Heavy Neutral Lepton at Same-Sign Muon Collider

TL;DR

The paper investigates heavy neutral leptons (HNLs) at a future same-sign muon collider, μTRISTAN, by analyzing two complementary signatures: lepton-flavor-violating (LFV) $\mu^+\mu^+ \to W^+\tau^+\bar{\nu}_\mu$ and lepton-number-violating (LNV) $\mu^+\mu^+ \to W^+W^+$. Employing a phenomenological HNL framework with active-sterile mixing $U_{\ell j}$ and Majorana mass $M_N$, the authors derive cross-section dependences $\sigma_{W^+\tau^+\bar{\nu}_\mu} \propto |\sum_I U_{\mu I}U_{\tau I}^*|^2$ and $\sigma_{W^+W^+} \propto |\sum_I U_{\mu I}^2|^2$, including high-energy scaling and a peak around $M_N \approx 0.74\sqrt{s}$. Monte Carlo simulations with $\sqrt{s}=10$ TeV and $\mathcal{L}=1~\mathrm{ab}^{-1}$ indicate that μTRISTAN can surpass current EWPO bounds on the mixing parameters over broad mass ranges, with the LFV channel primarily constraining the flavor structure and the LNV channel testing Majorana-induced lepton-number violation. The study highlights the complementarity of the two channels and points to future improvements from multivariate analyses and beam polarization. Overall, μTRISTAN would significantly extend the sensitivity to HNLs and offer a decisive test of whether neutrinos are Majorana particles.

Abstract

We explore the discovery potential of heavy neutral leptons (HNLs), motivated by models addressing the origin of neutrino masses, at the proposed high-energy same-sign muon collider known as $μ$TRISTAN. The study focuses on two complementary HNL-mediated signatures: (i) the lepton-flavor-violating (LFV) channel $μ^+μ^+ \to W^+τ^+\barν_μ$ and (ii) the lepton-number-violating (LNV) channel $μ^{+}μ^{+} \to W^{+}W^{+}$. The LNV process is the muon analogue of inverse neutrinoless double beta decay and, if observed, would provide strong evidence for Majorana neutrinos, while the LFV process offers a novel probe of flavor-changing neutral currents in the lepton sector. At the $μ$TRISTAN collider with $\sqrt{s} \sim \mathcal{O}(10)~\text{TeV}$, the resulting sensitivity to the HNL mixing with muon and tau neutrinos, as a function of mass, can surpass current bounds from the measurements of electroweak precision observables over a broad mass range. In particular, for the mixing with muon neutrinos, the collider bound can improve by an order of magnitude for $5$-$10$ TeV HNLs.

Figures (5)

• Figure 1: Feynman diagrams for the signal processes: the LFV process $\mu^+\mu^+\to W^+ \tau^+ \bar{\nu}_\mu$ (left) and the LNV process $\mu^+\mu^+\to W^+ W^+$ (right).
• Figure 2: The cross sections for $\mu^+\mu^+\to W^+\tau^+\bar{\nu}_\mu$ (blue dashed) and $\mu^+\mu^+\to W^+W^+$ (red) at $\sqrt{s}=10\,{\rm TeV}$, with $|\sum_{I=1}^{\tilde{n}} U_{\mu I}U_{\tau I}^*|=|\sum_{I=1}^{\tilde{n}} U_{\mu I}^2|=1$.
• Figure 3: Distributions of various kinematic observables for the individual SM background processes and the LFV signal channel corresponding to $|\sum_{I=1}^{\tilde{n}} U_{\mu I}U_{\tau I}^*| = 6.3\times10^{-4}$. The panels correspond to the observables (top-left) $P_T^{\tau \, \text{jet}}$, (top-right) $M_{\text{tot}}$, (bottom-left) $P_T^{\text{non-}\tau \, \text{jet}}$, and (bottom-right) $M_{\text{non-}\tau \, \text{jet}}$, all in GeV. The distribution of $M_{\text{non-}\tau \, \text{jet}}$ is shown after applying the cuts for $p^{\tau-\text{jet}}$, $M_{\text{tot}}$ and $p_T^{\text{non-}\tau\text{-jet}}$.
• Figure 4: Distributions of various kinematic observables (top) $P_T^{\text{jet1}}$, (bottom-left) $P_T^{\text{jet2}}$ and (bottom-right) $M_{\text{tot}}$, all in GeV, for the individual SM background processes and the LNV signal channel corresponding to $|\sum_{I=1}^{\tilde{n}}U_{\mu I}^2| = 6.5\times10^{-5}$, $M_N=1$ TeV.
• Figure 5: The projected sensitivity (blue lines) in the $M_N-|\sum_{I=1}^{\tilde{n}} U_{\mu I}U_{\tau I}|$ (left) and $M_N-|\sum_{I=1}^{\tilde{n}} U_{\mu I}^2|$ (right) planes. The current exclusion limit from the measurements of the EWPOs is indicated by the black dashed line.