Understanding Excited Baryon Resonances: Results from polarization experiments at CLAS

Transcription

1 Understanding Excited Baryon Resonances: Results from polarization experiments at CLAS Volker Credé Florida State University, Tallahassee, FL JLab Users Group Workshop Jefferson Lab 6/4/24

2 Outline Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π and π + Mesons off the Proton 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 and π + Mesons off the Proton 4

4 Quarks, QCD, and Confinement Structure of Baryon Resonances Non-Perturbative Quantum Chromodynamics (QCD) QCD is the theory of the strong nuclear force which describes the interactions of quarks and gluons making up hadrons. Strong processes at larger distances and at small (soft) momentum transfers belong to the realm of non-perturbative QCD. Quarks are confined within hadrons. Confinement of quarks and gluons within nucleons is a non-perturbative phenomenon, and QCD is extremely hard to solve in non-perturbative regimes: Knowledge of internal structure of nucleons is still limited. This is particularly true for excited nucleons.

5 Non-Perturbative QCD Quarks, QCD, and Confinement Structure of Baryon Resonances How does QCD give rise to excited 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 98% of observed matter in the universe? Excited Baryons: What are the fundamental degrees of freedom inside a proton or a neutron? How do they change with varying quark masses?

6 3 Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Spectrum of N Resonances (PDG < 22) S. Capstick and N. Isgur, Phys. Rev. D34 (986) ** *** ** 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-

7 3 Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Spectrum of N Resonances (PDG < 22) 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-

8 Quarks, QCD, and Confinement Structure of Baryon Resonances Mass [MeV] Spectrum of N Resonances N J P (L 2I,2J ) N(44) /2 + (P ) N(52) 3/2 (D 3 ) N(535) /2 (S ) N(65) /2 (S ) ** N(675) 5/2 (D 5 ) 25 N(68) 5/2 + (F 5 ) N(685) *** N(7) 3/2 (D 3 ) N(7) /2 (P ) + N(72) 3/2 + (P 3 ) ** 2 N(86) 5/2 + ** * N(875) 3/2 * S ** ** N(88) S /2 + ** S N(895) /2 N(9) *** 3/2 + (P 3 ) *** 5 N(99) 7/2 + (F 7 ) N(2) 5/2 + (F 5 ) N(28) D 3 N(29) S N(24) 3/2 + N(26) 5/2 N(2) /2 + (P ) N(22) 3/2 J π /2+ 3/2+ 5/2+ 7/2+ 9/2+ /2+ 3/2+ N(29) /2-7/2 3/2- (G 7 5/2- ) 7/2-9/2- /2-3/2- N(22) D 5 V.C. & W. Roberts, Rep. Prog. Phys. 76 (23) N(222) 9/2 + (H 9 )

9 Quarks, QCD, and Confinement Structure of Baryon Resonances Mass [MeV] Spectrum of N Resonances N J P (L 2I,2J ) N(44) /2 + (P ) N(52) 3/2 (D 3 ) N(535) /2 (S ) N(65) /2 (S ) ** N(675) 5/2 (D 5 ) 25 N(68) 5/2 + (F 5 ) N(685) *** N(7) 3/2 (D 3 ) N(7) /2 (P ) + N(72) 3/2 + (P 3 ) ** 2 N(86) 5/2 + ** * N(875) 3/2 * S ** ** N(88) S /2 + ** S N(895) /2 N(9) *** 3/2 + (P 3 ) *** 5 N(99) 7/2 + (F 7 ) N(2) 5/2 + (F 5 ) N(28) D 3 N(29) S N(24) 3/2 + N(26) 5/2 N(2) /2 + (P ) N(22) 3/2 J π /2+ 3/2+ 5/2+ 7/2+ 9/2+ /2+ 3/2+ N(29) /2-7/2 3/2- (G 7 5/2- ) 7/2-9/2- /2-3/2- N(22) D 5 V.C. & W. Roberts, Rep. Prog. Phys. 76 (23) N(222) 9/2 + (H 9 )

10 Quarks, QCD, and Confinement Structure of Baryon Resonances Polarization Transfer in γp K + Λ : Cx, C z C x, C z without N(9)P 3 C x, C z with N(9)P N(9)P 3, N(2)F 5, N(99)F cos θ K Bonn-Gatchina PWA requires N(9)P 3 No quark-diquark oscillations! Both -.5 oscillators.5 need to be excited. cos θ K R. Bradford et al. [CLAS Collaboration], Phys. Rev. C 75, 3525 (27) Fits: BoGa-Model, V. A. Nikonov et al., Phys. Lett. B 662, 245 (28)

11 CLAS g9b GRAAL Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances Complete Experiment for K + Y :.65 < W < 2.2 GeV W + Target Asym. [.9, for K Λ (.9<W<.95.95] GeV) GeV γp K + Λ K + Σ W [2., 2.5] GeV + Target Asym. for K Σ (2.<W<2.5 GeV).5 TTx T T z cos θ cm W [.85,.9] GeV + T x for K Λ (.85<W<.9 GeV) W [.9,.95] GeV + T z for K Λ (.9<W<.95 GeV) T x cos θ cm W [.95, 2.] GeV + T x for K Σ (.95<W<2. GeV) cos θ cm N. Walford et al. [CLAS Collaboration], FROST run group cos θ cm BoGa MAID cos θ cm

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 = 396 MeV Exhibits broad features expected of SU(6) O(3) symmetry Counting of levels consistent with non-rel. quark model, no parity doubling

13 Gluonic Excitations on the Lattice Quarks, QCD, and Confinement Structure of Baryon Resonances J. J. Dudek and R. G. Edwards, Phys. Rev. D 85, 546 (22) m π = 524 MeV m π = 396 MeV m u = m d = m s m π = 72 MeV The mass scale is m m ρ for mesons and m m N for baryons. Common scale of.3 GeV for gluonic excitation, but hybrid baryons are difficult to identify experimentally.

14 Quarks, QCD, and Confinement Structure of Baryon Resonances Helicity Amplitudes for the Roper Resonance A /2 ( 3 GeV /2 ) Q 2 (GeV 2 ) S /2 ( 3 GeV /2 ) 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. V. Mokeev et al., PRC 86, 3523 (22); PRC 8, 4522 (29). Quark-model calculations: - q 3 radial excitation q 3 G hybrid state

15 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 and π + Mesons off the Proton 4

16 Electromagnetic Probes Mission Goal: Complete Experiments E γ [GeV] Reaction Thresholds 3, W [GeV] In addition: SPring-8 J-PARC.66.6 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

17 Electromagnetic Probes Mission Goal: Complete Experiments The CLAS Spectrometer at Jefferson Laboratory g8b linear beam polarization FROST double polarization

18 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.

19 Electromagnetic Probes Mission Goal: Complete Experiments Why are Polarization Observables Important? For single-meson production: γp pπ E γ [MeV] dσ 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)} Box 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. Box Eight well-chosen measurements are needed to fully determine production amplitudes F, F 2, F 3, and F 4.

20 Example: Ambiguities in γ p pπ Electromagnetic Probes Mission Goal: Complete Experiments Bonn-Gatchina (2-2) SAID (SN, CM2) MAID σ /2 Example: E = N N 2Λ z δ N + N σ 3/

21 Electromagnetic Probes Mission Goal: Complete Experiments Helicity Asymmetry E in γ p ELSA E E = σ /2 σ 3/2 σ /2 +σ 3/2 E γ [.6, 2.2] GeV CBELSA/TAPS Maid Said (CM2) BoGa (2_2) Angular distributions sensitive to interference between resonances. M. Gottschall et al., Phys. Rev. Lett. 2, 23 (24) cosθ π

22 Outline Introduction Photoproduction of π and π + Mesons off the Proton Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π and π + Mesons off the Proton 4

23 Asymmetry G in γ p ELSA Photoproduction of π and π + Mesons off the Proton dσ 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)} Surprisingly, π production also not well understood at lower energies: BoGa - - SAID... MAID - cosθ π A. Thiel et al. [CBELSA/TAPS Collaboration], Phys. Rev. Lett. 9, 2 (22)

24 Asymmetry G in γ p ELSA θ π = 9±5 θ π = 3±5 A. Thiel et al. [CBELSA/TAPS Collaboration], Phys. Rev. Lett. 9, 2 (22) Photoproduction of π and π + Mesons off the Proton dσ 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)} Surprisingly, π production also not well understood at lower energies. Below GeV, discrepancies can be traced to the E + and E 2 multipoles, which are related to certain resonances: E +: N(535) 2, N(65) 2, (62) 2 E 2 : N(52) 3 2, (7) 3 2

25 Photoproduction of π and π + Mesons off the Proton Beam Asymmetry Σ in γ p CLAS (g8b) SAID DU3. SAID CM2 MAID 7... BoGa 2-2 dσ dω = σ { δ l Σ cos 2φ} M. Dugger et al. [CLAS Collaboration], Phys. Rev. C 88, 6523 (23)

26 Photoproduction of π and π + Mesons off the Proton Beam Asymmetry Σ in γ p CLAS (g8b) SAID DU3. SAID CM2 MAID 7... BoGa 2-2 M. Dugger et al. [CLAS Collaboration], Phys. Rev. C 88, 6523 (23) Largest changes in SAID DU3 Improved mapping of dip near 6 Couplings of (7) 3 2 (95) 5 + 2

27 Photoproduction of π and π + Mesons off the Proton Beam Asymmetry Σ in γ p nπ CLAS (g8b) SAID DU3. SAID CM2 MAID 7... BoGa 2-2 dσ dω = σ { δ l Σ cos 2φ} M. Dugger et al. [CLAS Collaboration], Phys. Rev. C 88, 6523 (23)

28 Photoproduction of π and π + Mesons off the Proton Helicity Difference E in γ p nπ CLAS (FROST) SAID ST4. MAID 7 BoGa Jülich 4 + N 2 pπ : 3 I = 3 2, I 3 = 2 3 I = 2, I 3 = 2 nπ + : 3 I = 3 2, I 3 = I = 2, I 3 = 2 S. Strauch SC, under CLAS collaboration review

29 Photoproduction of π and π + Mesons off the Proton Target Asymmetry T in γ p nπ + (CLAS FROST) MAID 7 SAID BoGA 2 Early-stage results (g9b) Transverse Target Polarization M. Dugger (ASU), CLAS g9b run group cosθ c.m. cosθ c.m.

30 Photoproduction of π and π + Mesons off the Proton Observable F in γ p nπ + (CLAS FROST-g9b) MAID 7 SAID 2 BoGa not shown dσ dω =σ { δ l Σ cos 2φ Box + 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)} M. Dugger (ASU), CLAS g9b run group cosθ c.m. Transv. target pol. & circ. beam pol. Early-stage analysis Reasonable agreement among predictions for W <.7 GeV Much to learn at the higher energies

31 Photoproduction of π and π + Mesons off the Proton Overview of CLAS Polarization Measurements Measurements off the proton γp pπ, nπ + γp pη, pη (I. Senderovich, R. Tucker et al.) Frozen-Spin Target (FROST) γp pπ + π, pφ, pω (CMU, CU, Florida State) Measurements off the neutron γn pπ γn nπ + π HD Ice Target Complete experiment possible in Strangeness Photoproduction (S. Fegan, N. Walford et al.) γp K Y (Y = Λ, Σ, Σ + ) γn K + Σ, K Λ Cross sections (K. Moriya, 2:3 PM) (also Λ(45), Λ(52))

32 Photoproduction of π and π + Mesons off the Proton 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) Double-Meson Final States (5 Observables) At higher excitation energies: Multi-meson final states important. Search for states in decay cascades!

33 Photoproduction of π and π + Mesons off the Proton W [.74; -..77] < cos( θπ GeV, ) < -.9 cosθ p -.9 < c.m. cos( ) < θπ > < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < Polarization Beam Observable Asymmetry I I s s < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < < cos( θπ + ) < Data of unprecedented statistical quality.7 < cos( θπ + ) <.8.8 < cos( θπ + ) <.9.9 < cos( θπ + ) < I = I {( + Λ i P)+ δ (I + Λ i P ) δ - - l [ sin 2β( I s + Λ - i P s ) φ π φ π + cos 2β( I c + Λ i P c )]} Charles Hanretty (FSU), approved by CLAS collaboration

34 Photoproduction of π and π + Mesons off the Proton s Polarization Beam Asymmetry Observable I I s - Fix & Arenhoevel CLAS-g9b CLAS-g8b -. < cos( θπ + ) < -.8. < cos(θπ ) < < cos( θπ + W [.76;.77] GeV.2 < cos( θπ + ) < -.6 ) < < cos( θπ +.4 < cos( θπ + ) < -.4 ) < < cos( θπ +.6 < cos( θπ + ) < -.2 ) < < cos( θπ +.8 < cos( θπ + ) <. ) < φ π + Priyashree Roy (Florida State), CLAS g9b (FROST)

35 Photoproduction of π and π + Mesons off the Proton Polarization Observable I Fix & Arenhoevel W. Roberts (FSU) CLAS-g9a CLAS-gc W =.4 GeV W =.6 GeV W = GeV W = 2. GeV W =.45 GeV W =.65 GeV W = GeV W = 2.5 GeV φ π + W =.5 GeV.3 W =.7 GeV W = GeV W = 2. GeV W =.55 GeV W 2=.75 3 GeV W = GeV Data of unprecedented statistical quality. Butanol I = I {( + Λ i P)+ δ (I + Λ i P ).2 Butanol(wQ) Phys.Rev.Lett. FSU-Model -. A.Fix-Model δ l [ sin 2β( I s + Λ i P s ) φ π + Sungkyun Park (FSU), under collaboration review cos 2β( I c + Λ i P c )]}

36 Photoproduction of π and π + Mesons off the Proton.2 Fix and Arenhoevel W. Roberts (FSU) CLAS Data (g9a) W =.4 GeV W =.45 GeV W =.5 GeV W =.55 GeV Polarization Observable P z W =.6 GeV W = GeV W = 2. GeV W =.65 GeV W = GeV W = 2.5 GeV φ π + W =.7 GeV W = GeV W = 2. GeV W =.75 GeV W = GeV Data of unprecedented statistical quality. I = I {( + Λ i P)+ δ (I + Λ i P ).2 Butanol(wQ) FSU-Model -. A.Fix-Model δ l [ sin 2β( I s + Λ i P s ) φ π + Sungkyun Park (FSU), under collaboration review cos 2β( I c + Λ i P c )]}

37 Photoproduction of π and π + Mesons off the Proton Fix and Arenhoevel W. Roberts (FSU) CLAS Data (g9a) W =.4 GeV W =.45 GeV W =.5 GeV W =.55 GeV Polarization Observable P z W =.6 GeV W =.8 GeV W = 2. GeV W =.65 GeV W =.85 GeV W = 2.5 GeV φ π + W =.7 GeV W =.9 GeV W = 2. GeV W =.75 GeV W = GeV Data of unprecedented statistical quality.8 Butanol(wQ) I = I {( + Λ i P)+ δ (I + Λ i P ) FSU-Model A.Fix-Model δ l [ sin 2β( -.8 I s + Λ i P s ) φ π + cos 2β( I c + Λ i P c )]} Sungkyun Park (FSU), under collaboration review

38 Outline Introduction Quarks, QCD, and Confinement Structure of Baryon Resonances 2 Electromagnetic Probes Mission Goal: Complete Experiments 3 Photoproduction of π and π + Mesons off the Proton 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 also be confirmed in photoproduction and that new states will be revealed: Goal of performing (almost) complete experiments has been (almost) achieved; significant contributions from (double-)polarization experiments. Still too early to nail down relevant degress of freedom in excited baryons. Some states might be generated dynamically... N(86) N(875) 3 2 N(88) + 2 N(895) 2 N(9) N(26) 5 2 (94) 3 2 πn γn πn γn ΛK ΣK πn γn ΛK ΣK πn γn ηn ΛK ΣK πn γn ηn ΛK ΣK π πn γn ηn ΣK πn γn η (!) New States in PDG 22.