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Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop

Measurement of helicity dependent total inclusive γ - 3 He cross section and the GDH sum r ule on the neutron. Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop “ Meson Production at Intermediate and High Energies ”

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Susanna Costanza (INFN Pavia) on behalf of the A2 Collaboration International Workshop

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  1. Measurementofhelicitydependent total inclusive γ-3He cross section and theGDH sum rule on the neutron Susanna Costanza (INFN Pavia) on behalfof the A2 Collaboration International Workshop “Meson Production at Intermediate and High Energies” Messina, 10-11 November 2011

  2. The GDH sum rule ProposedbyGerasimov – Drell – Hearn in 1966 Fundamental connection between the ground state propertiesof a particle and a moment of the entireexcitationspectrum Gives a prediction on the absorptionofcircularlypolarisedphotonsbylongitudinallypolarisednucleons/nuclei: Anomalousmagnetic moment Spin Mass Photonenergy th =  production threshold (nucleons) photodisintegrationthreshold (nuclei) Photonspin Baryonspin = p = a Itallowsto: checkourknowledgeof the N interaction and ofthe physicsofstronglyinteractingsystems accessnewexperimentalobservables and study the baryonresonancesthrough the helicitydependenceofpartialchannels test photoreactionmodels Messina, 11 /11/11

  3. The GDH sum rule∙∙ Could the GDH sum rulebeviolated? The only “weak” hypothesisis the assumptionthat the Compton scatteringN  ’N’ becomesspin-independentas   … a violationofthisassumption can notbeexplainedeasily … Possibleexplanationscouldbe: exchangeof a a1-like (J = 1+) mesonbetween and N non-pointlike (contituent) quarks? A measurementof the GDH integrandrepresents a fundamental test ofourknowledgeofboth the photon and the nucleon (nucleus) Messina, 11 /11/11

  4. GDH sum rule on the proton IGDH (p) = 211 ± 5 ± 12 b MAMI data: J. Ahrenset al., PRL 87 (2001) 022003 ELSA data: H. Dutzet al., PRL 91 (2003) 192001, H.Dutzet al., PRL 93 (2004) 032003 Messina, 11 /11/11

  5. GDH sum rule on the proton∙∙ IGDH (p) = 211 ± 5 ± 12 b Agreement (withinerrors) between the GDH sum rulevalue and the experimentalone: GDH sumeruleexperimentally (almost) proved!!! Messina, 11 /11/11

  6. GDH sum rule: theoreticalestimates … needofimproved N()analyses? Comparing the GDH sum rulevalue and the theoreticalpredictions… NO AGREEMENT for the proton… but … AGREEMENT for the neutron Messina, 11 /11/11

  7. GDH sum rule on the neutron Complication: lackof free neutrontargets Solution: useof2H (deuteratedbutanol) or high pressure gas 3He targets Directaccessto the free neutron cross sectionpreventedbynuclearstructureeffects and FSI Theoreticalmodelsneededtoevaluatetheseeffects System ofoneproton and oneneutronwithpairedspins, in relative sstates ( 96% probability) Deuteron (2H): Effectivenucleonpolarisation (D wave) Messina, 11 /11/11

  8. GDH sum rule on the deuteron Helicitydependent total inclusive cross section:  = p-a (b) GDH Mainz– PLB 672, 328 (09) GDH Bonn– PRL 94, 162001 (05) AFS model– PRL 93, 202301 (04) (AFS: Arenhoevel, Fix and Schwamb) 200 MeV < Eγ < 1800 GeV with: From GDH sum rule: Messina, 11 /11/11

  9. GDH sum rule on the neutron∙∙ Complication: lackof free neutrontargets Solution: useof2H (deuteratedbutanol) or high pressure gas 3He targets Directaccessto the free neutron cross sectionpreventedbynuclearstructureeffects and FSI Theoreticalmodelsneededtoevaluate System oftwoprotonswithspinspaired off and an “active” unpairedneutron, in relative sstates ( 90% probability) 3He: Effectivenucleonpolarisation (S’ and D waves) Messina, 11 /11/11

  10. GDH sum rule on the neutron∙∙∙ Frompreviousconsiderations, the most accurate evaluationofIGDHfor the neutroncomesfrom3He: the protoncontributionismuchsmallerthan in the deuteron case. 3He usedas a substitutefor a polarisedneutron target at MAMI Forth > m(photoproductionthreshold on free nucleon), IGDH(3He)  IGDH(n); For8 MeV <th < m(photodisintegrationregion), contributionofnuclearstructureeffectstoIGDH(3He): • Precise experimental data on 3He are requiredfrom break-up thresholdupwards: • @ HIS, fromphotodisintegrationthresholdto 60 MeV • @ MAMI, from photoproductionthreshold • to test the GDH sum rule on the neutron and 3He models • boththrough the inclusive and the partialchannelmeasurements. Messina, 11 /11/11

  11. Experimentalsetupfor3He exp. • Facility taggedphotonfacilityof the MAMIaccelerator in Mainz • Beam circularlypolarisedphotonsproducedbybremsstrahlungoflongitudinallypolarisedelectrons • Ee-= 525 MeV • 150 < E < 500 MeV • Target polarised3He gas • Detector the largeacceptance (93%) Crystal Ball (CB) photonspectrometer in combinationwith the TAPS detector (A. Thomas’s talk) CRYSTAL BALL MWPC PID TAPS Messina, 11 /11/11

  12. 3He polarisation MEOP: Metastability Exchange OpticalPumping 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 byatomiccollisions 11S0 Ground state Messina, 11 /11/11

  13. 3He target • Cylindricalcell: • Length: 20 cm • diameter: 6 cm • Madeofquartzglass (thickness: 2 mm) • Titaniumenter and exit windows (50 m) • provide the necessary gas tightness (4 bar) • give long relaxationtime (20 hrs) of the gas polarisation • 3He polarisationmeasurementscarried out via NMR technique; fieldprovidedbyHelmholtzcoils Developedby P.I. Mainz -beam Vacuumchamber Helmholtzcoils Messina, 11 /11/11

  14. 3He target ∙∙ Natoms 1021/cm2 102timeslessthan in a solid/liquid target 3He target isan “empty” target… Ti windows Allchargedparticleevents 3He gas Eventdifference (P – A) z–vertexfrom MWPC analysis 150 MeV < Eγ < 500 MeV Messina, 11 /11/11

  15. OUTLINE of the Section • Total inclusive cross section • No partialchannelseparation (just hadroncounting) • Partial/semi-inclusive channels: •  3He  ± X • ±/p separationbydE/dx – E technique (using PID & MWPC) •  3He  0 X •  invariant mass (usingonly CB) •  3He  ppn • ppn/ppn0separationbymissing mass/energyplots • Comparison: total inclusive vs sum ofpartialchannels Unpolarised cross sections on 3he …obtainedaftersubtractionof (the big) empty target contribution... Messina, 11/11/11

  16. Total inclusive cross section “Inclusive” analysismethod (NO partialchannelseparation) Extrapolationfromquasi-freepion production and MAID cross sections ExtrapolationfromSchwambmodelforppn Good agreement with DAPHNE data Messina, 11 /11/11

  17. Partialchannel I: 3He(γ,)X First data Preliminary Nuclearstructurecontribution (FSI, …) more importantfor the 0X partialchannel Messina, 11 /11/11

  18. Partialchannel II: γ3He  ppn First data Preliminary No model estimation available for 3He “Quasi-deuteron” approximation ( 3He  pnps) evaluated from the Schwambd  pnmodel Discrepancybetween data and Schwambmodel mostly due to 3-body absorptioneffects Messina, 11 /11/11

  19. Total inclusive vspartialchannels First data Preliminary Good agreement between inclusive method and the sum ofpartialchannels Messina, 11 /11/11

  20. OUTLINE of the Section • Total inclusive cross section • No partialchannelseparation • Partial/semi-inclusive channels: •  3He  ± X • ±/p separationbydE/dx – E technique (using PID & MWPC) •  3He  0 X •  invariant mass (usingonly CB) •  3He  ppn • ppn/ppn0separationbymissing mass/energyplots • Comparison: total inclusive vs sum ofpartialchannels Sameanalysesasforunpol polarised cross sections on 3he …nosubtractionofempty target contribution... Messina, 11/11/11

  21. Total inclusive cross section First data Fix + Schwamb MAID “Total Inclusive” analysis (NO partialchannelseparation) • Extrapolationfromquasi-freepion production and MAID cross sections • Model: • A. Fix (X - simplified treatment of FSI) + Schwamb (ppn) • Predictionbased on MAID Preliminary Reasonable agreement between data and empiricalprediction Messina, 11 /11/11

  22. Partialchannel I: 3He(γ,)X First data Preliminary Nuclearstructurecontribution (FSI, …) lessimportantthanfor the unpolarised case Messina, 11 /11/11

  23. Partialchannel II: γ 3He  ppn First data Preliminary No modelestimationavailablefor3He “Quasi-deuteron” approximation ( 3He  pnps) evaluatedfrom the Schwambd  pnmodel Discrepancybetween data and Schwambmodel mostly due to 3-body absorptioneffects Messina, 11 /11/11

  24. Roughderivationof IGDH(n) Messina, 11 /11/11

  25. Conclusions This first test hasproved the feasibilityof the useofpolarised3He gas target tocheck the GDH sum rule on the neutron Unprecedented data and preliminaryresultsforunpolarised and polarisedphotoabsorption cross section on 3He Good agreement for the unpolarised inclusive cross sectionbetween the CB-MAMI data and the DAPHNE data Importanceof the data in providingadditionalconstraintsfornuclear and subnuclearmodels Furthermeasurementstoimprovestatistics and to investigate a widerenergyrange are needed Thankyou! Messina, 11 /11/11

  26. Backup slides Messina, 11 /11/11

  27. ±/p separation dE/dx – E techinqueused Energy information from PID (correctedwith MWPC) and CB p  e Messina, 11 /11/11

  28. 0identification invariant mass Only CB information used Messina, 11 /11/11

  29. ppn/ppn0separation Messina, 11 /11/11

  30. Total inclusive vspartialchannels First data Preliminary Good agreement between inclusive method and the sum ofpartialchannels Messina, 11 /11/11

  31. GDH sum rule on the deuteron∙∙ Running GDH integralfor the deuteron ν0 = 200 MeV, Eγ = 1.8 GeV Assumingthat, in thismeasuredenergyregion, the incoherent, quasi-freemeson production processes dominate: with: GDH Mainz GDH Bonn AFS model From GDH sum rule: Messina, 11 /11/11

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