Vector-like lepton searches at a muon collider in the context of the 4321 model

The 4321 model [1−5] is an ultraviolet-complete model that extends the standard model (SM) gauge groups to a larger ${S U}(4) \times {S U}(3)' \times {S U}(2)_L \times {U}(1)'$ group. It is motivated by recent measurements ofBhadron decays that are in tension [1] with the SM and can provide a combined explanation for multiple anomalies observed inbhadron decays, which point to lepton flavor nonuniversality. This model also provides a novel and interesting final state topology to search for at colliders, with multi-jets and multi-tau leptons. A search for pair production of the lightest new particles in this model, such as vector-like leptons (VLLs), was recently performed [6] at the CMS experiment based on data collected in 2017 and 2018 corresponding to a total integrated luminosity of 96.5 ${ \mathrm{fb^{-1}}}$ . Interestingly, a mild excess has been found at the level of 2.8 standard deviations for a representative VLL mass point of 600 GeV [6]. In this study, we perform a similar investigation on a future TeV-scale muon collider [7], which should have great potential to explore VLLs and other predicted new particles in the 4321 model to a large extent.

A muon-muon collider with a center-of-mass energy at the multi-TeV scale has recently received much-revived interest [7] and has several advantages over both hadron-hadron and electron–electron colliders [8−10]. Because massive muons emit considerably less synchrotron radiation than electron beams, muons can be accelerated in a circular collider to higher energies with a much smaller circumference. Moreover, because the proton is a composite particle, muon-muon collisions are cleaner than proton-proton collisions and thus can lead to higher effective center-of-mass energies. However, owing to the short lifetime of the muon, the beam-induced background (BIB) from muon decays must be examined and properly reduced. Based on a realistic simulation at $ \sqrt{s}=1.5 $ TeV with BIB included, Ref. [11] found that the coupling between the Higgs boson and theb-quark can be measured at percent level with order