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JHEP02(2025)188Published for SISSA by SpringerReceived: December 4, 2024Accepted: January 27, 2025Published: February 27, 2025Axion couplings in heterotic string theoryPrateek Agrawal , a Michael Neeband Mario Reiga,ca Rudolf Peierls Centre for Theoretical Physics, University of Oxford,Parks Road, Oxford OX1 3PU, U.K.b Department of Physics, Harvard University,Cambridge, MA, 02138, U.S.A.c Department of Physics, Royal Holloway University of London,Egham, Surrey, TW20 0EX, U.K.E-mail: prateek.agrawal@physics.ox.ac.uk, mnee@fas.harvard.edu,mario.reiglopez@physics.ox.ac.ukAbstract: We study the coupling of axions to gauge bosons in heterotic string theory. Theaxion-gauge boson couplings in the low energy 4d theory are derived by matching mixedanomalies between higher-form global symmetries and the zero-form gauge symmetry inthe 10d theory. When the standard model gauge group is embedded in a single simplegroup in the 10d theory — as is the case for almost all heterotic models studied in theliterature — the ratio of the axion-photon coupling to the axion mass is bounded aboveby the QCD line. This bound is relevant for a large number of axion searches which havesensitivity to axion parameter space above this line. The discovery of an axion in thesesearches will rule out a large class of heterotic models, making such a signal challengingto explain within heterotic string theory.Keywords: Axions and ALPs, String Models, Grand Unif i cationArXiv ePrint: 2410.03820Open Access, © The Authors.Article funded by SCOAP 3 .https://doi.org/10.1007/JHEP02(2025)188