Electroexcitation of the Roper resonance from CLAS data Inna Aznauryan, Volker Burkert Jefferson Lab N * 2007, Bonn, September 7, 2007

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.

SU(6)xO(3) Classification of Baryons P 11 (1440)

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 )

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

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

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= 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

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:

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)).

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)

Analysis: Some points which are specific to high Q 2 From the analysis of the data at different Q 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:

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.

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

Results: Examples of cross sections at Q 2 =2.05 GeV 2 W-dependence φ-dependence at W=1.43 GeV

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)

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 = , 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

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

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

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) Capstick..PRD51(1995) Simula…PL B397 (1997)13 4. Riska..PRC69(2004) Aznauryan, PRC76(2007) Cano PL B431(1998)270

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

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)

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

Evidence for a P-wave resonance near 1700 MeV in η electroproduction with CLAS Volker Burkert Jefferson Lab N * 2007, Bonn, September 7, 2007

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, (2007), arXiv: [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 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.

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)

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

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.”

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, (2007), arXiv: [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 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, ….

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)

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

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

32 Single Quark Transition Model Predictions for [56,0 + ] → [70,1 - ] Transitions Proton