Nucleon resonance excitation with CLAS

Nucleon resonance excitation with CLAS

Raffaella De Vita (for the CLAS Collaboration)

Istituto Nazionale di Fisica Nucleare, Sezione di Genova Via Dodecaneso 33, 16146 Genova, Italy

ABSTRACT

The study of the baryon spectrum is a fundamental part of the scientific program in Hall B at Jefferson Laboratory. The so called N* program indeed concerns the measurement of the electromagnetic production of exclusive hadronic final states, with the purpose of extracting information on baryon excited states. CLAS, the CEBAF Large Acceptance Spectrometer, is explicitly designed for conducting a broad experimental program in hadronic physics, using the continuous electron beam provided by the laboratory. An overview of the most recent results is presented.

1 Introduction

The study of the nucleon structure is one of the primary interests in strong interaction physics and has been the subject of experimental and theoretical studies for several decades. One of the primary manifestations of the complex internal structure of the nucleon is the existence of its excited states, i.e. baryon resonances. These play an important role in intermediate energy phenomena and understanding their nature is a necessary step to reach a comprehensive picture of strong interaction physics.

The excited states of the nucleon were first observed in pN scattering in which their contribution was clearly evident as bumps in the total cross section. These measurements allowed a first classification of the excitation spectrum of the nucleon, providing measurement of the masses, widths, quantum numbers, and branching ratios of many baryon resonances [1]. In spite of the large amount of information collected by these experiments, the number of states that were identified was less than what predicted by the standard quark model. A possible explanation is that such ''missing'' states may decouple from the pN channel, making them undetectable in experiments with pion beams. Other explanations come from theoretical models that are able to predict a smaller number of states based on a reduced set of degrees of freedom [2]. Unraveling this problem requires measurements with probes different from pion beams.

The construction of new high intensity and high duty cycle electron and photon facilities opened new possibilities for the study of baryon resonances using electromagnetic probes. These provide information on the resonance and nucleon wavefunctions through the measurement of the helicity amplitudes, i.e. the photocouplings for gN ® N^* vertices. Furthermore electroproduction allows us to explore baryon structure for different distance scales by varying the photon virtuality.

Nowadays electroexcitation processes are a fundamental tool to pursue these studies. However due to the complexity of the baryon spectrum, the proximity and overlapping nature of the various excited states, the measurement of a single channel is not sufficient to complete this research program. On the contrary a thorough study of resonance properties requires the measurement of cross sections, angular distributions, as well as polarization observables for different final states.

A broad experimental program for the study of baryon resonances is in progress in Hall B of Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS) [3]. CLAS provides the large angular coverage that is necessary for the study of resonance decays. It is a magnetic spectrometer based on a six-coil torus magnet whose field is primarily oriented along the azimuthal direction. The particle detection system includes drift chambers for track reconstruction, scintillation counters for the time of flight measurement, Cerenkov counters for electron-pion discrimination, and electromagnetic calorimeters to identify electrons and neutrals. Charged particles can be detected and identified for momenta down to 0.2 GeV.

CLAS is the first large acceptance instrument that can measure exclusive electroproduction of mesons with sufficient resolution for a detailed study of resonance excitation. The entire resonance mass region and a wide range in the photon virtuality Q² can be covered simultaneously in a single measurement while detecting several meson final states. Data taking with the CLAS detector started in 1998 and since that time many different reactions has been studied. In the following I will discuss some of the most recent and relevant results obtained from these analyses.

2 Quadrupole deformation of the D(1232)

The D(1232) is the first excited state of the nucleon. It is a well identified and isolated resonance which dominates the photoproduction cross section and the electroproduction cross section at low momentum transfer Q². For this reason, this is probably the most studied of the nucleon excited states. In model with SU(6) spherical symmetry, the N ® D transition is simply due to a magnetic dipole M₁₊ mediated by a spin flip from the J = 1/2 nucleon ground state to the D with J = 3/2, while the electric and coulomb multipoles E₁₊ and S₁₊, which are allowed by parity and angular momentum conservation, are equal to 0. Non-zero values of E₁₊ and S₁₊ indicate a deviation from the SU(6) spherical symmetry that can be associated with a quadrupole deformation of the nucleon or of the D state as discussed in Ref. [4]. Dynamically such deformation may arise through interaction of the photon with the pion cloud [5,6] or through the one-gluon-exchange mechanism [7]. At large momentum transfer, helicity conservation directly implies R_EM = +1. An interpretation of R_EM in terms of quadrupole deformation can therefore only be valid at low momentum transfer.

Results of the multipole analysis of the CLAS data [8] are shown in Fig. 1, where data from previous experiments published after 1990 are included as well [9-11]. R_EM remains negative and small throughout the explored Q² range. There are no indications that leading pQCD contributions are important since they would result in a rise of R_EM = E₁₊/M₁₊ ® +1 [12]. R_SM behave quite differently. While it also remains negative, its magnitude tends to increase with Q². The comparison with theoretical models, from relativized quark models [13,14] to a chiral quark soliton model [15] and dynamical models [5,6,16], shows that simultaneous description of both R_EM and R_SM is achieved by dynamical models that include explicitly the pion cloud. This supports the claim that most of the quadrupole strength is due to meson effects that are not included in other models. Recently, calculation of R_EM and R_SM have been performed in quenched and unquenched lattice QCD in the Q² range of the CLAS results [17]. The full QCD results give R_EM values more negative than in the quenched approximation showing the contribution of the pion cloud to be negative, and causing an oblate deformation of the D(1232). The calculation at Q² = 0.52 GeV² is in agreement with the CLAS data for R_EM and R_SM.

3 The second resonance region

The mass region corresponding to the second enhancement in the inclusive gN cross section is known as the second resonance region. It covers the W range between approximately 1.4 GeV and 1.6 GeV and is dominated by the excitation of the S₁₁(1535) and D₁₃(1520) states.

3.1 The S₁₁(1535) and h production

The S₁₁(1535) resonance was found to have an unusually hard transition form factor. This in fact shows a slow fall-off with Q². This state has a significant branching ratio in ph final states where it shows up as a strong enhancement near the h threshold with very little background. Older data show some discrepancies in the values of the total width and photocoupling amplitude. In particular, analysis of pion photoproduction data [1] disagree significantly with the analysis of h photoproduction.

Data from CLAS [18] together with data from an earlier JLab experiment [19] now give a consistent picture of the Q² evolution of the form factor, confirming the slow fall-off with much improved data quality (see Fig. 2). Analysis of np⁺ and pp⁰ data at Q² = 0.4 GeV² gives a value of A_1/2 consistent with the analysis of the ph data [20].

So far the particular hardness of the form factor has been difficult to explain in theoretical models. However a recent calculation in the framework of the constituent quark model has shown that this behavior can be reproduced using an hypercentral potential [21].

3.2 Polarization Observables and Resonance amplitudes

The extraction of the helicity amplitudes in an unpolarized measurement requires a complex analysis of the full angular distribution. On the contrary polarization measurements provide direct information on the helicity amplitudes A_1/2 and A_3/2 through the measurement of the double spin asymmetry.

The Q² dependence of the double spin asymmetry asymmetry (A₁+ hA₂)/(1 + ÎR) for ® e'np⁺ is shown in Fig. 3 for four W ranges [24]. A₁ is the virtual photon helicity asymmetry A₁ = (|A_1/2|² – |A_3/2|²)/(|A_1/2|² + |A_3/2|²), A₂ is a longitudinal-transverse interference term, R is the longitudinal-transverse cross section ratio, while h and Î are kinematical factors. Due to the particular kinematics of the experiments as well as to the suppression of longitudinal terms in respect to transverse, the double spin asymmetry is expected to be dominated by the helicity asymmetry A₁. The sign and magnitude of the measured asymmetry indicate the dominance of the helicity-1/2 contribution. This is in contrast with the helicity-3/2 dominance observed at the photon point [1] and indicates that a transition occurs in between Q² = 0 and the measured Q² range. This feature is consistent with a strong change with Q² of the helicity structure of the D₁₃(1520) and F₁₅(1680) states that are predicted by constituent quark models [13,21,25,26] to vary from A₁ = –1 at the photon point to A₁ = 1 at high Q².

4 Higher mass states and missing resonances

The mass region above W = 1.5 – 1.6 GeV is characterized by the presence of several excited states. Many of these states are expected to strongly decouple from the pN final state. Therefore the measurement of other final states as ppN, hN, wN, ... is very important. Moreover many of the so-called '' missing states'' are predicted to couple strongly to the ppN channels [27]. Search for these states is of great importance for the understanding of the nucleon structure as alternative symmetry schemes do not predict nearly as many missing states [2].

4.1 Resonances in the pp⁺p^– channel

The cross section for the reaction g^*p ® pp⁺p^– has been measured at CLAS in Q² range from 0.5 to 1.5 GeV² [29]. The CLAS data are shown in Fig. 4. The most striking feature is the strong resonance peak near W = 1.72 GeV that was not seen in the DESY photoproduction data [30-32]. Further analysis of the CLAS data that includes the complete hadronic angular distribution and the pp⁺ and p⁺p^– mass distributions has allowed us to investigate the origin of this peak. This was found to be better described by a (1720) state. While there exists a state with such quantum numbers in this mass range, the P₁₃(1720), its hadronic properties as obtained from analysis of previous data seem to be inconsistent with what observed in this experiment. Keeping the hadronic coupling of this state within the limits imposed by previous data forces us to reduce its photocouplings and to introduce a second state with the same quantum numbers but very different hadronic couplings. Interpretation of this second state as a missing resonance is definitely possible. There are in fact model predictions of states with the same quantum numbers and reasonably close mass [27,33].

4.2 Photoproduction of h meson

The gp ® ph reaction has been measured with the CLAS detector covering the resonance region up to W = 2.15 GeV [34]. The differential cross section as been measured almost in the entire angular range and the total cross section has been extracted. This is shown in Fig. 5. Beyond the region of the well known S₁₁(1535), the data show a structure indicative of a higher mass resonance contribution to the ph channel. Preliminary calculations based on the cQM [35] shows that good agreement for W < 1.9 GeV is obtained when a new S₁₁ resonance with a mass of 1.79 GeV and width of 250-350 MeV is included. Further analysis of the angular distributions and the energy dependence are needed to come to a more definite conclusion.

5 Conclusion

Electroexcitation of baryon resonances has become an efficient tool to study the nucleon structure. The simultaneous measurement of several decay channels over a wide kinematics is necessary for a thorough study of resonance excitation. This is the goal of the measurements performed with the CLAS detector that provide new high quality data for many reactions covering the entire resonance region. These data have allowed us to significatly improve our understanding of the characteristics of known resonance states as the D(1232) or the S₁₁(1535). Furthermore they now provide indications for the existence of new states which may allow us to solve the puzzle of the ''missing'' states.

[17] C. Alexandrou et al., hep-lat/0309041.

[20] I. G. Aznaurian et al., to be submitted to Phys. Rev. C.

[35] B. Saghai, nucl-th/0202007

Received on 3 November, 2003

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Publication Dates

• Publication in this collection
  26 Oct 2004
• Date of issue
  Sept 2004

History

• Received
  03 Nov 2003