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→ P11(1440) • Discussion: What do we learn about the nature of the P11(1440) from

these results• Summary

– Comment on claims of a new P11(1650) resonance seen in nη and not seen in pη photoproduction.

SU(6)xO(3) Classification of Baryons

P11(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 P11(1440)≡[56,0+]r led to: large mass difference between nucleon and P11(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 P11(1440) and S11(1535) states Non-relativistic CQMs cannot explain sign of photo- coupling

amplitude A1/2 (S. Capstick, I. Aznauryan)

Introduction (continued)

However, right mass ordering between

P11(1440)≡ [56,0+]r and S11(1535) was observed in laterinvestigations:

• 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 P11(1440) prompted the development of alternative descriptions of this state:

– a q3G hybrid baryon state – a dynamically generated πN resonance – a nucleon-sigma molecule

The results for γ*p→ P11(1440) extracted from experiments in a wide Q2 range will allow us to discriminate between different descriptions of the state.

Due to the lack of predictions from the P11(πN) and P11(Nσ) resonance models we can compare only with the P11(q3G) model

Analysis: CLAS data

New ep→eπ+n electroproduction data Q2=1.72, 2.05, 2.44, 2.91, 3.48, 4.16 GeV2

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

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 P33(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 t1+

3/2(πN ) up to W=1.43 GeV provide strict constraints on the M1+

3/2,E1+3/2,S1+

3/2 multipoles of the P33(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 Q2

• From the analysis of the data at different Q2 = 1.7-4.2 GeV , we have obtained consistent results for fsub(t,Q2)

• fsub(t,Q2) has relatively flat behavior, in contrast with π

contribution:

Analysis: some points which are specific to high Q2 (continued)

The background of UIM we use at large Q2 consists of the Born term and t-channel ρ and ω contributions

•

At high Q2, a question can arise if there are additional t-channel contributions, which due to the gauge invariance, do not contribute at Q2=0, e.g. π(1300), π(1670), scalar dipole transitions for h1 (1170),

b1(1235), a1(1260) …

Such contributions are excluded by the data.

Analysis (continued)

Fitted parameters: amplitudes corresponding to: P33(1232),

P11(1440) , D13(1520) , S11(1535)

F15 (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 Q2=2.05 GeV2 • W-dependence • φ-dependence at W=1.43 GeV

Results: Legendre moments for σT+ε σL

DR UIM

Q2 = 2.05 GeV2

~cosθ ~(1 + bcos2θ)~ const.

DR w/o P11(1440)

Results: Multipole amplitudes for γ*p→ π+n

Q2 =0 Q2 =2.05 GeV2

ImRe_UIM Re_DR

At Q2=1.7-4.2, resonance behavior is seen in these amplitudes more clearly than at Q2 =0

DR and UIM give close results for real parts of multipole amplitudes

Results: Helicity amplitudes for the γp→ P11(1440) transition

DR UIM

RPP

Nπ, Nππ

Model uncertainties due to N, π, ρ(ω) → πγform factors

Nπ

CLAS

Comparison with quark models P11(1440)≡[56,0+]r

With increasing Q2, 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 3P0 model

Discussion: LF quark model predictions P11(1440)≡[56,0+]r

LF CQM predictions have common features, which agree with data: • Sign of A1/2 at Q2=0 is negative

• A1/2 changes sign at small Q2

• Sign of S1/2 is positive 1.Weber, PR C41(1990)2783 2.Capstick..PRD51(1995)3598

3.Simula…PL B397 (1997)13 4.Riska..PRC69(2004)0352125.Aznauryan, PRC76(2007)025212 6. Cano PL B431(1998)270

Discussion: P11(1440) as a hybrid baryon?

Suppression of S1/2 has its origin in the form of vertex γq→qG.It is practically independent of relativisticeffectsZ.P. Li, V. Burkert, Zh. Li, PRD46 (1992) 70

Gq3

In a nonrelativistic approximation A1/2(Q2) and S1/2(Q2) behave like the γ*NΔ(1232) amplitudes.

previous data previous data

Summary

We have extracted transverse and longitudinal amplitudes of the γ*p→ P11(1440) transition from experimental data at high Q2 using the nπ+ final state.

The DR analysis and the UIM analysis give consistent results

The results rule out the description of the P11(1440) as a q3G hybrid state due to the strong longitudinal response obtained from the experiment for γ*p→ P11(1440)

Summary (continued)

Comparison with quark model predictions provide evidence in favor of the P11(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→ P11(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

Q2 dependence of the S11(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.

Response Functions and Legendre Polynomials

Expansion in terms of Legendre Polynomials

Sample differential cross sections for Q2=0.8 GeV2, and selected W bins. Solid line: CLAS fit, dashed line: η-MAID.

4 resonance fit gives reasonable description including S11(1535), S11(1650), P11(1710), D13(1520)

1.6 1.7 1.8 1.6 1.7 1.8 1.6 1.7 1.8 1.6 1.7 1.8 1.6 1.7 1.8

S-wave dominance and s-p wave interference in ep→epη

S11(1535) is seen in angle-independent term A0, at all Q2. A1/A0 shows existence of P-wave strength interfering with the dominant s-wave. Good fit achieved with P11(1710) with Γ=100 MeV, and: ξP11(1710) /ξS11(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 E0+ and M1- multipoles because one of them is passing through resonance.

CLAS

Conclusions on γ*p→ P11(~1700)

• P-wave is needed to fit the data. Interference with S11 shows resonance near 1650 MeV in η production off proton.

• In a 4 resonance fit of S11(1535), D13(1520), S11(1650) and P11, a reasonable fit is obtained with P11 mass M ~ 1650 MeV, width Γ=100 MeV.

• There is no need for a new P11 state as long as P11(1710) parameters (mass, width, bηp) 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 Q2.”

Q2 dependence of the S11(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, ….

Response Functions and Legendre Polynomials

Expansion in terms of Legendre Polynomials

Sample diff. cross sections for Q2=0.8 GeV2, 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)

1.6 1.7 1.8 1.6 1.7 1.8 1.6 1.7 1.8 1.6 1.7 1.8 1.6 1.7 1.8

S-wave dominance and s-p wave interference in ep→epη

S11(1535) is seen in angle-independent term A0, at all Q2. A1/A0 shows existence of P-wave strength interfering with the dominant s-wave. Good fit achieved with P11(1710) with Γ=100 MeV, and: ξP11(1710) /ξS11(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 E0+ and M1- 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 S11 clearly shows resonance near 1650 MeV in η production off proton.

• In a 4 resonance fit of S11(1535), D13(1520), S11(1650), and P11 a good fit is obtained with mass M ~ 1650 MeV, width Γ=100 MeV.

• No need for a new P11 state as long as P11(1710) parameters (mass, width, bηp) are not well established.

All of this has been published

32

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

Proton