Terrestrial and Solar Limits on Long-Lived Particles in a Dark Sector

TL;DR

The paper analyzes light long-lived particles from a GeV-scale dark sector coupled to the SM via gauge kinetic mixing. It identifies two robust constraint channels—terrestrial beam-dump searches and solar DM annihilation signals—that probe LLP lifetimes from $10^{1}$ cm to $10^{15}$ cm. It emphasizes a Higgs-like dark scalar benchmark to illustrate how CHARM and solar gamma/electron observations carve out allowed regions in parameter space, and it proposes new searches such as displaced di-muons at B-factories, high-intensity beam dumps, precise solar gamma measurements, and milli-charge tests to cover remaining space.

Abstract

Dark matter charged under a new gauge sector, as motivated by recent data, suggests a rich GeV-scale "dark sector" weakly coupled to the Standard Model by gauge kinetic mixing. The new gauge bosons can decay to Standard Model leptons, but this mode is suppressed if decays into lighter dark sector particles are kinematically allowed. These particles in turn typically have macroscopic decay lifetimes that are constrained by two classes of experiments, which we discuss. Lifetimes of 10 cm < c tau < 10^8 cm are constrained by existing terrestrial beam-dump experiments. If, in addition, dark matter captured in the Sun (or Earth) annihilates into these particles, lifetimes up to 10^15 cm are constrained by solar observations. These bounds span fourteen orders of magnitude in lifetime, but they are not exhaustive. Accordingly, we identify promising new directions for experiments including searches for displaced di-muons in B-factories, studies at high-energy and -intensity proton beam dumps, precision gamma-ray and electronic measurements of the Sun, and milli-charge searches re-analyzed in this new context.

Figures (9)

• Figure 1: A pictorial summary of the two similar approaches discussed in this paper: A strong source, either a beam dump or dark matter annihilation in the Sun, produces dark sector particles. LLP's can penetrate the shield, either the Sun's interior or the beam dump shielding. Decays downstream over a baseline, $L_{\hbox{Baseline}}$, can be detected.
• Figure 2: Feynman diagrams for (a) $A'$ decay into a dark-sector higgs and (b) higgs$'$-strahlung process.
• Figure 3: Limits on $\epsilon^2 Br(X\rightarrow \ell^+\ell^-)$, as a function of decay length assuming resonant production of a vector $A'$, with subsequent decay to $X$. From bottom to top, curves correspond to ten events expected (none were observed) for $m_{A'}=$ 0.6, 1, 2, 3, 4 GeV and $m_{h_D} = 0.4 m_{A'}$. The regions above the curves are excluded. The gray band represents the expected band for the scalar model of Section \ref{['sec:Example']}, with the upper and lower lines corresponding to $(m_{A'},m_{h_D})=(0.6, 0.24)$ GeV and $(4,1.6)$ GeV, respectively. The width of the band comes predominantly from the opening of kaon decay channel at higher masses.
• Figure 4: Limits on $\epsilon^2 Br(X\rightarrow \ell^+\ell^-)$ as a function of decay length, assuming radiation of $X$ off an off-shell $A'$ as in the higgs$'$-strahlung process of Batell:2009yf. From bottom to top, curves correspond to ten events expected (none were observed) for $m_{A'}=$ 0.6, 1, 2, 3, 4 GeV and $m_{h_D} = 0.4 m_{A'}$. The regions above the curves are excluded. The gray band represents the expected band for the scalar model of Section \ref{['sec:orientation']}, with the upper and lower lines corresponding to $(m_{A'},m_{h_D})=(0.6, 0.24)$ GeV and $(4,1.6)$ GeV respectively. The width of the band comes predominantly from the opening of kaon decay channel at higher masses.
• Figure 5: The inelastic capture rate of DM in the Sun against the DM mass. The black (solid) curve correspond to an inelastic model with $\delta =125\,\mathrm{keV}$ and $\sigma_{n\chi}=10^{-39}~{\rm ~\mathrm{cm}^2}$, the blue (dashed) curve to $\sigma_{n\chi}=10^{-40}~{\rm ~\mathrm{cm}^2}$, and the purple (dash-dot) curve to $\sigma_{n\chi}=10^{-41}~{\rm ~\mathrm{cm}^2}$.