Deep inelastic $e-\tau$ and $\mu-\tau$ conversion in the NA64 experiment at the CERN SPS

TL;DR

This paper investigates lepton flavor violating $e- au$ and $- au$ conversion in deep inelastic scattering at CERN, deriving new limits on LFV operators and assessing experimental observability in NA64.

Contribution

It provides a model-independent analysis of LFV $e- au$ and $- au$ conversion in DIS, establishing new limits on LFV operator scales and exploring experimental detection prospects.

Findings

Derived lower limits on LFV operator mass scales $Lambda_k$.

Identified potential for new limits not previously reported.

Analyzed energy spectrum and experimental viability for NA64.

Abstract

We study the Lepton Flavor Violating (LFV) $e(\mu)-\tau$ conversion in Deep Inelastic Scattering (DIS) of electron (muon) on fixed-target nuclei. Our model-independent analysis is based on the set of the low-energy effective four-fermion LFV operators composed of leptons and quarks with the corresponding mass scales $\Lambda_{k}$ for each operator. Using the estimated sensitivity of the search for this LFV process in events with large missing energy in the NA64 experiment at the CERN SPS, we derive lower limits for $\Lambda_{k}$ and compared them with the corresponding limits existing in the literature. We show that the DIS $e(\mu)-\tau$ conversion is able to provide a plenty of new limits as yet non-existing in the literature. We also analyzed the energy spectrum of the final-state $\tau$ and discussed viability of the observation of this process in the NA64 experiment and ones akin to it. The case of polarized beams and targets is also discussed.