Muon collider experiments as electron/positron beam sources: case studies of new light-particle searches

Abstract

At muon colliders, muon decays naturally produce intense electrons and positrons with unique features, namely high energies, high repetition rates, and small intrinsic uncertainties, that are unavailable at existing accelerator facilities. We quantitatively study the feasibility of extracting such particles in two representative future muon collider designs, IMCC and $μ$TRISTAN. Using Monte Carlo simulations with the corresponding design parameters, we study the spatial, angular, and energy distributions of decay electrons and positrons in the curved sections of the collider ring. We find that typical deflections of $0.1-10~\mathrm{mrad}$ can be achieved even for high-energy electrons carrying large energy fractions ($\simeq 0.6 - 1.0$) of the muon beam energy, with the ring bending magnets (or magnets providing an equivalent field) effectively serving as a pre-septum magnet, that partially deflects the beam before the main septum magnet, suggesting that the extraction scheme could be practically feasible. Exploiting the distinct beam properties of IMCC and $μ$TRISTAN, we propose complementary search strategies, missing energy and momentum searches for dark matter at $μ$TRISTAN and visible-decay searches for axion-like particles and light scalars at IMCC, which probe parameter space beyond the reach of current and other proposed experiments.

Figures (10)

• Figure 1: Schematic illustration of the extraction region. Electrons produced from muon decays are bent by the magnetic field $B$ of the bending magnets (or magnets providing an equivalent magnetic field) installed along the curved section of the collider ring. The reference orbit denotes the stable muon trajectory in the absence of decay, and the extraction region has length $L_{\rm ext}$. At the downstream edge of the extraction region, we define the kinematic variables: the electron momentum $\vec{p}$ and the muon momentum in the absence of decay $\vec{p}_\mu$. The angle $\theta$ is defined, in the horizontal plane, as the angle between $\vec{p}$ and $\vec{p}_\mu$, and the momentum ratio is given by $r=p/p_\mu$ with $p=|\vec{p}|$ and $p_\mu=|\vec{p}_\mu|$. The transverse deviation from the reference orbit at the downstream edge is denoted by $(x, y)$, where $x$ is the horizontal displacement toward the curvature center and $y$ is the vertical displacement.
• Figure 2: Fluxes of particles $e^-$, $\bar{\nu}_e$, and $\nu_\mu$ produced by $\mu^-$ decays in the extraction region, obtained from Monte Carlo simulations using PHITS. The extraction region enclosed by black dotted lines corresponds to the same slice as in Fig. \ref{['fig:setup']}. For convenience, we newly introduce two axes, $x'$, and $z'$. A muon beam energy of 1.5 TeV and a magnetic field strength of 10 T are assumed, and $L_{\rm ext}=10$ m is taken as a representative extraction length. The gray region denotes the effective magnetic field region corresponding to that shown in Fig. \ref{['fig:setup']}.
• Figure 3: $x$-$y$ position distributions of decay products ($e^-$, $\bar{\nu}_e$, and $\nu_\mu$) at the downstream edge of the extraction region in Fig. \ref{['fig:setup']}, obtained via Monte Carlo simulations. We assume the same set of parameters as those in Fig. \ref{['fig:flux']}.
• Figure 4: Decay-electron probability distributions for the momentum ratio $r=p/p_\mu$ at $\theta = 0,\,3,\,6,\,9,\,12~\mathrm{mrad}$, obtained via Monte Carlo simulations. The same parameter settings as in Fig. \ref{['fig:flux']} are used.
• Figure 5: Schematic of the connection between the extraction region and the experimental area for new light particle searches. The electron/positron beams extracted from the region of Fig. \ref{['fig:setup']} are transferred to the experimental area, for example using septum magnets.