Electroproduction of the Roper resonance as a hybrid state

Abstract

We derive the Q2 dependence of the helicity amplitudes of the Roper resonance assuming that it is (1) a radially excited q 3 state, and (2) a q 3~ hybrid baryon. Our study shows that for a hybrid baryon assignment the magnitude of the transverse helicity amplitude decreases rapidly as Q 2 increases, and the longitudinal helicity amplitude vanishes. This behavior is quite different from the predictions of the q 3 quark potential model, which assumes a radially excited q 3 assignment. Comparison with data shows that the hybrid interpretation of the Roper resonance is favored. Future experiments at the Continuous Electron Beam Accelerator Facility should be able to clearly distinguish between these two possible assignments.PACS number(s1: 13.60. Rj, 12.40.Aa, 14.20.Gk Hybrid baryons are states presumably dominated by the state of three quarks oscillating against explicitly excited glue field configurations. We denote these states by q3G. Within the framework of QCD, such configurations are expected to exist in nature. However, because QCD is not well understood in the nonperturbative regime, little is known about the masses of hybrid states, and mass estimates have to rely on the use of models. Estimates within the bag-model approach [I] showed that the gluonic excitation energy should be about 550-700 MeV. This suggests that the masses of the lightest hybrid baryons could be as low as < 1500 MeV. Therefore, it is possible that some of the experimentally observed baryons are hybrid states rather than ordinary q 3 states. Recently, it has been suggested [2] that there should be a gluonic partner of the nucleon whose ratio of "proton" and "neutron" magnetic moments and photoproduction helicity amplitudes is -$. It is possible that the Roper resonance P I , ( 1440) and P33( 16001, which, in the quark model, are assigned to radial excitations of the nucleon and P j 3 ( 1232), respectively, are such hybrid states. The low mass of the Roper resonance as well as of the P33( 1600) are difficult to explain within the framework of the quark potential model [3]. Moreover, there are indications of the existence of P3,(1550) and P,,(1540) states, which are expected to exist as hybrid states. but have no place in the quark potential model.Three quark states and hybrid states may have the same quantum numbers; a study of the spectroscopic assignments will therefore not be sufficient to discriminate between the q and q 3~ states. However, because of their different internal structure (spatial wave function), as well as their different spin-flavor structure, studies of transition properties, especially of the photon transition amplitudes and the electromagnetic transition form factors, may prove very effective in distinguishing between these states. A hybrid state is excited in the spin-flavor space, and has an SU(6) spin-flavor wave function orthogonal to that of the nucleon, whereas the spin-flavor wave function of a radially excited state is identical to that of the nucleon. The difference between the spin-flavor excitation a...

Hadron structure at lowQ2

“…10. The results are in agreement with an earlier experiment by Liesenfeld et al (1999), but are complemented by a data point at the very low momentum transfer of Q 2 = 0.058 GeV 2 . An exact Rosenbluth separation is prerequisite, because the transverse cross section σ T is sensitive to G A and the longitudinal cross section to the pion form factor F π , which is discussed in subsection III.E.…”

Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

“…The importance of pion (cloud) contributions to the transition form factors has also been confirmed by the lattice calculations [29]. Alternative descriptions include the representation of N (1440)P 11 as a gluonic baryon excitation [30,31] and the possibility that nucleon resonances are meson-baryon molecules generated in chiral coupled-channel dynamics [32][33][34][35][36]. Relations between baryon electromagnetic form factors and generalized parton distributions (GPDs) have also been formulated that connect these two different notions to describe the baryon structure [37,38].…”

“…The thick curves correspond to the results obtained in the LF relativistic quark models assuming that N (1440)P 11 is a first radial excitation of the 3q ground state: [15] (dashed), [19] (solid). The thin dashed curves are obtained assuming that N (1440)P 11 is a gluonic baryon excitation (q 3 G hybrid state) [31]. The legend is partly as for Fig.…”

“…However, the difficulties of quark models in describing the low mass and large width of the N (1440)P 11 , and also its photocouplings to the proton and neutron, gave rise to numerous speculations. Alternative descriptions of this state as a gluonic baryon excitation [30,31], or a hadronic Nσ molecule [33], were suggested. The CLAS measurements, for the first time, made possible the determination of the electroexcitation amplitudes of the Roper resonance on the proton up to Q 2 = 4.5 GeV 2 .…”

Nucleon Resonance Excitation with Virtual Photons

“…Also the longitudinal Roper excitation exhibits very large values around Q 2 ≈ 0.5 GeV 2 , in fact this is the strongest longitudinal resonance excitation that we can find. This should be a clear answer to the question raised by Li and Burkert [38], whether the Roper resonance is a radially excited 3-quark state or a quark-gluon hybrid state. In the latter case it was argued that the longitudinal coupling would completely vanish.…”