Dark axion portal at $Z$ boson factories

Abstract

Dark axion portal connects an axion-like-particle (ALP), which can be the QCD axion, with the SM by the coupling to a photon and a dark photon, leading to a rich and distinct phenomenology related to dark matter, astrophysics, and cosmology. We note that due to the gauge invariance, the $Z$ boson-dark photon-ALP coupling should also be generated with sizable strength, and we study its phenomenological consequences at $Z$ boson factories: LEP, FCC-ee, and forward physics detectors at the LHC and FPF@FCC - FASER and MATHUSLA. Due to the large number of $Z$ bosons produced, DAP particles can be efficiently produced and detected by semi-visible displaced decays of the heavier DS species to $γ$+inv. or $f^+ f^-$+inv., and via missing energy signature with zero or one photon. Because of the complementarity of the two approaches, we find great prospects in exploring both short and long-lived lifetime regimes of DAP, especially when the heavier dark species has mass above $0.1\,$GeV.

Figures (4)

• Figure 1: Lifetime, $c\tau \,[$m$]$, (left) and branching ratios (right) as functions of the LLP mass for the two mass hierarchies of DAP. The top panels correspond to $m_{\gamma'} \gg m_a$, while bottom panels illustrate $m_a \gg m_{\gamma'}$. In the left panels, we fixed $g_{a\gamma\gamma'}=1/$GeV, while the general case is obtained by rescaling by $1/g_{a\gamma\gamma'}^2$.
• Figure 2: Sensitivity plot for HNL dipole portal obtained in Zhang:2023nxyOvchynnikov:2023wgg (solid lines), which we used to validate our simulation setup. Whenever relevant, we recovered (dotted lines) each of the limits. The red, blue, and black solid lines indicate exclusion bounds set by null observations at LEP DELPHI:1996drfOPAL:1994kgwDELPHI:2003dlq, while orange and green lines indicate predicted sensitivity of FCC-ee FCC:2018byv due to displaced decays and $\gamma$+inv. signature, respectively.
• Figure 3: Lepton colliders sensitivity to DAP with massless ALP (left) and massless DP (right). LEP exclusion bounds are shown in red, blue and black, while future sensitivity of FCC is shown in pink, orange and green. The simulation is analogous to the one employed for HNL, cf. Fig. \ref{['fig:validation_1']}
• Figure 4: Combined lepton colliders, beam dumps, and LHC (top) or FPF@FCC (bottom) forward physics detectors sensitivity to DAP with massless ALP (left) and massless DP (right). The limits set by lepton colliders - LEP and FCC-ee - were already shown in Fig. \ref{['fig:result_1']}. The limits shown in legends of each plot are results of this work, while other limits were obtained in deNiverville:2018hrcJodlowski:2023yne. Since at the FPF@FCC the $Z\to a \gamma^\prime$ production mode is dominant, we have shown resulting limits by dotted lines separately from those obtained by vector meson decays. For clarity, we have omitted astrophysical and cosmological bounds Hook:2021ousKalashev:2018braHook:2023smgHong:2023fcy relevant for $m \lesssim 1\,\rm GeV$ and small values of $g_{a\gamma \gamma^\prime}$.