Searching for UFOs from the early universe: direct detection prospects for relativistically decoupling dark matter

TL;DR

This paper evaluates the direct detection prospects of ultrarelativistically decoupling dark matter candidates, showing current experiments have already constrained significant parameter space and future experiments will probe more.

Contribution

It introduces a detailed analysis of UFO dark matter detection prospects, highlighting their potential visibility in upcoming experiments and comparing them to other candidates.

Findings

Current experiments exclude large UFO parameter space for 0.4 GeV to 1 TeV masses.

SuperCDMS SNOLAB will probe the 0.5-10 GeV mass range in the near future.

UFOs are more detectable than freeze-in candidates for heavy portal interactions.

Abstract

Particles that decouple relativistically from the Standard Model bath during reheating represent a versatile class of well-motivated cold dark matter candidates. In fact, ultrarelativistic decoupling ($T_{\rm FO}\gg m_\chi$) is quite generic for beyond the Standard Model (BSM) heavy portal interactions with strong couplings and relatively low reheating temperatures. In this work, we study the direct detection prospects for ultrarelativistically frozen-out (UFO) candidates, using $Z'$-portal dark matter as a case study. Although typical UFO cross sections are suppressed by a heavy mediator mass scale, we find that experiments such as LZ, XENONnT, PandaX, and DarkSide-50 have already excluded a large portion of the UFO parameter space and there remains viable space above the neutrino fog for $0.4 \text{ GeV} \lesssim m_{\rm DM}\lesssim 1$ TeV. Moreover, SuperCDMS SNOLAB, which is expected to begin collecting data in 2026, should access a large region of UFO parameter space in the 0.5-10 GeV mass range. For heavy BSM portal interactions ($M\gtrsim 1$ TeV), UFOs are typically more accessible to detection than freeze-in candidates due to the comparatively larger cross sections. We also carefully delineate regions of parameter space with degeneracy between UFO and non-relativistic freeze-out. In sum, UFOs are attractive candidates for ongoing and next-generation dark matter detection experiments in a looming post-WIMP era.