{
  "abstract": "The precision of the electromagnetic proton-neutron mass difference $δm_\\mathrm{QED}$ extracted from the Cottingham formula hinges on a subtraction function whose low-energy normalization $\\bar S(0)$ is fixed by the isovector combination of the proton and neutron polarizabilities, $(α_{E1}-β_{M1})^{p-n}$. We derive a dispersive sum rule for this combination in terms of $s$-channel photoabsorption cross sections and the product of $t$-channel $γγ\\toπη/K\\bar K_{I_t=1}$ and $πη/K\\bar K_{I_t=1}\\to N\\bar N$ amplitudes, without invoking Reggeon dominance. Combining empirical pion-photoproduction multipoles with coupled-channel Muskhelishvili-Omnès representations of the scalar-isovector $πη/K\\bar K$ amplitudes, we obtain $(α_{E1}-β_{M1})^{p-n}=-0.82(33)\\times10^{-4}\\,\\mathrm{fm}^3$, fixing its sign and reducing the uncertainty by one order of magnitude compared with the previously known value. This result yields $\\bar S(0)=-0.56(28)\\,\\mathrm{GeV}^{-2}$, leading to $δm_\\mathrm{QED}=0.75^{+0.02}_{-0.04}\\,\\mathrm{MeV}$, substantially more precise than previous Cottingham determinations. The negative $\\bar S(0)$ also provides a low-energy test of Reggeon dominance in the subtraction function.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.15154",
  "author_total": 3,
  "authors": [
    {
      "name": "Xiong-Hui Cao"
    },
    {
      "name": "Ling-Yun Dai"
    },
    {
      "name": "Feng-Kun Guo"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/b3tjzex3f4jjw.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.15154",
  "fulltext_url": null,
  "id": "b3tjzex3f4jjw",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.15154",
  "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-lat"
    ],
    [
      "arxiv",
      "hep-ph"
    ],
    [
      "arxiv",
      "nucl-ex"
    ],
    [
      "arxiv",
      "nucl-th"
    ],
    [
      "openalex",
      "3106|Quantum Chromodynamics and Particle Interactions"
    ]
  ],
  "openalex": "W7213348628",
  "pdf_url": "https://arxiv.org/pdf/2609.15154",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-25",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/b3tjzex3f4jjw.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:35:35Z",
      "native_id": "2609.15154",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.15154"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:hep-lat",
      "subfield": "high-energy"
    },
    {
      "field": "physics",
      "origin": "arxiv:nucl-ex",
      "subfield": "nuclear"
    }
  ],
  "title": "Precision Sum Rule for Nucleon Isovector Polarizabilities and the Proton-Neutron Mass Difference",
  "title_text": "Precision Sum Rule for Nucleon Isovector Polarizabilities and the Proton-Neutron Mass Difference",
  "type": "preprint",
  "updated_at": "2026-10-30T12:35:35Z",
  "url": "https://research.draconitic.com/paper/b3tjzex3f4jjw",
  "venue": "arXiv",
  "versions": []
}