Search

The background due to the direct diffractive dissociation of the photon into the $\pi^+\pi^-$-pair to the "elastic" diffractive $\rho^0$-meson production in electron-proton collisions is calculated. The amplitude for the background process $\gamma p \to \pi^+ \pi^- p$ is proportional to the $\pi$-meson - proton cross section. Therefore, describing the HERA data, we can estimate $\sigma (\pi p)$ at energy $s_{\pi p}\sim (2 - 3) 10^{3} GeV^2$ that is considerably higher the existing data. At large $Q^2$ the interference between resonant and non-resonant $\pi^+ \pi^-$ production leads also to the more slow increase of the $\sigma^L/\sigma^T$ ratio with the mass of the $2\pi$ (i.e. $\rho^0$-meson) state.

It is proposed that meson resonances are linear combinations of q-qbar and meson-meson (MM); baryon resonances are combinations of qqq and meson-baryon (MB). Mixing between these combinations arises via decays of confined states to meson-meson or meson-baryon. There is a precise analogy with the covalent bond in molecular physics; it helps to visualise what is happening physically. One eigenstate is lowered by the mixing; the other moves up and normally increases in width. Cusps arise at thresholds. At sharp thresholds due to S-wave 2-particle decays, these cusps play a conspicuous role in many sets of data. The overall pattern of light mesons is consistent with nearly linear Regge trajectories, hence q-qbar components. There is no obvious reason why this pattern should arise from dynamically generated states without q-qbar content., Comment: 15 pages, 4 figures. A major revision of an earlier paper.

It is proposed that meson resonances are linear combinations of q-qbar and meson-meson (MM); baryon resonances are combinations of qqq and meson-baryon (MB). Mixing between these combinations arises via decays of confined states to meson-meson or meson-baryon. There is a precise analogy with the covalent bond in molecular physics; it helps to visualise what is happening physically. One eigenstate is lowered by the mixing; the other moves up and normally increases in width. Cusps arise at thresholds. At sharp thresholds due to S-wave 2-particle decays, these cusps play a conspicuous role in many sets of data., Comment: 11 pages, 2 figures; extended version of a talk at the Hadron09 conference

The formulae needed to parametrise sigma, kappa, a0(980) and f0(980) are reviewed. For sigma and kappa, the Adler zero due to Chiral Symmetry Breaking plays a crucial role. The f0(980) and a0(980) are locked to the KK threshold by a cusp mechanism which leads to a sharp peak in attraction at the threshold. This mechanism may play a wider role. A novel suggestion is that the confinement potential involves a cooperative effect between QCD effects (coloured quarks and gluons) and conventional meson exchanges., 10 pages. Proc. Lisbon meeting on Scalar Mesons, Feb 10-16

The spectrum of qq states observed in Crystal Barrel data in flight is very simple: a set of parallel straight‐line trajectories of mass squared v. radial excitation number. There are 6 extra states, making candidates for 4 glueballs and 2 hybrids. Mass ratios of glueball candidates agree closely with the latest Lattice Gauge calculations. The evidence for 2−+ hybrids η2(1860) and π2(1880) is reviewed. Further data from a polarised target are highly desirable in channels 3π0, ηπ0, ωπ0 and ωη in order to separate cleanly triplet states with L = J ± 1. These data would provide precise measurements of tensor, spin‐orbit and spin‐spin splitting of masses.

Total cross sections of ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ mesons on protons and deuterons have been measured in a transmission experiment to relative accuracies of \ifmmode\pm\else\textpm\fi{}0.2% over the laboratory momentum range 0.46-2.67 GeV/c. The systematic error is estimated to be about \ifmmode\pm\else\textpm\fi{}0.5% over most of the range, increasing to about \ifmmode\pm\else\textpm\fi{}2% near both ends. Data have been obtained at momentum intervals of 25-50 MeV/c with a momentum resolution of \ifmmode\pm\else\textpm\fi{}0.6%. No new structure is observed in the ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}p$ total cross sections, but results differ in several details from previous experiments. From 1-2 GeV/c, where systematic erros are the smallest, the total cross section of ${\ensuremath{\pi}}^{\ensuremath{-}}$ mesons on deuterons is found to be consistently higher than that of ${\ensuremath{\pi}}^{+}$ mesons by (1.3\ifmmode\pm\else\textpm\fi{}0.3)%; about half of this difference may be understood in terms of Coulomb-barrier effects. The $\ensuremath{\pi}d$ and $\ensuremath{\pi}N$ total cross sections are used to check the validity of the Glauber theory. Substantial disagreements (up to 2 mb) are observed, and the conclusion is drawn that the Glauber theory is inadequate in this momentum range.

Theory of Meson Interactions with Nuclei. By Judah M. Eisenberg and Daniel S. Koltun. Pp.403. (Wiley-Interscience: 1980.) £21.25, $49.95.

Measurements are reported of pp total cross sections from 388 to 599 MeV/c in small momentum steps. Statistical errors are typically ±0.4%and the normalisation uncertainty is ±0.7%. There is no evidence for the “S-meson”.