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In this report, we discuss some aspects of the present theoretical status of the pentaquarks. In the first part we discuss the importance of the pion interaction due to spontaneous symmetry breaking of chiral symmetry, which affects the basic property such as the parity of Θ+. In the second part we show some calculations for the photoproduction of Θ+ and Λ(1520) in the effective Lagrangian approach. Within the minimal setup of the model, we make comments in comparison with the present controversial experimental results. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The production of baryon and mesons in the RHIC heavy-ion experiments has received a lot of attention lately. Although not widely known, the pp data measured concurrently with heavy ion collisions do not find a convincing explanation in terms of simple models. We present the results of an afterburner to Pythia and Hijing event generators, simulating radial flow which seems to qualitatively explain the experimental results when applied to the pp collision data from RHIC at 200 GeV center-of-mass energy. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We analyse second moments R2 of like-sign pion pairs in the two-dimensional (qL,qT) and three-dimensional (qO,qS,qL) decompositions of the three-momentum difference. Conventional fit parametrisations such as gaussian, exponential, power-law and Edgeworth fail miserably, while more elaborate ones such as Lévy do well but fail to yield a unique best-fit solution. A two-component model using a hard cut to separate small- and large-scale parts appears possible but not compelling. In all cases, the data exhibits a strong and hitherto unexplained peak at small momentum differences which exceeds current fits. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Pion-pion scattering amplitude is obtained from Chiral Perturbation Theory at one- and two-loop approximations. Dispersion relation formalism provides a more economic method, which was proved to reproduce the analytical structure of that amplitude at both approximation levels. This work extends the use of the formalism in order to compute further unitarity corrections to partial waves, including the D-wave amplitude. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We give the results of a simple model for the diagonal and off-diagonal valence quark distributions of a pion. We show that structure can be implemented in a gauge-invariant manner. This explicit model questions the validity of the momentum sum rule, and gives an explicit counter-example to the Wandzura-Wilczek ansatz for twist-3 GPD’s. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We explore the temperature dependence of the heavy-quarkonium interaction based on the Bhanot - Peskin leading order perturbative QCD analysis. The Wilson coefficients are computed solving the Schrödinger equation in a screened Coulomb heavy-quark potential. The inverse Mellin transform of the Wilson coefficients then allows for the computation of the 1S and 2S heavy-quarkonium gluon and pion total cross section at finite screening. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have evaluated forward dispersion relations for scattering amplitudes that follow from direct fits to several sets of ππ scattering experiments, together with the precise K decay results, and high to energy data. We find that some of the most commonly used experimental sets, as well as some recent theoretical analyses based on Roy equations, do not satisfy these constraints by several standard deviations. Finally, we provide a consistent ππ amplitude by improving a global fit to data with these dispersion relations. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The DIRAC experiment at CERN is presently measuring the lifetime of the pionium atom, a bound state of a π+ and π- meson. The isospin 0 and 2 scattering length difference will be determined unambiguously once the value of the lifetime is known. In what follows we describe the present status of the experiment, the experimental goals and the first results. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We study the behavior of the pion dispersion relation in a pion medium at finite density and temperature, introducing a chemical potential to describe the finite pion number density. Such description is particularly important during the hadronic phase of a relativistic heavy-ion collision, between chemical and thermal freeze-out, where the pion number changing processes, driven by the strong interaction, can be considered to be frozen. We make use of an effective Lagrangian that explicitly respects chiral symmetry through the enforcement of the chiral Ward identities. The pion dispersion relation is computed through the computation of the pion self-energy in a nonperturbative fashion by giving an approximate solution to the Schwinger-Dyson equation for this self-energy. The dispersion relation is described in terms of a density and temperature dependent mass and an index of refraction which is also temperature, density as well as momentum dependent. The index of refraction is larger than unity for all values of the momentum for finite μ and T. Given the strong coupling between ρ vectors and pions, we argue that the modification of the pion mass due to finite pion density effects has to be taken into account self-consistently for the description of the in-medium modifications of p's. [ABSTRACT FROM AUTHOR]

Measurements of the pp → ppπ[sup 0] reaction at 310, 320, 340, 360, 400 and 425 MeV and quasi-free pn → ppπ[sup -] production in pd collisions at 320 MeV have been carried out at the PROMICE/WASA facility at CELSIUS. The pp → ppπ[sup 0]cross sections have been parameterized and used to extract the poorly known σ[sub 01] cross section through the relation: σ[sub 01] = 2σ(pn → ppπ[sup -]) σ (pp → ppπ[sup 0]. The results obtained show the σ[sub 01] and σ[sub 11] to be of comparable size for CM energies between 8 and 48 MeV. [ABSTRACT FROM AUTHOR]

In this work, the properties of the p meson at rest in cold symmetric nuclear matter are studied. We make use of a chiral unitary approach to pion-pion scattering in the vector-isovector channel, calculated from the lowest order Chiral Perturbation Theory (χPT) lagrangian including explicit resonance fields. Low energy chiral constraints are considered by matching our expressions to those of one loop χPT. To account for the medium corrections, the p couples to ππ pairs which are properly renormalized in the nuclear medium, accounting for both p - h and δ - h excitations. The terms where the p couples directly to the hadrons in the p - h or δ - h excitations are also accounted for. In addition, the p is also allowed to couple to N[sup *](1520) - h components. [ABSTRACT FROM AUTHOR]

© 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]