Eur. Phys. J. C (2025) 85: 1182
https://doi.org/10.1140/epjc/s10052-025-14811-9

Regular Article - Experimental Physics

Search for high energy 5.5 MeV solar axions with the complete Borexino dataset

¹ Dipartimento di Fisica, Università degli Studi e INFN, 20133, Milan, Italy
² Chemical Engineering Department, Princeton University, 08544, Princeton, NJ, USA
³ INFN Laboratori Nazionali del Gran Sasso, 67010, Assergi (AQ), Italy
⁴ Physics Department, Princeton University, 08544, Princeton, NJ, USA
⁵ Dipartimento di Fisica, Università degli Studi e INFN, 16146, Genova, Italy
⁶ Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics, 119234, Moscow, Russia
⁷ St. Petersburg Nuclear Physics Institute NRC Kurchatov Institute, 188350, Gatchina, Russia
⁸ National Research Centre Kurchatov Institute, 123182, Moscow, Russia
⁹ APC, Université de Paris, CNRS, Astroparticule et Cosmologie, 75013, Paris, France
¹⁰ Gran Sasso Science Institute, 67100, L’Aquila, Italy
¹¹ Joint Institute for Nuclear Research, 141980, Dubna, Russia
¹² Institute of Physics and Excellence Cluster PRISMA+, Johannes Gutenberg-Universität Mainz, 55099, Mainz, Germany
¹³ M. Smoluchowski Institute of Physics, Jagiellonian University, 30348, Krakow, Poland
¹⁴ Institute for Nuclear Research of NAS Ukraine, 03028, Kyiv, Ukraine
¹⁵ Department of Physics, Royal Holloway, University of London, TW20 OEX, Egham, Surrey, UK
¹⁶ Institute of Nuclear Research (Atomki), Debrecen, Hungary
¹⁷ Institut für Kernphysik, Forschungszentrum Jülich, 52425, Jülich, Germany
¹⁸ RWTH Aachen University, 52062, Aachen, Germany
¹⁹ National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409, Moscow, Russia
²⁰ Physik-Department, Technische Universität München, 85748, Garching, Germany
²¹ Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi e INFN, 06123, Perugia, Italy
²² Amherst Center for Fundamental Interactions and Physics Department, UMass, 01003, Amherst, MA, USA
²³ Physics Department, Virginia Polytechnic Institute and State University, 24061, Blacksburg, VA, USA
²⁴ Department of Physics, Technische Universität Dresden, 01062, Dresden, Germany
²⁵ Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany
²⁶ IHEP Institute of High Energy Physics, 100049, Beijing, China
²⁷ INFN Laboratori Nazionali del Gran Sasso, 67010, Assergi (AQ), Italy
²⁸ Dipartimento di Fisica, Universita degli Studi e INFN Milano-Bicocca, 20126, Milan, Italy
²⁹ Department of Physics and Astronomy, University of California, Irvine, CA, USA
³⁰ GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291, Darmstadt, Germany
³¹ Institute of Physics, EC PRISMA +, Johannes Gutenberg Universität Mainz, Mainz, Germany
³² Dipartimento di Fisica, Università degli Studi e INFN, 20133, Milan, Italy
³³ Boston University, College of Arts and Sciences, Department of Physics, 02215, Boston, MA, USA
³⁴ Istituto Superiore per la Protezione e la Ricerca Ambientale, 00144, Rome, Italy
³⁵ GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291, Darmstadt, Germany
³⁶ National Research Centre Kurchatov Institute, 123182, Moscow, Russia

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Received: 28 April 2025
Accepted: 18 September 2025
Published online: 21 October 2025

Abstract

A search for solar axions and axion-like particles produced in the $p+d{\to }^3\text{He}+A(5.5\text{MeV})$ reaction was performed using the complete dataset of the Borexino detector (3995 days of measurement live-time). The following interaction processes have been considered: axion decay into two photons $(\text{A}\to 2\gamma )$, inverse Primakoff conversion on nuclei $(\text{A}+Z\to \gamma +Z$), the Compton conversion of axions to photons $(\text{A}+e\to e+\gamma )$ and the axio-electric effect $(\text{A}+e+Z\to e+Z$). Model-independent limits on product of axion–photon ($g_{A\gamma }$), axion–electron ($g_{\mathit{Ae}}$), and isovector axion–nucleon ($g_{3AN}$) couplings are obtained: $|g_{A\gamma }\times g_{3AN}|\le 2.3\times {10}^{-11}{\text{GeV}}^{-1}$ and $|g_{\mathit{Ae}}\times g_{3AN}|\le 1.9\times {10}^{-13}$ at $m_A<$ 1 MeV (90% c.l.). The Borexino results exclude new large regions of $g_{A\gamma }$, and $g_{\mathit{Ae}}$ coupling constants and axion masses $m_A$, and leads to constraints on the products $|g_{A\gamma }\times m_A|$ and $|g_{\mathit{Ae}}\times m_A|$ for the KSVZ- and the DFSZ-axion models.