Hall C Summer Workshop August 6, 2009 W. Luo Lanzhou University, China Analysis of GEp-III&2γ Inelastic Data --on behalf of the Jefferson Lab Hall C GEp-III collaboration

Outline  GEp-III & 2γ: π° Physics motivation Kinematics of π° production  SIMC simulation of π° photo-production  Preliminary result of π° production analysis Polarization transfer vs. E γ Induced polarization Py vs. E γ Induced polarization Py vs. θ π°  Conclusion

GEp-III & 2γ: π° The EM calorimeter is sensitive to e, e+ and γ, while the other particles have smaller amplitude. The BigCal threshold at 1/2 of elastic electron energy will further narrow down the reactions that could pass the trigger(10cm of absorber in front of BigCal also eliminate part of low energy particles). π° 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. GEp reaction: To get clean ep elastic events, at hardware level: BigCal – HMS coincidence trigger BigCal threshold ≈ 1/2 E ’ elastic 20cm(15cm) LH2 long target is equivalent to a 2.3% radiator. Two reactions may pass the trigger setting:

Physics motivation H Polarization transfer components: 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: PQCD predicts the HHC. The HHC predicts the transferred polarization Px and induced polarization Py both vanish; Pz becomes independent of beam energy in high E γ regime.

121.8cm 217.6cm 249.4cm Open distance HMS BigCal Beam line P γ θpθp θ 12 γ Target chamber Kinematics of π° production  π° decay into 2γ(fraction %)  Only if one gamma energy > 1/2 E ’ hits BigCal will form a trigger signal.  In 2 kinematics with BigCal close to the target we can measure both γs, and reconstruct the π° mass: E1E1 E2E2

E e, GeVp Ee’Ee’  p, deg  BigCal  γ  range π° θ C.M, Kinematics of π° production  GEp-2gamma:  dependence of R at 2.5 GeV 2  GEp-3: high Q 2 measurements E e, GeVp Ee’Ee’  p, deg  BigCal  γ  range π° θ C.M The polarization component of π° production has not been measured in this energy region. The polarization components of π° production was measured by Hall A experiment in range of 0.8GeV<E γ <4.0GeV. (K. Wijesooriya et al, Phys.Rev.C66: (2002)) Note: polarization of π° photo- and electro-production was expected and measured to be the same in the Hall A experiment.

3σ Cut SIMC simulation of π° photo-production 20cm(15cm) LH2 long target is equivalent to a 2.3% radiator. Assuming the photon comes from the bremsstrahlung reaction. An example of the data and the simulation comparison at E beam =2.839GeV, Q 2 =2.5GeV 2 Note: SIMC spectrum was normalized to the elastic peak of the data. The normalization parameters applied to elastic and π° are different. Goals of elastic and π° simulation: ep elastic simulation: radiation tail under inelastic spectrum ep elastic + π° simulation: π° events under elastic peak. Target wall cut performed

3σ Cut SIMC simulation of π° photo-production E e, GeV (GeV)  γ  range (GeV) π° θ C.M, R1 (%) R2 (%) Applied all the other elastic cuts(3σ) except pmiss_p, then evaluate the number of events left under the elastic peak. Estimated ratio of number of elastic tail events under π° events. Estimated ratio of number of π° events under elastic peak.

Preliminary result Note: Target wall cut and anti-elastic cut applied to the analysis. Only FPP1 data was analyzed. With the high statistic of π° data, we can split the data into several bins for each kinematics. Px must be reversed to be compared with the Hall A data. Same method applied to π° data as the ep elastic analysis. Helicity sum : Helicity diff : The asymmetry is related to the induced polarization. In Born approximation, Py = 0 for ep elastic ;

PRELIMINARY E γ (GeV)

PRELIMINARY E γ (GeV)

PRELIMINARY E γ (GeV)

PRELIMINARY θ C.M.

PRELIMINARY θ C.M.

Conclusion To do: Analysis of FPP2 will be done and that will increase the statistic; Subset of events at Q 2 =8.5GeV 2 and Q 2 =2.5GeV 2 lowest є which can be used to do π° identification; ep elastic correction False asymmetry correction to Py Systematic error  π° photo-production simulation agrees well with the data.  Measured π° production polarization components agrees with the Hall A data in GEp-2γ kinematics.  First measurement of π° production polarization components at E γ = GeV.