$W$-Boson Exotic Decay into Three Charged Leptons at the LHC

TL;DR

This paper explores the potential to detect and measure the rare exotic decay of the W boson into three charged leptons and a neutrino at the LHC, which could provide insights into Standard Model extensions.

Contribution

It demonstrates that the tiny branching ratio of this exotic decay can be measured with sub-percent precision at the HL-LHC, offering a new way to constrain new physics models.

Findings

Measurement of the decay with sub-percent precision is feasible.

Current bounds on the $L_-L_$ model can be significantly improved.

The decay channel can serve as a probe for Standard Model extensions.

Abstract

We investigate the $W$ boson's exotic decay channel, $W \rightarrow \ell\ell\ell \nu$, at the LHC. Although the four-body final states suppress the decay branching ratio, the large production of $W$ bosons makes detecting and precisely measuring this decay probability entirely feasible. Our simulation study indicates that this tiny branching ratio can be measured with sub-percent precision at the HL-LHC. This decay channel can also constrain Standard Model extensions. Using the $ L_\mu-L_\tau$ model as a benchmark, we find that the current bound on the gauge coupling for $Z'$ mass in the range of $[4,75]$ GeV can significantly improve.