Meson model for f_0(980) production in peripheral pion-nucleon reactions

The Juelich model for pion-pion-scattering, based on an effective meson-meson Lagrangian is applied to the analysis of the S-wave production amplitudes derived from the BNL E852 experiment pi^- p -> pi^0 pi^0 n for a pion momentum of 18.3 GeV. The unexpected strong dependence of the S-wave partial wave amplitude on the momentum transfer between the proton and neutron in the vicinity of the f_0(980) resonance is explained in our analysis as interference effect between the correlated and uncorrelated pi^0 pi^0 pairs.Comment: 6 pages, 7 figures, formulas added, typos removed, new figure

Two-photon final states in peripheral heavy ion collisions

We discuss processes leading to two photon final states in peripheral heavy ion collisions at RHIC. Due to the large photon luminosity we show that the continuum subprocess $\gamma \gamma \to \gamma \gamma$ can be observed with a large number of events. We study this reaction when it is intermediated by a resonance made of quarks or gluons and discuss its interplay with the continuum process, verifying that in several cases the resonant process ovewhelms the continuum one. It is also investigated the possibility of observing a scalar resonance (the $\sigma$ meson) in this process. Assuming for the $\sigma$ the mass and total decay width values recently reported by the E791 Collaboration we show that RHIC may detect this particle in its two photon decay mode if its partial photonic decay width is of the order of the ones discussed in the literature.Comment: 10 pages, 8 figure

Parton model versus color dipole formulation of the Drell-Yan process

In the kinematical region where the center of mass energy is much larger than all other scales, the Drell-Yan process can be formulated in the target rest frame in terms of the same color dipole cross section as low Bjorken-x deep inelastic scattering. Since the mechanisms for heavy dilepton production appear very different in the dipole approach and in the conventional parton model, one may wonder whether these two formulations really represent the same physics. We perform a comparison of numerical calculations in the color dipole approach with calculations in the next-to-leading order parton model. For proton-proton scattering, the results are very similar at low x_2 from fixed target to RHIC energies, confirming the close connection between these two very different approaches. We also compare the transverse momentum distributions of Drell-Yan dileptons predicted in both formulations. The range of applicability of the dipole formulation and the impact of future Drell-Yan data from RHIC for determining the color dipole cross section are discussed. A detailed derivation of the dipole formulation of the Drell-Yan process is also included.Comment: 20 pages, 5 figure

Unitarity and Interfering Resonances in pipi Scattering and in Pion Production piN->pipiN

Additivity of Breit-Wigner phases has been proposed to describe interfering resonances in partial waves in $\pi\pi$ scattering. This assumption leads to an expression for partial wave amplitudes that involves products of Breit-Wigner amplitudes. We show that this expression is equivalent to a coherent sum of Breit-Wigner amplitudes with specific complex coefficients which depend on the resonance parameters of all contributing resonances. We use analyticity of $\pi\pi$ partial wave amplitudes to show that they must have the form of a coherent sum of Breit-Wigner amplitudes with complex coefficients and a complex coherent background. The assumption of additivity of Breit-Wigner phases restricts the partial waves to analytical functions with very specific form of residues of Breit-Wigner poles. We argue that the general form provided by the analyticity is more appropriate in fits to data to determine resonance parameters. The partial wave unitarity can be imposed using the modern methods of constrained optimization. We discuss unitarity and the production amplitudes in $\pi N\to\pi\pi N$ and use analyticity in the dipion mass variable to justify the common practice of writing the production amplitudes as a coherent sum of Breit-Wigner amplitudes with free complex coefficients and a complex coherent background in fits to mass spectra with interfering resonances.Comment: 31 page

Couplings of light I=0 scalar mesons to simple operators in the complex plane

The flavour and glue structure of the light scalar mesons in QCD are probed by studying the couplings of the I=0 mesons $\sigma(600)$ and $f_0(980)$ to the operators $\bar{q}q$, $\alpha_s G^2$ and to two photons. The Roy dispersive representation for the $\pi\pi$ amplitude $t_0^0(s)$ is used to determine the pole positions as well as the residues in the complex plane. On the real axis, $t_0^0$ is constrained to solve the Roy equation together with elastic unitarity up to the K\Kbar threshold leading to an improved description of the $f_0(980)$. The problem of using a two-particle threshold as a matching point is discussed. A simple relation is established between the coupling of a scalar meson to an operator $j_S$ and the value of the related pion form-factor computed at the resonance pole. Pion scalar form-factors as well as two-photon partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive representations. Subtraction constants are constrained by chiral symmetry and experimental data. Comparison of our results for the $\bar{q}q$ couplings with earlier determinations of the analogous couplings of the lightest I=1 and $I=1/2$ scalar mesons are compatible with an assignment of the $\sigma$, $\kappa$, $a_0(980)$, $f_0(980)$ into a nonet. Concerning the gluonic operator $\alpha_s G^2$ we find a significant coupling to both the $\sigma$ and the $f_0(980)$.Comment: 31 pages, 5 figure

J/psi production in relativistic heavy ion collisions from a multi-phase transport model

Using A Multi-Phase Transport (AMPT) model, we study J/psi production from interactions between charm and anti-charm quarks in initial parton phase and between D and Dbar mesons in final hadron phase of relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). Including also the inverse reactions of J/psi absorption by gluons and light mesons, we find that the net number of J/psi from the parton and hadron phases is smaller than that expected from the superposition of initial nucleon-nucleon collisions, contrary to the J/psi enhancement predicted by the kinetic formation model. The production of J/psi is further suppressed if one includes the color screening effect in the parton phase. We have also studied the dependence of J/psi production on the charm quark mass and the effective charm meson mass.Comment: Figures redone with better statistic

Physics with the KLOE-2 experiment at the upgraded DAϕ\phiNE

Investigation at a $\phi$--factory can shed light on several debated issues in particle physics. We discuss: i) recent theoretical development and experimental progress in kaon physics relevant for the Standard Model tests in the flavor sector, ii) the sensitivity we can reach in probing CPT and Quantum Mechanics from time evolution of entangled kaon states, iii) the interest for improving on the present measurements of non-leptonic and radiative decays of kaons and eta/eta$^\prime$ mesons, iv) the contribution to understand the nature of light scalar mesons, and v) the opportunity to search for narrow di-lepton resonances suggested by recent models proposing a hidden dark-matter sector. We also report on the $e^+ e^-$ physics in the continuum with the measurements of (multi)hadronic cross sections and the study of gamma gamma processes.Comment: 60 pages, 41 figures; added affiliation for one of the authors; added reference to section