 ( ( Ω Ω /d /d 80 100 σ σ d d 70 80 60 60 50 First data First data 40 40 30 20 20 10 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 θ ( ° ) ( ) θ °

3 γ He → ppn Discrepancy between data and the quasi- deuteron model mostly due to 3-body absorption effects No model estimation available for this channel First data First data “Quasi-deuteron” approximation ( γ 3 He → pnp s ) evaluated from the Schwamb γ d → pn model

r r 3 Polarised data γ H → e X Δ σ = σ − σ ( b µ ) Δσ ( µ b) “Inclusive” analysis p a method CB-Inclusive method (NO partial channel separation) ˝ MAID Inspired ˝ 400 model Extrapolation from quasi-free pion production and MAID cross sections 200 Extrapolation from Schwamb model for ppn Extrapolation from quasi-free pion production and MAID cross 0 sections First data First data Model: Prediction based on MAID -200 Δ σ = ⋅ Δ σ − ⋅ Δ σ 0 . 87 0 . 05 200 300 400 500 n p E γ (MeV)

r r 3 γ H e → π X Δσ ( µ b) π 0 X Δ σ = σ − σ ( b µ ) 0 π X p a 300 A Fix model: � Nuclear 200 structure contribution 100 (FSI, …) less important 0 than for the unpolarised case Δσ ( µ b) σ ( µ b) π ± X 100 ± π X 0 -100 Δ σ = 0 . 87 ⋅ Δ σ − 0 . 05 ⋅ Δ σ MAID n p A. Fix (nuclear model) -200 First data First data MAID (free nucleons) 200 300 400 500 E γ (MeV)

r r Differential polarised 3 0 γ H e → π X cross section E γ = 400 MeV E γ = 418 MeV CB π 0 X CB A. Fix A. Fix θ LAB π MAID MAID E γ = 381 MeV E γ = 361 MeV First data First data

r r 3 Differential polarised ± γ H e → π X cross section diff_cross_sec_5 diff_cross_sec_6 E γ = 418 MeV E γ = 400 MeV b) b) 10 15 µ µ ( ( Ω 10 Ω /d /d 0 σ σ 5 d d -10 0 -5 -20 -10 -15 -30 -20 -40 -25 CB π 0 X CB 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 ( ) θ ° θ ( ° ) A. Fix A. Fix θ LAB π diff_cross_sec_7 diff_cross_sec_8 MAID MAID b) b) E γ = 381 MeV E γ = 361 MeV 20 µ µ 20 ( ( Ω Ω 15 /d /d σ σ 10 d d 10 5 0 0 -5 -10 -10 -15 First data First data -20 -20 -25 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 θ ( ° ) θ ( ° )

r r 3 γ H → e ppn ����� No model estimation available for this channel First data First data “Quasi-deuteron” approximation ( γ 3 He → pnp s ) evaluated from the Schwamb γ d → pn model

Very rough derivation of