First bounds on effective muon interactions using the NA64$\mu$ experiment at CERN

TL;DR

The NA64μ experiment at CERN can significantly advance the understanding of muon interactions by constraining previously unbounded operators in SMEFT and νSMEFT, bridging current experimental gaps and future collider potential.

Contribution

This work demonstrates NA64μ's ability to set new bounds on unbounded four-lepton and four-fermion operators in SMEFT and νSMEFT, expanding the experimental constraints on new physics models.

Findings

NA64μ can probe two previously unbounded four-lepton operators.

NA64μ can constrain three unbounded four-fermion operators in νSMEFT.

Future sensitivity estimates show potential to improve current bounds.

Abstract

We analyze how NA64$\mu$ can contribute to the global SMEFT program demonstrating that it can probe two effective four lepton operators completely unbounded so far and break one of the current flat directions. Furthermore, we also study an extension of SMEFT that includes fermion singlets of the SM gauge group in the low energy field content. This effective field theory, usually dubbed $\nu$SMEFT, is well motivated by the observation of light neutrino masses and leptonic mixing. We find that NA64$\mu$ can constrain three unbounded four fermion operators of the $\nu$SMEFT. We derive the current leading bounds on these operators and compute the future sensitivity. Our results fill the gap between the current experimental program and a possible future muon collider able to probe this type of New Physics.