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arXiv · 2611.03904

Constraints on a Long-Lived X17 Vector Boson with FASER

FASER Collaboration·Roshan Mammen Abraham·Xiaocong Ai·Saul Alonso Monsalve·John Anders·Emma Kate Anderson·Akitaka Ariga·Tomoko Ariga·Jeremy Atkinson·Florian U. Bernlochner·Jianming Bian·Tobias Boeckh·Eliot Bornand·Jamie Boyd·Lydia Brenner·Angela Burger·Franck Cadoux·Haichuan Cao·Roberto Cardella·David W. Casper·Charlotte Cavanagh·Shiyang Chen·Xin Chen·Xing Cheng·Dhruv Chouhan·Emily Chown·Andrea Coccaro·Fabio Cufino·Stephane Débieux·Ansh Desai·Sergey Dmitrievsky·Radu Dobre·Monica D'Onofrio·Sinead Eley·Yannick Favre·Jonathan L. Feng·Carlo Alberto Fenoglio·Didier Ferrere·Max Fieg·Wissal Filali·Elena Firu·Haruhi Fujimori·Edward Galantay·Ali Garabaglu·Stephen Gibson·Sergio Gonzalez-Sevilla·Yuri Gornushkin·Yotam Granov·Jinjing Gu·Carl Gwilliam·Elie Hammou·Daiki Hayakawa·Michael Holzbock·Shih-Chieh Hsu·Zhen Hu·Giuseppe Iacobucci·Tomohiro Inada·Luca Iodice·Sune Jakobsen·Cesar Jesus-Valls·Arash Jofrehei·Hans Joos·Enrique Kajomovitz·Hiroaki Kawahara·Alex Keyken·Felix Kling·Daniela Köck·Pantelis Kontaxakis·Jelle Koorn·Umut Kose·Peter Krack·Susanne Kuehn·Thanushan Kugathasan·Sebastian Laudage·Lorne Levinson·Botao Li·Jiaxi Liu·Yi Liu·Joern Mahlstedt·Toni Mäkelä·Yasuhiro Maruya·Anna Mascellani·Lawson McCoy·Josh McFayden·Andrea Pizarro Medina·Hiroaki Menjo·Théo Moretti·Toshiyuki Nakano·Laurie Nevay·Ken Ohashi·Hidetoshi Otono·Lorenzo Paolozzi·Annabelle Parry·Pawan Pawan·Brian Petersen·Titi Preda·Markus Prim·Junkai Qin·Michaela Queitsch-Maitland·Juan Rojo
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Abstract

The ATOMKI anomalies motivate the possibility of a new particle, the X17, with a mass of 17 MeV and weak couplings to nuclei and electrons. Such a particle may be produced at the LHC, travel 480 m in the far-forward direction, and decay to $e^+e^-$ in FASER. In this paper, previous FASER searches for dark photons and axion-like particles (ALPs) are combined and interpreted as searches for a candidate X17 particle, the protophobic vector boson. The searches use data collected from $pp$ collisions at $\sqrt{s} = 13.6$ TeV in LHC Run 3 from 2022 for the dark photon search and from 2022-2023 for the ALP search. No excess events are found above the expected background, and the combined null results exclude a previously-unconstrained "low-coupling window" with electron coupling $6 \times 10^{-5} < ε_e < 1.2 \times 10^{-4}$. A "high-coupling window" remains.

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FASER Collaboration, Roshan Mammen Abraham, Xiaocong Ai, Saul Alonso Monsalve, John Anders, Emma Kate Anderson, Akitaka Ariga, Tomoko Ariga, Jeremy Atkinson, Florian U. Bernlochner, Jianming Bian, Tobias Boeckh, Eliot Bornand, Jamie Boyd, Lydia Brenner, Angela Burger, Franck Cadoux, Haichuan Cao, Roberto Cardella, David W. Casper, Charlotte Cavanagh, Shiyang Chen, Xin Chen, Xing Cheng, Dhruv Chouhan, Emily Chown, Andrea Coccaro, Fabio Cufino, Stephane Débieux, Ansh Desai, Sergey Dmitrievsky, Radu Dobre, Monica D'Onofrio, Sinead Eley, Yannick Favre, Jonathan L. Feng, Carlo Alberto Fenoglio, Didier Ferrere, Max Fieg, Wissal Filali, Elena Firu, Haruhi Fujimori, Edward Galantay, Ali Garabaglu, Stephen Gibson, Sergio Gonzalez-Sevilla, Yuri Gornushkin, Yotam Granov, Jinjing Gu, Carl Gwilliam, Elie Hammou, Daiki Hayakawa, Michael Holzbock, Shih-Chieh Hsu, Zhen Hu, Giuseppe Iacobucci, Tomohiro Inada, Luca Iodice, Sune Jakobsen, Cesar Jesus-Valls, Arash Jofrehei, Hans Joos, Enrique Kajomovitz, Hiroaki Kawahara, Alex Keyken, Felix Kling, Daniela Köck, Pantelis Kontaxakis, Jelle Koorn, Umut Kose, Peter Krack, Susanne Kuehn, Thanushan Kugathasan, Sebastian Laudage, Lorne Levinson, Botao Li, Jiaxi Liu, Yi Liu, Joern Mahlstedt, Toni Mäkelä, Yasuhiro Maruya, Anna Mascellani, Lawson McCoy, Josh McFayden, Andrea Pizarro Medina, Hiroaki Menjo, Théo Moretti, Toshiyuki Nakano, Laurie Nevay, Ken Ohashi, Hidetoshi Otono, Lorenzo Paolozzi, Annabelle Parry, Pawan Pawan, Brian Petersen, Titi Preda, Markus Prim, Junkai Qin, Michaela Queitsch-Maitland, Juan Rojo. 2026-11-02. Constraints on a Long-Lived X17 Vector Boson with FASER. https://arxiv.org/abs/2611.03904

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