4th Workshop on Exclusive Reactions at High Momentum Transfer Beam Spin Asymmetry of the electroproduction of a Δ+ resonance and a photon on the proton with the CLAS detector Brahim Moreno 4th Workshop on Exclusive Reactions at High Momentum Transfer 05/18/2010 Jefferson Lab

4th Workshop on Exclusive Reactions at High Momentum Transfer Beam Spin Asymmetry of the electroproduction of a Δ+ resonance and a photon on the proton with the CLAS detector Introduction Analyses Results Conclusion 4th Workshop on Exclusive Reactions at High Momentum Transfer 05/18/2010 Jefferson Lab

Introduction Deeply Virtual Compton Scattering Processes Experimental context Experiment

Example of DVCS In the limit where: High and fixed AND low

Example of DVCS In the limit where: High and fixed AND low hard soft Handbag diagram

Example of DVCS hard soft In the limit where: High and fixed AND low Handbag diagram 2 DVCS BH +

Interference between the DVCS and Bethe-Heitler processes 2 DVCS BH + Interference between the DVCS and Bethe-Heitler processes Polarized lepton beam Beam Spin Asymmetry (BSA) leptonic plane hadronic plane parameterized as a function of Ф parameterization involving combinations of GPD BSA sensitive to GPDs

The ΔVCS process N-∆ transition GPD hard soft In the limit: high Q2, xb fixed t/Q2→0 et W2/Q2→0 N-Δ GPDs carries the same type of information as nucleon GPDs L.L. Frankfurt, M. V. Polyakov,M. Strikman, M.Vanderhaeghen Phys. Rev. Lett. 84: 2589-2592 (2000) P.A.M Guichon, L.Mossé, M.VanderhaeghenPhys. Rev. D68 034018 (2003)

In the limit: high Q2, xb fixed The ΔVCS process N-∆ transition GPD hard soft In the limit: high Q2, xb fixed t/Q2→0 et W2/Q2→0 Partonic description of the N-Δ transition In the "large Nc" approximation: N-Δ GPDs linked to nucleon GPDs

Interference between the ΔVCS and Bethe-Heitler processes 2 BH + ΔVCS Interference between the ΔVCS and Bethe-Heitler processes Calculations performed in the "large Nc" limit Experiment Non-resonant contributions taken into account BSA of ep → eNπγ (calculations) P.A.M Guichon, L.Mossé, M.Vanderhaeghen Phys. Rev. D68 034018 (2003)

Introduction Deeply Virtual Compton Scattering Processes Experimental context Experiment

Experimental context (DVCS) First observation (2001) Results of the 1st dedicated experiment (2006) Results of the 1st Hall B dedicated experiment (2008) (2007) (2009) (2003) (2005) CLAS (1) HERMES (2) H1 (3) Hall A (6) Hall A (7) CLAS (8) t (year) CLAS (9-10) COMPASS (11) ZEUS (4) H1 (5) Publications: (1) S. Stepanyan et al. (CLAS Collaboration) Phys. Rev. Lett. 87, 182002 (2001) (2) A. Airapetian et al. (HERMES Collaboration) Phys. Rev. Lett. 87, 182001 (2001) (3) C. Adloff et al. (H1 Collaboration) Phys. Lett. B 517, 47 (2001) (4) S. Chekanov et al. (ZEUS Collaboration) Phys. Lett. B 573, 46 (2003) (5) C. Aktas et al. (H1 Collaboration) Eur. Phys. J. C 44, 1 (2005) (6) C. Muñoz Camacho et al. (Hall A Collaboration) Phys. Rev. Lett. 97, 262002 (2006) (7) M. Mazouz et al. (Hall A Collaboration) Phys. Rev. Lett. 99, 242501 (2007) (8) F.X. Girod et al. (CLAS Collaboration) Phys. Rev. Lett. 100, 162002 (2008) (9) Proposal: http://www.jlab.org/exp_prog/proposals/06/PR06-003.pdf (10) Proposal: http://www.jlab.org/exp_prog/proposals/05/PR05-114.pdf (11) http://wwwcompass.cern.ch/compass/gpd/index.html ... Collaborations: CLAS (Jlab, USA) HERMES (DESY, Germany) H1 (DESY, Germany) ZEUS (DESY, Germany) HALL A (Jlab, USA) COMPASS (CERN)

Experimental context (ΔVCS) 1st attempt at measuring ΔVCS (2004) This analysis (2001) t (year) First observation of DVCS CLAS (2009) S. Bouchigny PhD thesis Collaborations: CLAS Publications:

Experimental context (ΔVCS) 1st attempt at measuring ΔVCS (2004) This analysis (2001) t (year) First observation of DVCS CLAS (2009) S. Bouchigny PhD thesis ep→eΔ+γ→enπ+(γ) ep→eΔ+γ→enπ+γ ep→eΔ+γ→epπ0γ non-detection of one of the final state particles makes it difficult to separate the signal from the background Detection of ALL final state particles required Analysis based on data of the first Hall B experiment dedicated to DVCS

Experimental context (ΔVCS) 1st attempt at measuring ΔVCS (2004) This analysis (2001) t (year) First observation of DVCS CLAS (2009) S. Bouchigny PhD thesis ep→eΔ+γ→enπ+γ ep→eΔ+γ→epπ0γ →epγγγ ep→eΔ+γ→enπ+(γ) Goal: FEASIBILITY study If possible: extraction of the beam spin asymmetry EXPLORATORY ANALYSIS

Introduction Deeply Virtual Compton Scattering Processes Experimental context Experiment

Thomas Jefferson Laboratory (Virginia, USA) Hall B Hall A Hall C Hall B

photon detector for γ/π0 separation (DVCS) The CLAS detector Torus magnet Electromagnetic calorimeters (EC, LAC) Drift chambers (DC) to detect showering particles to determine charged particles trajectories angular acceptance : Time of flight counters (TOF) Inner calorimeter (IC) angular acceptance: Cherenkov counters photon detector for γ/π0 separation (DVCS) for e/π separation

The e1-dvcs experiment FIRST Hall B experiment dedicated to DVCS Duration: from March to May 2005 Ebeam = 5.776 GeV Beam polarization: 75-80 % Beam current: 20-25 nA Target: lH2 (2.5 cm) Integrated luminosity: 3.3×107 nb-1 FIRST Hall B experiment dedicated to DVCS

4th Workshop on Exclusive Reactions at High Momentum Transfer Beam Spin Asymmetry of the electroproduction of a Δ+ resonance and a photon on the proton with the CLAS detector Introduction Analyses Results Conclusion 4th Workshop on Exclusive Reactions at High Momentum Transfer 05/18/2010 Jefferson Lab

ep→eNπγ in the Δ+ region Analyses ep→eNπγ in the Δ+ region ep→epπ0γ ep→enπ+γ Identification of eNπγX events epπ0γX enπ+γX Background subtraction (ππ)

Identification of ep→epπoγX → epγγγX events ALL final state particles detected 1 electron, 1 proton, 3 photons 1 Projected missing momentum Missing energy Missing mass 1 2 3 4 peak at 0 GeV 2 X 3 peak at 0 GeV2 4 Sideband method used to subtract combinatorial background under πo mass peak Before selection After selection

Identification of ep→enπ+γX events ALL final state particles detected 1 electron, 1 neutron, 1 π+, 1 photon 1 1 2 Peak at 0 GeV 2 X Projected missing momentum 3 Peak at 0 GeV2 4 3 4 Missing mass Missing energy Before selection After selection

Event selection : definition of the Δ+ region IMNπ invariant mass spectra after selection ep→epπ0γX 3rd region 2nd region ep→enπ+γX Δ+ Δ+

ep→eNπγ in the Δ+ region Analyses ep→eNπγ in the Δ+ region ep→epπ0γ ep→enπ+γ Identification of eNπγX events epπ0γX enπ+γX Background subtraction (ππ)

ππ background subtraction Background (B) Final state (S) Background Calculated and subtracted by using both SIMULATED and EXPERIMENTAL double pions production data Same procedure for both ep→eNπγ channels

ππ background subtraction Background (B) Final state (S) S+B S B S+B S B Experimental data Simulated data 1 photon detected all photons detected

Estimate of the contamination induced by ep→eNππ R= percentage of selected eNπγX events estimated to be coming from ep→eNππ Contamination of enπ+γX events Contamination of epπ0γX events 45% on average 30% on average

4th Workshop on Exclusive Reactions at High Momentum Transfer Beam Spin Asymmetry of the electroproduction of a Δ+ resonance and a photon on the proton with the CLAS detector Introduction Analyses Results Conclusion 4th Workshop on Exclusive Reactions at High Momentum Transfer 05/18/2010 Jefferson Lab

Results Beam spin asymmetry Interpretation

Beam spin asymmetry (BSA) of ep→eNπγ in the Δ+ region ep→epπ0γ ep→enπ+γ BSA Ф (deg) Q2 = 2.5 GeV2 W = 2.29 GeV t = -2 GeV2 xb = 0.38 Q2 = 2.44 GeV2 W = 2.37 GeV t = -1.63 GeV2 xb = 0.35 First experimental observation of an asymmetry for these channels Uncertainties shown are statistical

Results Beam spin asymmetry Interpretation

Experimental context (ΔVCS) 1st attempt at measuring ΔVCS (2004) DIS2009 HERMES This analysis (2001) (2009) t (year) First observation of DVCS CLAS First observation No data to compare with Collaborations: CLAS HERMES Publications:

Data interpretation BSA BSAs of opposite signs: ep→enπ+γ ep→epπ0γ BSA Ф (deg) BSA amplitude of the order of 20% pour ep→epπ0γ BSA amplitude of the order of 25% pour ep→enπ+γ First sign of observation of an interference between the ΔVCS and Bethe-Heitler processes Both channels are of different nature: the contribution from non-resonant events are not the SAME in both cases Data intergrated over the whole phase space: t/Q² ≈ 0, so non-leading-order terms may have a significant contribution

4th Workshop on Exclusive Reactions at High Momentum Transfer Beam Spin Asymmetry of the electroproduction of a Δ+ resonance and a photon on the proton with the CLAS detector Introduction Analyses Results Conclusion 4th Workshop on Exclusive Reactions at High Momentum Transfer 05/18/2010 Jefferson Lab

Conclusion Exploratory analysis of ep→eNπγ channels in the Δ+ region Feasibility demonstrated Extraction of the corresponding BSA Observation of an asymmetry of the order of twenty percent Around 20% ep→epπ0γ Around 25% ep→enπ+γ May be interpreted as the sign of an interference between the ΔVCS and Bethe-Heitler processes Outlook ΔVCS experimental status similar to that of DVCS in 2001 Independent experimental confirmation needed Dedicated study conceivable

Back up

anti-quark distribution What is a GPD? Quark lines x+ξ, x-ξ : quarks longitudinal momentum fractions p, p’ : 4-momenta of the initial and final nucleon of opposite signs x + - anti-quark distribution quark distribution distribution

Describing the nucleon Parton Distribution Functions Form factors GPDs Correlation between transverse position and longitudinal momentum fraction of quarks in the nucleon Quarks longitudinal momentum fraction distribution in the nucleon Transverse distribution of quarks in space coordinates

Describing the nucleon GPDs can be interpreted as being the probability amplitude of finding a quark inside the nucleon at given transverse position and longitudinal momentum fraction GPD = hybrid of FF and PDF

Example of DVCS Beam polarized, unpolarized target: leptonic plane hadronic plane Beam polarized, unpolarized target: Unpolarized beam, longitudinally polarized target: Unpolarized beam, transversally polarized target: Beam charge asymmetry:

N-Δ GPDs Sum rules: N-Δ GPDs entering ΔVCS description: In the “large Nc” limit: P.A.M Guichon, L.Mossé, M.VanderhaeghenPhys. Rev. D68 034018 (2003)

Identification of ep→epπoγX events Background/contamination → π0γ Combinatorial background Electroproduction of : ep→ep→epπ0γ ep → epπ0γ→epγγγ in the Δ+ region ep→epπ0γ(γ) Double electroproduction of pions: ep→epπ0π0→ epπ0γ(γ) Analysis subject Being described

Identification of ep→epπoγX → epγγγX events ALL final state particles detected 1 electron, 1 proton, 3 photons 1 2 Missing mass Missing mass γD 1 π0 2 3 4 proton 3 Angular cut Missing mass 4 Before selection After selection

Identification of ep→epπoγX events Background/contamination → π0γ Combinatorial background Electroproduction of : ep→ep→epπ0γ ep → epπ0γ→epγγγ in the Δ+ region Invariant mass distribution IM3γ fitted Rejection of « -resonant » events Analysis subject Being described

Identification of ep→epπoγX events Background/contamination → π0γ Combinatorial background Electroproduction of : ep→ep→epπ0γ ep → epπ0γ→epγγγ in the Δ+ region ep→epπ0γ(γ) Double electroproduction of pions: ep→epπ0π0→ epπ0γ(γ) Analysis subject Being described

Combinatorial background subtraction 6 Combinatorial background subtraction 3 Sideband method 2nd region Signal S+N Noise

Identification of ep→epπoγX events Background/contamination → π0γ Combinatorial background Electroproduction of : ep→ep→epπ0γ ep → epπ0γ→epγγγ in the Δ+ region ep→epπ0γ(γ) Double electroproduction of pions: ep→epπ0π0→ epπ0γ(γ) Analysis subject Identification of ep→epπoγ events Being described

Identification of ep→enπ+γX events ALL final state particle detected 1 electron, 1 neutron, 1 π+, 1 photon Masse manquante 1 2 Masse manquante γ 1 π+ 2 3 4 neutron 3 Coupure angulaire Masse manquante 4 Before selection After selection

Identification of ep→enπ+γX events Background/contamination ep→enπ+γ in the Δ+ region ep→enπ+γ(γ) Double electroproduction of pions: ep→enπ+π0→ enπ+γ(γ) Analysis

Comparison between experimental/simulated ep→epγγγX data Experimental data after selection Simulated data after selection 75% ep→eΔ+γ→epπ0γ (phase space) 25% ep→epπ0π0 (phase space) Mixture coming from background study Simulated data

Comparison between experimental/simulated ep→enπ+γX data Experimental data after selection Simulated data after selection 55% ep→eΔ+γ→enπ+γ (phase space) 45% ep→enπ+π0 (phase space) Mixture coming from background study Simulated data

ep→epπ0γ phase space Q2 (GeV2) -t (GeV2) Q2 (GeV2) xb W (GeV) xb

ep→enπ+γX phase space Q2 (GeV2) Q2 (GeV2) -t (GeV2) xb W (GeV) xb

Experimental uncertainties: estimate ep→epπ0γ ep→enπ+γ Statistical uncertainty dominates