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Electroexcitation of the Roper resonance from CLAS data Inna Aznauryan, Volker Burkert Jefferson Lab N * 2007, Bonn, September 7, 2007
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Outline Introduction: Puzzles of the Roper resonance Analysis: Dispersion Relations and Unitary Isobar Model Results: Helicity amplitudes for γ*p→ P 11 (1440) Discussion: What do we learn about the nature of the P 11 (1440) from these results Summary –Comment on claims of a new P 11 (1650) resonance seen in nη and not seen in pη photoproduction.
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SU(6)xO(3) Classification of Baryons P 11 (1440)
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Introduction: Puzzles of the Roper resonance The state attracted special attention since its discovery because of its unexpectedly low mass. In the quark and bag models, assumption that P 11 (1440)≡[56,0 + ] r led to: large mass difference between nucleon and P 11 (1440), which is several hundred MeV higher that the observed mass difference recent qLQCD simulations show even a much larger mass for first excited state of the nucleon wrong mass ordering between P 11 (1440) and S 11 (1535) states Non-relativistic CQMs cannot explain sign of photo- coupling amplitude A 1/2 ( S. Capstick, I. Aznauryan )
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Introduction ( continued ) However, right mass ordering between P 11 (1440)≡ [56,0 + ] r and S 11 (1535) wa s observed in later investigations: Chiral constituent QM with Goldstone-boson exchange between quarks Glozman, et al., Phys.Rep. 268, 263 (1996) in Lattice QCD Mathur, et al., Phys.Lett. 605, 137 (2005) …. but see talk by C. Gattringer
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Introduction ( continued ) Difficulties in the description of P 11 (1440) prompted the development of alternative descriptions of this state: –a q 3 G hybrid baryon state –a dynamically generated πN resonance –a nucleon-sigma molecule The results for γ*p→ P 11 (1440) extracted from experiments in a wide Q 2 range will allow us to discriminate between different descriptions of the state. Due to the lack of predictions from the P 11 (πN) and P 11 (Nσ) resonance models we can compare only with the P 11 (q 3 G) model
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Analysis: CLAS data New ep→eπ + n electroproduction data Q 2 =1.72, 2.05, 2.44, 2.91, 3.48, 4.16 GeV 2 W=1.15-1.70 GeV Differential cross sections Longitudinally polarized electron beam asymmetry Data have nearly full coverage in nπ + cm system for cosθ* and φ* > 33,000 differential cross sections, and > 3,000 electron beam asymmetries
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Analysis: Dispersion relations and Unitary Isobar Model Using two approaches allows us to draw conclusions on the model dependence of the extracted results. The main uncertainty of the analysis is related to the real parts of amplitudes which are built in DR and UIM in conceptually different way:
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Analysis ( continued ) The imaginary parts of the amplitudes are determined mainly by the resonance contributions: For all resonances, except P 33 (1232), we use relativistic Breit-Wigner parameterization with energy- dependent width ( Walker, PR 182 (1969) 1729 ) Combination of DR, Watson theorem, and the elasticity of t 1+ 3/2 (πN ) up to W=1.43 GeV provide strict constraints on the M 1+ 3/2,E 1+ 3/2,S 1+ 3/2 multipoles of the P 33 (1232) ( Δ(1232)).
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Fixed-t Dispersion Relations for invariant Ball amplitudes (Devenish & Lyth) Dispersion relations for 6 invariant Ball amplitudes: Unsubtracted Dispersion Relations Subtracted Dispersion Relation γ*p→Nπ (i=1,2,4,5,6)
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Analysis: Some points which are specific to high Q 2 From the analysis of the data at different Q 2 = 1.7-4.2 GeV, we have obtained consistent results for f sub (t,Q 2 ) f sub (t,Q 2 ) has relatively flat behavior, in contrast with π contribution:
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Analysis: some points which are specific to high Q 2 (continued) The background of UIM we use at large Q 2 consists of the Born term and t-channel ρ and ω contributions At high Q 2, a question can arise if there are additional t-channel contributions, which due to the gauge invariance, do not contribute at Q 2 =0, e.g. π(1300), π(1670), scalar dipole transitions for h 1 (1170), b 1 (1235), a 1 (1260) … Such contributions are excluded by the data.
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Analysis (continued) Fitted parameters: amplitudes corresponding to: P 33 (1232), P 11 (1440), D 13 (1520), S 11 (1535) F 15 (1680) Amplitudes of other resonances, in particular those with masses around 1700 MeV, were parameterized according to the SQTM or the results of analyses of previous data Including these amplitudes into the fitting procedure did not change the results
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Results: Examples of cross sections at Q 2 =2.05 GeV 2 W-dependence φ-dependence at W=1.43 GeV
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Results: Legendre moments for σ T +ε σ L DR UIM Q 2 = 2.05 GeV 2 ~cosθ~(1 + bcos 2 θ) ~ const. DR w/o P 11 (1440)
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Results: Multipole amplitudes for γ * p→ π + n Q 2 =0 Q 2 =2.05 GeV 2 Im Re_UIM Re_DR At Q 2 =1.7-4.2, resonance behavior is seen in these amplitudes more clearly than at Q 2 =0 DR and UIM give close results for real parts of multipole amplitudes
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Results: Helicity amplitudes for the γp→ P 11 (1440) transition DR UIM RPP Nπ, Nππ Model uncertainties due to N, π, ρ(ω) → πγ form factors NπNπ CLAS
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Comparison with quark models P 11 (1440)≡[56,0 + ] r With increasing Q 2, the proper treatment of relativistic effects becomes very important The consistent way to realize relativistic calculations of γN→N* transitions is to consider them in LF dynamics In LF calculations, the diagrams that violate impulse approximation are removed In the nonrel. approach of Cano et al., these diagrams are found using VDM and the 3 P 0 model
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Discussion: LF quark model predictions P 11 (1440)≡[56,0 + ] r LF CQM predictions have common features, which agree with data: Sign of A 1/2 at Q 2 =0 is negative A 1/2 changes sign at small Q 2 Sign of S 1/2 is positive 1.Weber, PR C41(1990)2783 2. Capstick..PRD51(1995)3598 3. Simula…PL B397 (1997)13 4. Riska..PRC69(2004)035212 5. Aznauryan, PRC76(2007)025212 6. Cano PL B431(1998)270
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Discussion: P 11 (1440) as a hybrid baryon? Suppression of S 1/2 has its origin in the form of vertex γq→qG. It is practically independent of relativistic effects Z.P. Li, V. Burkert, Zh. Li, PRD46 (1992) 70 G q3q3 In a nonrelativistic approximation A 1/2 (Q 2 ) and S 1/2 (Q 2 ) behave like the γ*NΔ(1232) amplitudes. previous data
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Summary We have extracted transverse and longitudinal amplitudes of the γ*p→ P 11 (1440) transition from experimental data at high Q 2 using the nπ+ final state. The DR analysis and the UIM analysis give consistent results The results rule out the description of the P 11 (1440) as a q 3 G hybrid state due to the strong longitudinal response obtained from the experiment for γ*p→ P 11 (1440)
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Summary ( continued ) Comparison with quark model predictions provide evidence in favor of the P 11 (1440) as a radial excitation of the nucleon Final confirmation of this conclusion requires a complete, and simultaneous description of the nucleon form factors and the γ*p→ P 11 (1440) amplitudes
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Evidence for a P-wave resonance near 1700 MeV in η electroproduction with CLAS Volker Burkert Jefferson Lab N * 2007, Bonn, September 7, 2007
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Q 2 dependence of the S 11 (1535) photocoupling and evidence for a P-wave resonance in η electro-production from protons. CLAS CLAS collaboration has recently published data on electroproduction of ep→epη. H. Denizli et al. (CLAS), Phys. Rev. C 76, 015204 (2007), arXiv:0704.2546 [nucl-ex] Integrated cross section shows peak structure near W=1.7 GeV or/and dip structure near W=1.66 GeV. We heard several times that the γn→nη, data show peak structure at 1650-1680 MeV, and γp→ηp did not show this structure. A new resonance is claimed that couples only to neutrons and not to protons: talks by: H. Shimizu, V. Kuznetsov, and others.
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Response Functions and Legendre Polynomials Expansion in terms of Legendre Polynomials Sample differential cross sections for Q 2 =0.8 GeV 2, and selected W bins. Solid line: CLAS fit, dashed line: η-MAID. 4 resonance fit gives reasonable description including S 11 (1535), S 11 (1650), P 11 (1710), D 13 (1520)
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1.6 1.7 1.8 S-wave dominance and s-p wave interference in ep → epη S 11 (1535) is seen in angle-independent term A 0, at all Q 2. A 1 /A 0 shows existence of P-wave strength interfering with the dominant s-wave. Good fit achieved with P 11 (1710) with Γ=100 MeV, and: ξ P11(1710) /ξ S11(1535) =0.22. Using only S11 and P11 partial waves the cross section can be qualitatively described. The observation is consistent with a rapid change in the relative phase of the E 0+ and M 1- multipoles because one of them is passing through resonance. CLAS
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Conclusions on γ*p→ P 11 (~1700) P-wave is needed to fit the data. Interference with S 11 shows resonance near 1650 MeV in η production off proton. In a 4 resonance fit of S 11 (1535), D 13 (1520), S 11 (1650) and P 11, a reasonable fit is obtained with P 11 mass M ~ 1650 MeV, width Γ=100 MeV. There is no need for a new P 11 state as long as P 11 (1710) parameters (mass, width, b ηp ) are not well established. Abstract of publication: “A sharp structure is seen near W ~ 1.7 GeV. The shape of the differential cross section is indicative of the presence of a P-wave resonance that persists to high Q 2.”
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Q 2 dependence of the S 11 (1535) photocoupling and evidence for a P-wave resonance in η electro-production from protons. CLAS CLAS collaboration has recently published data on electroproduction of ep→epη. H. Denizli et al. (CLAS), Phys. Rev. C 76, 015204 (2007), arXiv:0704.2546 [nucl-ex] Integrated cross section shows peak structure near W=1.7 GeV or/and dip structure near W=1.66 GeV. We heard several times that the γn→nη, data show peak structure at 1650-1680 MeV, and γp→ηp does not show this structure. A new resonance is claimed that couples only to neutrons and not to protons: talks by: H. Shimizu, V. Kuznetsov, ….
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Response Functions and Legendre Polynomials Expansion in terms of Legendre Polynomials Sample diff. cross sections for Q 2 =0.8 GeV 2, and selected W bins. Solid line: CLAS fit, dashed line: η-MAID. 4 resonance fit gives reasonable description: S11(1535), S11(1650), P11(1710), D13(1520)
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1.6 1.7 1.8 S-wave dominance and s-p wave interference in ep → epη S 11 (1535) is seen in angle-independent term A 0, at all Q 2. A 1 /A 0 shows existence of P-wave strength interfering with the dominant s-wave. Good fit achieved with P 11 (1710) with Γ=100 MeV, and: ξ P11(1710) /ξ S11(1535) =0.22. Using only S11 and P11 partial waves the cross section can be qualitatively described. The observation is consistent with a rapid change in the relative phase of the E 0+ and M 1- multipoles because one of them is passing through resonance. CLAS
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Conclusions on γ*p→ P + 11 (1650) P-wave is needed to fit the data. Interference with S 11 clearly shows resonance near 1650 MeV in η production off proton. In a 4 resonance fit of S 11 (1535), D 13 (1520), S 11 (1650), and P 11 a good fit is obtained with mass M ~ 1650 MeV, width Γ=100 MeV. No need for a new P 11 state as long as P 11 (1710) parameters (mass, width, b ηp ) are not well established. All of this has been published
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32 Single Quark Transition Model Predictions for [56,0 + ] → [70,1 - ] Transitions Proton
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