2009 Vol. 33, No. 12
Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on high Q2 nπ+ production better constrain the branching ratios βNπ and βNη. For the first time, the longitudinal transition amplitude to the S11(1535) was extracted from the nπ+ data. Also, new results on the transition amplitudes for the D13(1520) resonance are presented showing a rapid transition from helicity 3/2 dominance seen at the real photon point to helicty 1/2 dominance at higher Q2. I also discuss the status of the search for new excited nucleon states.
Recent physics results at Beijing Electron-Positron Collider (BEPC) are reviewed, especially on excited baryons. Substantial improvements are expected at the newly built BESIII experiment, which is operational since 2008. The detector performance is excellent and a total of 100 million ψ(2S) and 200 million J/ψ event are collected in 2009. Preliminary results shows that a new era of excited baryon physics will begin.
The present focus of the CBELSA/TAPS experiment is on meson-photoproduction off the nucleon using polarized photons and polarized targets. Here first preliminary results on the reactions γp→pπ0 and γp→pη for both circularly and linearly polarized photons and a longitudinally polarized target are presented. Preliminary results for the beam asymmetry ∑ and the double polarization observable G have been extracted for both reactions using the linear polarization data. From the circular polarization data the observable E has been determined. Due to the near 4π angular coverage of the detector system these results cover almost the full solid angle.
We discuss the analysis of data from πN elastic scattering and single pion photo- and electroproduction. The main focus is a study of low-lying non-strange baryon resonances. Here we concentrate on some difficulties associated with resonance identification, in particular the Roper and higher P11 states.
The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the
region below W=2 GeV and both single-Q2 and Q2 dependent transition form factors could be obtained for the Delta, Roper, D13(1520), S11(1535), S31(1620), S11(1650), D15(1675), F15(1680) and P13(1720). From the complete world data base, including also π－ data on the neutron, also Q2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transverse charge densities and our first results are given for the Roper, the S11 and D13 resonances.
We present an overview of a research program for the excited nucleon states in Excited Baryon Analysis Center (EBAC) at Jefferson Lab. Current status of our analysis of the meson production reactions based on the unitary dynamical coupled-channels model is summarized, and the N* pole positions extracted from the constructed scattering amplitudes are presented. Our plans for future developments are also discussed.
The present status of the Bonn-Gatchina partial wave analysis of the photoproduction and pion induced data is presented. An observation of signals which can be associated with new baryon resonances and possible interpretations of the baryon spectrum is discussed.
We present recent lattice results on the baryon spectrum, nucleon electromagnetic and axial form factors, nucleon to Δ transition form factors as well as the Δ electromagnetic form factors. The masses of the low lying baryons and the nucleon form factors are calculated using two degenerate flavors of twisted mass fermions down to pion mass of about 270 MeV. We compare to the results of other collaborations. The nucleon to Δ transition and Δ form factors are calculated in a hybrid scheme, which uses staggered sea quarks and domain wall valence quarks. The dominant magnetic dipole nucleon to Δ transition form factor is also evaluated using dynamical domain wall fermions. The transverse density distributions of the Δ in the infinite momentum frame are extracted using the form factors determined from lattice QCD.
The paper gives a brief introduction into the COSY-facility, the currently used detector systems (ANKE, TOF and WASA) and the targets (cluster jet, pellet and ABS), and it presents some recent results, which were obtained in the field of
baryon excited states.
Multi-meson final states such as γp→pπ0π0 and γp→pπ0η have been investigated with the Crystal-Barrel/TAPS experiment at ELSA. Baryon cascades via Δπ and Δη and also via higher mass baryon resonances are clearly observed. Based on this data and on data from other reactions a partial wave analysis has been performed from which partial decay widths of various N* and Δ* states into the different pπ0π0 and pπ0η decay channels have been determined, leading partly to unexpected results. In addition, polarisation observables have been investigated. The beam asymmetry ∑ has been determined for both reactions and double polarisation data using a longitudinally polarised target and a circularly or linearly polarised photon beam has been taken. Given the angular coverage of the Crystal-Barrel/TAPS experiment this data covers almost the full angular range and phase space. This data will provide new and important information for the partial wave analyses and bring us one step closer towards a complete experiment.
Complementary to the conventional experimental studies on N* from πN and γ(*)N reactions, the e+e－, pp and pp collisions can give novel insights into these N* resonances. While the e+e－ collisions through production and decay of vector charmonium ψ provide a nice isospin filter for a simultaneously study of N*, Δ*, Λ*, Σ* and Ξ*, the pp collisions should be the best place for producing those Δ*++ with large coupling to ρ+p though pp→nΔ*++ reaction, and the pp collisions should be the best place for looking for those N* with large coupling to σN.
The upgraded MAMI C accelerator is delivering electrons with an energy of 1558 MeV routinely. The A2 collaboration is doing experiments with energy marked polarised real photons produced via``Bremsstrahlung''. Recent results from the GDH-experiment using alongitudinal polarised frozen spin target in combination with the DAPHNE detector and from the Crystal Ball experiment at MAMI are
The analytic properties of scattering amplitudes provide a meeting point for
experimental and theoretical investigations of baryon resonances. Pole positions and residues allow for a parameterization of resonances in a well-defined way which relates different reactions. The recent progress made within the Jülich model is summarized.
In this talk I report on recent work related to the dynamical generation of
baryonic resonances, some made up from pseudoscalar meson-baryon, others from vector meson-baryon and a third type from two meson-one baryon systems. We can establish a correspondence with known baryonic resonances, reinforcing conclusions previously drawn and bringing new light on the nature of some baryonic resonances of higher mass.
We have developed an analytic continuation method for extracting parameters of nucleon resonances within a Hamiltonian formulation of meson-nucleon reactions. The method was tested for simple solvable models and then applied for our recent coupled channels model (πN, ηN, πΔ, ρN, and σN) of the πN and γ*N reactions. The resonance pole positions and their properties are studied for P33 and P11 channels.
Measurements of ω photoproduction of the Crystal-Barrel/TAPS experiment at the ELSA accelerator of Bonn University are presented which used linearly polarized tagged photon beam from threshold to Eγ=1700 MeV. The azimuthal asymmetries Σ and Σπ indicate s-channel resonance contributions on top of the established t-channel exchange processes. These findings are further enhanced by a very first measurement of the G-asymmetry which, in addition to the polarized photon beam, also requires a longitudinally polarized proton target. An intuitive interpretation of the specific sensitivity of the azimuthal asymmetries to the reaction mechanisms involved is given.
We discuss model-independent constraints on spin observables in exclusive and inclusive reactions, with special attention to the case of photoproduction.
The upgrade of the MAX-lab injector and the construction of MAX Ⅲ, provided the opportunity for upgrading the tagged-photon facility and thus lead to the possibility of more extensive program in nuclear physics research. This upgrade increased the injected electron energy to an eventual maximum of 250 MeV and allows for the extraction of electrons from the MAX Ⅰ ring operated in the stretcher mode. The first stretched beam was delivered in September 2005. The tagged-photon facility was commissioned in parallel with the commissioning of new experimental equipment. The PAC approved experimental program is current in progress, including measurements of pion photoproduction below the Δ(1232). The efforts at the tagged photon-facility are pursued within an international collaboration with around fifty members.
The study of exotic structure of hadrons is fully achieved when reaction dynamics of the associated hadron productions is well understood. We employ as the standard mechanism the effective Lagrangian method and investigate several physical observables within the framework. The parameters are constrained by microscopic description of hadrons. We discuss photoproductions of kaon associated with the ground state Λ(1116) and its resonances Λ(1405) and Λ(1520). In the former example we emphasize the meson cloud effect which significantly renormalizes the phenomenological parameters, while in the latter we discuss the features of the standard method. Finally we discuss briefly the production of ηπ associated with the nucleon resonance N(1535) for the study of chiral symmetry of baryons.
The reactions π－p→ηn and γp→ηp are investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region. The meson-baryon channels included are πN, ηN, πΔ, σN, and ρN. The direct η-photoproduction process is studied within a formalism based on a chiral constituent quark model approach, complemented with a one-gluon-exchange mechanism, to take into account the breakdown of the SU(6)⊙O(3) symmetry. In the models search, the following known nucleon resonances are embodied: S11(1535), S11(1650), P11(1440), P11(1710), P13(1720), D13(1520), D13(1700), D15(1675), and F15(1680). Data for the π－p→ηn reaction from threshold up to a total center-of-mass energy of W≈2 GeV are satisfactorily reproduced. For the photoproduction channel, two additional higher mass known resonances, P13(1900) and F15(2000), are also considered. However, reproducing the data for γp→ηp requires, within our approach, two new nucleon resonances, for which we extract mass and width.
The spectrum and properties of the excited states of the nucleon are still poorly established despite decades of study. These proceedings describe new measurements of pseudo-scalar meson photoproduction at the Crystal Ball at MAMI, employing a new large acceptance nucleon recoil polarimeter. The double- and single-polarisation observables obtained will provide valuable and unique data to be used as part of the world effort to improve our knowledge of this fundamental spectrum.
Two basic motivations for an upgraded JLab facility are the needs: to determine the essential nature of light-quark confinement and dynamical chiral symmetry breaking (DCSB); and to understand nucleon structure and spectroscopy in terms of QCD's elementary degrees of freedom. During the next ten years a programme of experiment and theory will be conducted that can address these questions. We present a Dyson-Schwinger equation perspective on this effort with numerous illustrations, amongst them: an interpretation of string-breaking; a symmetry-preserving truncation for mesons; the nucleon's strangeness σ-term; and the neutron's charge distribution.
It is unavoidable to deal with the quark and gluon momentum and angular momentum contributions to the nucleon momentum and spin in the study of nucleon internal structure. However we never have the quark and gluon momentum, orbital angular momentum and gluon spin operators which satisfy both the gauge invariance and the canonical momentum and angular momentum commutation relation. The conflicts between the gauge invariance and canonical quantization requirement of these operators are discussed. A new set of quark and gluon momentum, orbital angular momentum and spin operators, which satisfy both the gauge invariance and canonical momentum and angular momentum commutation relation, are proposed. The key point to achieve such a proper decomposition is to separate the gauge field into the pure gauge and the gauge covariant parts. The same conflicts also exist in QED and quantum mechanics and have been solved in the same manner. The impacts of this new decomposition to the nucleon internal structure are discussed.
A large part of the experimental program in Hall B of the Jefferson Lab is dedicated to baryon spectroscopy. Photoproduction experiments are essential part of this program. CEBAF Large Acceptance Spectrometer (CLAS) and availability of circularly and linearly polarized tagged photon beams provide unique conditions for this type of experiments. Recent addition of the Frozen Spin Target (FROST) gives a remarkable opportunity to measure double and triple polarization observables for different pseudo-scalar meson photoproduction processes. For the first time, a complete or nearly complete experiment becomes possible and will allow model independent extraction of the reaction amplitude. An overview of the experiment and its current status is presented.
Recent results on nucleon resonance studies in π+π－p electro- production off protons with the CLAS detector are presented. The analysis of CLAS data allowed us to determine all essential contributing mechanisms, providing a credible separation between resonant and non-resonant parts of the cross sections in a wide kinematical area of invariant masses of the final hadronic system 1.3
The measurements of exclusive single-meson and double-pion electro-production cross sections off the proton to study nucleon resonances will be extended to higher momentum transfers with the CLAS12 detector and the energy upgraded CEBAF beam. Based on new theoretical developments to extract and interpret the electromagnetic transition form factors and on the experience gained from the most recent results, the newly formed collaboration of experimentalists and theorists shall enable us to provide unprecedented high-precision data, high-quality analyses, and state-of-the-art model and QCD based calculations in a Q2 domain up to 10 GeV2. For the first time nucleon resonance structures will be studied at still unexplored distance scales, where the dressed quark contributions are the dominating degrees of freedom and their strong interaction is responsible for the ground and excited nucleon state formation. These studies also open up a promising opportunity to understand the origin of more than 98% of the nucleon mass that is created by strong fields predominantly at these distance scales by dressing the current quarks.
In this paper we discuss the reasons for our work towards establishing a new collaboration between Jefferson Lab (JLab) and the Institute of High Energy Physics (IHEP) in Beijing. We seek to combine experimentalists and theorists into a dedicated group focused on better understanding the current and future data from JLab and from the Beijing Electron Positron Collider (BEPC).
Recent JLab results on the extraction of single- and double-polarization observables in both the 1π- and 2π-channel show their high sensitivity to small production amplitudes and therefore their importance for the extraction of resonance parameters. The Beijing Electron Spectrometer (BES) at the BEPC has collected high statistics data on J/ψ production. Its decay into baryon-antibaryon channels offers a unique and complementary way of probing nucleon resonances. The CEBAF Large Acceptance Spectrometer, CLAS, has access to N* form factors at high Q2, which is advantageous for the study of dynamical properties of nucleon resonances, while the low-background BES results will be able to provide guidance for the search for less-dominant excited states at JLab. Moreover, with the recently approved experimental proposal Nucleon Resonance Studies with CLAS12 and the high-quality data streaming from BES-III and CLAS, the time has come for forging a new Trans-Pacific collaboration of theorists and experimentalists on NSTAR physics.
A summary of the contributions to this workshop is given. The status and future development of N* physics are discussed.
We show the lattice QCD results on the axial charge gAN*N* of negative parity nucleon resonances, N*(1535) and N*(1650), which are key clues to the chiral structure in baryon sector. The measurements are performed with up and down dynamical quarks employing the renormalization-group improved gauge action at β=1.95 and the O(a) improved clover quark action with the hopping parameters, κ=0.1375, 0.1390 and 0.1400. In order to properly separate signals of N*(1535) and N*(1650), we construct 2×2 correlation matrices and diagonalize them. Wraparound contributions in the correlator, which can be another source of signal contaminations, are eliminated by imposing the Dirichlet boundary condition in the temporal direction. We find that the axial charge of N*(1535) takes small values as gAN*N*~O(0.1), whereas that of N*(1650) is about 0.5, which is found independent of quark masses and consistent with the predictions by the naive nonrelativistic quark model.
In this proceeding, I review the attempts to calculate the Nucleon resonance (including Roper as first radially excited state of nucleon and other excited states) using lattice quantum chromodynamics (QCD). The latest preliminary results from Hadron Spectrum Collaboration (HSC) with mπ≈380 MeV are reported. The Sachs electric form factor of the proton and neutron and their transition with the Roper at large Q2 are also updated in this work.
The electromagnetic probe is an excellent tool to investigate the structure of the nucleon. The nearly 4π detector PANDA, will allow to make a precise determination of the electromagnetic form factors of the proton in the time-like region with unprecedented precision. In the one-photon exchange approximation, the center of mass unpolarized differential cross section of the reaction pp→e+e－ is a linear combination of the squared moduli of the electric GE and magnetic GM proton form factors. The precise measurement of the angular distribution over almost full angular range then directly gives these quantities. At present only two experiments have provided the ratio R=|GE|/|GM| but with large statistical uncertainties. It is shown that with strict PID cuts and a kinematic fit, the dominant background, pp→π+π－, can be supressed to much less than 1 % of the signal, without affecting the extraction of the ratio R. PANDA will therefore offer a unique opportunity to measure the ratio with a precision ranging from <1% at low q2 up to 30% for q2 = 14 (GeV/c)2.
The GRAAL experimental set-up consists of a polarized and tagged photon beam that covers an energy range from a minimum of 600 MeV up to a maximum of
1500 MeV, of a liquid Hydrogen or Deuterium target and of the 4π Lagrange detector optimized for photon detection. It allows the study of pseudo-scalar and vector meson photoproduction on the nucleon in the energy range corresponding to the second and the third resonance regions. In the following, the Σ beam asymmetries in η and π0 photoproduction on quasi-free nucleon are shown. Also single and double polarization observables in K+Λ photoproduction on free proton are shown; they are important to confirm the role of new or poorly known resonances in the 1900 MeV mass region.
We report the analysis status of single π0 electroproduction in the resonance region to study the electromagnetic excitation of the nucleon resonances. The study is aimed at understanding of the internal structure and dynamics of the nucleon. The experiment was performed using an unpolarized cryogenic hydrogen target and 2.0 and 5.8 GeV polarized electron beam during the e1e and e1-6 run periods with CLAS at Jefferson Lab. The new measurements will produce a data base with high statistics and large kinematic coverage for the hadronic invariant mass (W) up to 2.0 GeV in the momentum transfer (Q2) range of 0.3—6.0 GeV2. Preliminary results will be presented and compared with the various model calculations.
A group theoretical derivation of a relation between the N→Δ charge quadrupole transition and neutron charge form factors is presented.
We present recent results from Jefferson Lab on sum rules related to the spin structure of the nucleon. We then discuss how the Bjorken sum rule with its connection to the Gerasimov-Drell-Hearn sum, allows us to conveniently define an effective coupling for the strong force at all distances.
Constituent quark models based on two-body potentials systematically overpredict the mass of ΔD35(1930). A possible solution to this problem comes out from the application of a schematic hybrid model, containing three-quark as well as meson-baryon components, to the light-quark baryon spectrum. The ΔD35(1930) and its partners ΔD33(1940) and ΔS31(1900) are found to contain a significant ρΔ component. Then, through the use of the hidden gauge formalism, it is shown that these resonances can be dynamically generated from the ρ-Δ interaction. In particular ΔD35(1930) can be interpreted as being essentially a ρΔ bound state. This interpretation suggests that the inclusion of ρΔ as an effective inelastic channel in data analyses could improve the extraction and identification of the resonance.
In this talk, I present the results on the pole structure of pion-nucleon scattering in an analytic model based on meson exchange. The analytic properties of scattering amplitudes provide important information. Besides the cuts, the poles and zeros on the different Riemann sheets determine the global behavior of the amplitude on the physical axis. Pole positions and residues allow for a parameterization of resonances in a well-defined way, free of assumptions for the background and energy dependence of the resonance part. This is a necessary condition to relate resonance contributions in different reactions.
We present recent investigations on the vector and axial-vector transitions of the baryon antidecuplet within the framework of the self-consistent SU(3) chiral quark-soliton model, taking into account the 1/Nc rotational and linear ms corrections. The main contribution to the electric-like transition form factor comes from the wave-function corrections. This is a consequence of the generalized Ademollo-Gatto theorem. It is also found that in general the leading-order contributions are almost canceled by the rotational 1/Nc corrections. The results are summarized as follows: the vector and tensor $\rm K*NΘ coupling constants, gK*NΘ= 0.74—0.87 and fK*NΘ= 0.53—1.16, respectively, and ΓΘ→KN=0.71 MeV, based on the result of the KNΘ coupling constant gKnΘ=0.83. We also show the differential cross sections and beam asymmetries, based on the present results. We also discuss the connection of present results with the original work by Diakonov, Petrov, and Polyakov.
The six- and four-quark systems are studied in the framework of constituent quark models. It is emphasized that the color confinement used in multiquark system should be different from the one used in two- or three-quark system. For six-quark system, we look for ΔΔ and NΔ dibaryon resonances by calculating NN scattering phase shifts with explicit coupling to these dibaryon channels in quark delocalization and color screening model. The model gives a good description of low-energy NN properties and predicts IJP = 03+ and 01+ ΔΔ resonances, which can be promising candidates for the isoscalar ABC structure reported by the CELSIUS-WASA Collaboration. For tetraquark system, a flux-tube quark model with multi-body confinement interaction is employed to study Y(2175) as a tetraquark state. The Y(2175) with diquark-antidiquark structure has energy 2174 MeV which is very consistent with experimental data. The calculation shows that multi-body confinement potential may play a vital role in the multiquark system.
The η' photoproduction process on quasi-free proton and neutron and the reaction NN→η′NN are investigated within a relativistic effective Lagrangian approach to hadronic interactions. Resonances with spins 1/2 and 3/2 are considered together with the nucleonic and t-channel meson-exchange current contributions. In photoproduction processes, the S11 resonance is found to be responsible for the sharp rise of the cross sections near threshold. In pp→η′pp, it is found that the S11 resonance dominates the total cross section over the entire energy region considered. The spin observables, in particular the beam and target asymmetries, are shown to be very sensitive to the reaction mechanism and will help impose more stringent constraints on the model parameters.
Within the Excited Baryon Analysis Center we have performed a dynamical coupled-channels analysis of the available p(e,e' π)N data in the region of W≤ 1.6 GeV and Q2≤1.45 (GeV/c)2. The channels included are γ*N, πN, ηN, and ππN which has πΔ, ρN, and σN components. With the hadronic parameters of the model determined in our previous investigations of πN→πN reaction, we have found that the available data in the considered W≤1.6 GeV region can be fitted well by only adjusting the bare γ*N→N* helicity amplitudes for the lowest N* states in P33, P11, S11 and D13 partial waves. The meson cloud effect, as required by the unitarity conditions, on the γ*N→N* form factors are examined.
We have studied the ππN system and coupled channels by using of the Faddeev equations and two N* and one Δ states, all of them with JP=1/2+, have been found in the formalism as dynamically generated states. In addition, signatures for a new N* resonance with JP=1/2+ are found around an energy of 1920 MeV in the three-body center of mass system.
In this talk, we firstly overview the experimental status of NX(1625), which is an enhancement structure observed in K－Λ invariant mass spectrum of J/ψ→pK－Λ process. Then we present the result of the decay of NX(1625) under the two molecular assumptions, i.e. S-wave ΛK－ and S-wave Σ0K－ molecular states. Several experimental suggestions for NX(1625) are proposed.
We report our recent work on a hadronic molecule state of the KKN system with I=1/2 and JP=1/2+. We assume that the Λ(1405) resonance and the scalar mesons, f0(980), a0(980), are reproduced as quasi-bound states of KN and KK, respectively. Performing non-relativistic three-body calculations with a variational method for this system, we find a quasibound state of the KKN system around 1910 MeV below the three-body breakup threshold. This state corresponds to a new baryon resonance of N* with JP=1/2+. We find also that this resonance has the cluster structure of the two-body bound states keeping their properties as in the isolated two-particle systems. We also briefly discuss another hadronic molecular state composed by two K and one N, which corresponds to a Ξ* resonance.
We present our recent study of pion-nucleon charge exchange amplitudes above 2 GeV. We analyze the forward pion-nucleon charge exchange reaction data in a Regge model and compare the resulting amplitudes with those from the Karlsruhe-Helsinki and George-Washington-University partial-wave analyses. We explore possible high-energy constraints for theoretical baryon resonance analyses in the energy region above 2 GeV. Our results show that for the pion-nucleon charge exchange reaction, the appropriate energy region for matching meson-nucleon dynamics to diffractive scattering should be around 3 GeV for the helicity flip amplitude.
We study charmed baryon resonances that are generated dynamically from a coupled-channel unitary approach that implements heavy-quark symmetry. Some states can already be identified with experimental observations, such as Λc(2595), Λc(2660), Σc(2902) or Λc(2941), while others need a compilation of more experimental data as well as an extension of the model to include higher order contributions. We also compare our model to previous SU(4) schemes.
The ``good" diquark is employed to study Λc+ baryons within a mass loaded flux tube model. The study indicates that all Λc+ baryons candidates in the 2008 review by the Particle Data Group (PDG) are well described in the mass loaded flux model. The quantum numbers JP of these Λc+ candidates are assigned. If Λc(2765)+ is an orbitally excited Λc+, it is likely the JP=3/2+ one. If Λc(2765)+ is an orbitally excited Σc, there ought to be another JP=3/2+ Λc+ with mass ≈2770 MeV. In the model, there exists no JP=1/2+ Λc+(≈2700) predicted in existing literature. Λc(2940)+ is very possible the orbitally excited baryon with JP=5/2－.
Based on 58 million J/ψ data collected with the BESⅡ detector at the BEPC, Partial Wave Analysis(PWA) is performed on J/ψ→ppπ0. The new excited baryon N(2065) is confirmed. Clear signals for other N* states are observed and the corresponding masses, widths and spin-parity are also measured. Processes such as J/ψ→ppη, ppη′ are also studied and the branching ratios are measured.
The contributions of two-photon exchange in the process e++e－ →p+p including N and Δ intermediate states are estimated in a simple hadronic model. The corrections to the unpolarized cross section as well as to the polarized observables Px and Pz are evaluated. The results show the corrections to unpolarized cross section are small and the angle dependence becomes weak at small s after considering the N and Δ(1232) contributions simultaneously, while the correction to Pz is enhanced.
Photoproduction of mesons off the deuteron has been investigated at a tagged
photon beam of the Bonn ELSA accelerator with the combined Crystal Barrel - TAPS electromagnetic calorimeter for incident photon energies up to 2.5 GeV. The mesons have been detected in coincidence with recoil protons, neutrons and deuterons. This allow the measurement of meson production reactions off the quasifree nucleons bound in the deutron, as well as the coherent production off the deuteron. The comparison of quasifree proton reactions to free proton reactions can confirm or invalidate possible nuclear effects on the extracted cross section reactions. Furthermore the isospin composition of a resonance can be estimated from the comparison of quasifree proton and neutron reactions. The quasifree photoproduction of the η´ and π°η mesons off nucleons and the coherent photoproduction of π°η-pairs off the deuteron are discussed.
Quasi-free photoproduction of η-mesons off the neutron and off the proton has been studied using a deuterium target and bremsstrahlung photons produced by MAMI-C with incident energies up to 1.5 GeV. The η-mesons were detected in coincidence with the recoil nucleons thus a fully exclusive measurement was performed. Preliminary results show a bump-like structure in the excitation function for the neutron close to W≈1675 MeV which is not seen for the proton. Considering the experimental resolution and using a Breit-Wigner fit the width of this structure was approximated below 50 MeV.
Observables from vector meson photoproduction by linearly-polarized photons can be expressed in term of bilinear combinations of helicity amplitudes parameterized by the Spin Density Matrix Elements (SDMEs). These SDMEs give
straightforward relations for understanding the nature of the parity exchange at threshold energies, as well as for extracting signatures of the Okubo-Zweig-Iizuka violation. This paper will show preliminary measurements of SDMEs for γp→φp in the photon energy range of 1.7 to 1.9 GeV (momentum transfer squared t range of －1.2 to －0.25 GeV2) and 1.9 to 2.1 GeV (t range of －1.4 to －0.25 GeV2) from the g8b experimental data collected in the summer of 2005 in the Hall B of Jefferson Lab.
We analyze the left-right asymmetry in the semi-inclusive deep inelastic scattering (SIDIS) process using a method where no weighting function are used. Considering all flavor of quarks, we reanalyze the π± production and extend our calculation on the K± production. The predictions on HERMES, COMPASS and JLab kinematics with transversely polarized nucleon target are shown in this paper.
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+)Λ 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.
We report our investigation on the octet baryon spectrum in the nonrelativistic quark model by taking into account the two-gluon exchange effect. The calculated octet baryon masses agree better with the experimental data. It is also shown that the two-gluon exchange interactions bring a significant correction to the Gell-Mann−Okubo mass formula.
The dynamically generated resonances from vector meson-baryon decuplet are studied using Lagrangians of the hidden gauge theory for vector interactions. One shows that some of the generated states can be associated with some known baryon resonances in the PDG data, while others are predictions for new states. Furthermore, we calculate the radiative decay widths of these resonances into a photon and a baryon decuplet.
We present a preliminary analysis of the photon beam asymmetry observable
(Σ) from the photoproduction reaction channel γn→pπ－ in the invariant mass range 1.6—2.3 GeV. The measurement was obtained using the near-4π CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory, USA, employing a linearly polarised photon beam with an energy range 1.1—2.3 GeV, incident on a liquid deuterium target. The measurement will provide new data to address the poorly established neutron excitation spectrum and will greatly expand the sparse world data-set both in energy and angle.
A chiral quark-model approach is extended to the study of the KN scattering at low energies. The process of K－p→Σ0π0 at PK\≤800 MeV/c (i.e. the center mass energy W≤1.7 GeV) is investigated. The Λ(1405)S01 dominates the reactions over the energy region considered here. Around PK=400 MeV/c, the Λ(1520)D03 is responsible for a strong resonant peak in the cross section. Our analysis suggests that there exist configuration mixings within the Λ(1405)S01 and Λ(1670)S01 as admixtures of the [70,21,1/2] and [70,28,1/2] configurations. The Λ(1405)S01 is dominated by [70,21,1/2], and Λ(1670)S01 by [70,28,1/2]. The non-resonant background contributions, i.e. u-channel and $t$-channel, also play important roles in the explanation of the angular distributions due to amplitude interferences.
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.
In an effective Lagrangian model we find that the N*(1535) resonance contribution might be important to the interpretation of the present data of the pp→ppη´ and pn→dφ reactions. The strong coupling strength of N*(1535) to η´ and φ are indicated, and the possible implication to the intrinsic component of N*(1535) is explored. These results may provide hints to the real origin of the OZI rule violation. It is stressed that further measurements could be performed at the Cooling Storage Ring (CSR) at Lanzhou of China.
In this talk, we give a short review of our recent works on studying the singly heavy baryon, doubly heavy baryon, and triply heavy baryon spectra from QCD sum rules.
In this report we investigate η-meson productions on the proton via electromagnetic and hadron probes in a chiral quark model approach. The observables, such as, differential cross section and beam asymmetry for the two productions are calculated and compared with the experiment. The five known resonances S11(1535), S11(1650), P13(1720), D13(1520), and F15(1680) are found to be dominant in the reaction mechanisms in both channels. Significant contribution from a new S11 resonances are deduced. For the so-called ``missing resonances'', no evidence is found within the investigated reactions. The partial wave amplitudes for π－p→ηn are also presented.
Here we employ the extended chiral constituent quark model to investigate the five-quark components in the N*(1535) resonance. The axial charge of N*(1535) and the electromagnetic transition γ* N→N*(1535) are also analyzed. The results show that there may be sizable strangeness component in N*(1535).
In order to describe Kaon-Nucleon scattering data, the mixing of scalar meson σ0 and σ8 must be introduced in the chiral SU(3) quark model. Inspired by this, now the mixing of scalar meson is further considered to study some interesting dibaryons in the chiral SU(3) quark model. The results show that the mixing of scalar meson has different effects on these dibaryons.
The results of two-boson-exchange effects in the parity-violating elastic electron-proton scattering are reported based on a simple hadronic model. The corrections are calculated including the nucleon and Δ(1232) intermediate states. And the numerical results are also compared with the recent results reported by other group and other methods.
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