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Proton polarization measurements in π° photo-production --on behalf of the Jefferson Lab Hall C

Proton polarization measurements in π° photo-production --on behalf of the Jefferson Lab Hall C GEp-III and GEp-2 γ collaboration. 2010 Annual Fall Meeting of the APS Division of Nuclear Physics Santa Fe, New Mexico Wei Luo , Lanzhou University, China

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Proton polarization measurements in π° photo-production --on behalf of the Jefferson Lab Hall C

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  1. Proton polarization measurements in π° photo-production --on behalf of the Jefferson Lab Hall C GEp-III and GEp-2γcollaboration 2010 Annual Fall Meeting of the APS Division of Nuclear Physics Santa Fe, New Mexico Wei Luo, Lanzhou University, China Presented by Mark Jones, Jefferson Lab TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAA

  2. Overview of Gep3 and Gep2g experiments • Experiments were run in Hall C at Jefferson Lab accelerator Hall A Hall C Hall B

  3. Overview of Gep3 and Gep2g experiments Calorimeter (BigCal) 121.8cm Elastic BigCal 217.6cm P θp Polarized electron beam Target chamber HMS FPP Electron Hall A Hall C Hall B

  4. Overview of Gep3 and Gep2g experiments Elastic BigCal P θp Polarized electron beam Focal Plane Polarimeter (FPP) in High Momentum Spectrometer Target chamber HMS FPP Electron • Measure proton GE/GM • from ratio of PT/PL • Gep3 to Q2 = 8.5 GeV2 • Gep2g covered 3e at Q2 = 2.5GeV2 • Talks Sat ND.0002 & ND.0003 by • Andrew Puckett. Hall A Hall C Hall B

  5. π° photo-production in GEp-III & 2gexperiments HMS Spectra with no BIGCAL cuts • BigCal sensitive to electrons, positrons and photons • Trigger is coincidence between proton and cluster in BigCal with more than ½ the elastic electron energy • 20cm LH2 target, 2.3% radiator Counts Large Background Elastics Two reactions have the possibilities to pass the trigger settings: π°photo-production: the cross section is much larger than ep elastic at high Q2, most of the background comes from this reaction. Real Compton Scattering: Cross section is much smaller than the π°production at high Q2.

  6. Physics motivation Polarization transfer components at target: Px, Pz (PT, PL) Induced polarization: Py(PN) Helicity independent variable. 1. Evidence of baryon resonances: In low photon energy range(Eγ<2GeV), the π° production is dominated by the baryon resonances. This has been largely tested by the measurement of the cross section* and the induced polarization observable Py. One would expect the polarization components to behave smoothly above the baryon resonance regime. 2. HadronHelicity Conservation(HHC) Rule**: As a consequence of pQCD, the HHC predicts that transferred polarization Px and induced polarization Pybothvanish and Pz should become independent of beam energy at high beam energy. ** S.J. Brodsky et. al., Phy. Rev. D 24, 2848(1981) * D. Menze, et. al., Compilation of Pion Photoproduction Data (Physikalisches Institut der Universitat, bonn, 1977)

  7. Kinematics of π° photo-production EM Calorimeter: BigCal In some kinematics settings, BigCalcan detect both γs(depends on θ12 and distance from target to BigCal). Polarization of proton are measured by Focal Plane Polarimeter inside HMS hu in the ideal dipole approximation: High Momentum Spectrometer (HMS) γ BigCal γ θ12 P θp Polarized electron beam GEp-III & 2γ kinematics table*: Target chamber *only the kinematics which can be used for π° analysis are shown.

  8. Identification of reaction • Oneg detected from p0 decay: • high statistics • high background contamination • Calculate • Initial photon energy • Outgoing pion angle and energy • Measure • Proton • Energy & angle • Use both and assume • π° photo-production • kinematics HMS: Acceptance cut • Measure • decay photon • Energy & angle • Use photon angle and pion energy & angle • π° decay kinematics BigCal: Acceptance cut Coin TDC cut • Calculate • decay photon energy

  9. π° event identification: 1 photondetected π° event identification: one cluster found in BigCal In case only one photon hit BigCal, to select π°event one can use the energy correlation between the energy measured by BigCaland photon energy predicted by proton measured in HMS. E1 Cut: Ethreshold <E1< Eelastic Ethreshold <E1’ < Eelastic E1’ E1/E1’ The E1/E1’ resolution is similar to energy resolution of ep elastic data. Fitting the E1/E1’ distribution as a gauss function for π°, and Landau function for background. The total background contamination is about 7.7%.

  10. Identification of reaction • Two photonsdetected from p0 decay: • low statistics • lowbackground contamination • Calculate • Initial photon energy • Outgoing pion angle and energy • Measure • Proton • Energy & angle • Use both and assume • π° photo-production • kinematics HMS: Acceptance cut • Measure • Both decay photons • Energy & angle • Use one photon’s angle and pion energy & angle • π° decay kinematics BigCal: Acceptance cut Coin TDC cut • Calculate photon energy • and 2nd photon’s energy and angle

  11. π° event identification: 2 photonsdetected GEp-III: Beam energy 5.71 GeV, BigCalat closest position to target, two photons decayed from π° could be detected. BigCal γ2 Δφ cut φ2 φ1 γ1 π° Target Simulation Data Estimated background contamination: < 4.2%

  12. Asymmetry at Focal Plane Polarimeter Helicity-dependent distribution: Helicity-independent distribution: Θπ°c.m. = 122°, χ =176.5° (1γ detected) • Θπ°c.m. = 137°, χ =262.8° • (2γ detected) In the ideal dipole approximation: , Helicity-independent components at focal plane , Helicity-dependent components at focal plane Ay Analyzing power of CH2 h Beam helicity Total number of helicity(+/-) events/number of bins

  13. Preliminary results for Px Px • FPP analyzing power • and false asymmetry are • calibrated by ep elastic events; • Using COSY spin transport matrix and maximum likelihood method • Estimated maximum systematic error < 3% for each bin PRELIMINARY Eγ (GeV)

  14. Preliminary results for Pz Pz • FPP analyzing power • and false asymmetry are • calibrated by ep elastic events; • Using COSY spin transport matrix and maximum likelihood method • Estimated maximum systematic error < 3% for each bin PRELIMINARY Eγ (GeV)

  15. Preliminary results for Py Py • FPP analyzing power • and false asymmetry are • calibrated by ep elastic events; • Using COSY spin transport matrix and maximum likelihood method • Estimated maximum systematic error < 3% for each bin PRELIMINARY Eγ (GeV)

  16. Conclusion • Identifying π° with two different methods give consistent results. • First measurement of π° photo-production polarization components in range of 5.0 GeV< Eγ < 5.7 GeV. • Preliminary results show: • Consistency with the results of previous experiment at low Eγ • Non-zero induced polarization Py at large Eγ • Transferred Polarization component Pz changes dramatically as a function of θπ°cm when Eγ > 5.0 GeV

  17. Backup slides

  18. Polarization at Focal Plane Polarimeter: ep elastic GEp-III: Q2=8.5 GeV2 : ep elastic data Analyzing power:

  19. Q2 = 8.5 GeV2 background correction Two γs identified in BigCal One γ identified in BigCal

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