Search

The study of the spectrum and structure of excited nucleon states employing the electroproduction of exclusive reactions is an important avenue for exploring the nature of the non-perturbative strong interaction. The CLAS detector in Hall B has provided the dominant part of the available world data on most relevant meson electroproduction channels off the nucleon in the resonance region for Q2 up to 5 GeV2. Analyses of CLAS data for the exclusive channels πN, ηN, and π+π−p on a proton target have provided the only results available on the Q2 evolution of the electro-excitation amplitudes for the transitions from the initial photon-proton to the final N* states in the mass range up to W =1.8 GeV. These electrocouplings allow for exploration of the internal structure of the produced excited nucleon states. This work has made it clear that consistent results from independent analyses of several exclusive channels with different resonance hadronic decay parameters and non-resonant backgrounds but the same N* electro-excitation amplitudes, is essential to have confidence in the extracted results. Starting in early 2018, a program to study the spectrum and structure of N* states in various exclusive electroproduction channels using the new CLAS12 spectrometer commenced. These studies will probe the structure of N* states in the mass range up to W =3 GeV and for Q2 as low as 0.05 GeV2 and as high as 10-12 GeV2, thus providing a means to access N* structure information spanning a broad range of distance scales. Quasi-real photoproduction studies are also planned to search for additional N* states, the so-called hybrid baryons, for which the glue serves as an active structural component. In this talk the N* programs from both CLAS and CLAS12 will be reviewed. [ABSTRACT FROM AUTHOR]

One of the most challenging topics in Quantum Chromo-Dynamics (QCD) for decades is the study of nucleon structure in terms of the fundamental QCD picture of quarks and gluons. Performing this study over a range of energies helps understand the dynamics of strong interaction, and gives a reasonable description of the transition from colored confined partons to the ordinary colorless hadrons. One of the best tools to study this transition is to search for the onset of Color Transparency (CT), one of the predicted phenomena of QCD. Color transparency refers to the suppression of final (and/or initial) state interactions caused by the cancellation of color fields in a special configuration of quarks and gluons with small transverse separation. I will give an overview of the CT measurements that were carried through the production of different hadrons at various energies, and highlight the future experiments planned for the 12 GeV upgrade at Jefferson Lab. [ABSTRACT FROM AUTHOR]

Meson-baryon scattering and photon-nucleon reactions are described in a coupled-channel approach for energies below 2 GeV. Poles and residues of the reaction amplitudes are obtained by analytical continuation. The dependence of the resonance poles on the pion mass is investigated. [ABSTRACT FROM AUTHOR]

We present the current status of a combined and simultaneous analysis of meson production reactions based on a dynamical coupled-channels (DCC) model, which is conducted at Excited Baryon Analysis Center (EBAC) of Jefferson Lab. [ABSTRACT FROM AUTHOR]

We demonstrate the equivalence of the light-front, equal-time and covariant formulations in meson-baryon interactions. In particular, we discuss the self-energy Σ of a nucleon dressed by pion loops with the pseudovector πNN coupling. It is shown that the leading nonanalytic behavior of Σ is equivalent whichever formulations are used for the derivation. [ABSTRACT FROM AUTHOR]

We report the current status of exploring the dynamical aspect of the excited nucleon states through the comprehensive coupled-channels analysis of meson production reactions at the Excited Baryon Analysis Center of Jefferson Lab. [ABSTRACT FROM AUTHOR]

We investigate the scattering of heavy-light K and D mesons by nucleons at low energies. The short-distance part of the interaction is described by quark-gluon interchange and the longdistance part is described by a one-meson-exchange model that includes the contributions of vector (ρ, ω) and scalar (σ) mesons. The microscopic quark model incorporates a confining Coulomb potential extracted from lattice QCD simulations and a transverse hyperfine interaction consistent with a finite gluon propagator in the infrared. The derived effective meson-nucleon potential is used in a Lippmann-Schwinger equation to obtain s-wave phase shifts. Our final aim is to set up a theoretical framework that can be extended to finite temperatures and baryon densities. [ABSTRACT FROM AUTHOR]

We discuss the physics of Bs0 mesons focusing on CP violation in Bs0→J/ψ[lowercase_phi_synonym] decays at the Tevatron. We summarize measurements of the properties of bottom baryons at the Tevatron including the Σb states and the Ξb- baryon. We also discuss the discovery of the Ωb- baryon. [ABSTRACT FROM AUTHOR]

Leading neutron production measurements from the H1 and ZEUS experiments at HERA are presented and compared to models. Standard string fragmentation models do not describe the data. Models including also baryon production via virtual meson exchange and accounting for absorption give a good description of the data. In the exchange picture, leading neutron data are used to extract the pion structure function. [ABSTRACT FROM AUTHOR]

Understanding quark and gluon confinement in Quantum Chromodynamics is one of the main issue in hadronic physics. Electromagnetic probes, real and virtual photons, are a powerful tool to investigate how the QCD-partons manifest themselves in strong interaction at the energy scale of the nucleon mass (GeV). The CLAS Collaboration in Hall B at Jefferson Lab (USA) has a broad scientific program ranging from the static and dynamic properties of constituent quarks to the investigation of possible exotic baryon configurations. After a brief description of the CLAS detector, some selected results will be outlined showing the richness of this non-perturbative physics. [ABSTRACT FROM AUTHOR]

In this work, the strong coupling constants of the octet baryons to the K and π mesons are studied within the Light Cone QCD sum rule method. It is well known that in the SU(3)f limit, these couplings can be expressed in terms of only 2 constants, so called F and D constants. In this work, the SU(3)f symmetry violating effects are also studied and it is shown that all the coupling constants can be expressed in terms of four analytical functions of the quark parameters. [ABSTRACT FROM AUTHOR]

The Cronin effect is studied with identified particle spectra from 200GeV d+Au collision at RHIC. The nuclear modification factor RdAu of baryons (proton etc.) rise faster than those of mesons (pions, kaons and phi etc.) and show a baryon/meson scaling behavior similar to that found at Au+Au collisions at same energy. The particle-species dependence of the Cronin effect is observed to be significantly smaller than that at lower energies. The eta asymmetry is also reported and shows contrary trends to prediction based on initial multiple parton scattering model. These measurements indicate that the final state effect plays an important role in the Cronin effect. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Identified particle measurements in 200 GeV Au+Au and d+Au collisions in STAR are presented. The importance of using identified particles to probe properties of medium formed at RHIC is emphasized. Magnitude of the nuclear modification factor is observed to depend on particle type (baryon vs meson) instead of particle mass at intermediate transverse momentum. This observation supports recombination/coalescence processes as dominant dynamics for particle production in this transverse momentum region. We argue that [lowercase_phi_synonym] and heavy quark yields are sensitive to the medium properties at the early stage. The significant suppression of identified particle production observed at high transverse momentum in central Au+Au collisions indicates the formation of strongly-interacting medium at RHIC. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present a precision measurement of the mass of the Λc+ and studies of the production and decay of the Ωc0 and [cap_xi]c0 charm baryons using data collected by the BABAR experiment. To keep the systematic uncertainty as low as possible, the Λc+ mass measurement is performed using the low Q-value decays, Λc+ → Λ0KS0K+ and Λc+ → Σ0KS0K+. Several hadronic final states involving an Ω- and a [cap_xi]- hyperon are analyzed to reconstruct the [cap_xi]c0 and the Ωc0. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We study S-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons using the lowest order chiral Lagrangian. We find two bound states in the SU(3) limit corresponding to the octet and decuplet representations. These are found to split into eight different trajectories in the complex plane when the SU(3) symmetry is broken gradually. Finally, we are able to provide a reasonable description for a good number of 4-star 3/2- resonances listed by the Particle Data Group. In particular, the [cap_xi](1820), the Λ(1520) and the Σ(1670) states are well reproduced. We predict a few other resonances and also evaluate the couplings of the observed resonances to the various channels from the residues at the poles of the scattering matrix from where partial decay widths into different channels can be evaluated. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A combined analysis of new π0 and η photoproduction data from CB-ELSA collaboration was performed including the earlier data from SAID data base, new data on beam asymmetry from the GRAAL collaboration and data on γp → KΛ and KΣ. Three baryon resonances have a substantial coupling to ηN, the well known N(1535)S11, N(1720)P13 and the newly observed N(2070)D15 resonance. A new P11 state with mass 1840 MeV and width 140 MeV was observed contributing to most of the fitted reactions. The data also demand the presence of two D13 states at 1870 and 2170 MeV. The properties of observed states and their contributions to different observables are discussed. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We study the Λ(1520) resonance in a coupled channel approach involving the πΣ(1385), K[cap_xi](1530), KN and πΣ channels. Implementing unitarity in coupled channels, we make an analysis of the relative importance of the different mechanisms which contribute to the dynamical structure of this resonance. From experimental information on some partial wave amplitudes and constraints imposed by unitarity, we get a comprehensive description of the amplitudes and hence the couplings to the different channels. We test these amplitudes in different reactions like K-p → Λππ, γp → K+K-p, γp → K+π0π0Λ and π-p → K0K-p and find a fair agreement with the experimental data. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The K-p → π0π0Σ0 reaction is studied within a chiral unitary model. The distribution of π0Σ0 states forming the Λ(1405) shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of Γ = 38 MeV. We use these data in combination with those of the π-p → K0πΣ reaction and elements of chiral unitary theory to prove that there are two Λ(1405) states instead of one as so far assumed. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We study the mass spectrum of baryons with two and three charmed quarks. For double charm baryons the spin splitting is found to be smaller than standard quark-model potential predictions. This splitting is not influenced either by the particular form of the confining potential or by the regularization taken for the contact term of the spin-spin potential. We consistently predict the spectra for triply charmed baryons. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A program to measure the two-pion production in nucleon-nucleon collisions exclusively and in all channels from threshold up to Tp = 1360 MeV has been started at CELSIUS-WASA. First preliminary results for ppπ+π¯ and ppπ0π0 channels are presented. In the near-threshold region they are consistent with a dominance of the Roper excitation and its successive decay into Nσ and Δπ channels. At energies Tp > 1 GeV we observe a very different behavior, which is at variance with theoretical predictions. At COSY-TOF exclusive measurements of the pp → ppπ+π¯ reaction have been carried out with polarized beam. Preliminary results show non-zero analyzing powers which point to a sizeable admixture of l ≠ 0 partial waves indicating possibly an influence of the ρ-channel. [ABSTRACT FROM AUTHOR]

We compute magnetic moments of baryons with a heavy quark in the bound state approach for heavy baryons. In this approach the heavy baryon is considered as a heavy meson bound to a light baryon. The latter is represented as a soliton excitation of light meson fields. We obtain the magnetic moments by sandwiching pertinent components of the electromagnetic current operator between the bound state wave-functions. We extract this current operator from the coupling to the photon field after extending the action to be gauge invariant. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Sufficiently fast chemical equilibration of (strange) antibaryons in an environment of nucleons, pions and kaons during the course of a relativistic heavy ion collision can be understood by a 'clustering' of mesons to buildt up baryon-antibaryon palrs. This multi-mesonic (fusion-type) process has to exist in medium due to the principle of detailed balance. Novel numerical calculations for a dynamical setup are presented. They show that - at maximum SPS energies - yields of each antihyperon specie are obtained which are consistent with chemical saturated populations of T ≈ 150 -- 160 MeV, in line with popular chemical freeze-out parameters extracted from thermal model analyses. [ABSTRACT FROM AUTHOR]

It is interesting that the fields of meson and baryon spectroscopy have taken divergent paths in the last decade. It is time the fields were more aware of the similarities and this author hopes the fields will benefit from each other's accomplishments. This note will point out the similarities and the differences. Sometimes, the differences are in nomenclature. However, there are also practical reasons for the divergences. The typical meson experiment has a more complicated final state than baryon experiments and the most complete theoretical treatments limit the number of final state particles to three. There are also important differences in sociology. [ABSTRACT FROM AUTHOR]

Coherent π[sup +]&pi[sup -]π[sup -] production in the interactions of a beam of 600 GeV π[sup -] mesons with C, Cu and Pb nuclei has been studied with the SELEX facility (Experiment E781 at Fermilab). The a[sub 2] (1320) meson signal has been detected in the Coulomb (low q[sup 2]) region. The Primakoff formalism used to extract radiative decay width of this meson yields F = 284 ± 25 ± 25 keV, which is the most precise measurement to date. [ABSTRACT FROM AUTHOR]

The relativistic string with massive ends describing the meson and four various string baryon models (quark-diquark, linear, three-string and triangle) are considered. For all these systems the rotational motions or planar uniform rotations are used for describing orbitally excited hadron states on the Regge trajectories. We analyze small disturbances of these rotations (quasirotational states) with different methods. This study demonstrates that the classical rotational motions for the meson-like model and the triangle string baryon model are stable, but they are unstable for the linear configuration (q-q-q) and for the three-string (Y). For the latter two models the spectrum of the quasirotational oscillations contains exponentially growing modes. But for the string with massive ends we have two types of these modes describing stationary waves without growth. The obtained quasirotational states are useful for quantization and describing higher radial hadron excitations. [ABSTRACT FROM AUTHOR]

The I=1 and I=0 kaon-nucleon s, p, d, f, g-waves phase shifts have been calculated in a non relativistic quark potential model using the resonating group method (RGM). The interquark potential includes gluon exchanges with a spin-orbit interaction. This force was determined to reproduce as well as possible the meson and baryon spectra. The same force is employed for the cluster and intercluster dynamics and the relative KN wave function is calculated without any approximation. While some channels are correctly described, the theory is still unable to explain others. [ABSTRACT FROM AUTHOR]

A significant amount of experimental data for particle production in high-energy heavy ion collisions (10 - 200 GeV/A at center of mass) has been accumulated during last years. Many different theoretical attempts have tried to describe these data using thermal models in the approximation of global thermal equilibrium considering only one freeze-out temperature. However the thermal models often are not able to describe adequately the whole multiplicities of hadrons. For instance, the abundance of strange particles is overestimate and the pion yields are underestimated. In this work is presented a thermal hadronic model with two different temperatures in order to describe the baryonic and mesonic chemical freeze-out in ultra-relativistic heavy ion collisions. The model is used to fit the particle population ratios of the hadrons produced in the reaction. The proposal is not merely to incorporate one additional degree of freedom in the adjustment procedure of data, but to present and alternative scenario for the freeze out stage in the collisional proces s. This new reformulated version of thermal model was applied to a set of data, offering a rather good improvement in the fitting of the calculated particle ratios to the data. The results suggest that the introduced model makes the thermal approach more robust to handle with a larger number of colliding systems and a more comprehensive set of reaction observables. [ABSTRACT FROM AUTHOR]

Six-quark dibaryon model for fundamental NN- and 3N-forces at intermediate and short ranges are discussed briefly. The structure of the intermediate dibaryon dressed with σ-meson field is similar to those of Roper resonance where the σ-meson field plays a very important role. The t-channel σ-exchange between dibaryon and third nucleon is shown to result in new strong scalar attractive 3N force which contributes at least a half of nuclear binding in 3N system. This dibaryon induced σ-exchange leads also to a new spin-orbit attractive 3N-force. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We study two distinct phases of nuclear matter, a baryon-meson phase and a quark-gluon phase (QGP). For the baryon-meson phase we develop an equation of state (EoS) using a quark-meson formulation based on a new version of the fuzzy bag model with scalar-isoscalar, vector-isoscalar and vector-isovector meson-quark couplings and leptonic degrees of freedom as well as the constraints of chemical equilibrium, baryon number and electric charge conservation. For the QGP phase we model an EoS for asymptotically free massless quarks and gluons using the MIT approach and a temperature and baryon chemical potential dependent bag constant, B(T,μ), which allows an isentropic equilibrium phase transition from a QGP to a hadron gas. Our main results indicate the EoS and static global properties of neutron stars and protoneutron stars at low and moderate values of temperature are slightly modified in comparison to the predictions based on the MIT bag model with a constant B. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]