Heavy Neutral Leptons in Gauged $U(1)_{L_μ-L_τ}$ at Muon Collider

Abstract

Heavy neutral leptons $N$ are the most appealing candidates to generate the tiny neutrino masses. In this paper, we study the signature of heavy neutral leptons in gauged $U(1)_{L_μ-L_τ}$ at a muon collider. Charged under the $U(1)_{L_μ-L_τ}$ symmetry, the heavy neutral leptons can be pair produced via the new gauge boson $Z'$ at muon collider as $μ^+μ^-\to Z^{\prime *}\to NN$ and $μ^+μ^-\to Z^{\prime (*)} γ\to NNγ$. We then perform a detailed analysis on the lepton number violation signature $μ^+μ^-\to NN\to μ^\pmμ^\pm W^\mp W^\mp$ and $μ^+μ^-\to NN γ\to μ^\pmμ^\pm W^\mp W^\mp γ$ at the 3 TeV muon collider, where the hadronic decays of $W$ boson are treated as fat-jets $J$. These lepton number violation signatures have quite clean backgrounds at the muon collider. Our simulation shows that a wide range of viable parameter space is within the reach of the 3 TeV muon collider. For instance, with new gauge coupling $g'=0.6$ and an integrated luminosity of 1000 fb$^{-1}$, the $μ^\pmμ^\pm JJ$ signal could probe $m_{Z'}\lesssim 12.5$ TeV. Meanwhile, if the gauge boson mass satisfies $2 m_N<m_{Z'}<\sqrt{s}$, the $μ^\pmμ^\pm JJγ$ signature would be more promising than the $μ^\pmμ^\pm JJ$ signature.