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

TL;DR

This paper investigates whether introducing a new boson can explain the muonic fine-structure anomaly in heavy atoms, concluding that such Beyond-the-Standard-Model solutions are generally disfavoured and excluded.

Contribution

The study systematically analyzes various types of hypothetical bosons and finds that none can simultaneously account for the observed anomaly across different atomic states.

Findings

Spin-dependent couplings are disfavoured due to nuclear angular momentum.

Spin-independent interactions cannot fit multiple atomic states simultaneously.

A single new boson cannot resolve the anomaly within the considered models.

Abstract

The leading-order contribution of a new boson to the muonic fine-structure anomaly, which refers to a discrepancy between the predicted transition energies 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 by pseudoscalars or pseudovectors are disfavoured as solutions to the anomaly due to the nuclei in question having vanishing angular momentum. Spin-independent interactions resulting from scalar or vector exchange are also disfavoured because no parameter space exists to simultaneously fit different atomic states of the same nucleus. Therefore, we conclude that a `Beyond-the-Standard-Model' resolution of the muonic fine-structure anomaly is generally disfavoured, and the first-order solution by a single new boson is excluded.