Probing Hidden Leptonic Scalar Portals using the NA64 Experiment at CERN

TL;DR

This paper explores NA64's potential to detect a new scalar boson mediating lepton-flavor-changing processes, which could shed light on dark sector physics and the muon g-2 anomaly.

Contribution

It develops a detailed Monte Carlo simulation for NA64 to assess its sensitivity to a new leptonic scalar portal involving $e ightarrow\mu$ transitions.

Findings

NA64 can probe parameter space relevant to muon g-2 anomaly

Simulation shows potential to detect scalar boson with $10^{11}$ EOT

Minor setup optimizations can significantly enhance sensitivity

Abstract

In this study, we demonstrate the potential of the NA64 experiment at CERN SPS to search for New Physics processes involving $e\rightarrow\mu$ transitions after the collision of 100 GeV electrons with target nuclei. A new Dark Sector leptonic portal in which a scalar boson $\varphi$ could be produced in the lepton-flavor-changing bremsstrahlung-like reaction, $eN\rightarrow \mu N\varphi$, is used as benchmark process. In this work, we develop a realistic Monte Carlo simulation of the NA64 experimental setup implementing the differential and total production cross-section computed at exact tree-level and applying the Weisz\"{a}cker-Williams phase space approximation. Using this framework, we investigate the main background sources and calculate the expected sensitivity of the experiment. The results indicate that with minor setup optimization, NA64 can probe a large fraction of the available parameter space compatible with the muon $g-2$ anomaly and the Dark Matter relic predictions in the context of a new Dark Sector leptonic portal with $10^{11}$ EOT. This result paves the way to the exploration of lepton-flavour-changing transitions in NA64.