Leptophilic (pseudo)scalars with TWIST data for polarized muon decays

Abstract We study the production of (pseudo-) scalar particles, in general coined as axion-like particles (ALPs) in association with electrons and neutrinos in the muon decay process. For this purpose, we compute the decay width of the muon to a four-body channel using a $$d=7$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>d</mml:mi> <mml:mo>=</mml:mo> <mml:mn>7</mml:mn> </mml:mrow> </mml:math> effective operator that couples the ALP to the Standard Model fermions, namely leptons and neutrinos. Assuming a dominant coupling of the ALP to the dark sector, we only consider ALP decays to invisible final states. To obtain constraints on our model using the existing measurements, we leverage data from the TRIUMF Weak Interaction Symmetry Test (TWIST) experiment and obtain bounds on the ALP-lepton coupling for masses in the range of $$0&lt; m_{\phi } &lt; m_{\mu }/4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0</mml:mn> <mml:mo>&lt;</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>ϕ</mml:mi> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>μ</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:math> , as allowed by kinematics. Using the precision of current TWIST measurements, we obtain an order of magnitude estimation necessary for future searches to further constrain the parameter space for such a setup. Furthermore, we find that keeping realistic considerations, the new physics contribution can possibly be enhanced even with a minimalistic modification to the fiducial area used in the experiment potentially allowing for stronger constraints. At the end, in an attempt to relax the assumption that ALP decays to invisible only, we also investigate its stability and find potential longevity within collider environments for the mass range considered in this study.