{
  "id": "nucl-th/9806013",
  "version": "v3",
  "published": "1998-06-04T11:24:27.000Z",
  "updated": "1999-01-05T07:23:04.000Z",
  "title": "Novel approach to pion and eta production in proton-proton collisions near threshold",
  "authors": [
    "V. Bernard",
    "N. Kaiser",
    "Ulf-G. Meißner"
  ],
  "comment": "28 pp, LaTeX, 9 figs, uses epsf, updated version. To be published in Eur. Phys. J. A (1999). **Title changed again**",
  "journal": "Eur.Phys.J. A4 (1999) 259-275",
  "doi": "10.1007/s100500050229",
  "categories": [
    "nucl-th"
  ],
  "abstract": "We evaluate the threshold matrix-element for the reaction $pp \\to pp\\pi^0$ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S-wave $pp$ final-state interaction. The experimental value for the threshold amplitude ${\\cal A} = (2.7 - i 0.3)$ fm$^4$ can be reproduced by contributions from tree level chiral (long-range) pion exchange and short-range effects related to vector meson exchanges, with $\\omega$ exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN-system above the pion production threshold due to the large external momentum, $|\\vec p | \\simeq \\sqrt {Mm_\\pi}$, with $M$ and $m_\\pi$ the nucleon and the pion mass, respectively. We furthermore investigate the reaction $pp\\to p n \\pi^+$ near threshold within the same approach. We extract from the data the triplet threshold amplitude as ${\\cal B}= (2.8 -i 1.5)$ fm$^4$. Its real part can be well understood from (relativistic) tree level meson-exchange diagrams. In addition, we investigate the process $pp \\to pp \\eta$ near threshold. We use a simple factorization ansatz for the $pp\\eta$ final-state interaction and extract from the data the modulus of the threshold amplitude, $|{\\cal C}| = 1.32 $fm$^4$. With $g_{\\eta N}=5.3$, this value can be reproduced by (relativistic) tree level meson-exchange diagrams and $\\eta$-rescattering, whose strength is fixed by the $\\eta N$ scattering length. We also comment on the recent near threshold data for $\\eta'$-production.",
  "revisions": [
    {
      "version": "v3",
      "updated": "1999-01-05T07:23:04.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "eta production",
      "proton-proton collisions",
      "novel approach",
      "tree level meson-exchange diagrams",
      "threshold amplitude"
    ],
    "tags": [
      "journal article"
    ],
    "note": {
      "typesetting": "LaTeX",
      "pages": 28,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 471412
    }
  }
}