Exploring Scalar Leptoquarks at Muon Collider via Indirect Signatures and Right-Handed Neutrino-Assisted Decays

Abstract

Scalar leptoquarks (sLQs) appear in a wide range of ultraviolet-motivated extensions of the Standard Model and provide a natural link between the quark and lepton sectors. In this work, we investigate the discovery potential of an sLQ doublet $\widetilde{R}_2(\mathbf{3},\mathbf{2},1/6)$ that couples to light quarks and right-handed neutrinos (RHNs) at the proposed muon collider. We analyze both indirect probes arising from $t$-channel sLQ exchange that affects the high-$p_T$ behavior of dijet spectra and direct searches exploiting pair and single production of the sLQs, incorporating the full interplay of kinematic thresholds and decay topologies. We find that indirect probes at muon colliders deliver remarkably robust sensitivity to the sLQ-quark-muon coupling over a broad mass range. Assuming a sub-$\mathcal{O}(1)$ Yukawa coupling, we achieve a $5σ$ sensitivity up to sLQ masses $\sim 4.0$ TeV ($7.0$ TeV) at $\sqrt{s}=5$ (10) TeV center-of-mass energy with $\mathcal{L}=3~\mathrm{ab}^{-1}$ ($10~\mathrm{ab}^{-1}$) integrated luminosity. Direct production channels provide complementary reach: pair production dominates below threshold, while single production, driven by the sLQ-quark-muon/RHN interaction, decisively extends the mass reach well into the multi-TeV regime. We demonstrate that with $\mathcal{O}(1)$ Yukawa couplings, the single production channel can probe sLQ masses up to $3.0$ TeV ($6.0$ TeV) for $\sqrt{s}=5$ TeV ($10$ TeV). Together, these channels enable a unified exploration of parameter space far beyond the projected capabilities of the HL-LHC, including regions where conventional charged-lepton signatures are subdominant.

Figures (11)

• Figure 1: Representative Feynman diagrams for various decay modes of $\widetilde{R}^{2/3}_2(\widetilde{R}^{-1/3}_2) \rightarrow N u(Nd)$ and $\widetilde{R}^{2/3}_2(\widetilde{R}^{-1/3}_2) \rightarrow \mu d (\nu_{\mu} d)$ are shown in Figs. (a) and (b), respectively. In Figs. (c) and (d), we show the different decay modes of $N$. $N$ can decay to muon/neutrino and light quarks via an off-shell $\widetilde{R}_2$ or $W$ boson. In addition, $N$ can also decay to charged leptons and neutrinos. There are similar decay modes of $N$ mediated by $Z$ and $H$ bosons which we do not show here. However, all decay modes have been considered in the computation of BRs.
• Figure 2: (a) BRs of $\widetilde{R}^{2/3}_2 \rightarrow \mu d$ and $\widetilde{R}^{2/3}_2\rightarrow N u$ as functions of the coupling $Y_{12}$, evaluated for three benchmark values of $Z_{11}$ ($Z_{11}=0.2, 0.5, 1.0$), keeping $M_N = 50$ GeV and $M_{\widetilde{R}^{2/3}_2} = 1.0$ TeV. Panels (b), (c), and (d) show the BRs of different decay modes of $N$ for $M_N = 50~\text{GeV}$, $500~\text{GeV}$, and $2.0~\text{TeV}$, respectively, as functions of the mass of $\widetilde{R}_2$, for three benchmark values of the active–sterile mixing angle $V_{\mu N}$ ($V_{\mu N} = 1.0$, $10^{-3}$, and $10^{-6}$).
• Figure 3: Variation of sLQ pair production cross-section times ${\rm BR}^2$ versus sLQ mass. Panels (a) and (b) show the results for $\mu\mu jj$ and $\mu\nu jj$ channels, respectively. The observed limits are indicated by the solid black lines, from the ATLAS search for $pp \to \mu \mu jj$ATLAS:2020dsk and from the CMS search for $\mu\nu jj$CMS:2018lab, respectively. The blue and red lines (dotted and solid) represent the theoretical cross-sections times ${\rm BR}^2$ for various benchmark values of $M_N$, $Y_{12}$ and $Z_{11}$.
• Figure 4: Representative Feynman diagrams illustrating the pair and single production of $\widetilde{R}_2$. The last diagram represents dijet production at a muon collider, mediated via an sLQ.
• Figure 5: Variation of cross-section for different $\widetilde{R}_2$ production modes as a function of $M_{\widetilde{R}_2}$. Left and right panel correspond to the muon collider C.O.M. energy $\sqrt{s} = 5$ TeV and $10$ TeV, respectively. The Yukawa couplings are set to $Z_{11}=Y_{12}=1$. We explain the labels in detail in the text.