{
  "abstract": "An enhancement in the three-pion energy at around $\\sqrt{s}\\approx 1.42~\\textrm{GeV}$ with $a_1$ quantum numbers was observed at the COMPASS experiment. This was later attributed to the triangle singularity mechanism involving an on-shell $K^*(892)$, $K$ and $\\bar K$ intermediate states. The alignment of the decay $K$ with the spectator $\\bar K$ produces an $f_0(980)$, resulting in a kinematic enhancement, which is classically explained by the Landau equations. However, this one-loop process forms only part of a non-diagonal transition in a much larger coupled-channel framework. This study demonstrates the feasibility of embedding one-loop triangle-singularity calculations into a unitary three-body amplitude allowing one to consistently incorporate final-state interactions and their potentially substantial effect. For this, up to $P$-wave isobars and all sub-channel isospins are combined in a nine-channel production amplitude that is fitted to COMPASS lineshapes at different momentum transfers. The fitted amplitude reproduces the narrow enhancement in the $(πf_0)_P$ channel near $\\sqrt{s}\\approx1.42$ GeV. This implies that the triangle singularity mechanism sufficiently explains the observed enhancement, and an additional genuine $a_1(1420)$ pole is not required. Incidentally, the parameters of the ground state axial vector resonance (the $a_1(1260)$) are also extracted from that data.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": "Journal reference: J. High Energ. Phys. 2026, 45 (2026)",
  "arxiv": "2606.24709",
  "author_total": 4,
  "authors": [
    {
      "name": "Ajay S. Sakthivasan"
    },
    {
      "name": "Yuchuan Feng"
    },
    {
      "name": "Michael Döring"
    },
    {
      "name": "Maxim Mai"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/fjg7s3yr65ecw.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2606.24709",
  "fulltext_url": null,
  "id": "fjg7s3yr65ecw",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2606.24709",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "hep-ph"
    ],
    [
      "arxiv",
      "nucl-th"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2606.24709",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-06",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/fjg7s3yr65ecw.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:27:22Z",
      "native_id": "2606.24709",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2606.24709"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:hep-ph",
      "subfield": "high-energy"
    },
    {
      "field": "physics",
      "origin": "arxiv:nucl-th",
      "subfield": "nuclear"
    }
  ],
  "title": "The $a_1(1420)$ in a Unitary Coupled-Channel Three-Body Approach",
  "title_text": "The a_1(1420) in a Unitary Coupled-Channel Three-Body Approach",
  "type": "preprint",
  "updated_at": "2026-10-30T12:27:22Z",
  "url": "https://research.draconitic.com/paper/fjg7s3yr65ecw",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.1007/jhep10(2026)045",
      "pid": "74s167mt67qm1",
      "relation": "published-as",
      "title": "The a1(1420) in a unitary coupled-channel three-body approach",
      "venue": "Journal of High Energy Physics"
    }
  ]
}