Axionlike-particle contributions to the μ→e conversion

We study the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>μ</a:mi><a:mo stretchy="false">→</a:mo><a:mi>e</a:mi></a:math> conversion process in nuclear targets arising in models of axionlike particles (ALPs) with hadronic and charged lepton flavor violating (CLFV) interactions. Contributions to this process generally fall into two categories: spin-independent (SI) and spin-dependent (SD). While the SI contribution can be generated by a dipole operator through purely leptonic ALP interactions, the SD contribution can also be present through ALP-quark interactions at tree-level. It is naively anticipated that the SI contribution would be dominant due to its coherent enhancement. In this paper, we show that is not generically the case; in particular, for naturally sized ALP couplings to quarks of order <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:mo>∼</d:mo><d:msub><d:mrow><d:mi>m</d:mi></d:mrow><d:mrow><d:mi>q</d:mi></d:mrow></d:msub><d:mo>/</d:mo><d:msub><d:mrow><d:mi>f</d:mi></d:mrow><d:mrow><d:mi>a</d:mi></d:mrow></d:msub></d:mrow></d:math>, the SD interaction induced by ALP-<f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:msup><f:mi>π</f:mi><f:mn>0</f:mn></f:msup></f:math> mixing turns out to be the leading contribution to <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mi>μ</h:mi><h:mo stretchy="false">→</h:mo><h:mi>e</h:mi></h:math> conversion. Intuitively, this stems from the suppressed dipole contribution by the QED one-loop factor which counters the effect of SI coherent enhancement. Our study highlights the importance of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>μ</k:mi><k:mo stretchy="false">→</k:mo><k:mi>e</k:mi></k:math> conversion searches in exploring the parameter space of generic ALP models, and demonstrates the competitiveness of these searches in probing the CLFV ALP parameter space in the heavy mass range of <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msub><n:mi>m</n:mi><n:mi>a</n:mi></n:msub><n:mo>≳</n:mo><n:msub><n:mi>m</n:mi><n:mi>μ</n:mi></n:msub></n:math>.