Light Baryon Spectroscopy What have we learned about excited baryons?

Transcription

1 Light Baryon Spectroscopy What have we learned about excited baryons? Volker Credé Florida State University, Tallahassee, FL The 9th Particles and Nuclei International Conference MIT, Cambridge, USA, 7/27/2

2 Outline Introduction Quarks, QCD, and Confinement Why do we study excited baryons? 2 3 Polarization Experiments Hadron Structure with Electromagnetic Probes 4

3 Outline Introduction Quarks, QCD, and Confinement Why do we study excited baryons? Introduction Quarks, QCD, and Confinement Why do we study excited baryons? 2 3 Polarization Experiments Hadron Structure with Electromagnetic Probes 4

4 QCD and Confinement Quarks, QCD, and Confinement Why do we study excited baryons? From about 6 s on, all quark and anti-quarks became confined inside of hadronic matter. Only protons and neutrons remained after about s. 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?

5 Non-Perturbative QCD Quarks, QCD, and Confinement Why do we study excited baryons? Courtesy of Craig Roberts, Argonne How does QCD give rise to hadrons? Interaction between quarks 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.

6 Quarks, QCD, and Confinement Why do we study excited baryons? The (Experimental) Issues with Hadrons Baryons What are the fundamental degrees of freedom inside a proton or a neutron? How do they change with varying quark masses? CQM CQM+flux tubes Quark diquark clustering 2 Mesons What is the role of glue in a quark-antiquark system and how is this related to the confinement of QCD? What are the properties of predicted states beyond simple quark-antiquark systems (hybrids, glueballs, multi-quark states,...)? Need to map out new states (Session 3C): BES III, BELLE, COMPASS, Panda@GSI, GlueX@JLab,...

7 Quarks, QCD, and Confinement Why do we study excited baryons? Components of the Experimental N Program The excited baryon program has two main components: 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. 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 confining (effective) forces of the 3-quark system.

8 Quarks, QCD, and Confinement Why do we study excited baryons? One of the Goals of the Excited N Program is the search for missing or yet unobserved baryon resonances. Quark models predict many more baryons than have been observed. N Spectrum Spectrum Particle Data Group (J. Phys. G 37, 752 (2)) little known (many open questions left) Are the states missing in the predicted spectrum because our models do not capture the correct degrees of freedom? 2 Or have the resonances simply escaped detection?

9 Quarks, QCD, and Confinement Why do we study excited baryons? One of the Goals of the Excited N Program is the search for missing or yet unobserved baryon resonances. Quark models predict many more baryons than have been observed. N Spectrum Spectrum Particle Data Group (J. Phys. G 37, 752 (2)) cross section [mb] Broad, overlapping resonances π + p total π + p elastic Have not been observed, yet. Nearly all existing data on baryons result from πn scattering experiments. If the resonances did not couple to πn, they would not have been discovered!!

10 3 Introduction Quarks, QCD, and Confinement Why do we study excited baryons? Spectrum of Nucleon Resonances S. Capstick and N. Isgur, Phys. Rev. D34 (986) 289 many predicted states missing ** 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, ) 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 Why do we study excited baryons? 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, ) 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 Why do we study excited baryons? Excited-State Baryon Spectroscopy from Lattice QCD R. Edwards et al., arxiv:4.552 [hep-ph] Missing states? (7) N(938) (232) (62) m π = 4 MeV C. Morningstar Session 2C Exhibits broad features expected of SU(6) O(3) symmetry Counting of levels consistent with non-rel. quark model, no parity doubling

13 Quarks, QCD, and Confinement Why do we study excited baryons? 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 EBAC, Jülich, Gießen, etc.

14 Outline Introduction Introduction Quarks, QCD, and Confinement Why do we study excited baryons? 2 3 Polarization Experiments Hadron Structure with Electromagnetic Probes 4

15 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. Experiments partially complementary All facilities have started polarization programs., ELSA CLAS MAMI-C GRAAL

16 in Photoproduction: γp K Y Photon beam Target Recoil Target - Recoil 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 the amplitude 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 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 6 observables will be measured with CLAS Allows many cross checks e.g. γp KΛ published to be published data taken data taken, being analyzed

17 Comparison of Different Data for γp K + Λ [µb] (µb) dσ d cos θ K c.m. dσ d cos θ c.m. K.5 c.m cosθ K < c.m cosθ K < c.m cosθ K < -.5 c.m..5.5 cosθ K < c.m cosθ K < c.m..65 cosθ K < c.m cosθ K c.m cosθ K c.m. -.5 cosθ K c.m..5 cosθ K c.m..45 cosθ K c.m..75 cosθ K < -.65 < -.35 < -.5 <.25 <.55 <.85 s [GeV] s (GeV) c.m cosθ K c.m cosθ K c.m. -.5 cosθ K c.m..25 cosθ K c.m..55 cosθ K c.m..85 cosθ K < -.55 < -.25 <.5 <.35 <.65 < Significant improvement of the data quality in recent years Much more precise data with larger kinematic coverage High-statistics data samples allow for many different topologies to be analyzed Confirmation of CLAS 6 results CLAS 2 CLAS Collaboration, Phys. Rev. C 8, 252 (2) CLAS 26 CLAS Collaboration, Phys. Rev. C 73, 3522 (26) SAPHIR 24 SAPHIR Collaboration, Eur. Phys. J. A 9, 25 (24)

18 Polarization Transfer in γp K + Λ C x, C z without N(9)P 3 C x, C z with N(9)P N(9)P , N(2)F 5, N(99)F cos θ K Bonn-Gatchina PWA requires N(9)P 3. No246 quark-diquark 2454 oscillations! Both oscillators cos θ K need to be excited. 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)

19 3 Introduction Spectrum of Nucleon Resonances S. Capstick and N. Isgur, Phys. Rev. D34 (986) 289 many predicted states missing ** 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, ) 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-

20 Isospin Filter: γp N (I = /2) 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.

21 Isospin Filter: γp N (I = /2) 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) 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. Hints for a missing state!

22 Isospin Filter: γp N (I = /2) p ω M. Williams et al. [CLAS Collaboration], Phys. Rev. C 8, 6529 (29) 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.

23 Photoproduction of π Mesons from the Proton Reaction γp p π remains important for our understanding of baryons. ] µb sr [ dσ dω At ELSA, excellent data with good statistics in the forward direction. Forward region is very sensitive to higher-spin resonances: Observation of N(29)G 7 within the Bonn-Gatchina PWA framework (Important to confirm high-mass states first observed in πn scattering) MeV MeV θ c.m. V. C. et al. [CBELSA/TAPS Collaboration], arxiv: MeV CBELSA/TAPS CB-ELSA CLAS, GRAAL older Bonn data

24 Beam Asymmetry Σ in γp p π 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. M. Dugger (ASU), CLAS g8b run group, to be published

25 Beam Asymmetry Σ in γp p π 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

26 Beam Asymmetry Σ in γp p π and γp n π + M. Dugger (ASU), CLAS g8b run group, to be published

27 Why are Polarization Observables Important? η γγ dσ/dω η 3π Broad, overlapping resonances PWA γp pη: Only nucleon (i.e. N ) resonances can contribute S (535), D 3 (52), S (65), F 5 (68), P 3 (72), D 3 (28) N(535)S +..., + N(52)D ρ -, ω -t-channel 3, N(65)S exchange, N(68)F 5, N(72)P 3,..., ρ- and ω-t-channel exchange + new D 5 : m = 268 ± 22 MeV, New resonance N(27)D Γ = 295 ± 4 MeV 5 : m = (268 ± 22) MeV/c 2 (Bonn-Gatchina (needed: PWA) confirmation in Γ = polarisation (295 ± 4) exp.) MeV/c 2 (needs confirmation in polarization experiments) No need for a 3rd S!

28 Analysis of γp p η: Total Cross Section σ tot, µb CBELSA/TAPS data BoGa fit S P P 3 D 5 Isospin Filter Only N resonances can contribute! Bonn-Gatchina (PWA) group: Hint for N resonance (27)D 5 (Phys. Rev. Lett. 94, 24 (25)) Confirmed in 29 analysis! M(γp) [MeV] Resonances dominantly contributing: 2 N(72)P 3 p η? η-maid: N(7)P p η significant! N(535)S, (N(72)P 3 )?, N(27)D 5

29 Beam Asymmetry Σ in the Reaction γp p η P 3 (72) D 3 (52) BoGa-PWA 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)} E γ = 25 MeV D 3 (52) P 3 (72) η-maid Further spin observables are available. G and E from experiments with longitudinal target polarization at MAMI-C, ELSA, CLAS Data being analyzed. H, F, T, P from experiments with transverse target polarization (program completed at CLAS@JLab, soon at ELSA and MAMI) [CBELSA/TAPS Collaboration], EPJ A 33, 47 (27)

30 Helicity-Dependent Cross Section for γ p p η E = σ /2 σ 3/2 σ /2 + σ 3/2 M. Gottschall et al. [CBELSA/TAPS Collaboration]

31 Helicity-Dependent Cross Section for γ p p η Very preliminary: Data are positive M. Gottschall et al. [CBELSA/TAPS Collaboration] B. Morrison et al. [CLAS Collaboration]

32 Double-Polarization: Toward Calorimeter system at ELSA is optimized for neutral particles. Close to 4π coverage Frozen Spin Target: Butanol (C 4 H 9 OH). target pol. axis photon pol. x y z unpolarized σ T linear Σ H P G circular F E

33 Double-Polarization at ELSA: Target Asymmetry T D. Elsner Session C direction of target pol.: β = 99 N(φ) = f P target N N N + N = T sin (φ β) Unprecedented statistical quality. target pol. axis photon pol. x y z unpolarized σ T linear Σ H P G circular F E

34 Double-Polarization at ELSA: Observables P and H angle of lin. pol. plane: α = 45 direction of target pol.: β = 99 N(φ) = C (N N ) (N N ) (N + N ) + (N + N ) = P (sin(φ β) cos (2(φ α)) + H (cos (φ β) sin(2(φ α)) target pol. axis photon pol. x y z unpolarized σ T linear Σ H P G circular F E

35 Double-Polarization at JLab: CLAS-FROST E. Pasyuk Session C FRozen-Spin Target (FROST) P z 8% Relaxation time 2, h Holding mode (B =.5 T, T 28 mk) γp p η (Dugger, Morrison et al.) Arizona State University γp p ω (Collins, Vernarsky et al.) Catholic University, Carnegie Mellon γp n π + (E) (S. Strauch et al.) University of South Carolina γp n π + (G) (J. McAndrew et al.) University of Edinburgh γp p π (H. Iwamoto et al.) George Washington University γp p π + π (S. Park et al.) Florida State University γp K + Y (S. Fegan et al.) University of Glasgow

36 Helicity Difference E in γp n π + S. Strauch (University of South Carolina) X

37 Helicity Difference E in γp n π + S. Strauch (University of South Carolina) X

38 Helicity Difference E in γp n π + Preliminary results for E from FROST About 7 data points covering a wide energy and angular range: S. Strauch (University of South Carolina).9 < cosθ π + <.9.25 GeV < W < 2.25 GeV Average uncertainty for E: ±.8 (stat.) and < % (sys.) W <.7 GeV: SAID PWA solution describes main features of the preliminary data remarkably well. W >.7 GeV: Partial-wave analyses currently ambiguous; new data will provide additional constraints and stringent tests. X

39 Outline Introduction Polarization Experiments Hadron Structure with Electromagnetic Probes Introduction Quarks, QCD, and Confinement Why do we study excited baryons? 2 3 Polarization Experiments Hadron Structure with Electromagnetic Probes 4

40 Polarization Experiments Hadron Structure with Electromagnetic Probes No recoil polarization for non-strange channels More observables for vector mesons: ω, φ, etc. published acquired and being analyzed acquired at Jefferson Lab being taken at ELSA, MAMI planned This is not boring stamp collection. We do not want to observe all resonances, but we need to find a pattern!

41 Polarization Experiments Hadron Structure with Electromagnetic Probes Hadron Structure with Electromagnetic Probes Study structure of the nucleon spectrum in domain where dressed quarks are the major active degree of freedom. Explore formation of excited nucleon states in interactions of dressed quarks and their emergence from QCD.

42 A /2 ( -3 GeV -/2 ) Introduction Polarization Experiments Hadron Structure with Electromagnetic Probes 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: 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 ruled out! Data from CLAS A /2 and S /2 amplitudes: e.g. I. Aznauryan et al., PRC 78, 4529 (28)

43 Polarization Experiments Hadron Structure with Electromagnetic Probes Helicity Amplitudes for γp 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.

44 Outline Introduction Quarks, QCD, and Confinement Why do we study excited baryons? 2 3 Polarization Experiments Hadron Structure with Electromagnetic Probes 4

45 The quest to understand confinement and the strong force is about to make great leaps forward: Progress in theory and computing will allow us to solve QCD and understand the baryon spectrum and the role of glue. New results from the current polarization programs worldwide will (soon) give us new insight on the observed and missing baryons. New candidates for baryon resonances have been proposed. The definitive experiments to confirm or refute current expectations on the role of glue are being built, e.g. Lab. Conclusions Advances in both areas will allow us to finally understand QCD and confinement.