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Two-lepton tales: Dalitz decays of heavy quarkonia

P. Colangelo ¹, F. De Fazio ¹, and R. Pinto ²

¹Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Via Orabona 4, 70126 Bari, Italy
²Dipartimento Interateneo di Fisica “Michelangelo Merlin”, Università degli Studi di Bari, via Orabona 4, 70126 Bari, Italy

Phys. Rev. D 113, 073007 – Published 24 April, 2026

DOI: https://doi.org/10.1103/6wdr-tr4f

Abstract

We study the Dalitz decays of heavy quarkonia, which result from the internal virtual photon conversion into an $ℓ^{+} ℓ^{-}$ lepton pair. Heavy-quark symmetries allow us to establish systematic relations between transitions of different quarkonium states and to precisely determine the branching fractions for several charmonium and bottomonium decay modes. For charmonium, existing data on $χ_{c J} (1 P) \to J / ψ ℓ^{+} ℓ^{-}$ and $ψ (2 S) \to χ_{c J} (1 P) ℓ^{+} ℓ^{-}$ enable us to determine the parameters of the transition form factors and to predict the rates of yet-unobserved modes. The Dalitz transitions of $χ_{c 1} (3872)$ are important, as they can help assessing the structure of this meson. For bottomonium, recent LHCb measurements allow us to predict the branching fractions of $χ_{b J} (n P) \to ϒ (1 S) ℓ^{+} ℓ^{-}$ and $h_{b} (n P) \to η_{b} (1 S) ℓ^{+} ℓ^{-}$ ( $n = 1$ , 2). We also investigate the sensitivity of heavy quarkonia Dalitz modes to the contribution of a new light vector mediator, such as the putative $X (17)$ .

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References (75)

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https://github.com/picolangelo/files/tree/DN14208.

Phys. Rev. D 113, 073007– Published 24 April, 2026

Vol. 113, Iss. 7 — 1 April 2026

Received 24 January 2026
Accepted 6 April 2026

DOI: https://doi.org/10.1103/6wdr-tr4f

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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Two-lepton tales: Dalitz decays of heavy quarkonia

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