Muonphilic asymmetric dark matter at a future muon collider

TL;DR

This work investigates muonphilic portals to fermionic asymmetric dark matter (ADM) via both effective field theory (EFT) operators and two UV completions based on gauged $U(1)_{L_-L_ au}$, distinguishing vector and axial couplings. It enforces the ADM condition that the symmetric relic component is subdominant and combines this with direct-detection, collider, neutrino-trident, and muon $g-2$ constraints to map viable parameter spaces. It then projects the sensitivity of future muon colliders at $\sqrt{s}=3$ TeV and $10$ TeV with $1~\mathrm{ab}^{-1}$, focusing on the mono-photon channel $\mu^+\mu^- \to \chi\bar{\chi}\gamma$, and includes neutron-star heating considerations for EFT scenarios. The main findings show that the vector $L_-L_ au$ model is highly constrained and near-resonant regions are finely tuned, while the axial model allows larger viable regions at higher DM masses that could be probed by a muon collider, illustrating the complementary roles of collider and astrophysical probes in exploring muonphilic ADM.

Abstract

We explore phenomenological constraints on, and future muon collider sensitivities to, the parameter spaces of various muonphilic portals to fermionic asymmetric dark matter (ADM). Both WEFT-level dimension-6 effective operators and two UV models based on gauged $L_μ- L_τ$ are considered. One of the latter features a vector coupling to the dark matter and the other an axial vector coupling. The ADM criterion that at least $99\%$ of the dark matter relic density is asymmetric is also imposed. We identify which of these scenarios are currently allowed by direct detection and collider constraints, and then determine how much more of the parameter space could be probed by 3 and 10 TeV muon colliders with 1 ab$^{-1}$ of data. For the UV models, the constraints from $g-2$ of the muon are included. The future sensitivity curves due to neutron star heating considerations are also depicted. We present results for both the few-GeV dark matter mass regime motivated by ADM approaches to the $Ω_b \simeq Ω_\text{DM}/5$ coincidence problem, and for larger masses in the context of more general ADM.

Figures (9)

• Figure 1: A few representative Feynman diagrams of the mono-photon production process in the SM (left), UV-completions with $Z^\prime$ mediator (centre), and EFT (right) at a muon collider. The blobs represents new physics interactions.
• Figure 2: Distribution of the photon energy fraction in mono-photon events. The left panel corresponds to the EFT case for $\mathcal{O}_{vv}$, while the right panel displays the vector-like UV model. For the EFT case we choose the benchmark values $\rm \Lambda=1~TeV$ and $m_\chi=5$ GeV, while for the UV case we use $g_\chi=1$, $g^{\prime}=1$, $m_\chi=5$ GeV and $m_{Z^\prime}=10$ GeV.
• Figure 3: Limits on $\Lambda$ for $\mathcal{O}_{ss}\equiv \bar{\mu}\mu\,\bar{\chi}\chi$ (left) and $\mathcal{O}_{sp}\equiv\bar{\mu}\mu\,\bar{\chi}\gamma^5\chi$ (right) type interaction of DM with muons from different experimental observations and the ADM condition of Eq. \ref{['eq:constraint']}, labeled on the respective regions with darker shades. The striped green and yellow region corresponds to the muon collider (MuC) 3$\sigma$ reach for $\sqrt{s}=3$ and 10 TeV respectively, with $\mathcal{L}=1~\text{ab}^{-1}$. DD corresponds to limits from DM-nucleon scattering experiments PandaX:2022xqxPandaX:2023xglLZ:2024zvoPICO:2019vsc. The experimental exclusions/reach extends up to the bottom of the plots and overlapping regions are implicit. For the muon collider, partial-wave unitarity of $\mu^+\mu^- \to \chi\bar{\chi}$ implies that the EFT description is not reliable for $\Lambda \lesssim 0.4~\text{TeV}$ at $\sqrt{s}=3~\text{TeV}$ and $\Lambda \lesssim 1.4~\text{TeV}$ at $\sqrt{s}=10~\text{TeV}$ in the light-DM limit (up to $\mathcal{O}(1)$ Lorentz-structure factors), and the corresponding unitarity bound drops following the trend of the muon-collider sensitivity at $m_\chi$ close to $\sqrt{s}$. The neutron star sensitivity regions are as discussed in Sec. \ref{['sec:NS']}.
• Figure 4: Similar to Figure \ref{['fig:EFT_exclusion1']} for $\mathcal{O}_{ps}\equiv \bar{\mu}\gamma^5\mu\bar{\chi}\chi$ (left) and $\mathcal{O}_{pp}\equiv\bar{\mu}\gamma^5\mu\,\bar{\chi}\gamma^5\chi$ (right) type interaction.
• Figure 5: Similar to Figure \ref{['fig:EFT_exclusion1']} for $\mathcal{O}_{vv}\equiv \bar{\mu}\gamma_\alpha\mu\,\bar{\chi}\gamma^\alpha\chi$ (left) and $\mathcal{O}_{va}\equiv\bar{\mu}\gamma_\alpha\mu\,\bar{\chi}\gamma^\alpha\gamma^5\chi$ (right) type interaction.