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The leading-order contribution of a new boson to the muonic fine-structureanomaly, which refers to a discrepancy between the predicted transitionenergies and spectroscopic measurements of $\mu-^{90}$Zr, $\mu-^{120}$Sn, and$\mu-^{208}$Pb, is investigated. We consider bosons of scalar, vector,pseudoscalar, and pseudovector type. Spin-dependent couplings sourced bypseudoscalars or pseudovectors are disfavoured as solutions to the anomaly dueto the nuclei in question having vanishing angular momentum. Spin-independentinteractions resulting from scalar or vector exchange are also disfavouredbecause no parameter space exists to simultaneously fit different atomic statesof the same nucleus. Therefore, we conclude that a `Beyond-the-Standard-Model'resolution of the muonic fine-structure anomaly is generally disfavoured, andthe first-order solution by a single new boson is excluded.

Challenging Beyond-the-Standard-Model Solutions to the Fine-Structure Anomaly in Heavy Muonic Atoms