Baryon Spectroscopy at Jefferson Lab What have we learned about excited baryons?

Transcription

1 Baryon Spectroscopy at Jefferson Lab What have we learned about excited baryons? Volker Credé Florida State University, Tallahassee, FL Spring Meeting of the American Physical Society Atlanta, Georgia, 4//22

2 Outline Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions 4

3 Outline Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions 4

4 Non-Perturbative QCD Quarks, QCD, and Confinement Structure of Baryon Resonances Courtesy of Craig Roberts, Argonne How does QCD give rise to hadrons? Interaction between quarks fairly unknown throughout > 98 % of a hadron s volume. Explaining the excitation spectrum of hadrons is central to our understanding of QCD in the low-energy regime (Hadron Models, Lattice QCD, etc.) Complementary to Deep Inelastic Scattering (DIS) where information on collective degrees of freedom is lost.

5 Non-Perturbative QCD Quarks, QCD, and Confinement Structure of Baryon Resonances How does QCD give rise to hadrons? What is the origin of confinement? 2 How are confinement and chiral symmetry breaking connected? 3 Would the answers to these questions explain the origin of 99 % of observed matter in the universe? Baryons: What are the fundamental degrees of freedom inside a proton or a neutron? How do they change with varying quark masses? CQM CQMflux tubes Quark diquark clustering

6 Quarks, QCD, and Confinement Structure of Baryon Resonances Components of the Experimental N Program The excited baryon program has two main components: Probe resonance transitions at different distance scales Electron beams are ideal to measure resonance form factors and their corresponding Q 2 dependence. Provides information on the structure of excited nucleons and on the confining (effective) forces of the 3-quark system. Establish the systematics of the spectrum Current medium-energy experiments use photon beams to map out the baryon spectrum (JLab, ELSA, MAMI, SPring-8, etc.). Provides information on the nature of the effective degrees of freedom in strong QCD and also addresses the issue of previously unobserved or so-called missing resonances.

7 A /2 ( -3 GeV -/2 ) Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Helicity Amplitudes for the Roper Resonance sign change LCQM Q 3 G Q 2 (GeV 2 ) S /2 ( -3 GeV -/2 ) N N (prel.) N, N Q 2 (GeV 2 ) Consistency between both channels (Nππ, Nπ): sign change, magnitude,... At short distances (high Q 2 ), Roper behaves like radial excitation. Low Q 2 behavior not well described by LF quark models: e.g. meson-baryon interactions missing Gluonic excitation likely ruled out! Data from CLAS A /2 and S /2 amplitudes: e.g. I. Aznauryan et al., PRC 78, 4529 (28); PRC 8, 5523 (29). Quark-model calculations: - q 3 G hybrid state - q 3 radial excitation - - -

8 Quarks, QCD, and Confinement Structure of Baryon Resonances Helicity Amplitudes for γ p N(52)D 3 Transition A /2 ( -3 GeV -/2 ) -5 A 3/2 ( -3 GeV -/2 ) 5 S /2 ( -3 GeV -/2 ) Q 2 (GeV 2 ) Q 2 (GeV 2 ) Q 2 (GeV 2 ) There is clear evidence for helicity switch from λ = 3/2 (at photon point) to λ = /2 at high Q 2 : Rapid change in helicity structure when going from photo- to electroproduction of a nucleon resonance Stringent prediction of the CQM! -.75 N(52)D A /2 2 A 3/2 2 Q 2 (GeV 2 ) A hel = A /2 2 A 3/2 2 A hel L. Tiator et al.

9 Quarks, QCD, and Confinement Structure of Baryon Resonances Components of the Experimental N Program The excited baryon program has two main components: Probe resonance transitions at different distance scales Electron beams are ideal to measure resonance form factors and their corresponding Q 2 dependence. Provides information on the structure of excited nucleons and on the confining (effective) forces of the 3-quark system. Establish the systematics of the spectrum Current medium-energy experiments use photon beams to map out the baryon spectrum (JLab, ELSA, MAMI, SPring-8, etc.). Provides information on the nature of the effective degrees of freedom in strong QCD and also addresses the issue of previously unobserved or so-called missing resonances.

10 3 Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Spectrum of Nucleon Resonances S. Capstick and N. Isgur, Phys. Rev. D34 (986) 289 Perhaps only the tip of the iceberg has been discovered? ** 25 *** **** ** **** **** Mass [MeV] 2 5 * S *** **** ** **** ** ** **** 2. Excitation Band: (56, 2 ), (56, 2 2 ) (7, 2 ), (7, 2 2 ) ( ) (2, 2 )? * ** S S *** **** **** **** ****. Excitation Band: (7, ) Theory Experiment **** J π /2 3/2 5/2 7/2 9/2 /2 3/2 /2-3/2-5/2-7/2-9/2- /2-3/2-

11 3 Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Spectrum of Nucleon Resonances S. Capstick and N. Isgur, Phys. Rev. D34 (986) 289 Perhaps only the tip of the iceberg has been discovered? ** 25 *** **** ** **** **** Mass [MeV] 2 5 * S *** **** ** **** ** ** **** 2. Excitation Band: (56, 2 ), (56, 2 2 ) (7, 2 ), (7, 2 2 ) ( ) (2, 2 )? * ** S S *** **** **** **** ****. Excitation Band: (7, ) Theory Experiment **** J π /2 3/2 5/2 7/2 9/2 /2 3/2 /2-3/2-5/2-7/2-9/2- /2-3/2-

12 Quarks, QCD, and Confinement Structure of Baryon Resonances Excited-State Baryon Spectroscopy from Lattice QCD R. Edwards et al., Phys. Rev. D 84, 7458 (2) Missing states? (7) N(938) (232) (62) m π = 4 MeV Exhibits broad features expected of SU(6) O(3) symmetry Counting of levels consistent with non-rel. quark model, no parity doubling

13 Outline Introduction Electromagnetic Probes Mission Goal: Complete Experiments Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions 4

14 Electromagnetic Probes Mission Goal: Complete Experiments E γ [GeV] Reaction Thresholds 3, W [GeV] In addition: LEGS SPring γp pηη γp pπ ω γp pπ η γp pη γp pπππ γp pππ γp pπ 2, γp KΛ KΣ,.9.7 Common efforts at ELSA, JLab, and MAMI (Double-)polarization measurements, γp & γn reactions, etc.., ELSA CLAS MAMI-C GRAAL

15 Electromagnetic Probes Mission Goal: Complete Experiments Extraction of Resonance Parameters Double-polarization measurements Measurements off neutron and proton to resolve isospin contributions: A(γN π, η, K) I=3/2 2 A(γN π, η, K) I=/2 N Re-scattering effects: Large number of measurements (and reaction channels) needed to extract full scattering amplitude. Coupled Channels Maid Said BoGa... Jülich, Gießen, EBAC, etc.

16 Electromagnetic Probes Mission Goal: Complete Experiments The CLAS Spectrometer at Jefferson Laboratory More talks on CLAS results in Session Q g8b linear beam polarization FROST double polarization

17 Electromagnetic Probes Mission Goal: Complete Experiments Double-Polarization: Frozen Spin Targets Horizontal cryostat with integrated solenoid to freeze the proton spin. DNP at high B-field of 5 T Holding Mode: B T, T 3 mk, τ 2 h 2 Carbon Target Holding Magnets (P z 8 %) Race-track coil - Dipole Magnet Polyethlene Butanol 2 Solenoid

18 Electromagnetic Probes Mission Goal: Complete Experiments Why are Polarization Observables Important? From π threshold up to (232) region d σ dω = σ { δ l Σ cos 2φ Λ x ( δ l H sin 2φ δ F ) s- & p-wave approximation Λ y ( T δ l P cos 2φ) Fermi-Watson Theorem Λ z ( δ l G sin 2φ δ E)} Two observables sufficient, e.g. dσ/dω, Σ. Chiang & Tabakin, Phys. Rev. C55, 254 (997) 2 Above the ππ threshold More observables needed. In order to determine the full scattering amplitude without ambiguities, one has to carry out eight carefully selected measurements: four double-spin observables along with four single-spin observables. Eight well-chosen measurements are needed to fully determine production amplitudes F, F 2, F 3, and F 4.

19 Electromagnetic Probes Mission Goal: Complete Experiments Why are Polarization Observables Important? From π threshold up to (232) region d σ dω = σ { δ l Σ cos 2φ Λ x ( δ l H sin 2φ δ F ) s- & p-wave approximation Λ y ( T δ l P cos 2φ) Fermi-Watson Theorem Λ z ( δ l G sin 2φ δ E)} Two observables sufficient, e.g. dσ/dω, Σ. Chiang & Tabakin, Phys. Rev. C55, 254 (997) 2 Above the ππ threshold More observables needed. In order to determine the full scattering amplitude without ambiguities, one has to carry out eight carefully selected measurements: four double-spin observables along with four single-spin observables. Eight well-chosen measurements are needed to fully determine production amplitudes F, F 2, F 3, and F 4.

20 Electromagnetic Probes Mission Goal: Complete Experiments Complete Experiment in K Λ Photoproduction Photon beam Target Recoil Target - Recoil x' y z x' x' x' y y y z z z' x y z x y z x y z x y z unpolarized T P T x L x T z L z CLAS: 6 observables linearly P H P G O x T O z L z C z T z E F L x C x T x circular P F E C x C z O z G H O x Chiang & Tabakin, Phys. Rev. C55, 254 (997) In order to determine the full scattering amplitude without ambiguities, one has to carry out eight carefully selected measurements: four double-spin observables along with four single-spin observables. Eight well-chosen measurements are needed to fully determine production amplitudes F, F 2, F 3, and F 4.

21 Electromagnetic Probes Mission Goal: Complete Experiments Helicity-Dependent Cross Section for γ p p η η count rate difference [µb] σ tot 2 5 E γ [GeV] (535)S 5 - /2 (72)P 3 (27)D 5 3/2-5/2 ρ-ω W [GeV] CBELSA/TAPS Collaboration E γ E = σ /2 σ 3/2 σ /2 σ 3/2

22 Electromagnetic Probes Mission Goal: Complete Experiments Helicity-Dependent Cross Section for γ p p η CBELSA/TAPS Collaboration E γ E = σ /2 σ 3/2 σ /2 σ 3/2

23 Electromagnetic Probes Mission Goal: Complete Experiments Helicity-Dependent Cross Section for γ p p η η count rate difference B. Morrison et al. [CLAS Collaboration] E E XXXXXXXXXXXXXX cos θ η cos θ η Very preliminary: Data are positive CBELSA/TAPS Collaboration E γ

24 Outline Introduction Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions 4

25 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Isospin Filter: γ p N (I = /2) p ω A. Wilson et al. [CBELSA/TAPS], Ph.D. thesis CBELSA/TAPS CLAS (29) dσ/dω dσ/dω [ µb [µb/sr] / ] Preliminary M. Williams et al. [CLAS Collaboration] E γ [ GeV ] Good agreement between experiments Excellent statistics Great progress in our understanding of the differences between experiments.

26 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Baryon Resonances in the Reaction γ p p ω M. Williams et al. [CLAS Collaboration], Phys. Rev. C 8, 6529 (29) PWA fit includes resonances t-channel amplitudes. Strong evidence for (W < 2 GeV): (3/2) N(7) (5/2) N(68) Only nucleon resonances can contribute (isospin filter) First-time PWA of ω photoproduction channel High statistics data sets are key to pull out signals. CLAS at JLab can provide statistics, but there are also limitations in the acceptance.

27 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Baryon Resonances in the Reaction γ p p ω M. Williams et al. [CLAS Collaboration], Phys. Rev. C 8, 6529 (29) PWA fit includes resonances t-channel amplitudes. Strong evidence for (W > 2 GeV): (5/2) N(68) (5/2) N(95) (7/2) N(29) J π /2 3/2 5/2 7/2

28 Polarization Observables in γ p p ω Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Asymmetry Σ for γp p ω (P. Collins et al., CUA) Strong evidence for (W > 2 GeV): (5/2) N(68) (5/2) N(95) (7/2) N(29) Oh et al. Paris et al. Sarantsev et al.

29 Beam Asymmetry Σ in γ p p π Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions d σ dω = σ { δ l Σ cos 2φ Λ x ( δ l H sin 2φ δ F ) Λ y ( T δ l P cos 2φ) Λ z ( δ l G sin 2φ δ E)} SAID MAID CLAS (E γ < 2 GeV,.85 < cosθ π <.35) Serious discrepancies between models and data above.4 GeV. Photoproduction of π mesons still not very well understood. M. Dugger (ASU), CLAS g8b run group, to be published

30 Beam Asymmetry Σ in γ p p π Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions SAID MAID CLAS (E γ < 2 GeV,.35 < cosθ π <.85) Combination of p π and n π final states can help distinguish between and N resonances: M. Dugger (ASU), CLAS g8b run group, to be published

31 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Beam Asymmetry Σ in γ p p π and γp n π M. Dugger (ASU), CLAS g8b run group, to be published

32 Helicity Difference E in γp n π Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions S. Strauch et al., CLAS (FROST) X

33 Helicity Difference E in γp n π Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions S. Strauch et al., CLAS (FROST) X

34 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Beam-Target Polarization Observables in γ p p ππ I = I {( Λ i P ) δ (I Λ i P ) δ l [ sin 2β ( I s Λ i P s ) cos 2β ( I c Λ i P c )]} W. Roberts et al., Phys. Rev. C 7, 552 (25) N*, * Double-Meson Final States (5 Observables) At higher excitation energies: Multi-meson final states important. γ Proton π π mmm Clea N* *, Search for states in decay cascades!

35 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Photoproduction of ππ Pairs off the Proton: Kinematics Two mesons in the final state require 5 independent variables! For example: E γ, Θ c.m., φ, θ, M p meson Single-meson reactions: p-meson system in the reaction plane Two-meson reactions: Reaction and decay plane form angle φ

36 I Introduction Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions I s in γ p p π π from JLab: < E γ < 5 MeV I s -. < cos( θπ ) < < cos( θπ ) < Data < cos( θπ of ) < unprecedented < cos( ) < -.6 θπ stat. -.6 quality < cos( ) < - θπ Here: 3-dim. phasespace (E γ, θ π, φ π ) - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( ) <.7 θπ.7 < cos( θπ Preliminary ) <.8.8 < cos( θπ ) <.9 C. Hanretty et al., CLAS-g8b run group, under review.9 < cos( θπ -2 ) <. φ π

37 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions Beam Asymmetry I in γ p p π π from JLab I Analysis of butanol target challenging:.2 Determination of dilution factor (event-based dilution factors possible) W =.7 [GeV] W =.75 [GeV] W =.7 [GeV] Topology : γ p -> p π ( π- ) Butanol data Bound Nucleon data Proton data Miss-Mass [MeV] W =.9 [GeV] Preliminary W = 2. [GeV] 2 4 S. Park et al., CLAS (FROST), to be published φ π CLAS-FROST (butanol) data Phys. Rev. Lett. 95, 623 (25) (CLAS Collaboration) The interpretation of these ππ data has only just begun.

38 Outline Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π, η, and ω Mesons Observables in the Photoproduction of Two Pions 4

39 Our understanding of baryon resonances has made great leaps forward. There is good evidence that most of the known states (listed in the PDG) will be confirmed in photoproduction and that new states will be revealed: The current (6 GeV) N program at Jefferson Laboratory is almost complete, but the data need to be analyzed in the coming years. Goal of performing (almost) complete experiments has been (almost) achieved; program on neutron ongoing. Baryon spectroscopy at Jefferson Lab will continue in the 2 GeV era (e.g. Ξ, Ω states): GlueX and CLAS2 Conclusions Advances in both theory and experiment will allow us to finally understand QCD and confinement.

40 I s in γ p p π π from JLab: < E γ < 5 MeV I I s -. < cos( θπ ) < < cos( θπ ) < Data < cos( θπ of ) < unprecedented < cos( ) < -.6 θπ stat. -.6 quality < cos( ) < - θπ Here: 3-dim. phasespace (E γ, θ π, φ π ) - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

41 I s in γ p p π π from JLab: 5 < E γ < 2 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

42 I s in γ p p π π from JLab: 2 < E γ < 25 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

43 I s in γ p p π π from JLab: 25 < E γ < 3 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

44 I s in γ p p π π from JLab: 3 < E γ < 35 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

45 I s in γ p p π π from JLab: 35 < E γ < 4 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

46 I s in γ p p π π from JLab: 4 < E γ < 45 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

47 I s in γ p p π π from JLab: 45 < E γ < 5 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

48 I s in γ p p π π from JLab: 5 < E γ < 55 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

49 I s in γ p p π π from JLab: 55 < E γ < 6 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

50 I s in γ p p π π from JLab: 6 < E γ < 65 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

51 I s in γ p p π π from JLab: 65 < E γ < 7 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

52 I s in γ p p π π from JLab: 7 < E γ < 75 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

53 I s in γ p p π π from JLab: 75 < E γ < 8 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

54 I s in γ p p π π from JLab: 8 < E γ < 85 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

55 I s in γ p p π π from JLab: 85 < E γ < 9 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

56 I s in γ p p π π from JLab: 9 < E γ < 95 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

57 I s in γ p p π π from JLab: 95 < E γ < 2 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

58 I s in γ p p π π from JLab: 2 < E γ < 25 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π

59 I s in γ p p π π from JLab: 25 < E γ < 2 MeV I I s -. < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < - - < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) < < cos( θπ ) <. s X. < cos( θπ ) <.. < cos( θπ ) <.2.2 < cos( θπ ) <.3.3 < cos( θπ ) <.4.4 < cos( θπ ) < < cos( θπ ) <.6.6 < cos( θπ ) <.7.7 < cos( θπ ) <.8.8 < cos( θπ ) <.9.9 < cos( θπ ) <. C. Hanretty et al., CLAS-g8b run group, under review -2 φ π