Exclusive π 0 electroproduction in the resonance region. Nikolay Markov, Maurizio Ungaro, Kyungseon Joo University of Connecticut Hadron spectroscopy meeting September 26, 2009

2 Outline Motivation Experiment Analysis Results Conclusion

The experimental N* program has as main component the accurate measurements of transition form factors (A 3/2, A 1/2, S 1/2 ) of known states as function of the photon virtuality (Q 2 ) to probe their internal structure and confining mechanism Exclusive pion electroproduction from protons is proven to be an especially sensitive tool for the study of the transition from the hadronic picture to the quark-gluon picture of nucleon resonance excitations. Motivation 3 e e’ γvγv N N’N’ N*,△ A 3/2, A 1/2, S 1/2 M l+/-, E l+/-, S l+/- 

E1e run Beam energy: GeV Beam polarization: ~ 70% Current: 10nA Target: Liquid Hydrogen, thickness 2 cm, radius 0.2 – 0.6 cm Torus current: 2250 A Mini-torus current: 6000 A Data taking period: – Number of triggers: 1.5*10 9 4

Data analysis: procedure  Particle ID Electron ID Proton ID  Good runs selection  Electron momentum correction  Proton momentum correction  π 0 selection: BH subtraction  Cherenkov cut efficiency  Simulation and acceptance correction  Radiative and bin centering correction  Systematic studies 5  Normalization Elastic process Inclusive process

Electron ID X versus Y cut Minimum momentum cut P el > GeV Sampling fraction cut Number of photoelectrons E inner > 50MeV Particle ID 6 NPE X, cm Y, cm P, GeV Etot/P

Electron fiducial cut Momentum and sector dependent The regions of uniform acceptance: TOF inefficiencies Particle ID Sector 5 7 φ θ θ θ φ φ P, GeV θ

Proton ID Particle ID Timing correction For the loosely identified protons 8 T measured - T theoretical for each scintillator calculated Resulting correction is then applied to the events. Based on the ΔT = T measured -T theoretical -2ns < ΔT < 4ns P p,GeV M 2,GeV 2 P, GeV ΔT, ns P p,GeV M 2,GeV 2 P p,GeV Δ T, s P p,GeV β

Proton fiducial cut Particle ID 0.4 < P < 0.6 GeV, Sector 1 8 bins in momentum 0.0 GeV < P < 0.4 GeV 0.4 GeV < P < 0.6 GeV 0.6 GeV < P < 0.8 GeV 0.8 GeV < P < 1.0 GeV 1.0 GeV < P < 1.2 GeV 1.2 GeV < P < 1.4 GeV 1.4 GeV < P < 1.6 GeV 1.6 GeV < P < 1.8 GeV θ bins

Electron momentum correction Momentum correction  Kinematical coverage of elastic events is quite different from π 0,while BH lies in similar region; Since we have good statistics in the elastic region, they will be used as additional W bin with its own kinematics. Overview and reaction selection 10 BH selection, pre- and post-radiative processes Preradiative BH selection, emitted photon is aligned with the beam direction. θ P, GeV ep -> epX all BH pre-radiative post-radiative mm 2 Elastic eventsBH eventsπ 0 events Electron kinematics coverage in case of

Electron momentum correction Momentum correction 11 Elastic events BH events Peak position σ Data Corrected Peak position σ Data Corrected is calculated for each event and stored in sector - W - θ e - φ e bins. Gaussian fit to obtain a peak position is performed and peak positions are fitted with 2 nd order polynomial a + bx + cx 2 as a function of φ e. Coefficients of the polynomial fit are interpolated as a function of W, giving as a correction to be applied. Correction applied

Based on the ratio of the number of events in the specific reaction in the run to the faraday cup charge for this run, 4 runs were excluded from the further consideration. Data set selection 12

Proton momentum correction Based on the ep->epπ 0 kinematics. 13 Based on the ratio of generated and reconstructed proton momentum. Energy loss Result, peak position Result, σ Data | Corrected Before correction After correction Uses electron momentum and angles and proton angles Before correction After correction φ φ ΔP/P P, GeV ΔP/P ΔP/P

BH separation 14 φ e – φ p θ 1p – θ p θ 2p – θ p φe-φpφe-φp φe-φpφe-φp φe-φpφe-φp Θ 1p – θ p mm 2, GeV 2 All events π 0 events

Binning and kinematical coverage Binning: ΔW = 25 MeV ΔQ 2 = 0.1 GeV 2 Δcosθ = 0.2 Δφ = 30 0 [15 o ] 15

Overview Using MAID 2007 model with radiative effects, 130M events were generated (10M data events) GSIM processing was based on e1e configuration GPP was used to include effects for:  DC wire inefficiency  TOF smearing and DC smearing Same reconstruction code was used for both data and simulation Same cuts applied to data and simulation 16 Simulation

TOF mass σ GPP 17 TOF mass 2, GeV 2 mm 2, GeV 2 simulation Simulation

Cherenkov cut efficiency Events with identifies electron (no CC cut) divided in the bins of P e, θ e and φ e, Fit the npe spectrum with the Poisson function. 18 Electrons Problem: Efficiency distribution for Sector 2 NPE ←φ→ ←θ→

Elastic -Elastic events with realistic radiative tail were generated -Momentum correction was applied to the simulation; -Electron or both electron and proton were detected in the final state. Normalization 19 Electron detection only Electron and proton detection Cross-sections comparison to Bosted parametereization Cross-sections ratio to Bosted parameterization Electron detection only Electron and proton detection Bosted parameterezation

Inclusive Events were generated using keppel_rad generator; Momentum correction, which uses the generated events as a precise measurements, were applied Radiative correction based on the ratio of keppel_rad/keppel_norad was applied; Bin centering correction based on the keppel_rad model was applied; Data is compared to the Keppel and Brasse parametrization Normalization Bin centering correction Radiative correction Result Data Brasse Keppel 20 μB/GeV 3

Acceptance correction < W< GeV, 0.4 < Q 2 < 0.5 GeV < W< GeV, 0.8 < Q 2 < 0.9 GeV < W< 1.55 GeV, 0.5 < Q 2 < 0.6 GeV < W< 1.75 GeV, 0.4 < Q 2 < 0.5 GeV 2 Corrections Acceptance

Radiative correction Bremmstrahlung Vertex correction Vacuum polarization Radiative processes: exclurad code MAID 07 model as an input < W < GeV 0.5 < Q 2 < 0.6 GeV 2 22 Corrections

Bin centering correction leave the point, shift the value leave the value, shift the point < W < GeV 0.5 < Q 2 < 0.6 GeV 2 MAID 2007 Each W-Q 2 -θ-φ bin is divided in 10 sub-bins 23 Corrections

Cross-section 24 Data MAID 03 MAID < W < GeV 0.6 < Q 2 < 0.7 GeV 2 Statistical error only φ μb Results Preliminary

Data MAID 03 MAID 07 Structure functions extraction Very preliminary Statistical error only 25 φ < W < GeV 0.6 < Q 2 < 0.7 GeV 2 Results μb Preliminary

26 Structure functions Preliminary Data MAID 03 MAID 07 Statistical error only < W < GeV 0.6 < Q 2 < 0.7 GeV 2 Results μb

Overview 27 Sampling fractionElectron fiducial cut Proton fiducial cutMissing mass cutProton timing Vertex cut Systematics

28 Data regilar cut Dat strict Cut Data strictest Cut MAID 03 MAID < W < GeV 0.6 < Q 2 < 0.7 GeV 2 Electron fiducial cut θ φ Systematics μb Preliminary

29 N. Markov 29 Data regular cut Data strict Cut Data strictest Cut MAID 03 MAID < W < GeV 0.6 < Q 2 < 0.7 GeV 2 Missing mass cut mm 2, GeV 2 Systematics μb Preliminary

30 Preliminary cross-sections and structure functions were obtained in wide kinematic range with high statistics. Systematic studies are partially finished. Detailed partial wave analysis combined with other channels using JANR will be performed. Conclusions

31 N. Markov 31 More details

Narrow φ* binning 32 No BC and radiative corrections Data MAID 03 MAID 07 Statistical error only < W < GeV 0.6 < Q 2 < 0.7 GeV 2 φ bin size 15 o μb/sr*GeV 3

backup 33 Data 100% elastic events 90% elastics events Elastic generated events