Muon Collider Physics Summary

TL;DR

This work surveys the physics potential of high-energy muon colliders, arguing that they uniquely combine multi-TeV energy with unprecedented measurement precision. It demonstrates both direct reach for new electroweak states and indirect sensitivity to 100 TeV-scale physics via high-energy measurements and vector-boson fusion, highlighting Higgs physics and Higgs portal scenarios. The document also explores muon-specific opportunities related to lepton-flavor phenomena, the role of electroweak radiation, and the necessity of advanced detector technologies to mitigate beam-induced backgrounds. Together, these elements present a compelling case for a muon collider program as a complementary and potentially superior pathway to future discoveries compared to proton colliders, while outlining the substantial technical challenges and research directions required to realize this vision.

Abstract

The perspective of designing muon colliders with high energy and luminosity, which is being investigated by the International Muon Collider Collaboration, has triggered a growing interest in their physics reach. We present a concise summary of the muon colliders potential to explore new physics, leveraging on the unique possibility of combining high available energy with very precise measurements.

Figures (7)

• Figure 1: Equivalent proton collider energy. The left plot Delahaye:2019omf, assumes that $qq$ and $gg$ partonic initial states both contribute to the production. In the orange and blue lines, $\beta=1$ and $\beta=10$, respectively. In the right panel AlAli:2021let, production from $qq$ and from $gg$ are considered separately.
• Figure 2: Left panel: the number of expected events (from Ref. Buttazzo:2020uzc, see also Costantini:2020stv) at a $10$ TeV muon collider, with $10$ ab$^{-1}$ luminosity, for several BSM particles. Right panel: $95\%$ CL mass reach, from Ref. EuropeanStrategyforParticlePhysicsPreparatoryGroup:2019qin, at the HL-LHC (solid bars) and at the FCC-hh (shaded bars). The tentative discovery reach of a 10, 14 and 30 TeV muon collider are reported as horizontal lines.
• Figure 3: Left panel: exclusion and discovery mass reach on Higgsino and Wino Dark Matter candidates at muon colliders from disappearing tracks, and at other facilities. The plot is adapted from Ref. Capdevilla:2021fmj. Right: exclusion contour AlAli:2021let for a scalar singlet of mass $m_\phi$ mixed with the Higgs boson with strength $\sin\gamma$
• Figure 4: Left panel: schematic representation of vector boson fusion or scattering processes. The collinear $V$ bosons emitted from the muons participate to a process with hardness $\sqrt{\hat{s}}\ll E_{\rm{cm}}$. Right panel: number of expected events for selected SM processes at a muon collider with variable $E_{\rm{cm}}$ and luminosity scaling as in eq. (\ref{['lums']}).
• Figure 5: Left panel: $1\sigma$ sensitivities (in %) from a 10-parameter fit in the $\kappa$-framework at a $10$ TeV muon collider with $10$ ab$^{-1}$MuonCollider:2022xlm, compared with HL-LHC. The effect of measurements from a $250$ GeV $e^+e^-$ Higgs factory is also reported. Right panel: sensitivity to $\delta\kappa_\lambda$ for different $E_{\rm{cm}}$. The luminosity is as in eq. (\ref{['lums']}) for all energies, apart from $E_{\rm{cm}}\space=\space3$ TeV, where doubled luminosity (of 1.8 ab$^{-1}$) is assumed MuonCollider:2022xlm.