A search for an excited muon decaying to a muon and two jets in $pp$ collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

TL;DR

This paper reports a search for excited muons decaying into a muon and two jets using 8 TeV proton-proton collision data from ATLAS, setting new limits on their mass and production cross section.

Contribution

It introduces a novel search channel for excited muons via contact interactions, extending the mass exclusion limits beyond previous gauge-mediated decay searches.

Findings

No evidence of excited muons was observed.

Excluded excited muons with masses below 2.9 TeV.

Set upper limits on cross section times branching ratio between 0.6 and 1 fb.

Abstract

A new search signature for excited leptons is explored. Excited muons are sought in the channel $pp \to \mu\mu^* \to \mu \mu\textrm{ jet jet}$, assuming both the production and decay occur via a contact interaction. The analysis is based on 20.3 fb$^{-1}$ of $pp$ collision data at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV taken with the ATLAS detector at the Large Hadron Collider. No evidence of excited muons is found, and limits are set at the 95% confidence level on the cross section times branching ratio as a function of the excited-muon mass $m_{\mu^*}$. For $m_{\mu^*}$ between 1.3 TeV and 3.0 TeV, the upper limit on $\sigma B(\mu^* \to \mu q \bar{q}$) is between 0.6 and 1 fb. Limits on $\sigma B$ are converted to lower bounds on the compositeness scale $\Lambda$. In the limiting case $\Lambda = m_{\mu^*}$, excited muons with a mass below 2.9 TeV are excluded. With the same model assumptions, these limits at larger $\mu^*$ masses improve upon previous limits from traditional searches based on the gauge-mediated decay $\mu^* \to \mu \gamma$.