{
  "abstract": "Searches for new physics in low-background experiments infer a non-negative signal strength from few events and often report an upper limit. Nominal frequentist coverage requires both a valid interval construction and an adequate data model. We study how controlled model departures affect lower- and upper-endpoint coverage for six interval procedures in an exact Poisson counting experiment, dark-matter recoil spectra, and a neutrinoless-double-beta-decay peak search. We introduce the Poisson–Fisher degeneracy index $$\\mathcal I_{\\textrm{PF}}(\\delta \\nu ;\\vartheta _0)=(\\beta ,\\gamma )$$, which maps a specified expected-count deformation, after projection onto the complete fitted tangent space, to $$\\beta $$, the signed fitted signal shift in profiled standard-error units, and $$\\gamma $$, the Poisson–Fisher norm of the unabsorbed residual. Locally, the sign of $$\\beta $$ identifies the threatened endpoint, while $$\\gamma $$ provides a local indication of detectability by the saturated-Poisson goodness-of-fit test used here at a $$5\\%$$ nominal type-I error rate. Across the studied deformations, positive signal-like bias degrades discovery-side coverage while making upper limits conservative; negative signal bias from overestimated signal efficiency can make the upper endpoint undercover. Calibration under the nominal simulator does not protect against misspecification of that simulator relative to the data-generating process. A plausible deformation with large $$|\\beta |$$ and small $$\\gamma $$ may therefore evade diagnosis and should be represented by a nuisance constrained with auxiliary information or included in a defensible envelope. In the exactly collinear constrained-nuisance benchmark, modelling the deformation restores coverage at the evaluated grid points, at a quantifiable cost in interval sensitivity.",
  "abstract_shown": true,
  "affiliations": [
    "University of Brescia",
    "Istituto Nazionale di Fisica Nucleare (INFN)"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 1,
  "authors": [
    {
      "affs": [
        0,
        1
      ],
      "name": "Davide Pagano",
      "orcid": "0000-0003-0333-448X"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/a3p0bd9b6te5a.bib",
  "cited_by": 0,
  "doi": "10.1140/epjc/s10052-026-16570-7",
  "fulltext_url": null,
  "id": "a3p0bd9b6te5a",
  "issns": [
    "1434-6052"
  ],
  "landing_url": "https://link.springer.com/article/10.1140/epjc/s10052-026-16570-7",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0",
  "metadata_licence": null,
  "native_subjects": [
    [
      "feed-journal",
      "physics/high-energy"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://link.springer.com/content/pdf/10.1140/epjc/s10052-026-16570-7.pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-06",
  "publisher": "Springer Science and Business Media LLC",
  "ris": "https://research.draconitic.com/paper/a3p0bd9b6te5a.ris",
  "sources": [
    {
      "first_seen": "2026-11-06T12:20:44Z",
      "native_id": "10.1140/epjc/s10052-026-16570-7",
      "source": "feed-springer",
      "url": "https://link.springer.com/article/10.1140/epjc/s10052-026-16570-7"
    },
    {
      "first_seen": "2026-11-06T12:20:49Z",
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      "source": "crossref",
      "url": "https://link.springer.com/article/10.1140/epjc/s10052-026-16570-7"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "feed-journal:physics/high-energy",
      "subfield": "high-energy"
    }
  ],
  "title": "Silent coverage failures in rare-event searches and a degeneracy index that predicts them",
  "title_text": "Silent coverage failures in rare-event searches and a degeneracy index that predicts them",
  "type": "peer-reviewed",
  "updated_at": "2026-11-08T00:21:04Z",
  "url": "https://research.draconitic.com/paper/a3p0bd9b6te5a",
  "venue": "The European Physical Journal C",
  "versions": []
}