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Measurement of helicity dependent total inclusive γ-3He cross section and the GDH sum rule on the neutron Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop “Meson Production at Intermediate and High Energies” Messina, November 2011
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Anomalous magnetic moment
The GDH sum rule Proposed by Gerasimov – Drell – Hearn in 1966 Fundamental connection between the ground state properties of a particle and a moment of the entire excitation spectrum Gives a prediction on the absorption of circularly polarised photons by longitudinally polarised nucleons/nuclei: Anomalous magnetic moment Spin Mass Photon spin Baryon spin = a = p Photon energy th = production threshold (nucleons) photodisintegration threshold (nuclei) Relates the anomalous magnetics moment (AMM) of a particle of spin S and mass M to the integral over the weighted helicity asymmetry of the total absorption cross section for circularly polarised photons on a longitudinally polarised target It allows to: check our knowledge of the N interaction and of the physics of strongly interacting systems access new experimental observables and study the baryon resonances through the helicity dependence of partial channels test photoreaction models Susanna Costanza Messina, 11 /11/11
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The GDH sum rule ∙∙ Could the GDH sum rule be violated?
The only “weak” hypothesis is the assumption that the Compton scattering N ’N’ becomes spin-independent as … a violation of this assumption can not be explained easily … Possible explanations could be: exchange of a a1-like (J = 1+) meson between and N non-pointlike (contituent) quarks? A measurement of the GDH integrand represents a fundamental test of our knowledge of both the photon and the nucleon (nucleus) Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the proton
IGDH (p) = 211 ± 5 ± 12 b MAMI data: J. Ahrens et al., PRL 87 (2001) ELSA data: H. Dutz et al., PRL 91 (2003) , H.Dutz et al., PRL 93 (2004) Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the proton ∙∙
IGDH (p) = 211 ± 5 ± 12 b Eγ (GeV) Who IGDH (b) < 0.20 MAID/SAID -28.5 ± 2 0.20 – 2.90 MAMI+ELSA (measured) 254 ± 5 ± 12 > 2.90 (Regge approach) Simula et al. Bianchi–Thomas -13 -14 Total 211 ± 5 ± 12 GDH sum rule 205 Agreement (within errors) between the GDH sum rule value and the experimental one: GDH sume rule experimentally (almost) proved!!! Susanna Costanza Messina, 11 /11/11
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GDH sum rule: theoretical estimates
Partial channels Models IGDH (p) [b] IGDH (n) [b] g p ® Np SAID-FA07K [MAID07] 172 [164] 147 [131] g p ® Npp Fix, Arenhoevel EPJA 25, 114 (2005) 94 82 p ® Nh MAID -8 -6 g p ® KL (S) Sumowidagdo et al. PRC 65, (02) -4 2 g p ® Nr(w) Zhao et al. PRC 65, (03) Regge contribution Bianchi-Thomas PLB 450, 439(99) -14 20 Total 239 [231] 244 [231] GDH sum rule 205 233 … need of improved N() analyses? Comparing the GDH sum rule value and the theoretical predictions… NO AGREEMENT for the proton … but … AGREEMENT for the neutron Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the neutron
Complication: lack of free neutron targets Solution: use of 2H (deuterated butanol) or high pressure gas 3He targets Direct access to the free neutron cross section prevented by nuclear structure effects and FSI Theoretical models needed to evaluate these effects System of one proton and one neutron with paired spins, in relative s states ( 96% probability) Deuteron (2H): Effective nucleon polarisation (D wave) Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the deuteron
Helicity dependent total inclusive cross section: = p-a (b) GDH Mainz – PLB 672, 328 (09) GDH Bonn – PRL 94, (05) AFS model – PRL 93, (04) (AFS: Arenhoevel, Fix and Schwamb) with: From GDH sum rule: 200 MeV < Eγ < 1800 GeV Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the neutron ∙∙
Complication: lack of free neutron targets Solution: use of 2H (deuterated butanol) or high pressure gas 3He targets Direct access to the free neutron cross section prevented by nuclear structure effects and FSI Theoretical models needed to evaluate System of two protons with spins paired off and an “active” unpaired neutron, in relative s states ( 90% probability) 3He: Effective nucleon polarisation (S’ and D waves) Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the neutron ∙∙∙
From previous considerations, the most accurate evaluation of IGDH for the neutron comes from 3He: the proton contribution is much smaller than in the deuteron case. 3He used as a substitute for a polarised neutron target at MAMI For th > m ( photoproduction threshold on free nucleon), IGDH(3He) IGDH(n); For 8 MeV < th < m (photodisintegration region), contribution of nuclear structure effects to IGDH(3He): Precise experimental data on 3He are required from break-up threshold upwards: @ HIS, from photodisintegration threshold to 60 MeV @ MAMI, from photoproduction threshold to test the GDH sum rule on the neutron and 3He models both through the inclusive and the partial channel measurements. Susanna Costanza Messina, 11 /11/11
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Experimental setup for 3He exp.
Facility tagged photon facility of the MAMI accelerator in Mainz Beam circularly polarised photons produced by bremsstrahlung of longitudinally polarised electrons Ee- = 525 MeV 150 < E < 500 MeV Target polarised 3He gas Detector the large acceptance (93%) Crystal Ball (CB) photon spectrometer in combination with the TAPS detector (A. Thomas’s talk) CRYSTAL BALL MWPC PID TAPS Susanna Costanza Messina, 11 /11/11
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MEOP: Metastability Exchange Optical Pumping
3He polarisation MEOP: Metastability Exchange Optical Pumping B = 0 B 0 mF = +½ (F = ½) 23P0 Excited state mF = -½ Simplified! 1083 nm + transition mF = +½ (F = ½) 23S1 Metastable state mF = -½ Polarisation transfer to the 3He ground state by atomic collisions 11S0 Ground state Susanna Costanza Messina, 11 /11/11
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3He target Cylindrical cell: Length: 20 cm
diameter: 6 cm Made of quartz glass (thickness: 2 mm) Titanium enter and exit windows (50 m) provide the necessary gas tightness (4 bar) give long relaxation time (20 hrs) of the gas polarisation 3He polarisation measurements carried out via NMR technique; field provided by Helmholtz coils Developed by P.I. Mainz -beam Vacuum chamber Helmholtz coils Susanna Costanza Messina, 11 /11/11
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3He target is an “empty” target… z–vertex from MWPC analysis
Natoms 1021/cm2 102 times less than in a solid/liquid target 3He target is an “empty” target… All charged particle events Ti windows 3He gas Event difference (P – A) z–vertex from MWPC analysis 150 MeV < Eγ < 500 MeV Susanna Costanza Messina, 11 /11/11
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Unpolarised cross sections on 3he
OUTLINE of the Section Total inclusive cross section No partial channel separation (just hadron counting) Partial/semi-inclusive channels: 3He ± X ±/p separation by dE/dx – E technique (using PID & MWPC) 3He 0 X invariant mass (using only CB) 3He ppn ppn/ppn0 separation by missing mass/energy plots Comparison: total inclusive vs sum of partial channels …obtained after subtraction of (the big) empty target contribution... Susanna Costanza Messina, 11/11/11
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Total inclusive cross section
“Inclusive” analysis method (NO partial channel separation) Extrapolation from quasi-free pion production and MAID cross sections Extrapolation from Schwamb model for ppn Good agreement with DAPHNE data Susanna Costanza Messina, 11 /11/11
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Partial channel I: 3He(γ,)X
First data Preliminary Nuclear structure contribution (FSI, …) more important for the 0X partial channel Susanna Costanza Messina, 11 /11/11
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Partial channel II: γ 3He ppn
First data Preliminary 1.68 diversa densità e numero di quasi-deutoni. Effetti a 3 corpi = circa 30 % No model estimation available for 3He “Quasi-deuteron” approximation ( 3He pnps) evaluated from the Schwamb d pn model Discrepancy between data and Schwamb model mostly due to 3-body absorption effects Susanna Costanza Messina, 11 /11/11
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Total inclusive vs partial channels
First data Preliminary Good agreement between inclusive method and the sum of partial channels Susanna Costanza Messina, 11 /11/11
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polarised cross sections on 3he
OUTLINE of the Section Total inclusive cross section No partial channel separation Partial/semi-inclusive channels: 3He ± X ±/p separation by dE/dx – E technique (using PID & MWPC) 3He 0 X invariant mass (using only CB) 3He ppn ppn/ppn0 separation by missing mass/energy plots Comparison: total inclusive vs sum of partial channels Same analyses as for unpol …no subtraction of empty target contribution... Susanna Costanza Messina, 11/11/11
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Total inclusive cross section
First data Fix + Schwamb MAID “Total Inclusive” analysis (NO partial channel separation) Extrapolation from quasi-free pion production and MAID cross sections Model: A. Fix (X - simplified treatment of FSI) + Schwamb (ppn) Prediction based on MAID Preliminary Reasonable agreement between data and empirical prediction Susanna Costanza Messina, 11 /11/11
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Partial channel I: 3He(γ,)X
First data Preliminary Nuclear structure contribution (FSI, …) less important than for the unpolarised case Susanna Costanza Messina, 11 /11/11
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Partial channel II: γ 3He ppn
First data Preliminary No model estimation available for 3He “Quasi-deuteron” approximation ( 3He pnps) evaluated from the Schwamb d pn model Discrepancy between data and Schwamb model mostly due to 3-body absorption effects Susanna Costanza Messina, 11 /11/11
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Rough derivation of IGDH(n)
Ipotesi nucleoni liberi e effective nuclear polarisation Susanna Costanza Messina, 11 /11/11
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Conclusions This first test has proved the feasibility of the use of polarised 3He gas target to check the GDH sum rule on the neutron Unprecedented data and preliminary results for unpolarised and polarised photoabsorption cross section on 3He Good agreement for the unpolarised inclusive cross section between the CB-MAMI data and the DAPHNE data Importance of the data in providing additional constraints for nuclear and subnuclear models Further measurements to improve statistics and to investigate a wider energy range are needed Ipotesi nucleoni liberi e effective nuclear polarisation Thank you! Susanna Costanza Messina, 11 /11/11
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Backup slides Susanna Costanza Messina, 11 /11/11
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dE/dx – E techinque used
±/p separation dE/dx – E techinque used Energy information from PID (corrected with MWPC) and CB p e Susanna Costanza Messina, 11 /11/11
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Only CB information used
0 identification invariant mass Only CB information used Susanna Costanza Messina, 11 /11/11
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ppn/ppn0 separation Susanna Costanza Messina, 11 /11/11
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Total inclusive vs partial channels
First data Preliminary Good agreement between inclusive method and the sum of partial channels Susanna Costanza Messina, 11 /11/11
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GDH sum rule on the deuteron ∙∙
Running GDH integral for the deuteron ν0 = 200 MeV, Eγ = 1.8 GeV GDH Mainz GDH Bonn AFS model Assuming that, in this measured energy region, the incoherent, quasi-free meson production processes dominate: with: Nu0 = lowest measured photon energy measured From GDH sum rule: Susanna Costanza Messina, 11 /11/11
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