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 Presented by Mark Jones, Jefferson Lab TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAA A A

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

Elastic Hall A Hall C Hall B BigCal Polarized electron beam P θpθp Target chamber HMS Electron FPP 217.6cm Calorimeter (BigCal)121.8cm

Elastic Hall A Hall C Hall B BigCal Polarized electron beam P θpθp Target chamber HMS Electron Focal Plane Polarimeter (FPP) in High Momentum Spectrometer FPP Overview of Gep3 and Gep2g experiments Measure proton G E /G M from ratio of P T /P L Gep3 to Q 2 = 8.5 GeV 2 Gep2g covered 3  at Q 2 = 2.5GeV 2 Talks Sat ND.0002 & ND.0003 by Andrew Puckett.

π° photo-production in GEp-III & 2 g experiments π° photo-production: the cross section is much larger than ep elastic at high Q 2, most of the background comes from this reaction. Real Compton Scattering: Cross section is much smaller than the π° production at high Q 2. Two reactions have the possibilities to pass the trigger settings: 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 HMS Spectra with no BIGCAL cuts Large Background Elastics Counts

Physics motivation Polarization transfer components at target: Px, Pz (P T, P L ) Induced polarization: Py (P N ) Helicity independent variable. 1. Evidence of baryon resonances: In low photon energy range(E γ <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. Hadron Helicity Conservation(HHC) Rule**: As a consequence of pQCD, the HHC predicts that transferred polarization Px and induced polarization Py both vanish 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)

In some kinematics settings, BigCal can detect both γ s(depends on θ 12 and distance from target to BigCal). Polarization of proton are measured by Focal Plane Polarimeter inside HMS hu in the ideal dipole approximation: BigC al Polarized electron beam P γ θpθp θ 12 γ Target chamber EM Calorimeter: BigCal High Momentum Spectrometer (HMS) Kinematics of π° photo-production ExperimentE e (GeV) P HMS (GeV/c) Θ HMS (°) R BigCal (cm) Χ (°) E γ (GeV) θ C.M, π° (°) GEp-2 γ GEp-III GEp-III & 2 γ kinematics table*: *only the kinematics which can be used for π° analysis are shown.

Measure Proton Energy & angle Calculate Initial photon energy Outgoing pion angle and energy Use both and assume π° photo- production kinematics Measure decay photon Energy & angle Use photon angle and pion energy & angle π° decay kinematics Calculate decay photon energy HMS: Acceptance cut BigCal: Acceptance cut Coin TDC cut Identification of reaction One  detected from  0 decay: high statistics high background contamination

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 BigCal and photon energy predicted by proton measured in HMS. The E 1 /E 1 ’ resolution is similar to energy resolution of ep elastic data. Fitting the E 1 /E 1 ’ distribution as a gauss function for π °, and Landau function for background. The total background contamination is about 7.7%. Cut: E threshold <E 1 < E elastic E threshold <E 1 ’ < E elastic π° event identification: 1 photon detected E 1 /E 1 ’ E1’E1’ E1E1

Measure Proton Energy & angle Calculate Initial photon energy Outgoing pion angle and energy Use both and assume π° photo- production kinematics Measure Both decay photons Energy & angle Use one photon’s angle and pion energy & angle π° decay kinematics Calculate photon energy and 2 nd photon’s energy and angle HMS: Acceptance cut BigCal: Acceptance cut Coin TDC cut Identification of reaction Two photons  detected from  0 decay: low statistics low background contamination

π° γ1γ1 φ1φ1 φ2φ2 Target BigCal γ2γ2 GEp-III: Beam energy 5.71 GeV, BigCal at closest position to target, two photons decayed from π° could be detected. π° event identification: 2 photons detected Δ φ cut SimulationData Estimated background contamination: < 4.2%

Asymmetry at Focal Plane Polarimeter Helicity-independent distribution: Helicity-dependent distribution: Θ π° c.m. = 122°, χ =176.5° (1 γ detected ) Θ π° c.m. = 137°, χ = 262.8° (2 γ 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

Preliminary results for P x E γ (GeV) PxPx PRELIMINARY  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 results for P z E γ (GeV) PzPz PRELIMINARY  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 results for P y E γ (GeV) PyPy PRELIMINARY  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

Conclusion  Identifying π° with two different methods give consistent results.  First measurement of π° photo - production polarization components in range of 5.0 GeV< E γ < 5.7 GeV.  Preliminary results show: Consistency with the results of previous experiment at low E γ Non-zero induced polarization P y at large E γ Transferred Polarization component P z changes dramatically as a function of θ π° cm when E γ > 5.0 GeV

Backup slides

GEp-III: Q 2 =8.5 GeV 2 : ep elastic data Analyzing power: Polarization at Focal Plane Polarimeter: ep elastic

Two γs identified in BigCal One γ identified in BigCal Q2 = 8.5 GeV 2 background correction