Excitation Spectrum Of Nucleon Research Articles

Role of chiral symmetry in the nucleon excitation spectrum

The origin of the low-lying nature of the $N$*(1440), or Roper resonance, has been the subject of significant interest for many years, including several investigations using lattice QCD. The majority of lattice studies have not observed a low-lying excited state energy level in the region of the Roper resonance. However, it has been claimed that chiral symmetry could play an important role in our understanding of this resonance. The purpose of this study is to systematically examine the role of chiral symmetry in the low-lying nucleon spectrum by directly comparing the clover and overlap fermion actions. To ensure any differences in results are attributable to the choice of fermion action, simulations are performed on the same set of gauge field configurations at matched pion masses. Correlation matrix techniques are employed to determine the excitation energy of the first positive-parity excited state for each action. The clover and overlap actions show a remarkable level of agreement. We do not find any evidence that fermion action chiral symmetry plays a significant role in understanding the Roper resonance on the lattice.

Study of pp → 𝚵+ ΛK- with the PANDA Detector

For a deeper insight into the mechanisms of non-perturbative QCD it is essential to understand the excitation pattern of baryons. Up to now only the nucleon excitation spectrum has been subject to systematic experimental studies, while very little is known on excited states of double or triple strange baryons. In studies of antiproton-proton collisions the PANDA experiment is well-suited for a comprehensive baryon spectroscopy program in the multi-strange sector. A large fraction of the inelastic pp cross section is associated to final states with a baryon-antibaryon pair together with additional mesons, giving access to excited states both in the baryon and the antibaryon channel. For final states containing a 𝚵+ 𝚵- pair, cross sections up to µb are expected, corresponding to production rates of ~ 106/d at a luminosity L = 1031 cm-2 s-1. This study focuses on excited 𝚵- states decaying into ΛK-. A strategy to reconstruct the reaction pp → 𝚵+ 𝚵* and its charge conjugate channel with the PANDA detector will be presented.

Doubly-polarised pion photoproduction on the nucleon at MAMI

The A2 Collaboration at MAMI (Mainz) carried out new measurements of the helicity dependence of the total inclusive photo-absorption cross section and of the partial cross sections for several reaction channels on the proton and on the neutron in the photon energy region 200 He targets. Hadronic reaction products were detected by the large acceptance Crystal Ball-TAPS spectrometer, complemented by plastic scintillators and vertex detectors for charged particle tracking and identification.These new, high-quality doubly-polarised pion-photoproduction data sets provide a valuable input to the study of the nucleon structure and excitation spectrum by significantly constraining the electromagnetic multipole evaluation performed by the different available partial wave analysis models. Furthermore, the helicity dependent observables provide the main ingredient for the verification of the well-known Gerasimov-Drell-Hearn (GDH) sum rule, which relates the helicity-dependent photoasborption process to the main static nucleon properties (mass, charge, spin).

Study of Excited Ξ Baryons in p̅p-Collisions with the P̄ANDA Detector

Understanding the excitation pattern of baryons is indispensable for the understanding of non-perturbative QCD. Up to now only the nucleon excitation spectrum has been subject to systematic experimental studies, while very little is known on excited states of double or triple strange baryons. In studies of antiproton-proton collisions, the P̅ANDA experiment is well-suited for a comprehensive baryon spectroscopy program in the multi-strange and charm sector. In the present study we focus on excited Ξ- states. For final states containing a Ξ- Ξ̅+ pair, cross sections of the order of μb are expected, corresponding to production rates of ~ 106/d at a luminosity L = 1031 cm-2 s-1. Here we present the reconstruction of the reaction p̅p → Ξ (1820)- Ξ̅+ with Ξ (1820)- → Λ K- and its charged conjugate channel with the P̅ANDA detector.

Double polarisation experiments in meson photoproduction

One of the remaining challenges within the standard model is to gain a good understanding of QCD in the non-perturbative regime. A key step towards this aim is baryon spectroscopy, investigating the spectrum and the properties of baryon resonances. To gain access to resonances with small $\pi N$ partial width, photoproduction experiments provide essential information. Partial wave analyses need to be performed to extract the contributing resonances. Here, a complete experiment is required to unambiguously determine the contributing amplitudes. This involves the measurement of carefully chosen single and double polarisation observables. In a joint endeavour by MAMI, ELSA, and Jefferson Laboratory, a new generation of experiments with polarised beams, polarised proton and neutron targets, and $4\pi$ particle detectors have been performed in recent years. Many results of unprecedented quality were recently published by all three experiments, and included by the various partial wave analysis groups in their analyses, leading to substantial improvements, e.g. a more precise determination of resonance parameters. An overview of recent results is given, with an emphasis on results from the CBELSA/TAPS experiment, and their impact on our understanding of the nucleon excitation spectrum is discussed.

Measurement of theH1(γ⃗, p⃗)π0Reaction Using a Novel Nucleon Spin Polarimeter

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon, pioneering a novel polarimetry technique with wide application to future nuclear and hadronic physics experiments. The commissioning measurement of polarization transfer in the $^{1}H$($\vec{\gamma}$,$\vec{p}$)$\pi^{0}$ reaction in the range $0.4<E_{\gamma}<1.4$ GeV is highly selective regarding the basic parameterizations used in partial wave analyses to extract the nucleon excitation spectrum. The new data strongly favor the recently proposed Chew-Mandelstam formalism.

Measurement of the double polarization observable E in η′-photoproduction

The nucleon excitation spectrum is investigated at the Crystal Barrel experiment through different photoproduction reactions. The photoproduction of single η′ mesons off protons is of particular importance since it offers the opportunity to study the high-lying resonances and therefore the part of the nucleon excitation spectrum where many missing resonances are predicted. Preliminary results of the double polarization observable E and the decomposition of the total cross section into its helicity-dependent components σ1/2 and σ3/2 in η′-photoproduction are presented for beam energies from 1450 MeV to 2320 MeV.

THE BGO-OD EXPERIMENT AT ELSA

Meson photoproduction is a key tool for the experimental investigation of the nucleon excitation spectrum. To disentangle the specific couplings of resonances, in addition to the rather well measured pion and eta photoproduction channels it is mandatory to obtain information on channels involving strange and vector mesons and higher mass pseudoscalar mesons, and the associated multi-particle final states with both charged and neutral particles. In this respect, the new BGO-OD experiment at the ELSA accelerator of the University of Bonn's Physikalisches Institut provides unique instrumentation. We describe the experiment, present its status and the initial program of measurements.

Polarization ObservablesTandFin singleπ0- andη-Photoproduction off quasi-free Nucleons

Meson photoproduction has developed into a powerful tool to study the nucleon excitation spectrum and test effective quark models which operate in the non-perturbative regime of QCD. An insight into the JP configurations and isospin decompositions of the contributing resonances is gained by measuring a minimal set of polarization observables on both the proton and the neutron.

Evidence of new nucleon resonances from electromagnetic meson production

The study of nucleon resonances in electromagnetic meson production with the CLAS detector is discussed. The electromagnetic interaction is complementary to pion scattering in the exploration of the nucleon excitation spectrum. Higher mass states often decouple from the $N\pi$ channel and are not seen in $\pi N \to \pi N$. Photoproduction of mesons, such as $K\Lambda$, $\omega p$ and $\eta^\prime p$ may be more sensitive to many of these states. The CLAS detector, combined with the use of energy-tagged polarized photons and polarized electrons, as well as polarized targets and the measurement of recoil polarization, are the tools needed for a comprehensive nucleon resonance program. Some of the recently published high statistics data sets had significant impact on further clarifying the nucleon excitation spectrum.

Measurement of the double polarization observable E in the reactionγp→pη′

The main goal of the CBELSA/TAPS experiment is the investigation of the nucleon excitation spectrum, which consists of many overlapping resonances. In order to disentangle the different resonance contributions, a partial wave analysis is necessary. In the field of photoproduction of single pseudoscalar mesons, the measurement of a well chosen set of at least eight single and double polarization observables allows for the determination of an unambiguous solution. Of particular interest is the η ′ meson since it couples only to resonances with isospin , thus reducing the number of overlapping resonances. Additionally, its comparatively high mass gives access to the poorly understood regime of high-lying resonances.With the CBELSA/TAPS experiment at the electron stretcher accelerator ELSA, double polarization observables such as E can be obtained by studying photoproduction reactions using a circularly polarized photon beam in combination with a longitudinally polarized butanol target. The decay mode η′ → γγ was analyzed for a beam photon energy range of 1447-2350 MeV. The preliminary results for the double polarization observable E are shown.

Results from the N* Program at Jefferson Lab

We discuss the results on the fundamental degrees of freedom underlying the nucleon excitation spectrum and how they evolve as the resonance transitions are investigated with increasingly better space-time resolution of the electromagnetic probe. Improved photocouplings for a number of resonant states, those for the N(1720)P13 being significantly changed, have been determined and entered into the 2008 edition of the RPP. Strong sensitivity to the N(1900)P13 state, listed now as a 2-star state in the same edition of RPP, has been observed in KΛ and KΣ photoproduction. None of the earlier observations of a Θ+5(1540) was confirmed in a series of three Jefferson Lab high statistics dedicated measurements, and stringent upper limits on production cross sections were placed in several channels. For the four lowest excited states, the Δ(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11, the transition amplitudes have been measured in a wide range in photon virtuality Q2. The amplitudes for the Δ(1232) show the importance of the pion-cloud contribution and do not show any sign of approaching the pQCD regime for Q2 < 7 GeV2. For the Roper resonance, N(1440)P11, the data provide strong evidence for this state as a predominantly radial excitation of the nucleon as a 3-quark ground state. For the N(1535)S11, comparison of the results extracted from π and η photo- and electroproduction data allowed one to specify the branching ratios of this state to the πN and ηN channels; they entered into the 2010 edition of the RPP. Measured for the first time, the longitudinal transition amplitude for the N(1535)S11 became a challenge for quark models and can be indicative of large meson-cloud contributions or alternative representations of this state. The N(1520)D13 clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2 > 0.5 GeV2 confirming a long-standing prediction of the constituent quark model. The search for undiscovered but predicted states continues to be pursued with a vigorous experimental program. While recent data from Jefferson Lab and elsewhere provide intriguing hints of new states, final conclusions will have to wait for the results of the broad experimental effort currently underway with CLAS, and subsequent analyses involving the EBAC at Jefferson Lab.

Precision measurements of strangeness photoproduction at threshold energies with the Crystal Ball at MAMI-C

The photoproduction of K+ mesons from the nucleon provides important constraints on the nucleon excitation spectrum and at threshold energies challenges effective field theories based on chiral perturbation in the strange quark sector. Preliminary cross-section measurements for γ(p, K+)A are presented at an unprecedented beam energy resolution. The data was collected at the MAMI-C facility in Mainz using the Crystal Ball Detector. A new method of K+ detection was used in which the K+ is tagged from its weak decay products in the detector crystals. This technique has application with other calorimeters at present and future hadron facilities.

Recoil polarimetry in meson photoproduction at MAMI

The nucleon excitation spectrum remains poorly known, with the masses, widths, EM couplings and even existence of many states not well established. A program of experiments using meson photoproduction off the nucleon is being carried out to improve this situation. A new large acceptance recoil polarimeter has been developed by the Edinburgh group for use in such reactions with the Crystal Ball at MAMI. This work summarizes the procedure used to measure recoil polarization with the new device and presents some preliminary results for the double polarization observable Cx in the reaction γp → pπ0, compared to the current partial wave analysis.

MEASUREMENT OF POLARISATION TRANSFER IN HYPERON PHOTOPRODUCTION AT MAMI

The photoproduction of K+ mesons is an important challenge to recent QCD based chiral perturbation theories in the strange quark sector and is an important constraint on the nucleon excitation spectrum. We present preliminary data from a new high precision measurement using the Crystal Ball detector. The measurement pioneers a new technique for tagging strangeness using detailed cluster analysis in segmented calorimeters which has potential wider application at present and future hadron physics facilities.

Two-photon physics

It is reviewed how Compton scattering sum rules relate low-energy nucleon structure quantities to the nucleon excitation spectrum. In particular, the GDH sum rule and recently proposed extensions of it will be discussed. These extensions are sometimes more calculationally robust, which may be an advantage when estimating the chiral extrapolations of lattice QCD results, such as for anomalous magnetic moments. Subsequently, new developments in our description of the nucleon excitation spectrum will be discussed, in particular a recently developed chiral effective field theory framework for the Δ(1232)-resonance region. Within this framework, we discuss results on N and Δ masses, the γNΔ transition and the Δ magnetic dipole moment.

Two-photon physics

It is reviewed how Compton scattering sum rules relate low-energy nucleon structure quantities to the nucleon excitation spectrum. In particular, the GDH sum rule and recently proposed extensions of it will be discussed. These extensions are sometimes more calculationally robust, which may be an advantage when estimating the chiral extrapolations of lattice QCD results, such as for anomalous magnetic moments. Subsequently, new developments in our description of the nucleon excitation spectrum will be discussed, in particular a recently developed chiral effective field theory framework for the $\Delta(1232)$-resonance region. Within this framework, we discuss results on $N$ and $\Delta$ masses, the $\gamma N \Delta$ transition and the $\Delta$ magnetic dipole moment.

The helicity structure of the nucleon excitation spectrum

The helicity dependence of the total and partial photoproduction reaction channels has been measured for the first time in an energy range 200 MeV ≤ ν ≤ 800 MeV at MAMI using circularly polarized photons together with a longitudinally polarized proton target and a 4π detector system. The reaction γp → p π 0 is predominantly sensitive to the D 13(1520) resonance and the data are used to adjust its parameters.

Inelastic nucleon contributions in(e,e′)nuclear response functions

We estimate the contribution of inelastic nucleon excitations to the $(e,e^\prime)$ inclusive cross section in the CEBAF kinematic range. Calculations are based upon parameterizations of the nucleon structure functions measured at SLAC. Nuclear binding effects are included in a vector-scalar field theory, and are assumed have a minimal effect on the nucleon excitation spectrum. We find that for $q\lsim 1$ GeV the elastic and inelastic nucleon contributions to the nuclear response functions are comparable, and can be separated, but with roughly a factor of two uncertainty in the latter from the extrapolation from data. In contrast, for $q\rsim 2$ GeV this uncertainty is greatly reduced but the elastic nucleon contribution is heavily dominated by the inelastic nucleon background.